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Abstract

Reacting numerical simulations today are often based on either fitted global reaction schemes, comprised of a few empirical reactions, or pre-tabulated laminar flame solutions computed with detailed chemistry. Although both methods can accurately predict global quantities such as laminar flame speed and burnt gas composition, they have significant limitations. In particular, neither are able to directly and adequately describe the complexity of pollutant chemistry. In the context of reducing harmful emissions of the next generation of aeronautical combustors, however, including these needed additional kinetic details in combustion simulations is becoming essential. Direct integration of detailed chemistry in accurate turbulent combustion models is not a viable option in the foreseeable future, because of excessive computational demands and numerical stiffness. In this context, Analytically Reduced Chemistry (ARC) represents an attractive compromise between accuracy and efficiency, and is already employed in relatively complex Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES). ARCs are knowledge-based compact mechanisms retaining only the most relevant kinetic information as extracted directly, and without fitting, from detailed chemical models using specialized reduction techniques (important species identification through graph search, lumping of species with similar features, short-living species identification, etc.). In recent years, several multi-step efficient and automated reduction tools have been developed, enabling the easy generation of ARCs with minimum input and knowledge from the user. The main objective of this paper is to present a review of ARCs for fuels ranging from methane to aviation kerosene surrogates, recently derived with such a multi-step automated reduction tool: YARC. Information about the applicability and range of validity of each derived mechanism are given, along with further references. Each one was specifically derived to be convenient to use in CFD; in particular, the stiffness was regarded as a key factor and the final number of transported species never exceeds thirty. In a final section, the great potential of the methodology is illustrated in a multi-phase, reactive LES application where the fuel is a real multi-component transportation fuel. To that end, an ARC based on a Jet A described by the novel Hybrid Chemistry (HyChem) approach is coupled with the Dynamically Thickened Flame LES (DTFLES) model and directly integrated into the LES solver AVBP. A Lagrangian spray description is used. Results are compared to experimental data in terms of temperature and major species (CO2, H2O, CO, NO) mass fractions, leading to very satisfying results.

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... These QSS species are kept in the chemical scheme but they are not computed with a transport equation: their concentration is calculated from algebraic relations issuing from the condition of zero net chemical source term. The ARC approach has already shown promising results on diverse applications [13]. Also, compared to other chemistry reduction approaches such as neural networks [14,15] or virtual chemistry concept [16,17], ARC has the advantage to keep true reaction pathways. ...
... Also, compared to other chemistry reduction approaches such as neural networks [14,15] or virtual chemistry concept [16,17], ARC has the advantage to keep true reaction pathways. Thus, ARCs are known to keep a realistic description of the true chemistry [13,18,19], even if it usually means a higher cost compared to the other mentioned techniques. ARC schemes already exist in the literature for methane-air combustion [19,20]. ...
... They confirm that the flame is substantially stretched but stays far from quenching. Note that ARC remains valid for strain rates largely exceeding its range of derivation presented in Section 2: this is a known advantage of ARC as demonstrated in [13]. The two AVBP simulations give very same results, which are however different from the laminar CANTERA solution. ...
Article
Methane-oxygen burning is considered for many future rocket engines for practicality and cost reasons. As this combustion is slower than hydrogen-oxygen, flame ignition and stability may be more difficult to obtain. To address these questions, numerical simulation with realistic chemistry is appropriate. However the high pressure and turbulence intensity encountered in rocket engines enhance drastically the stiffness of methane oxy-combustion. In this work, Analytically Reduced Chemistry (ARC) is proposed for accurate chemistry description at a reasonable computational cost. An ARC scheme is specifically derived for typical rocket engine conditions. It is validated by comparison with its parent skeletal mechanism on a series of laminar flames. Then the numerical stiffness of chemistry is overcome with an original approach for time integration, allowing to run simulations close to the acoustic time step whatever the chemical stiffness. It is demonstrated on laminar cases that the flame structure is well preserved, and that numerical stability is ensured while decreasing significantly the computational cost. The performance of ARC with the fast time integration method is finally demonstrated in a 3D Large-Eddy Simulation of a lab-scale Liquid Rocket Engine combustion chamber, where a detailed flame analysis is conducted.
... Alternatively, reduced versions of these mechanism (the so-called skeletal mechanisms) can be used in LES, 16 but still with a relatively high computational cost. Analytically reduced mechanism (ARM) 17,18 allows reducing the computational cost by identifying the species that do not need to be transported with the flow. However, different from a skeletal mechanism, the production rate of the transported species in an ARM is not expressed as a combination of elementary reaction in the Arrhenius form but rather as a combination of complex analytical relations. ...
... However, different from a skeletal mechanism, the production rate of the transported species in an ARM is not expressed as a combination of elementary reaction in the Arrhenius form but rather as a combination of complex analytical relations. 17 This complexity, together with the number of transported species that could still be more than twenty, makes ARM not the first option for applications where lower computational cost is needed due to computational power limitations or a high number of conditions required to be simulated. In such cases, global mechanisms are an alternative that reduces the computational cost. ...
Article
Full-text available
The effects of hydrogen addition on the flame dynamics of a bluff-body stabilized methane-hydrogen turbulent flame is studied with large eddy simulation (LES). The LES are carried out with the thickened flame model and global kinetic mechanisms calibrated for the methane-hydrogen mixtures. A data-based calibration of the global mechanisms is done with a methodology based on reproducing the net species production rates computed with a detailed kinetic mechanism. An improvement of this methodology is proposed to increase its accuracy and reliability. The calibrated mechanisms accurately describe the variation of the laminar flame speed and the thermal flame thickness with hydrogen addition and equivalence ratio in a freely propagating premixed flame. The variations of the consumption speed and the thermal flame thickness with strain rate in a symmetric counterflow premixed flame are also well predicted. The numerical simulations reproduce the transition from V to M-shape flame induced by hydrogen addition, and the axial distribution of the heat release agrees with the experimental measurements of OH chemiluminescence. The unit impulse response and the flame transfer function are computed from the LES data using system identification (SysID). The flame transfer functions show a remarkable agreement with the experimental data, demonstrating that the LES-SysID approach using properly calibrated global mechanisms can predict the response of turbulent methane-hydrogen flames to velocity fluctuations. A comparison of the unit impulse response for the various hydrogen additions is presented and the effect of hydrogen in the flow-flame interaction of the burner evaluated is discussed.
... It should be pointed out that in the framework of ignition, a more detailed description of chemistry should be preferred. In particular, reduction techniques, based on Articially Reduced Chemistry (ARC) [50] or virtual chemistry [51], can be employed since they include some radical species that are important in the first stages of the flame development. However, in the present study, the attention is focused on the light-round process and not specifically on the flame initiation. ...
... Ultimately, the simplified two step chemical description represents a further element to be improved. A study to investigate the effects of detailed chemistry would be interesting and advanced techniques such as ARC [50] or virtual chemistry [51] could be considered in this context. The combination of the dynamic closure with these improvements will be pursued in the next future to obtain a better agreement with the experiments. ...
Article
Ignition of annular gas turbine combustors, also called light-round, is a challenging problem for aeronautic reliability and safety. Large Eddy Simulation (LES), based on massively parallel computations, has been already assessed as a reliable tool to analyse and predict such a phenomenon. The present study focuses on the effect of the flame subgrid scale wrinkling factor entering the combustion model. Large Eddy Simulations of the annular MICCA-Spray laboratory-scale chamber are performed with constant and dynamic strategies for the wrinkling parameter and are compared against experimental data. A bi-modal distribution of the model exponent during light-round is obtained with a different behaviour between the flames stabilized around each injector, and the flame fronts propagating in a circumferential direction. The impact of such locally defined and time-dependent evolution is analysed by studying several quantities associated to the flame propagation mechanisms. It is shown that the dynamic procedure entails a reduction of the wrinkling factor that is not balanced by an increase of the resolved flame surface. After considering the exhaust of burnt gases that affects the ignition in such a semi-confined system, the dynamic formulation leads to a moderate slowing down of the flame. Surprisingly, the sounder dynamic model does not yield better predictions of the experimental data, revealing the need to address other modelling improvements in the future. However, this does not temper the need for a dynamic combustion model in numerical simulation of light-round.
... Tracking fuel combustion chemistry 'on-the-fly' (as opposed to tabulating it, for example) in Large Eddy Simulation (LES) is a computationally expensive prospect [8,9] and requires careful treatment to ensure accurate computations of trace species. When higher hydrocarbon (C2+) fuel is present, the number of species required for an accurate description of intermediate chemistry greatly increases. ...
... When higher hydrocarbon (C2+) fuel is present, the number of species required for an accurate description of intermediate chemistry greatly increases. Smaller mechanisms also invariably require accepting some assumptions such as quasisteady-state [9] which may not always hold in low-temperature chemistry. A review of existing literature indicates that high fidelity simulations of autoignition are mainly restricted to DNS and LES in simplified setups. ...
Conference Paper
Full-text available
The minimization of autoignition risk is critical to the design of premixers of high power aeroderivative gas turbines as an increased use of highly reactive future fuels (for example, hydrogen or higher hydrocarbons) is anticipated. Safety factors based on ignition delays of homogeneous mixtures, are generally used to guide the choice of a residence time for a given premixer. However, autoignition chemistry at aeroderivative conditions is fast (0.5–2 milliseconds) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period necessarily involves the study of low-temperature autoignition precursor chemistry, but precursors can change with fuel and local reactivity. Chemical Explosive Modes are a natural alternative to study this as they can provide a measure of autoignition risk by considering the whole thermochemical state in the framework of an eigenvalue problem. When transport effects are included by coupling the evolution of the Chemical Explosive Modes to turbulence, it is possible to obtain a measure of spatial autoignition risk where both chemical (e.g. ignition delay) and aerodynamic (e.g. local residence time) influences are unified. In this article, we describe a method that couples Large Eddy Simulation to newly developed, reduced autoignition chemical kinetics to study autoignition precursors in an example pre-mixer representative of real life geometric complexity. A blend of pure methane and dimethyl ether (DME), a common fuel used for experimental autoignition studies, was transported using the reduced mechanism (38 species / 238 reactions) at engine conditions at increasing levels of DME concentration until exothermic autoignition kernels were formed. The resolution of species profiles was ensured by using a thickened flame model where dynamic thickening was carried out with a flame sensor modified to work with multi-stage heat release. The paper is outlined as follows: First, a reduced mechanism is constructed and validated for modeling methane as well as di-methyl ether (DME) autoignition. Second, sensitivity analysis is used to show the need for Chemical Explosive Modes. Third, the thickened flame model modifications are described and then applied to an example premixer at 25 bar / 890K preheat. The Chemical Explosive Mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentration and identifies where the premixer is sensitive and flame anchoring is likely to occur.
... This mechanism consists of seven species including CH 4 , O 2 , N 2 , H 2 O, CO 2 , CO, and NO. Regarding the nitrogen reaction, there are three generally accepted mechanisms for NO x formation such as thermal NO x [28][29][30], prompt NO x [28], and fuel NO x [31]. Since the fuel NO x is formed by the direct oxidation of organ nitrogen compounds in fuel, which is not important in natural gas oxidation; thus, in the present work, thermal and prompt mechanisms are considered for NO x formation. ...
... This mechanism consists of seven species including CH 4 , O 2 , N 2 , H 2 O, CO 2 , CO, and NO. Regarding the nitrogen reaction, there are three generally accepted mechanisms for NO x formation such as thermal NO x [28][29][30], prompt NO x [28], and fuel NO x [31]. Since the fuel NO x is formed by the direct oxidation of organ nitrogen compounds in fuel, which is not important in natural gas oxidation; thus, in the present work, thermal and prompt mechanisms are considered for NO x formation. ...
Article
In this study, the thermal performance of an unsteady, one-dimensional model, and irreversible of a CH4-air reacting process is investigated. Due to the fundamental differences between the free flame and porous structures such as flame thickness and the length of the preheated zone, it is necessary to employ a chemical kinetic which in accordance with the structure of a porous duct made of cellular ceramic, even if it is very simple. To achieve this aim, CH4 oxidation with the five-step reaction mechanism is considered to simulate the combustion phenomenon in the porous burner. In order to solve the governing equations, a finite volume method is used to discretize the governing conservation equations of the problem. Transient, displacement and diffusion terms were respectively solved using completely implicit schemes, Upwind, and discretized central difference and the resulting algebraic equations by the TDMA repeated procedure. In this research, the segregated method was used to solve the set of equations, in which the suitable convergence condition for the governing variables of the problem was used. After validation of the obtained results with available experimental data, the effects of the solid matrix porosity density, porosity, length, extinction coefficient as well as the effects of fire rate, equilibrium ratio, and inlet temperature are investigated. The results show any changes in solid matrix properties and inlet nificant changes in combustion structure.
... Finally, the wall heat flux is underestimated in the flame tip region. Future work will account for a refined mesh, as well as for a detailed chemistry [12] to improve this result. ...
Conference Paper
Full-text available
Liquid propellants rocket engines feature a liquid injection which atomises to feed the turbulent flame. This atomisation has a strong impact on the subsequent flame structure and should be accurately described in numerical models. To represent liquid injection and atomisation in a computationally-efficient way in 3D calculations, a diffuse interface method relying on the 3-equations multi-fluid model proposed in [24] and accounting for surface tension forces [18] has been implemented in the unstructured, parallel combustion code AVBP using Large Eddy Simulation (LES). This paper presents the application of the above injection and combustion model to LOx/GCH4 turbulent flames in rocket engines under realistic conditions. The considered configuration was experimentally studied at the Technical University of Munich (TUM) [37]. It consists in a square combustion chamber of 290 mm long and 12 mm side length, with co-axial injection. A preliminary study without combustion is first presented to demonstrate the robustness and the relevance of the diffuse interface model. A second simulation is then analysed, in which combustion is activated and described with the infinitely fast chemistry model (IFCM). Results are compared to both measurements and a previous simulation with a correlation-based injection model [2]. Good agreement is found between all cases and the improvement brought by the diffuse interface model is demonstrated.
... The ability to predict flame properties as well as the onset and characteristics of instabilities depends on the accuracy of the reaction models which are employed. The development of such reaction models is a subject of a number of resent studies [6,7]. ...
... Taking into account detailed kinetic schemes implies solving stiff systems of hundreds and thousands of differential equations, which makes calculations not feasible at present [32]. In this regard, implementing reduced chemical kinetics mechanisms proves to be an optimal and reliable method for achieving a balance between essential chemical representation and saving computational costs [10,33]. Conventionally, mechanism reduction is usually carried out based on a set of targets, such as ignition delay times (IDT), laminar burning velocities (LBV), burnt gas temperature, heat release rate and concentrations of the main products (CO, CO 2 and H 2 O). ...
Article
Full-text available
Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.
... The main objective of this thesis is to investigate on the capabilities of a semi-deterministic Lagrangian approach for soot prediction in terms of accuracy; and to confront this approach to the existing techniques in terms of computational cost. Another objective is to combine a detailed soot model to an Analytically Reduced Chemistry (ARC) [39,40], which ensures a good description of the flame structure and dynamics and species involved in the soot model considered. ...
Thesis
Full-text available
Lagrangian tracking of soot particles within the LES framework: development and implementation of lagrangian methods for high density particles flow in a multi-parallel compressible Navier-Stokes solver. Simulation of two aeronautical combustors: 1) DLR combustor FIRST burning gaseous C2H4 with high level of experimental characterization 2) UTIAS combustor burning liquid real fuel (Jet A) with soot measurements. Thesis deals with combustion, lagrangian formalism, soot particles, soot chemistry, finite-rate chemistry, lagrangian spray, radiative transfer in complex configuration.
... Fully detailed chemistry involving thousands of reactions and chemical species remain however out of reach and reduced mechanisms have been introduced [18]. Such mechanisms, like Analytically Reduced Chemical (ARC) schemes [19,20], contain 10 to 30 species and have been shown to accurately describe combustion phenomena and predict pollutant formation in realistic 3D configurations, as for example in [4,21] where NOx and CO were computed in a methane-air burner. The derivation of reduced schemes is more difficult for transportation fuels, which are complex blends of a large number of hydrocarbons where only average properties are known. ...
Preprint
Full-text available
This work uses Large Eddy Simulation (LES) combined with an accurate chemical description to predict soot formation in a turbulent spray flame burning real fuel at atmospheric conditions. Understanding and being able to predict soot formation in practical configurations burning complex liquid fuel is essential for the design of engines meeting present and future environmental requirements. The prediction of soot formation with numerical simulations has been mostly limited to academic configurations burning light gaseous fuel. This is explained by the numerical cost of (i) the fuel oxidation chemistry including soot precursors like Polycyclic Aromatic Hydrocarbons (PAH), and (ii) the modeling of two dispersed phases, i.e., the liquid fuel spray and the soot particles. In this work, an Analytically Reduced Chemistry (ARC) for real fuels is proposed to allow a direct integration of accurate combustion chemistry including PAH in the compressible LES solver AVBP. The ARC model is coupled with a Lagrangian particle tracking approach for both the fuel droplets and the soot, including for the latter the description of the complex physical and chemical processes driving the particle evolution. Validation is first performed in a one-dimensional ethylene/air flame configuration, experimentally studied in the literature at several operating points. The numerical profiles of both the soot volume fraction and the soot diameter are in good agreement with measurements. This allows to apply the LES methodology to the sooting swirled turbulent liquid JetA-1/air combustor measured at UTIAS. Very satisfying predictions for both the flow dynamics and the soot production are obtained. The analysis of the results brings valuable new insights on the interaction between fuel droplets, turbulent combustion, PAH and soot evolution in such complex flames.
... Variety methodologies in mechanism reduction have been developed in decades of research to minimize the mechanism sizes and stiffness. It can be characterized into roughly four primary groups based on its functions, including skeletal (Xue et al., 2020;Wu et al., 2020), lumping (Till et al., 2019;Brunialti et al., in press), time-scale analysis (Koniavitis et al., 2017;Chang et al., 2020), and stiffness reduction (Felden et al., 2019;Malpica Galassi et al., 2022), in which they employ the user-specified tolerance for regulating accuracy. Several public mechanisms are usually constructed in a restricted combustion condition, such as a flow regime, to obtain a more downsize, resulting in a narrow range of usability (Zhao et al., 2022). ...
Preprint
Full-text available
The main objective of this work is to obtain the reduced reaction mechanism, which is consistent with a benchmark case in modeling a 0-D ignition delay, 1-D laminar flame speed, and 2-D simulated flame result and spent less processing time. In achieving this, the ten reduced reaction mechanisms developed for methane combustion were assessed, whereas the GRI-Mech 3.0 is considered a Benchmark. The result showed that only a reaction mechanism named SK30 was satisfactory. Still, the processing time in simulating the simple 2-D of a premixed model at the microscale was overly substantial. Subsequently, SK30 was further lessened using the two reduction steps. Firstly, the automatic algorithm based on a direct relation graph with the error propagation aided sensitivity analysis using ignition delays as a criterion in automatic reduction was applied. By doing this, the accuracy of ignition delays was maintained, and the flame speed was distorted. Accordingly, sensitivity analysis was employed to obtain the influential reaction in the benchmarking mechanism in the second step. The significant species and reactions on flame speed but less in ignition delay, which was missing in the current development, were considered to retrieve back manually as few as possible. Finally, the novel mechanism consisting of 25 species 132 reactions was proposed for methane-air combustion. In validation, the 1-D flame speed and the 2-D premixed flame model were agreement with the benchmark model. In addition, the processing time of this reduced mechanism was 50% faster than the SK30.
... These reduced mechanisms often consist of one or two global reactions and a few species as derived by e.g., Jones and Lindstedt (1988) for hydrocarbon fuels. Other techniques can involve intrinsic low-dimensional manifolds (ILDM) (Maas and Pope, 1992), the flame prolongation of ILDM (known as "FPI", Gicquel et al. (2000)), flamelet-generated manifolds (FGM) (van Oijen and de Goey, 2000), the flame-progress variable model (FPV) (Pierce and Moin, 2004), in-situ adaptive tabulation (ISAT) (Pope, 1997), in-situ adaptive manifolds (ISAM) (Lacey et al., 2021), directed relation graph with error propagation methods (DRGEP) (Pepiot-Desjardins and Pitsch, 2008) or analytically reduced chemistries (ARC) (Felden et al., 2019). A recent review article with focus on approaches for LES can also be found in (Fiorina et al., 2015). ...
Thesis
The forthcoming transition in aviation burner technology towards renewable energy sources and reduced emissions requires aero-engine combustors to operate on increasingly cleaner fuels and new designs without compromising on safety.Engine restart in particular is of paramount importance, as its success must be ensured under a variety of operating conditions, which are specified by certification authorities. Restart scenarios involve (among others) quick relight in climb, or restart from windmilling at high altitude, equivalent to a large range of inflow temperatures, restart delay periods, and combustor wall temperatures.In quick relight, engine parts will remain at an elevated temperature as there is not enough time for the air flow to cause a noticeable cooling effect. Conversely, restart from windmilling at high altitude after extended delay periods is likely to be performed with substantially cooled combustor walls, impeding fuel evaporation and successful ignition. Academic test facilities which can emulate real engine conditions in terms of both temperature and pressure are extremely scarce due to their complexity and cost. However, the impact of wall temperatures on flame propagation during light-round, the final phase in a complex four-step process of forced ignition in annular combustors, has been revealed in a lab-scale setting: faster flame propagation and shorter light-round durations were observed experimentally at increased wall temperatures over ambient temperature walls.Despite its first order impact, the role of heat transfer was not fully clarified in a comprehensive analysis. Therefore, we study light-round ignition numerically and theoretically in the annular spray-flame combustor MICCA-Spray in two configurations to enhance the effect of heat transfer: (i) ambient temperature walls, approximating restart from windmilling, and (ii) preheated combustor walls, approximating quick relight. Large-Eddy Simulations are preformed in a unique setup including Lagrangian particle tracking for the polydisperse liquid fuel spray, a dynamic combustion model, and a novel tabulated wall model with a detailed description of thermophysical properties in the boundary layer.Predicted light-round durations agree remarkably well with experimental data.It is shown that the volumetric expansion of burnt gases induces a flow acceleration in azimuthal direction which constitutes the main driving mechanism of flame propagation in the first case. Droplet accumulations in the wake of swirling jets are generated ahead of the propagating flame fronts, which in turn cause a characteristic sawtooth propagation mode of the leading point.A cooling effect of the combustor walls on burnt gases is particularly pronounced downstream, diminishing the resulting flame propagation speed. For the second case, precursor Conjugate Heat Transfer simulations are carried out to obtain realistic wall temperature profiles in stationary operating conditions, which are not readily available from the experiment. These temperature profiles are subsequently imposed as boundary conditions for prefueling and final light-round simulations in preheated conditions. Results suggest that preheating diminishes the effect of the liquid phase, and enhances the azimuthal flow acceleration. Fresh gas preheating in the second case causes a substantial increase of the laminar flame speed over the first case, outweighing the observed decrease of the density ratio. These observations are supported by a theoretical analysis by means of a low-order model, capable of predicting average flame propagation speeds from LES data.It is also used to emphasize the importance of detailed modeling, and proves that all governing mechanisms must be accurately modeled in LES, which would otherwise be corrupted by compensating errors. The comprehensive analysis also clarifies the role of heat transfers during light-round.
... Introducing a sub-grid efficiency model to take into account the unresolved filtered flame front wrinkling, this approach then allows to solve unsteady turbulent flames taking into account heat losses, liquid fuels or dilution without further modeling. The TF approach may be combined to various descriptions of the flame structure, such as direct chemistry integration [26,27] , tabulated flamelets [28][29][30] or equilibrium [31] . It represents the flame-turbulence interaction with the resolved contribution and a subgrid-scale contribution in the form of an efficiency function, i.e., totally different from pdf-based or viscosity-based (ILES) methods. ...
Article
Modeling turbulent non-premixed combustion remains a challenge in the context of Large Eddy Simulation (LES) in complex geometries and for realistic conditions, taking into account all physical phenomena impacting the flame such as heat loss, dilution, or liquid fuel atomization and evaporation. In this work, the Thickened Flame concept, which allows to resolve the flame front on the LES grid while preserving the consumption speed, and initially derived for premixed combustion, is adapted to diffusion flames. It is demonstrated that the concept holds for these flames, with however, a different formulation of the model due to but their specific nature and properties. In particular, in the high-Damköhler regime, the thickening factor is applied only to the diffusion coefficients. The behavior of thickened diffusion flames is illustrated on laminar steady strained flames for both simple and complex chemistry, showing how the Thickened Flame concept applies. Based on these results, an expression for the thickening factor related to mesh coarsening is derived. For a complete turbulent combustion model, the thickening factor should also describe the sub-grid scale flame-turbulence interaction, which is left for future work.
... reduced chemistry (ARC) mechanism (Felden et al., 2018(Felden et al., , 2019. From a reduced synthetic paraffinic kerosene (SPK) mechanism (48 species and 416 reactions) offering the possibility to model lean premixed combustion of air and iso-octane, a reduction procedure was applied using the Yark tool (Pepiot-Desjardins and which includes skeletal reductions and quasi-steady state (QSS) approximations. ...
Thesis
Car manufacturers aim to develop new technologies for reducing the CO2 emissions of Spark Ignition (SI) engines as emission regulations get increasingly stringent. One of the solutions found by the car manufacturers is the downsizing concept applied to SI engines, which is to reduce the displacement of the engine while increasing the specific power. Nonetheless, this concept increases the occurrence of abnormal combustions, like knock and super-knock. So, to avoid such phenomena, the combustion is diluted through exhaust gas recirculation (EGR), the rate of which is increased from 5% to 20% or even 30%. However, this recirculation increases the combustion cycle-to-cycle variability. Nowadays, the car manufacturers rely on CFD tools for designing and optimizing SI engines. However, the current models of turbulent combustion lose their predictivity when they are used to simulate a highly diluted combustion involving high turbulent intensities. Indeed, the current models were built based on the assumptions of the flamelet regime. Yet, the combustion in a diluted boosted SI engine drifts from the flamelet regime to the thin reaction zone (TRZ) regime in a Peters diagram. Thus, a numerical study using the AVBP code is proposed in this Ph.D thesis to highlight the differences between flames in the flamelet regime and flames in the TRZ regime, the aim of which is to determine a combustion model suitable for combustion in the TRZ regime based on the formalism of the coherent flame model (CFM). Direct numerical simulations (DNS) of a premixed C8H18/air flame are conducted. First, a set of flame-vortex interactions is performed to investigate the modelling of the tangential strain rate through the definition of an efficiency function. Then, interactions between a planar flame and a three-dimensional turbulent field are studied considering a forced turbulent field using a spectral method. These simulations are analysed to investigate the validity of the flame surface density (FSD) concept in the TRZ regime through the analyses of the fame surface, the flame structure, the characteristics of the flame stretch and of the displacement speed in the TRZ regime, which are considered as key factors in the modelling of highly diluted turbulent combustion. First the reaction zone is shown to remains thin for each flame, leading to focus this study on a specific iso-surface in the reaction zone and how it is affected by turbulence. Second, the displacement speed on this iso-surface shows a differentiate dependency on tangential strain rate and curvature. This dependency is modelled through two effective Markstein lengths, which depend on both turbulence intensity and preferential diffusion. Models for these lengths with respect to the Karlovitz number are proposed. This analysis of the DNS yields an extension of the CFM model to TRZ regime through the definition of a new progress variable and using a fine-grained flame surface density.Then, the transport equations of both the new progress variable and the fine-grained FSD are closed with existing models and new ones for the source terms involved. Each sub-model is compared to the DNS results through a specific post-processing method, leading to adjustment of the proposed models. The model proposed is implemented in the AVBP code. Therefore, a new relationship between the filtered and the resolved progress variable is developed. Then, a set of one-dimensional turbulent flames is performed to evaluate the proposed model and the behaviour of the sub-models with each other. The results from these simulations are compared to the DNS through the previously proposed post-processing methodology. Adjustments of the models are deduced from this analysis. Finally, a discussion on the applicability of this model to engine application is drawn.To conclude, the encouraging results from a priori and a posteriori validation show the potential of the extension of the CFM model proposed in this thesis.
... Three reduction techniques are available in the literature to prevent prohibitive computational costs while preserving accuracy in turbulent flames. They are: (1) skeletal reduction (Lu and Law, 2005a;Pepiot-Desjardins and Pitsch, 2008); (2) optimization-based techniques including analytical reduced chemistry (ARC) (Felden et al., 2019) and the recent virtual chemistry (Cailler et al., 2017); (3) the tabulation techniques. The ARC techniques have been successfully applied in purely gaseous and spray flames (Jaravel et al., 2017(Jaravel et al., , 2018a n di nas o o t i n gfl a m e coupled with a C 2 H 2 -based model Gallen et al., 2018). ...
Thesis
Predicting soot production in industrial systems using a Large Eddy Simulation (LES) approach represents a great challenge for many reasons. First, in- depth knowledge and modeling of the complex physico-chemical processes involved in soot production are still lacking. Second, multiple combustion regimes coexist in technical devices, presenting an additional difficulty for high fidelity simulations. Third, soot particles are con- fined in very thin structures interacting with turbulence. Because of their size, these structures may not be re- solved on the LES grids, so specific soot subgrid-scale models are required. Finally, the validation of newly developed models is difficult due to the non-linear inter- actions between such multi-physical phenomena and the massive computational resources required for a reliable statistical representation. In the framework of the SO- PRANO European Project this thesis aims to investigate the reliability of the LES formalism for the prediction of soot production in an aero-engine model combustor, the DLR burner. First, an improved soot model with a new soot surface reaction mechanism is proposed, enabling the reproduction of the experimental soot yield in lami- nar premixed and non-premixed flames, which is essen- tial for multi-regime turbulent configurations as the DLR burner. Then the reliability of the LES formalism, based on a tabulated chemical model and a three-equation soot model, is evaluated through statistical analysis. This analysis reveals that soot occurrence is rare as it requires specific flow conditions rarely observed in the studied configuration. Therefore, numerical convergence is quite challenging to be achieved when considering soot pro- duction. Since unaffordable CPU cost is required for a reliable soot prediction, a new strategy based on a unique LES transporting a duplicated set of soot equations is proposed to rigorously investigate soot models in tur- bulent flames. One set accounts for the soot reference model, while the other is treated with the model un- der the scope. Therefore, both sets experience the same unique temporal and spatial gas phase evolution allow- ing the analysis of the instantaneous model response to rare gaseous events leading to soot production without attaining convergence. Thanks to this approach, the first indications of the soot intermittency model contribution to soot prediction in the DLR burner are proposed.
... To reduce the computational cost, a reduced chemical mechanism was developed using Analytically Reduced Chemistry (ARC) [39]. This was undertaken 180 using GRI 3.0 chemical kinetics mechanism as the reference mechanism with flame speed, autoignition delay time and CO mass fraction chosen as target parameters. ...
Article
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Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Nitrogen Oxides (NOx) in industrial applications. In gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products is investigated using direct numerical simulation. A three-Dimensional (3D) turbulent V-flame in a channel with isothermal hot and cold walls is simulated. It is shown that a main reaction zone in the central region between two walls supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. The cold wall leads to a longer flame, thereby having disproportionately large contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localises the near-wall CO distribution. High mean CO mass fraction develops in the free-stream while a high magnitude of the peak RMS CO mass fraction is present closer to the wall. It is also shown that one-dimensional flame solutions can reasonably describe the changes of CO mass fraction as a function of temperature in the free-stream region and some parts of the near-wall region but not close to the wall. Turbulent mixing and diffusion effects contribute to this deviation. The results highlight the complexities involved in CO modelling for diluted flames and set a benchmark for future work.
... A canonical example of the utilization of QSSA is the Michaelis-Menten kinetic formula, which is still widely adopted to formulate enzyme reactions in biochemistry. Nowadays, QSSA is still widely employed in numerical simulations of reaction-transport systems to remove chemical stiffness and enable the explicit time integration with relatively large time steps [23,24]. Moreover, imposing QSSA also reduces the number of state variables and transport equations by eliminating the fast species such that the computational cost can be greatly reduced. ...
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Recently developed physics-informed neural network (PINN) has achieved success in many science and engineering disciplines by encoding physics laws into the loss functions of the neural network, such that the network not only conforms to the measurements, initial and boundary conditions but also satisfies the governing equations. This work first investigates the performance of PINN in solving stiff chemical kinetic problems with governing equations of stiff ordinary differential equations (ODEs). The results elucidate the challenges of utilizing PINN in stiff ODE systems. Consequently, we employ Quasi-Steady-State-Assumptions (QSSA) to reduce the stiffness of the ODE systems, and the PINN then can be successfully applied to the converted non/mild-stiff systems. Therefore, the results suggest that stiffness could be the major reason for the failure of the regular PINN in the studied stiff chemical kinetic systems. The developed Stiff-PINN approach that utilizes QSSA to enable PINN to solve stiff chemical kinetics shall open the possibility of applying PINN to various reaction-diffusion systems involving stiff dynamics.
... The elimination process is refined by evaluating the influence of any specie or reaction on the target of interest, typically a fuel or a product, by propagating local influences throughout the graph. An extensive literature documents the advantages and successes of the DRG and DRGEP methods [15][16][17][18]. ...
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Detailed kinetics mechanisms for plasma-assisted combustion contain numerous species and reactions that model the interplay between non-equilibrium plasma processes and hydrocarbon oxidation. While physically accurate and comprehensive, such detailed mechanisms are impractical for simulations of unsteady multi-dimensional plasma discharges and their effect on reactive mixtures in practical devices. In this work, we develop and apply a novel methodology for the reduction of large detailed plasma-assisted combustion mechanisms to smaller skeletal ones. The methodology extends the Directed Relation Graph with Error Propagation (DRGEP) approach in order to consider the energy branching characteristics of plasma discharges during the reduction. Ensuring tight error tolerances on the relative proportions of energy lost by the electrons to the various classes of impact processes (i.e. vibrational and electronic excitation, ionization, and impact dissociation) is key to preserving the correct discharge physics in the skeletal mechanism. To this end, new targets that include energy transfers are defined and incorporated in DRGEP. The performance of the novel framework, called P-DRGEP, is assessed for the simulation of ethylene-air ignition by nanosecond repetitive pulsed discharges at conditions relevant to supersonic combustion and flame holding in scramjet cavities, i.e. from 600 K to 1000 K, 0.5 atm, and equivalence ratios between 0.75 and 1.5. P-DRGEP is found to be greatly superior to the traditional reduction approach applied to plasma-assisted ignition in that it generates a smaller skeletal mechanism with significantly lower errors. For ethylene-air ignition at the target conditions, P-DRGEP generates a skeletal mechanism with 54 species and 236 reactions, resulting in a 84% computational speed-up for ignition simulations, while guaranteeing errors below 10% on the time required for ignition following the first pulse, 1% on the mean electron energy, between 4 and 35% on electron energy losses depending on the process, and 5% on the laminar flame speed.
... Introducing a sub-grid efficiency model to take into account the unresolved filtered flame front wrinkling, this approach then allows to solve unsteady turbulent flames taking into account heat losses, liquid fuels or dilution without further modeling. The TF approach may be combined to various descriptions of the flame structure, such as direct chemistry integration [26,27] , tabulated flamelets [28][29][30] or equilibrium [31] . It represents the flame-turbulence interaction with the resolved contribution and a subgrid-scale contribution in the form of an efficiency function, i.e., totally different from pdf-based or viscosity-based (ILES) methods. ...
Chapter
The flamelet approach for non-premixed combustion is based on the description of the turbulent flame as a collection of laminar flame elements embedded in a turbulent flow and interacting with it. The local structure of the flame at each point of the flame front is supposed to be similar to a laminar flamelet, while the interaction with turbulence is reduced to the front evolution. This view is supported by the introduction of the mixture fraction, which allows to decouple the turbulent transport and the flame structure. One key parameter of the flamelet structure is the scalar dissipation rate, which controls the reactant fluxes to the reaction zone and is related to the flow velocity gradients. Probability density functions or flame surface density are then used to describe the turbulent flame and relate the flamelet description to the turbulent flame front. As unsteady effects may become significant, various transient flamelet approaches also exist to take into account the flame history. The flamelet approach may be used either in the RANS or LES context and is still being developed to account for additional complexities such as heat losses and sprays.
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Simplified chemistry models are commonly used in reactive computational fluid dynamics (CFD) simulations to alleviate the computational cost. Uncertainties associated with the calibration of such simplified models have been characterized in some works, but to our knowledge, there is a lack of studies analyzing the subsequent propagation through CFD simulation of combustion processes. This work propagates the uncertainties - arising in the calibration of a global chemistry model - through direct numerical simulations (DNS) of flame-vortex interactions. Calibration uncertainties are derived by inferring the parameters of a two-step reaction mechanism for methane, using synthetic observations of one-dimensional laminar premixed flames based on a detailed mechanism. To assist the inference, independent surrogate models for estimating flame speed and thermal thickness are built taking advantage of the Principal Component Analysis (PCA) and the Polynomial Chaos (PC) expansion. Using the Markov Chain Monte Carlo (MCMC) sampling method, a discussion on how push-forward posterior densities behave with respect to the detailed mechanism is provided based on three different calibrations relying (i) only on flame speed, (ii) only on thermal thickness, and (iii) on both quantities simultaneously. The model parameter uncertainties characterized in the latter calibration are propagated to two-dimensional flame-vortex interactions using 60 independent samples. Posterior predictive densities for the time evolution of the heat release and flame surface are consistent, being that the confidence intervals contain the reference simulation. However, the two-step mechanism fails to reproduce the flame response to stretch as it was not considered in the calibration. This study highlights the capabilities and limitations of propagating chemistry-models uncertainties to CFD applications to fully quantify posterior uncertainties on target quantities.
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The minimization of autoignition risk is critical to premixer design. Safety factors based on ignition delays of homogeneous mixtures, are generally used to guide the choice of a residence time for a given premixer. However, autoignition chemistry at aeroderivative conditions is fast (0.5-2 milliseconds) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period involves the study of low-temperature precursor chemistry. By coupling the evolution of the Chemical Explosive Modes to turbulence, it is possible to obtain a measure of spatial autoignition risk where both chemical (e.g. ignition delay) and aerodynamic (e.g. local residence time) influences are unified. In this article, we describe a method that couples Large Eddy Simulation to newly developed, reduced autoignition chemical kinetics to study autoignition precursors in an example premixer representative of real life geometric complexity. A blend of pure methane and dimethyl ether (DME), a common fuel used for experimental autoignition studies, was transported using the reduced mechanism (38 species / 238 reactions) at engine conditions at increasing levels of DME concentration until exothermic autoignition kernels were formed. The Chemical Explosive Mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentration and identifies where the premixer is sensitive and flame anchoring is likely to occur.
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A large-eddy simulation is presented of a challenging high-pressure jet flame case that is representative of state of the art, dry low-NOx and low-CO real gas turbine combustion. A reaction scheme is developed for lifted lean premixed high pressure methane jet flames, and tested by three-dimensional large-eddy simulation of an experiment, for which very detailed data are available. Auto-ignition-delay times of different mixtures of fresh gas and products have been introduced as a novel optimization criterion for the mechanism development. The new mechanism has been developed by a genetic algorithm-based reduction and optimization, and consists of 15 species and 18 reactions. The large-eddy simulations are performed using a finite rate chemistry (FRC) approach and the dynamic thickened flame (DTF) model to investigate a lifted jet flame at high pressure in a gas turbine model combustor. In the simulations, the novel mechanism is compared to a similar mechanism that was generated without this criterion, and the well-established Lu19 mechanism. With the new mechanism, the LES predicts the flame as accurately as with Lu19, at a significantly lower cost. Further post processing with Lagrangian tracer particles confirmed that ignition events occur in the region corresponding to the liftoff height estimated in the experiment, which is corroborated by a chemical explosive mode analysis (CEMA). Overall, the newly developed mechanism with the novel optimization criterion was found to provide a better agreement with the experiments than previous mechanisms of similar cost, or a comparable agreement to a mechanism of significantly higher cost.
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Large Eddy Simulation (LES) is progressively becoming a crucial design tool for the next generation of aeronautical combustion chambers. However, further improvements of the predictive capability of LES is required especially for predictions of pollutant formation. In general, the exact description of real fuel combustion requires to take into account thousands of unique chemical species involved in complex and highly non-linear chemical reaction mechanisms, and the direct integration of such chemistry in LES is not a viable path because of excessive computational demands and numerical stiffness. Modeling of real aeronautical transportation fuel is further complicated by the fact that kerosenes are complex blends of a large number of hydrocarbon compounds and their exact composition is very difficult to determine. In this work, we propose a new framework relying upon the Hybrid Chemistry (HyChem) approach and Analytically Reduced Chemistry (ARC) to allow a direct integration of real fuel chemistry in the compressible LES solver AVBP. The HyChem-ARC model is coupled with the Dynamically Thickened Flame LES model (DTFLES) and a Lagrangian description of the spray to investigate the turbulent two-phase flow flame in a lean direct injection combustor, fueled with Jet-A. The LES results are compared to experimental data in terms of gas velocity, temperature and species (CO2, H2O, CO, NO) mass fractions. It is found that the proposed methodology leads to very satisfying predictions of both the flow dynamics and the NOx levels. Additionally, the refined level of chemistry description enables to gain valuable insights into flame/spray interactions as well as on the NOx formation mechanism in such complex flame configurations. To improve further the results, a more detailed experimental characterization of the liquid fuel injection should be provided.
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Large-eddy simulation (LES) of a swirl-stabilized non-premixed ethylene/air aero-engine combustor experimentally studied at DLR is performed, with direct integration of Analytically Reduced Chemistry (ARC). Combined with the Dynamic Thickened Flame model (DTFLES), the ARC-LES approach does not require specific flame modeling assumptions and naturally adapts to any flow or geometrical complexity. To demonstrate the added value of the ARC methodology for the prediction of flame structures in various combustion regimes, including formation of intermediate species and pollutants, it is compared to a standard tabulation method (FPI). Comparisons with available measurements show an overall good agreement with both chemistry approaches, for the velocity and temperature fields. However, the flame structure is shown to be much improved by the inclusion of explicitly resolved chemistry with ARC. In particular, the ability of ARC to respond to strain and curvature, and to intrinsically contain CO/O2 chemistry greatly influences the flame shape and position, as well as important species production and consumption throughout the combustion chamber. Additionally, since both chemistry descriptions are able to account for intermediate species such as OH and C2H2, soot formation is also investigated using a two-equations empirical soot model with C2H2 as the sole precursor. It is found that, in the present configuration, this precursor is strongly impacted by differential diffusion and partial premixing, not included in the FPI approach. This leads to a strong under-prediction of soot levels by about one order of magnitude with FPI, while ARC recovers the correct measured soot concentrations.
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This paper describes Large Eddy Simulations of a turbulent premixed flame (the VOLVO rig) comparing Analytically Reduced Chemistry (ARC) with globally reduced chemistry for propane-air combustion, a dynamic Thickened Flame (TFLES) model with the usual non-dynamic TFLES model and a high-order Taylor Galerkin numerical scheme with a low-order Lax–Wendroff scheme. Comparisons with experimental data are presented for a stable case in terms of velocity and temperature fields. They show that going from two-step to ARC chemistry changes the flame stabilization zone. Compared to the usual non-dynamic TFLES model, the dynamic formulation allows to perform a parameter-free simulation. Finally, the order of accuracy of the numerical method is also found to play an important role. As a result, the high-order numerical method combined with the ARC chemistry and the dynamic TFLES model provides the best comparison with the experimental data. Since the VOLVO data base is used in various benchmarking exercises, this paper suggests that these three elements (precise chemistry description, dynamic parameter-free turbulent combustion model and high-order numerical methods) play important roles and must be considered carefully in any LES approach .
Technical Report
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Large Eddy Simulation (LES) of an aeronautical burner is performed with two combustion models and a reduced chemical scheme able to accurately describe the combustion of a real multi-component kerosene aviation fuel. The accuracy of the reduced scheme is first assessed on laminar flame cases through comparison with detailed chemistry mechanism. Subsequently, the chemical mechanism is employed in 3D simulations, demonstrating its ability to correctly predict combustion chemistry in turbulent flames.
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Real fuels may contain thousands of hydrocarbon components. This paper examines how nature simplifies the problem. We will discuss the internal structure of the fuel oxidation process at high temperatures. Over a wide range of conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low-molecular weight fragments, and in the case of conventional petroleum-derived fuels, the composition variation in the decomposition products is reduced by the law of large numbers. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it will be shown also that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer composition and fuel composition within the range of temperatures of direct relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling high-temperature oxidation of real fuels: the kinetics of thermal or oxidative thermal decomposition of the fuel is lumped using kinetic parameters derived from experiments, while the oxidation of the decomposed fragments is described by a detailed reaction model. Sample results will be given that supports this modeling approach.
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Towards the implementation of alternative jet fuels in aviation gas turbines, testing in combustor rigs and engines is required to evaluate the fuel performance on combustion stability, relight, and lean blow-out (LBO) characteristics. The objective of this work is to evaluate the effect of different fuel candidates on the operability of gas turbines by comparing a conventional petroleum-based fuel with two other alternative fuel candidates. A comparative study of fuel properties is first conducted to identify physico-chemical processes that are affected by these fuels. Subsequently, large-eddy simulations (LES) are performed to examine the performance of these fuels on the stable condition close to blow-out in a referee gas turbine combustor. LES results are compared to available experimental data to assess their capabilities in reproducing observed fuel effects. It is shown that the simulations correctly predict the spray main characteristics as well as the flame position. The change in OH *-emissions for different fuel candidates is also qualitatively captured. An analysis of the flame anchoring mechanisms highlights the fuel effects on the flame position. Finally, the LBO-behavior is examined in order to evaluate the LBO-limit in terms of equivalence ratio and identify fuel effects on the blow-out behavior.
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Accurate characterization of swirled flames is a key point in the development of more efficient and safer aeronautical engines. The task is even more challenging for spray injection systems. On the one side, spray interacts with both turbulence and flame, eventually affecting the flame dynamics. On the other side, the structure of turbulent spray flame is highly complex due to equivalence ratio inhomogeneities caused by evaporation and mixing processes. The first objective of this work is to numerically characterize the structure and dynamics of a swirled spray flame. The target configuration is the experimental benchmark named MERCATO, representative of an actual turbojet injection system. Due to the complex nature of the flame, a detailed description of chemical kinetics is necessary and is here obtained by using a 24-species chemical scheme, which has been developed for numerical simulations of spray flames. The first Large Eddy Simulation (LES) of a swirled spray flame using such a detailed chemical description is performed here and results are analyzed to study the complex interactions between the spray, the turbulent flow and the flame. It is observed that this coupling has an effect on the flame structure and that flame dynamics are governed by the interactions between spray, precessing vortex core and flame front. Even if such a detailed kinetic description leads to an accurate characterization of the flame, it is still highly expensive in terms of CPU time. Tabulated techniques have been expressly developed to account for detailed chemistry at a reduced computational cost in purely gaseous configurations. The second objective is then to verify the capability of the FPI tabulated chemistry method to correctly reproduce the spray flame characteristics by performing LES. To do this, results with the FPI method are compared to the experimental database and to the results obtained with the 24-species description in terms of mean and fluctuating axial gas velocity and liquid phase characteristics (droplet diameter and liquid velocity). Moreover, the flame characterization obtained with the FPI approach is compared to the results of the 24-species scheme focusing on the flame structure, on major and minor species concentrations as well as on pollutant emissions. The potential and the limits of the tabulated approach for spray flame are finally assessed.
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This numerical study deals with the distinction between autoignition and propagation driven reaction zones using an autoignition index (AI). It allows a clear identification of the two burning regimes based on the relative contribution of two reactions for hydroperoxyl (HO2) chemistry. AI was applied to a lifted methane–air jet in a hot (1350 K) vitiated coflow, namely the Cabra flame configuration. Large Eddy Simulation (LES) were performed using the Dynamic Thickened Flame model (DTF) with an Analytically Reduced Chemistry (ARC) mechanism with 22 transported species, as well as 18 species in Quasi-Steady State (QSS) approximation. A detailed validation of the numerical methods is presented. Comparisons with experimental data are in good agreement for mixture fraction, temperature and species mass fractions for both a fine and a coarse mesh. In a detailed analysis of the flame structure, AI identifies autoignition as dominant over propagation at the flame base. Autoignition pockets are close to the lean most reactive mixture fraction. Lean and rich propagation is recognized to dominate in regions located at higher mixture fractions closer to the centerline with significantly higher heat release rates compared to autoignition.
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The paper describes the results of a computational study of the auto-ignition of a fuel spray under Exhaust Gas Recirculation (EGR) conditions, a technique used to reduce the production of NOx. Large Eddy Simulation (LES) is performed, and the stochastic field method is used for the solution of the joint sub-grid probability density function (pdf) of the chemical species and energy. The fuel spray is n-heptane, a diesel surrogate and its chemical kinetics are described by a reduced mechanism involving 22 species and 18 reaction steps. The method is applied to a constant volume combustion vessel able to reproduce EGR conditions by the ignition of a hot gas mixture previously introduced into the chamber. Once the prescribed conditions are reached the fuel is then injected. Different EGR conditions in terms of temperature and initial ambient chemical composition are simulated. The results are in good overall agreement with measurements both regarding the ignition delay times and the lift-off heights.
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This paper describes the application of a coupled acoustic model/large-eddy simulation approach to assess the effect of fuel split on combustion instabilities in an industrial ultra-low-NOx annular combustor. Multiphase flow LES and an analytical model (analytical tool to analyze and control azimuthal modes in annular chambers (ATACAMAC)) to predict thermoacoustic modes are combined to reveal and compare two mechanisms leading to thermoacoustic instabilities: (1) a gaseous type in the multipoint zone (MPZ) where acoustics generates vortex shedding, which then wrinkle the flame front, and (2) a multiphase flow type in the pilot zone (PZ) where acoustics can modify the liquid fuel transport and the evaporation process leading to gaseous fuel oscillations. The aim of this paper is to investigate these mechanisms by changing the fuel split (from 5% to 20%, mainly affecting the PZ and mechanism 2) to assess which mechanism controls the flame dynamics. First, the eigenmodes of the annular chamber are investigated using an analytical model validated by 3D Helmholtz simulations. Then, multiphase flow LES are forced at the eigenfrequencies of the chamber for three different fuel split values. Key features of the flow and flame dynamics are investigated. Results show that acoustic forcing generates gaseous fuel oscillations in the PZ, which strongly depend on the fuel split parameter. However, the correlation between acoustics and the global (pilot + multipoint) heat release fluctuations highlights no dependency on the fuel split staging. It suggests that vortex shedding in the MPZ, almost not depending on the fuel split, is the main feature controlling the flame dynamics for this engine.
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Flame ignition, stabilization and extinction or pollutant predictions are crucial issues in Large Eddy Simulations (LES) of turbulent combustion. These phenomena are strongly influenced by complex chemical effects. Unfortunately, despite the rapid increase in computational power, performing turbulent simulations of industrial configurations including detailed chemical mechanisms will still remain out of reach for a long time. This article proposes a review of commonly-used approaches to address fluid/chemistry interactions at a reduced computational cost. Several chemistry modeling routes are first examined with a focus on tabulated chemistry techniques. The problem of coupling chemistry with LES is considered in a second step. Examples of turbulent combustion simulations are presented in the final part of the article. Three LES applications are analyzed: a lean swirled combustor, a non-adiabatic turbulent stratified flame and a combustion chamber where internal recirculations promote the dilution of fresh gases by burnt gases.
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An eddy-viscosity based, subgrid-scale model for large eddy simulations is derived from the analysis of the singular values of the resolved velocity gradient tensor. The proposed sigma-model has, by construction, the property to automatically vanish as soon as the resolved field is either two-dimensional or two-component, including the pure shear and solid rotation cases. In addition, the model generates no subgrid-scale viscosity when the resolved scales are in pure axisymmetric or isotropic contraction/expansion. At last, it is shown analytically that it has the appropriate cubic behavior in the vicinity of solid boundaries without requiring any ad-hoc treatment. Results for two classical test cases (decaying isotropic turbulence and periodic channel flow) obtained from three different solvers with a variety of numerics (finite elements, finite differences, or spectral methods) are presented to illustrate the potential of this model. The results obtained with the proposed model are systematically equivalent or slightly better than the results from the Dynamic Smagorinsky model. Still, the sigma-model has a low computational cost, is easy to implement, and does not require any homogeneous direction in space or time. It is thus anticipated that it has a high potential for the computation of non-homogeneous, wall-bounded flows.
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Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and cannot correctly predict the rich flammability limit. However, the best choice of rate parameters suitably reproduces rich and lean flammability limits as well as the dependence of the flame speed on pressure and equivalence ratio for all of the fuels examined. Two-step and quasi-global approaches also yield information on flame temperature and burned gas composition. However, none of the simplified mechanisms studied accurately describes the chemical structure of the flame itself.
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Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach.
Article
We propose and test an alternative approach to modeling high-temperature combustion chemistry of multicomponent real fuels. The hybrid chemistry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel pyrolysis products. The pyrolysis (or oxidative pyrolysis) process is modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients are derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three conventional jet fuels and two rocket fuels as examples. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all five fuels. Sensitivity analysis shows that for conventional, petroleum-derived real fuels, the uncertainties in the experimental measurements of C2H4 and CH4 impact model predictions to an extent, but the largest influence of the model predictability stems from the uncertainties of the foundational fuel chemistry model used (USC Mech II). In addition, we introduce an approach in the realm of the HyChem approach to address the need to predict the negative-temperature coefficient (NTC) behaviors of jet fuels, in which the CH2O speciation history is proposed to be a viable NTC-activity marker for model development. Finally, the paper shows that the HyChem model can be reduced to about 30 species in size to enable turbulent combustion modeling of real fuels with a testable chemistry model.
Conference Paper
In this work we introduce an unconventional approach to modeling the high-temperature combustion chemistry of multicomponent real fuels. The hybrid chemistry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel decomposition intermediates. The thermal decomposition and oxidative thermal decomposition processes are modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients may be derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three petroleum-derived fuels: JP-8, Jet A and JP-5 as examples. The experimental observations show only a small number of intermediates are formed during thermal decomposition under pyrolysis and oxidative conditions, and support the hypothesis that the stoichiometric coefficients in the lumped reaction steps are not a strong function of temperature, pressure, or fuel-oxidizer composition, as we discussed in a companion paper. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all three fuels.
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Complying with stringent pollutant emission regulations requires a strong optimization of modern gas turbine combustors, for which Large Eddy Simulation (LES) is a promising tool at the design stage. Yet the accurate prediction of pollutant formation remains a challenge because of the complex flame structure in this type of configuration. The strategy retained for the present LES study is to employ analytically reduced mechanism (ARC) with accurate pollutant chemistry in combination with the Dynamic Thickened Flame model (TFLES) in the SGT-100 burner. The reduction of the mechanism is first presented and validated in the burner operating conditions on canonical cases. Then, comparisons of LES results with the experimental data show the excellent agreement of velocity statistics and a good agreement in terms of flame shape and exhaust pollutant prediction. The turbulent flame structure is further analyzed and compared with laminar unstrained and strained flames. Unmixedness and strain are found to significantly impact pollutant formation and flame stabilization. The ARC/TFLES strategy accounts for these effects with a very good compromise between cost and accuracy.
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In this work a transported joint scalar probability density function method is combined with the flamelet generated manifolds (FGM) tabulated chemistry approach for large eddy simulation (LES) modeling of turbulent combustion. This strategy accounts for the turbulence-chemistry interaction at reasonable computational costs and allows the usage of detailed chemistry information by tabulation. Apart from the details regarding the solution procedure, a technique for an improved stability of the proposed approach is introduced and validated using a one-dimensional test case. Next, a two-dimensional flame configuration is considered in order to perform an in-depth analysis regarding the laminar and turbulent behavior of the model. Here, transient and time-averaged simulation data is used to provide insight into the predicted flame shape and its dynamics, where the implemented approach is compared with the well-established artificially thickened flame (ATF) combustion model. Moreover, the sensitivity of the results to different modeling approaches and model parameters is investigated. Finally, the method is applied to a three-dimensional turbulent stratified burner. Here, in addition to the ATF model, the suggested approach is compared to measurements of the velocity and scalar quantities to evaluate its prediction capability. Consequently, the investigation conducted in this work aims to provide a complete picture on the ability of the proposed method to reproduce the flame propagation and the resulting flow conditions within complex premixed and stratified turbulent flames.
Article
A swirling ethanol spray flame in conditions close to blow-off has been simulated using Large Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model aiming to further validate the capability of the LES/CMC approach to capture local extinctions in turbulent spray flames. A detailed chemical mechanism was used and a transport equation of the mixture fraction sub-grid variance, with spray interaction terms included, was solved. Numerical results are in good agreement with the experiment in terms of both instantaneous and mean flame shape and droplet velocity and size. Local extinctions were detected in the region around the bluff- body, resulting in a fluctuating lift-off of the flame there, and the probability density function of the lift-off height was in very good agreement with the experiment, suggesting that the degree of local extinction is captured quantitatively. Analysis of the CMC equation suggested that local extinction was influenced by both transport in physical space and high scalar dissipation rate. The modelling of the latter needs development in areas where the spray evaporation is strong enough to increase significantly the sub-grid mixture fraction fluctuations and their small-scale gradients, possibly leading to deviations from the present usual approach of relating the sub-grid scalar dissipation to the sub-grid mixture fraction variance.
Article
Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large eddy simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion simulations as the transient nature of the flow is resolved. In most situations; however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here we examine the predictive capabilities between LES flamelet models, such as the flamelet progress variable (LES-FPV) model, and LES finite rate chemistry models, such as the thickened flame model (LES-TFM), the eddy dissipation concept (LES-EDC) model, and the partially stirred reactor model (LES-PaSR). The different models are here used to examine a swirl-stabilized premixed flame in a laboratory gas turbine combustor, featuring the triple annular research swirler (TARS), for which high-quality experimental data is available. The comparisons include velocity and temperature profiles as well as combustor dynamics and NO formation.
Article
Une nouvelle methode pour la reduction de mecanismes detailles de cinetique chimique est presentee. Elle est basee sur le calcul des flux atomiques et l'analyse des chemins reactionnels. La methode a ete appliquee a un mecanisme detaille complet pour le systeme CH 4 /O 2 /N 2 , incluant les reactions des NO x , et precedemment valide par comparaison avec un grand nombre de donnees experimentales obtenues dans des reacteurs a roulement, des tubes a choc et des flammes laminaires. Un mecanisme global pour la combustion du methane et la formation de NO a ete mis au point. Il est constitue de six reactions chimiques impliquant 10 especes differentes
Chapter
Combustion is a widely used technique in energy transformation and is encountered in many practical systems such as heaters, domestic or industrial furnaces, thermal power plants, automotive and aeronautic engines, rocket engines,... In most applications, combustion occurs in turbulent gaseous flows. Accordingly, the interaction between turbulence and combustion has to be described. Combustion phenomena may be characterized by: a strong and irreversible heat release. Heat release occurs in very thin zones (typical flame thicknesses δ L are about 0.1 to 1 mm) and induces strong temperature gradients (temperature ratio between burnt and unburnt gases, T b /T u , are about 5 to 7) leading to strong heat transfers and large density variations. a stiff highly non linear reaction rate (Arrhenius law).
Conference Paper
This work reports an experimental study of the behavior and structure of a liquid reacting spray immersed in a strong swirling field. A vane type swirler, a pressure swirl atomizer and a rectangle chamber were integrated to perform the experiments. The vane-type swirler with 60-degree vane angle combined with a converging-diverging nozzle at the swirler exit was used to produce strong swirling flow inducing a strong recirculation zone in the model combustor chamber. Properties of the dispersed phase such as velocity, size distribution, and drop flux were measured at several locations within the swirling flow field. In addition, mean velocity and turbulent properties were obtained for the gas phase. The measurements were performed using a two-component Phase Doppler Particle Analyzer (PDPA) system. Flow visualization was conducted with a laser sheet to gain further understanding of the spray distribution and the influence of the swirling flow on the spray. The results indicate that the significant slip velocities between the continuous phase and the droplets reflect a strong momentum exchange between the phases. The spray velocity distribution is significantly affected by the swirling flow and the induced reversal flow. The configuration of the converging-diverging nozzle has strong effect on the spray distributions. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Conference Paper
An experimental investigation was conducted to study the aerodynamic characteristics of the confined, non-reacting, swirling flow field. The flow was generated by a helicoidal axial-vaned swirler with a short internal convergent-divergent venturi, which was confined within 2-inch square test section. A series of helicoidal axial-vaned swirlers have been designed with tip vane angles of 40°, 45°, 50°, 55°, 60° and 65°. The swirler with the tip vane angle of 60° was combined with several simulated fuel nozzle insertions of varying lengths. A two-component Laser Doppler Velocimetry (LDV) system was employed to measure the three-component mean velocities and Reynolds stresses. Detailed data are provided to enhance understanding swirling flow with different swirl degrees and geometries and to support the development of more accurate physical/numerical models. The data indicated that the degree of swirl had a clear impact on the mean and turbulent flow fields. The swirling flow fields changed significantly with the addition of a variety of simulated fuel nozzle insertion lengths.
Article
This paper proposes a method for modeling soot when performing Large Eddy Simulation of complex geometries. To obtain a good trade-off between CPU cost and accuracy, soot chemistry is included via a tabulated flamelet approach, combined to a turbulent combustion model for Large Eddy Simulation based on a simplified description of chemistry. A semi-empirical soot model is chosen and validated on laminar premixed and counterflow diffusion flames. A proposed procedure enables to calculate radiation with a Discrete Ordinates Method approach and optimized spectral models. The developed soot model is applied to a real configuration, being the combustion chamber of a helicopter engine. To evaluate the importance of radiative heat losses, two cases are studied, using either adiabatic conditions or accounting for radiative heat gains/loss.
Article
The ignition transient is a critical fundamental phase in combustion systems that has strong practical implications. While this phenomenon has been extensively studied on single injector configurations, the burner-to-burner propagation of a full annular combustor is rarely investigated, due to the size and complexity of the geometry involved. To this purpose, an annular experimental setup has been developed at EM2C, featuring sixteen swirl injectors and quartz tubes providing a direct optical access to the flame. Ignition has been investigated systematically on this device, thus providing a large experimental database. In this work, this experiment is computed in the Large Eddy Simulation (LES) framework by carrying out massively parallel computations. This constitutes a unique comparison between experiments and calculations of a critical process for gas turbines. The ability of turbulent combustion models to properly retrieve the flame structure and propagation at the largest scales is not yet fully assessed and is investigated in this paper by comparing two conceptually different combustion modeling approaches, namely the filtered tabulated chemistry (F-TACLES) and the flame thickening with reduced chemistry (TFLES). Qualitative and quantitative comparisons between both simulations and experiment show an overall excellent agreement.
Article
Large Eddy Simulations (LESs) for a lean-direct injection (LDI) combustor are performed and compared with experimental data. The LDI emissions characteristics, and radiation-spray coupling effect on the predictions are analyzed. The flamelet progress variable approach is employed for chemistry tabulation coupled with a stochastic secondary breakup model. Good comparisons are shown with the experimental data mean and root mean square for both the gas phase and spray droplets profiles. The effect of combustion is found to change the shape and structure of the central recirculation zone to be more compact in length but larger in diameter in the transverse direction. In-addition the results show that the gas phase radiation alters the spray dynamics by changing the local gas-phase temperature distribution. This impacts the spray evaporation rate, the local mixture fraction, and consequently the combustion heat released rate and the predicted emissions. The simulation with no radiation modeling shows over prediction in the temperature distribution, pollutants emissions, higher fuel evaporation rate, and narrower range of droplet size distribution with lower number density for the smaller size particles. The current study suggests that, even for low pressure systems, radiation modeling can be important for accurate emissions prediction.
Article
Mathematical models are utilized to approximate various highly complex engineering, physical, environmental, social, and economic phenomena. Model parameters exerting the most influence on model results are identified through a 'sensitivity analysis'. A comprehensive review is presented of more than a dozen sensitivity analysis methods. This review is intended for those not intimately familiar with statistics or the techniques utilized for sensitivity analysis of computer models. The most fundamental of sensitivity techniques utilizes partial differentiation whereas the simplest approach requires varying parameter values one-at-a-time. Correlation analysis is used to determine relationships between independent and dependent variables. Regression analysis provides the most comprehensive sensitivity measure and is commonly utilized to build response surfaces that approximate complex models.
Article
Tabulated chemistry is a popular technique to account for detailed chemical effects with an affordable computational cost in gaseous combustion systems. However its performances for spray combustion have not completely been identified. The present article discusses the chemical structure modeling of spray flames using tabulated chemistry methods under the hypothesis that the chemical subspace accessed by a two-phase reactive flow can be mapped by a collection of gaseous flamelets. It is shown that tabulated chemistry methods based either on pure premixed flamelets or on pure non-premixed flamelets fail to capture the structure of spray combustion. The reason is the complexity of the chemical structure of spray flames which exhibits both premixed-like and non-premixed-like reaction zones. To overcome this issue, a new multi-regime flamelet combustion model (called Partially-Premixed Flamelet Tabulation 2PFT) is presented in this paper. Information from premixed, partially-premixed and diffusion flames are stored in a 3-D look-up table parametrized as a function of the progress variable Y-c, describing the progress of the reaction, the mixture fraction Y-z, denoting the local equivalence ratio, and the scalar dissipation chi*, which identifies the combustion regime. The performances of the 2PFT method are evaluated on counterflow laminar spray flames for different injection conditions of droplet diameter, liquid volume fraction and velocity. The 2PFT tabulation method better describes the chemical structure of spray flames compared to the classical techniques based on single archetypal flamelets. These results also confirm that the chemical structure of laminar spray flame can be modeled by a multi-regime flamelet combustion model based on gaseous flamelets.
Article
The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained ‘turbulence-chemistry’ interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information is leading to new physical insight, providing benchmark data for assessing model assumptions, suggesting new closure hypotheses, and providing interpretation of statistics obtained from lower-dimensional measurements. In this paper the various roles of petascale DNS are illustrated through selected examples related to lifted flame stabilization, premixed and stratified flame propagation in intense turbulence, and extinction and reignition in turbulent non-premixed jet flames. Extending the DNS envelope to higher Reynolds numbers, higher pressures, and greater chemical complexity will require exascale computing in the next decade. The future outlook of DNS in terms of challenges and opportunities in this regard are addressed.
Article
Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and fundamental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers…. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabilities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs.
Article
A turbulent premixed swirl burner is simulated using the sgs-pdf evolution equation approach in conjunction with the Eulerian stochastic field solution method in the context of Large Eddy Simulation. Simple gradient diffusion models are adopted for the sub-grid stresses and eight stochastic fields were utilised to characterise the influence of the sub-grid fluctuations. The chemistry was represented by an augmented reduced mechanism derived from GRI 3.0 with 15 reaction steps and 19 species. Statistical means and instantaneous quantities show overall good agreement with the experimental data and demonstrate the capability of the pdf method in LES to simulate premixed combustion in complex flame configurations.
Article
In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.
Article
Recent advances in automation of systematically reduced mechanisms are reported here with the aim to accelerate the development process. A computer algorithm has been developed enabling fast generation and testing of reduced chemistry. This algorithm has been used to develop various reduced mechanisms of methane-air combustion for modelling of turbulent combustion. A 10-step reduced chemistry has been extensively tested showing good performances in predicting a wide range of flame phenomena, including general flame characteristics, flame extinction limits, flame propagation speeds, and auto-ignition delay times. Strategies for using such extensive reduced chemistry for modelling turbulent combustion are briefly discussed.
Article
Chemical mechanisms have been employed in hydrocarbon combustion as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This chapter provides an introduction to most of the mathematical methods that have been used for the construction, investigation, and reduction of combustion mechanisms. The use of algebraic manipulation in techniques, such as the quasi-steady-state approximation (QSSA) and lumping, make the production of a reduced mechanism essential and make subsequent calculations as simple as possible. Computational singular perturbation (CSP) is an alternative to the rate-of-production and sensitivity methods for mechanism reduction and provides an automatic selection of the important reactions as well as time-scale analysis. The simplest and most widely used technique involving the separation of time scales is the QSSA; however, a possible limitation is that it may not provide the minimum low-order system. Chemical lumping can prove very useful in areas, such as the combustion of hydrocarbon mixtures or soot formation. Several programs are available for the investigation and reduction of combustion mechanisms, including MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUN1DL, PSR, SHOCK and EQLIB. Because models in combustion are expected to cover a wide range of conditions, it is natural to expect that a different approach might be used for different cases.
Article
Surrogate mixtures of one to 10 hydrocarbons that have similar properties to aviation fuels are desirable for experimental and computational tractability and reproducibility, However, aviation fuels, such as Jet A, JP-8, and JP-5, contain hundreds of hydrocarbons, This paper describes appropriate "surrogate" mixtures to reproduce the behavior of multicomponent aviation fuels. Surrogate mixtures from the literature and their applicability to various situations are summarized.
Article
A novel approach to the lumping of species in large chemical kinetic mechanisms is presented. Species with similar composition and functionalities are lumped into one single representative species. Simulations using the detailed scheme are used to gather statistical information on the distribution of the isomers within each lump group. These distributions are functions of space and time. Closure is performed in state space by approximating these distribution functions as the conditional averages depending on the independent state space variables of the lumped scheme. This approach is simplified further, so that the resulting chemical mechanisms can be used directly in standard chemistry packages. For this purpose, only the dependence of the isomer distributions on the temperature is retained, and optimal correcting factors are incorporated into the Arrhenius form of the rate coefficients of lumped reactions. Validation is performed using two comprehensive mechanisms for n-heptane and iso-octane oxidation. In all cases, a very good agreement is observed between the predictions obtained using the detailed and the lumped mechanisms. Effects of the lumping procedure on sensitivities of the kinetic scheme and on isomer concentrations are studied. Also, integration of this lumping approach into a multi-stage reduction strategy is discussed and illustrated.
Article
A palette of computational techniques are employed for the analysis and reduction of a detailed 53-species methane-air mechanism including nitrogen chemistry in a perfectly stirred reactor (PSR). The analysis of the mechanism for a PSR operating at an equivalence ratio φ = 0.5, inlet temperature Tin = 400°C and pressure p = 1 bar, enables a first reduction to a 26-species skeletal mechanism. Computational singular perturbation is then applied to find the species at quasi-steady state. The resulting 11-species reduced mechanism is tested at different operating conditions in the PSR, as well as in a reactor network of a PSR followed by a plug flow reactor (PSR), showing good aggreement with the full mechanism over a range of equivalence ratios up to stoichiometric.
Article
Direct numerical simulation of a three-dimensional spatially developing turbulent slot-burner Bunsen flame has been performed with a new reduced methane–air mechanism. The mechanism, derived from sequential application of directed relation graph theory, sensitivity analysis and computational singular perturbation over the GRI-1.2 detailed mechanism is non-stiff and tailored to the lean conditions of the DNS. The simulation is performed for three flow through times, long enough to achieve statistical stationarity. The turbulence parameters have been chosen such that the combustion occurs in the thin reaction zones regime of premixed combustion. The data is analyzed to study possible influences of turbulence on the structure of the preheat and reaction zones. The results show that the mean thickness of the turbulent flame, based on progress variable gradient, is greater than the corresponding laminar flame. The effects of flow straining and flame front curvature on the mean flame thickness are quantified through conditional means of the thickness and by examining the balance equation for the evolution of the flame thickness. Finally, conditional mean reaction rate of key species compared to the laminar reaction rate profiles show that there is no significant perturbation of the heat release layer.
Article
For modeling the combustion of aviation fuels, consisting of very complex hydrocarbon mixtures, it is often necessary to use less complex surrogate mixtures. The various surrogates used to represent kerosene and the available kinetic data for the ignition, oxidation, and combustion of kerosene and surrogate mixtures are reviewed. Recent achievements in chemical kinetic modeling of kerosene combustion using model-fuels of variable complexity are also presented. (c) 2005 Elsevier Ltd. All rights reserved.
Article
The structure of this review its main themes may be summarised as follows : to establish what is required of kinetics models and to discuss the experimental background for their validation; to introduce comprehensive kinetics models and to describe formal methods for deriving reduced forms; to assess the achievements of analytical approaches and how they link into numerical work by use of skeleton kinetic models; to trace the development of models that represent the oxidation of alkanes; to show what has been achieved in application to combustion in reciprocating engines
Article
Flamelet-generated manifolds have been restricted so far to premixed or diffusion flame archetypes, even though the resulting tables have been applied to nonpremixed and partially premixed flame simulations. By using a projection of the full set of mass conservation species balance equations into a restricted subset of the composition space, unsteady multidimensional flamelet governing equations are derived from first principles, under given hypotheses. During the projection, as in usual one-dimensional flamelets, the tangential strain rate of scalar isosurfaces is expressed in the form of the scalar dissipation rates of the control parameters of the multidimensional flamelet-generated manifold (MFM), which is tested in its five-dimensional form for partially premixed combustion, with two composition space directions and three scalar dissipation rates. It is shown that strain-rate-induced effects can hardly be fully neglected in chemistry tabulation of partially premixed combustion, because of fluxes across iso-equivalence-ratio and iso-progress-of-reaction surfaces. This is illustrated by comparing the 5D flamelet-generated manifold with one-dimensional premixed flame and unsteady strained diffusion flame composition space trajectories. The formal links between the asymptotic behavior of MFM and stratified flame, weakly varying partially premixed front, triple-flame, premixed and nonpremixed edge flames are also evidenced.
Article
The flame index was originally proposed by Yamashita et al. as a method of locally distinguishing between premixed and non-premixed combustion. Although this index has been applied both passively in the analysis of direct numerical simulation data, and actively using single step combustion models, certain limitations restrict its use in more detailed combustion models. In this work a general flamelet transformation that holds in the limits of both premixed and non-premixed combustion is developed. This transformation makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter. The transformation is used to produce a model for distinguishing between premixed and non-premixed combustion regimes. The new model locally examines the term budget of the general flamelet transformation. The magnitudes of each of the terms in the budget are calculated and compared to the chemical source term. Determining whether a flame burns in a premixed or a non-premixed regime then amounts to determining which sets of these terms most significantly contribute to balancing the source term. The model is tested in a numerical simulation of a laminar triple flame, and is compared to a recent manifestation of the flame index approach. Additionally, the model is applied in a presumed probability density function (PDF) large eddy simulation (LES) of a lean premixed swirl burner. The model is used to locally select whether tabulated premixed or tabulated non-premixed chemistry should be referenced in the LES. Results from the LES are compared to experiments.
Article
The re-examination of the classical droplet vaporization model is made in order to develop the simple but sufficiently accurate calculation algorithm which can be used in spray combustion calculations. The new model includes the effects of variable thermophysical properties, non-unitary Lewis number in the gas film, the effect of the Stefan flow on heat and mass transfer between the droplet and the gas, and the effect of internal circulation and transient liquid heating. To evaluate the competing simplified models of the droplet heating, the more-refined, extended model of heat transfer within a moving circulating droplet is considered. A simplified, one-dimensional 'effective conductivity' model is formulated for the transient liquid heating with internal circulation. As an illustration, the dynamic and vaporization histories of the droplets injected into the steady and fluctuating hot air streams are analyzed.
Article
Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3-dimethylbenzene (m-xylene), and 1-methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. [Combust. Flame 156 (2009) 588–607]. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.
Article
The large Eddy simulations (LES)-conditional moment closure (CMC) method with detailed chemistry is applied to a bluff-body stabilized flame. Computations of the velocity and mixture fraction fields show good agreement with the experiments. Temperature and major species are well-predicted throughout the flame with the exception of the flow regions in the outer shear layer close to the nozzle where the pure mixing between hot recirculating products and fresh oxidizer cannot be captured. LES-CMC generally improves on results obtained with RANS-CMC and on LES that uses one representative flamelet to model the dependence of reactive species on mixture fraction. Simulated CO mass fractions are generally in good agreement with the experimental data although a 10% overprediction can be found at downstream positions. NO predictions show a distinct improvement over the flamelet approach, however, simulations overpredict NO mass fractions at all downstream locations due to an overprediction of temperature close to the nozzle. The potential of LES-CMC to predict unsteady finite rate effects is demonstrated by the prediction of endothermic—or “flame cooling”—regions close to the neck of the recirculation zone that favours ethylene production via the methane fuel decomposition channel.
Article
Comprehensive descriptions of chemical systems frequently contain large numbers of species and reactions, making the prediction of chemical kinetics computationally expensive, particularly when the aim is to embody them within computational fluid dynamic models. There is therefore a need for the development of mathematical representations of chemical kinetics that maintain the important features of full schemes, but with higher computational efficiency and reduced numbers of variables. The present work concerns the use of reduction techniques such as sensitivity and principal component analysis, for the production of a skeleton chemical mechanism. A novel species lumping approach is then developed and applied to the skeleton scheme to achieve further reduction. Application is illustrated for a model describing the oxidation of fuel-rich methane mixtures in a closed vessel under isothermal conditions over a range of reactor temperatures and feedstocks.
Article
A systematic approach for mechanism reduction was developed and demonstrated. The approach consists of the generation of skeletal mechanisms from detailed mechanism using directed relation graph with specified accuracy requirement, and the subsequent generation of reduced mechanisms from the skeletal mechanisms using computational singular perturbation based on the assumption of quasi-steady-state species. Both stages of generation are guided by the performance of PSR for high-temperature chemistry and auto-ignition delay for low- to moderately high-temperature chemistry. The demonstration was performed for a detailed ethylene oxidation mechanism consisting of 70 species and 463 elementary reactions, resulting in a specific skeletal mechanism consisting of 33 species and 205 elementary reactions, and a specific reduced mechanism consisting of 20 species and 16 global reactions. Calculations for laminar flame speeds and nonpremixed counterflow ignition using either the skeletal mechanism or the reduced mechanism show very close agreement with those obtained by using the detailed mechanism over wide parametric ranges of pressure, temperature, and equivalence ratio.