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HyChem Model: Application to Petroleum-Derived Jet Fuels
Abstract
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.
... The resulting lumped reactions were added to a core kinetic mechanism for small hydrocarbon molecules. Recently, Wang et al. [124,125] proposed an approach that uses experimental data to define and calibrate small sets of lumped reactions to describe the high-temperature decomposition of transportation fuel mixtures. Again, those reactions were then appended onto a detailed C 0 − C 4 kinetic mechanism, yielding chemical models of reasonable sizes, even for very complex realistic fuels. ...
... As expected from previous analyses of high temperature n-dodecane decomposition [137], C 2 H 4 is the most dominant species produced in cracking reactions. CH 4 , a species assumed to be a fuel decomposition product in [137] and [124], is not selected as a lumped reaction product due to a low rate of formation in R f . Nevertheless, CH 4 is predicted accurately in lumped simulations because it is primarily produced in core reactions. ...
... drogen and methane combustion [153]. Indeed, the simulations performed in [153] using a lumped jet fuel decomposition description generated with HyChem [124] were some of the first to describe both the evaporation and combustion of a realis- ...
... Surrogate models are thus computationally expensive, if not impossible, for implementation in computational fluid dynamics (CFD) simulations. A newly proposed modeling concept, Hybrid Chemistry (termed HyChem) approach, seeks to advance a compact fuel combustion chemistry model for jet and rocket fuels through a physics-based understanding of the primary reaction pathways in fuel combustion[5,6]. It combines an experimentally constrained, lumped fuel pyrolysis model with a detailed foundational chemistry model for the oxidation of pyrolysis products to describe and predict the combustion behaviors of real, multicomponent liquid fuels. ...
... The A2 fuel is a typical petroleum-based jet fuel, where the major decomposition product is ethylene, and the C1 fuel is a unique bio-derived one with two highly branched alkanes (shown inFigure 1), with isobutene being the dominant decomposition product. For jet fuels studied thus far[5][6][7], ethylene and isobutene appear to be the only major products of fuel pyrolysis. It is thus reasonable to hypothesize, at least from the evidence available, that any jet fuel can be represented by a mixture of A2 and C1. ...
... Detailed discussions on the HyChem approach have been provided elsewhere[5,6]Briefly, HyChem is a pathway-centric, top-down approach to describe and predict the combustion behavior of real, multicomponent liquid fuels. A HyChem model consists of an experimentally constrained fuel pyrolysis model with a detailed foundational chemistry model for the oxidation of pyrolysis products. ...
The hybrid chemistry modeling approach, termed HyChem, was used to explore the combustion chemistry of blended petroleum and bio-derived jet fuels. The pyrolysis products of conventional petroleum derived-fuels, such as Jet A, are dominated by ethylene and propene, whereas in many bio-derived fuels, such as alcohol to jet (ATJ) fuels, the fuel comprises highly branched alkanes and produces isobutene as a main pyrolysis product. We report here an investigation of blends of Jet A (designated A2) and an ATJ fuel (designated C1) with the central question of whether the HyChem models for neat fuels can be combined to model the blend combustion behaviors. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at 1 atm, 1030 and 1140K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements of blended fuel pyrolysis were performed at 1 atm from 1025 to 1325 K. Good agreement between measurements and model predictions was found showing that formation of the products in the blended fuels were predicted by a simple combination of the HyChem models for the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions are “additive.”
... Furthermore, combustion of jet fuels results in myriad intermediate species during the pyrolysis and oxidation processes, such that it is highly challenging to model the chemical kinetic behaviors of real jet fuels. Recently, a HyChem approach was proposed to model high-temperature combustion of practical jet fuels[4,5], and compact real fuel HyChem models have been developed for multiple jet fuels, such as POSF10325 (Jet-A), POSF11498 and POSF12345, also known in the NJFCP naming convention as Cat A2, Cat C1 and Cat C5, respectively. Note that Cat A2 is a conventional jet fuel, while Cat C1 and Cat C5 are derived from alternative sources. ...
... Cat C5 features similar chemical properties and vastly different physical properties as those of Cat A2[3]. The key aspects of these three fuels are listed inTable 1.Table 1. Summary of the target fuels in the present study Fuel ID Key aspects Fuel composition Cat A2/POSF10325 average/nominal, Jet A C11H22 Cat C1/POSF11498 highly-branched iso-paraffinic kerosene, extremely low cetane, unusual boiling range C13H28 Cat C5/POSF12345 very " flat " boiling range C10H19 The HyChem approach is based on the assumption that, fuel cracking through beta-scission is fast at high-temperature conditions compared with the subsequent oxidation of the small molecular fragments[4][5][6][7][8], such that the fuel cracking process can be approximated as a quasi-steady state process and is lumped into a few semi-global reaction steps. This assumption has been extensively investigated in auto-ignition, perfectly stirred reactors (PSR), 1-D laminar flames and turbulent premixed flames, and was found valid not only in the 0-D and 1-D systems, but also at highly turbulent flame conditions[6]. ...
Reduced kinetic models are developed for combustion of different jet fuels based on detailed HyChem models, each of which consists of 119 species, 7 semi-global reactions for fuel cracking, and detailed oxidation pathways for the fuel cracking products. In the present study, fuel-specific reduced models and a universal reduced model are developed. The fuel-specific reduced models are obtained primarily through skeletal reduction using directed relation graph (DRG) and sensitivity analysis, and timescale reduction using the linearized quasi-steady state approximations (LQSSA). The QSS species are removed from the transported equations and solved analytically with internal algebraic equations. This two-stage reduction approach is applied on three different jet fuels, namely POSF10325 (Cat A2), POSF11498 (Cat C1) and POSF12345 (Cat C5), resulting in fuel-specific skeletal models with 41, 34 and 41 species for Cat A2, C1 and C5, respectively, and the fuel-specific reduced models consist of 31, 26, and 31 species, respectively. The fuel-specific skeletal models are combined to further obtain a universal skeletal model with 48 species and 270 reactions, and a universal reduced model with 35 transported species. The universal reduced model features programmable fuel thermodynamic and transport properties and fuel cracking reaction parameters, as well as a shared reduced oxidation core for the fuel cracking products. The fuel-specific and universal reduced models are validated against the detailed HyChem models for auto-ignition, perfectly stirred reactors (PSR), 1-D laminar premixed flame speed, and extinction of premixed and non-premixed counterflow flames.
... The HyChem approach[4,7,8]assumes that fuel pyrolysis is decoupled from the oxidation of decomposition intermediates. The thermal and oxidative thermal decomposition of the fuel is modeled by seven lumped reaction steps in which the stoichiometric parameters and reaction rate coefficients are derived from selected experiments. ...
... While C2H4 is dominant for A2, iC4H8 is the major product of C1 decomposition. Key information from the flow reactor experiment, including the ratios of 1-C4H8and C3H6-to-C2H4 and of C6H6 to the sum of C6H6 and C7H8 (toluene) and the absolute concentrations of CH4 and H2 were used to derive the HyChem model parameters[8]. The rapid rise measured for the concentrations for many species during the early stage of reaction was caused by finite-rate fluid mixing at the reactor entrance. ...
With increasing use of alternative fuels, approaches that can efficiently model their combustion chemistry are essential to facilitate their utilization. The hybrid chemistry (HyChem) method incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the surrogate fuel approach. The present work focused on a comparative study of one conventional, petroleum-derived Jet A fuel (designated as A2), with an alternative, bio-derived fuel (designated as C1), using a variety of experimental techniques as well as HyChem modeling. While A2 is composed of a mixture of n-, iso-, and cyclo-alkanes, and aromatics, C1 is composed primarily of two highly branched C12 and C16 alkanes. Upon decomposition, A2 produces primarily ethylene and propene, while C1 produces mostly isobutene. HyChem models were developed for each fuel, using both shock tube and flow reactor speciation data. The developed models were tested against a wide range of experimental data, including shock tube ignition delay time and laminar flame speed. The stringent validations and agreement between the models and experimental measurements highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of liquid hydrocarbon fuels.
... He showed that a set of representative species can be selected for different fuels. One of the key strategies in the development of HyChem [1][2][3][10][11][12][13][14] and the data-based hybrid chemistry approach [5,7] is based on modeling fuel and fragments during the pyrolysis stage, during which fuel consumption occurs. The fast pyrolysis followed by the slower oxidation of fragments during HTO in complex hydrocarbon fuels was observed by You et al. [15]. ...
The oxidation of complex hydrocarbons is a computationally expensive process involving detailed mechanisms with hundreds of chemical species and thousands of reactions. For low-temperature oxidation, an accurate account of the fuel-specific species is required to correctly describe the pyrolysis stage of oxidation. In this study, we develop a hybrid chemistry framework to model and accelerate the low-temperature oxidation of complex hydrocarbon fuels. The framework is based on a selection of representative species that capture the different stages of ignition, heat release, and final products. These species are selected using a two-step principal component analysis of the reaction rates of simulation data. Artificial neural networks (ANNs) are used to model the source terms of the representative species during the pyrolysis stage up to the transition time. This ANN-based model is coupled with C0
–C4
foundational chemistry, which is used to model the remaining species up to the transition time and all species beyond the transition time. Coupled with the USC II mechanism as foundational chemistry, this framework is demonstrated using simple reactor homogeneous chemistry and perfectly stirred reactor (PSR) calculations for n-heptane oxidation over a range of composition and thermodynamic conditions. The hybrid chemistry framework accurately captures correct physical behavior and reproduces the results obtained using detailed chemistry at a fraction of the computational cost.
... fuel pyrolysis and significantly influences subsequent oxidation chemistry [151,152]. Similarly, C 2 H 4 is a pyrolysis product of many modern ablative thermal protection systems for atmospheric entry vehicles and influences boundary layer flow fields, kinetics, and radiation [153][154][155]. Moreover, C 2 H 4 is a constituent of the atmosphere of many substellar objects such as brown dwarfs, gas giants, and super-earths [156,157]. ...
Absorption spectroscopy is an important branch of spectroscopy that quantitatively measures the level of attenuation on electromagnetic radiation by a test sample. It offers the promise of in-situ, non-intrusive, fast, and sensitive diagnostics for application to transient harsh environments, such as exoplanets, flames, combustion systems, and hypersonic flows. In the endeavor to expand upon existing spectroscopic knowledge of infrared absorption and offer optical sensing solutions to the practical challenges in these complex environments, better experimental strategies of measurement and calibration for the associated high-temperature gas-phase atoms and molecules are warranted. This dissertation describes the development of two experimental approaches for the studies of potassium line shapes and broadband molecular absorption using state-of-the-art lasers at previously unexplored temperature conditions that are made possible by a shock tube.
... Several applications have already been conducted [181], showing the potential of the HyChem method. Pyrolysis products are found to be between 6 and 10, with ethylene being the major intermediate for all conventional jet fuels. ...
With the climate change emergency, pollutant and fuel consumption reductions are now a priority for aircraft industries. In combustion chambers, the chemistry and soot modeling are critical to correctly quantify engines soot particles and greenhouse gases emissions. This thesis aimed at improving aircraft numerical pollutant tools, in terms of computational cost and prediction level, for engines high fidelity simulations. It was achieved by enhancing chemistry reduction tools, allowing to predict CO emissions of an aircraft engines at affordable cost for the industry. Next, a novel closure model for unresolved terms in the LES filtered transport equations is developed, based on neural networks (NN), to propose a better flame modeling. Then, an original soot model for engine high fidelity simulations is presented, also based on NN. This new model is applied to a one-dimensional premixed sooted flame, and finally to an industrial combustion chamber LES with measured soot comparison.
... The standing distance between the flame and stagnation surface/sampling probe was varied by changing the unburned gas flow rate. The flame is pseudo-one dimensional and amenable to numerical simulation using the OpenSMOKE++ code [7] and the HyChem model for Jet A combustion [8,9] with boundary conditions appropriate for the current stagnation flame problem. Soot mobility size distributions were measured at the stagnation surface along the center axis of the flame. ...
Real jet fuels are complex mixtures of many organic components, some of which are aromatic compounds. Towards the high-temperature end of the distillation curve, some of the aromatics may contain multiple rings. A trace amount of these high molecular weight species in the fuel would directly allow for soot nucleation in practical engines especially when the fuel is injected as a spray. This work aims to highlight the variation of the sooting propensity as a function of distillate fractions. Evolution of soot particle size distribution functions (PSDFs) is studied in a series of premixed stretch-stabilized ethylene-oxygen-nitrogen flames doped with Jet A or a certain distillate fraction. Mobility particle size distributions functions, soot volume fraction, and number density are determined by a Scanning Mobility Particle Sizer (SMPS). The results show that the tail end of the distillation curve can produce significantly more soot than the light molecular fraction and the not distilled jet fuel. The implication of the experimental finding on modeling soot formation from real jet fuel combustion will be discussed.
During their high-temperature oxidation, complex hydrocarbons and their early fragments are short-lived and figure prominently only during the pyrolysis stage. However, they are quickly replaced by smaller hydrocarbons at the onset of the oxidation stage, resulting in simpler chemistry requirements past pyrolysis. In this study, we develop a data-based hybrid chemistry approach to accelerate chemistry integration for complex fuels. The approach is based on tracking the evolution of chemistry through representative species for the pyrolysis and coupling their reactions with simpler foundational chemistry. The selection of these representative species is implemented using principal component analysis (PCA) based on simulation data. The description of chemistry for the representative species is implemented using an artificial neural network (ANN) model for their reaction rates followed by the description of their chemistry using a foundational chemistry model. The selection of the transition between these models is trained a priori using an ANN pattern recognition classifier. This data-based hybrid chemistry acceleration model is demonstrated for three fuels: n-dodecane, n-heptane and n-decane and investigated with two foundational chemistry, C0–C2 and C0–C4, models. The hybrid scheme results in computational saving, up to one order of magnitude for n-dodecane, two orders of magnitudes for n-heptane, and three orders of magnitudes for n-decane. The accuracy and saving in computational cost depend on the number of selected species and the size of the used foundational chemistry. The hybrid model coupled with the more detailed C0–C4 foundational performs, overall, better than the one coupled with the C0–C2 foundational chemistry.
An extended hybrid chemistry approach for complex hydrocarbons is developed to capture high-temperature fuel chemistry beyond the pyrolysis stage. The model may be constructed based on time-resolved measurements of oxidation species beyond the pyrolysis stage. The species’ temporal profiles are reconstructed through an artificial neural network (ANN) regression to directly extract their chemical reaction rate information. The ANN regression is combined with a foundational C 0 -C 2 chemical mechanism to model high-temperature fuel oxidation. This new approach is demonstrated for published experimental data sets of 3 fuels: n-heptane, n-dodecane and n-hexadecane. Further, a perturbed numerical data set for n-dodecane, generated using a detailed mechanism, is used to validate this approach with homogeneous chemistry calculations. The results demonstrate the performance and feasibility of the proposed approach.
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.
The oxidation chemistry of complex hydrocarbons involves large mechanisms with hundreds or thousands of chemical species and reactions. For practical applications and computational ease, it is desirable to reduce their chemistry. To this end, high-temperature fuel oxidation for large carbon number fuels may be described as comprising two steps, fuel pyrolysis and small species oxidation. Such an approach has recently been adopted as ‘hybrid chemistry’ or HyChem to handle high-temperature chemistry of jet fuels by utilizing time-series measurements of pyrolysis products. In the approach proposed here, a shallow Artificial Neural Network (ANN) is used to fit temporal profiles of fuel fragments to directly extract chemical reaction rate information. This information is then correlated with the species concentrations to build an ANN-based model for the fragments’ chemistry during the pyrolysis stage. Finally, this model is combined with a C0-C4 chemical mechanism to model high-temperature fuel oxidation. This new hybrid chemistry approach is demonstrated using homogeneous chemistry calculations of n-dodecane (n-C12H26) oxidation. The experimental uncertainty is simulated by introducing realistic noise in the data. The comparison shows a good agreement between the proposed ANN hybrid chemistry approach and detailed chemistry results.
A methodology for the measurement of mid-infrared absorption cross sections of gaseous molecules at high temperatures and pressures is presented. A rapid-tuning, broad-scan external cavity quantum cascade laser with a tuning rate in excess of 30000 cm−1s−1 has been employed in the measurement of full vibrational bands (> 100 cm−1) in shock-heated test gases. The approach is demonstrated with measurements of the absorption cross section of ethylene (C2H4) in the 8.5 µm to 11.7 µm region for temperatures from 8.0 to 1.00 K and pressures from 1 to 5 atm. Decreasing peak strength with temperature is observed as well as pressure-insensitivity. The measurements are compared with existing experimental, empirical, and ab initio databases. Additionally, illustrative absorption cross section measurements of propene (C3H6), 1-butene (1-C4H8), 2-butene (2-C4H8), 1,3-butadiene (1,3-C4H6), and methanol (CH3OH) are presented near 1000 K and 2.3 atm.
Combustion devices such as rocket engines, gas turbines and HCCI engines frequently operate at a pressure higher than the critical pressure of the fuel or the oxidizer. This significantly limits the transferability of existing chemical kinetics models as they are developed and validated at low pressure/temperature conditions, considering only temperature dependence on the reaction rates while neglecting pressure dependence on combustion pathways. Since the experiments are difficult to perform at the supercritical region, in this study, we demonstrate the capability of ReaxFF reactive force field method simulations to study combustion kinetics of fuels and fuel mixtures at these conditions with an objective to investigate how the presence of a highly reactive fuel can alter the properties of a much less reactive fuel during pyrolysis. We consider two different fuel mixtures, namely JP-10/toluene and n-dodecane/toluene and find that they behave differently at different mixing conditions and densities. We also compare our results with continuum simulation results using a detailed chemical kinetic model and elaborate why the continuum results fail to capture the phenomena predicted by the ReaxFF simulations. Lastly, with the help of product distribution of decomposition of different fuel components, we explain the reasons behind the observed behavior. Additionally, this study enables us to identify the pressure/temperature regime and the mixing conditions where the simple first order kinetics and Arrhenius type relations do not prevail. This study reveals that the overall pyrolysis characteristics of a fuel mixture do not only depend on the activation energy of its most reactive component. In fact, the product of pyrolysis plays a more significant role in helping other, less reactive, molecules to decompose. Overall, ReaxFF force field based molecular dynamics simulations can provide important atomistic insights on the pyrolysis properties of fuel mixtures at supercritical conditions.
In the present study, reduced kinetic models, including fuel-specific reduced models and a universal reduced foundational fuel chemistry model for jet fuel combustion, are developed based on the recently developed HyChem models. The HyChem approach takes advantage of the de-coupling between fuel pyrolysis and oxidation of the pyrolysis products that underlies the basic physics of real, liquid fuel combustion processes and the diagnostic capabilities currently available. The resulting HyChem model of real jet fuels is comprised of a " 1-species " lumped model of a jet fuel and a detailed foundational reaction model for the pyrolysis and oxidation H2/CO/C1-4/one-ring aromatics, and is thus already substantially reduced in size. The foundational fuel chemistry model may be further reduced through skeletal reduction using directed relation graph (DRG) and sensitivity analysis, and timescale reduction using the linearized quasi-steady state approximations (LQSSA). This two-stage reduction approach is applied on one conventional and two alternative jet fuels, resulting in fuel-specific reduced models with 31, 26, and 31 species, respectively. A universal reduced model with 35 species is further proposed for the three fuels, which features programmable fuel thermodynamic and transport properties and fuel cracking reaction parameters, as well as a shared reduced oxidation core for the fuel cracking products. The fuel-specific and universal reduced models are validated against the detailed HyChem models for auto-ignition, perfectly stirred reactors (PSR), 1-D laminar premixed flame speed, and extinction of premixed and non-premixed counterflow flames.
The development of a compact HyChem reaction mechanisms for jet fuels requires datasets both for pyrolysis products yields to constrain the model and for kinetic targets to evaluate the model. To this end, we have measured selected species time-histories during fuel pyrolysis using laser absorption, and ignition delay times using multiple methods behind reflected shock waves in a heated shock tube. Measurements were performed for three jet fuels over a temperature range of 1000-1400 K and pressures from 12 to 40 atm, for equivalence ratios of 0.5 to 1 and diluted in nitrogen or argon. Fuel loading was measured using an IR He-Ne laser at 3391 nm; ethylene with a CO 2 gas laser at wavelengths of 10532 nm and 10674 nm; and methane with a tunable diode laser at wavelengths of 3175 nm and 3177 nm. Ignition delay times were measured three ways: by monitoring fuel removal with laser absorption, by sidewall pressure, and by OH* emission. Particular care was taken in this study in mixture preparation and transfer of the gaseous fuel mixture to the shock tube. The current HyChem model shows good agreement with these data.
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