Article

A physics-based approach to modeling real-fuel combustion chemistry – IV. HyChem modeling of combustion kinetics of a bio-derived jet fuel and its blends with a conventional Jet A

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Abstract

A Hybrid Chemistry (HyChem) approach has been recently developed for the modeling of real fuels; it incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the conventional surrogate fuel approach. The present work extends this approach to modeling the combustion behaviors of a two-component bio-derived jet fuel (Gevo, designated as C1) and its blending with a conventional, petroleum-derived jet fuel (Jet A, designated as A2). The stringent tests and agreement between the HyChem models and experimental measurements for the combustion chemistry, including ignition delay and laminar flame speed, of C1 highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of complex liquid hydrocarbon fuels. Another aspect of the present study aims at answering a central question of whether the HyChem models for neat fuels can be simply combined to model the combustion behaviors of fuel blends. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at pressure of 1 atm, temperature of 1030 K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements were performed for the blended fuel pyrolysis at 1 atm from 1025 to 1325 K. Ignition delay times were also measured using a shock-tube. Good agreement between measurements and model predictions was found showing that formation of the products as well as combustion properties of 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.”

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... Other groups [5][6][7] have also simulated the same configuration under NJFCP, but with key differences in modeling and analysis of the results as further discussed in [8] . A finite-rate chemistry (FRC) with a partially stirred reactor (PaSR) turbulence-chemistry closure and a non-stiff moderately reduced mechanism (26-31 species, 182-202 reactions) [12] is used here to capture any possible extinctions/re-ignitions, whereas flamelet [5] , zonal [6] , or quasi-laminar [7] approaches by the other groups. Pressure-based solvers are faster for low-Mach problems [5,8] , but a density-based solver [13] is used here as it can capture acoustic interactions. ...
... Simpler swirl combustors show a vortex breakdown bubble (VBB) caused by the swirling motion, burning in the shear layers, and the flame is typically anchored in a premixed manner with non-premixed burning away from it [9,14] . However, the referee rig flow-field is highly complex and the real fuels that are considered here go through a pyrolysis step [12] before burning at the flame surface, both of which could possibly affect the flame stabilization mechanism requiring further analysis. Previous studies of the referee rig combustor revealed importance of hot products in the recirculation zone in stabilizing the flame [5,6] . ...
... As noted earlier, A2 is a nominal Jet-A fuel with average properties (Derived Cetane Number (DCN) = 48.3, T 20 = 447 K), and C1, selected as an alternate fuel under NJFCP [8] , is an alcoholto-jet with slower chemistry but faster vaporization (DCN = 17.1, T 20 = 445 K) [12] . These fuels contain thousands of hydrocarbon ...
Article
Understanding combustion and flame stabilization of alternate fuels is necessary to establish their use in mainstream aviation. An Eulerian-Lagrangian (EL) large eddy simulation (LES) study is conducted using a fully compressible solver for a Referee Rig combustor, developed within the National Jet Fuel Combustion Program, (NJFCP). The Referee Rig contains multiple swirlers, dilution jets, and effusion holes, mimicking the complex flow-field in a real gas turbine combustor. Conditions as in the experiments for near blowout (NBO) operation are simulated for two fuels identified by NJFCP: a baseline fuel (A2) and an alternate fuel (C1). Time-averaged LES results are shown to provide a reasonable match against the experimental droplet statistics and OH* chemiluminescence data for both the fuels. Several post-processing tools, such as, conditional averaging, flame index, chemically explosive mode analysis (CEMA) are used to understand flame stabilization mechanism and burning behavior under NBO condition. Flame anchor point is shown to be premixed, but most heat release results from fuel pockets burning in the shear layer in a non-premixed manner. Pressure signatures at various locations and other time-varying properties show fuel-sensitive effects, even though the time-averaged behavior is similar between A2 and C1. When compared to A2, C1 shows a higher premixed burning and it is shown to be dependent on the slower burning behavior of C1 at NBO.
... Under NJFCP, multiple groups [6][7][8] simulated LBO for this configuration, and a summary of similarities and differences can be found in [9] . This work uses finite-rate chemistry (FRC, 26-31 species, 182-202 reactions [10] ) with a partially stirred reactor (PaSR) turbulence-chemistry closure, wheres other groups used flamelet [6] , quasi-laminar [8] , or a zonal approach [7] . The LBO was captured by all, but the flamelet model showed a reversed fuel sensitive trend, while the zonal simulations captured it for certain chemical kinetic mechanisms but not all. ...
... A density-based Eulerian formulation is used for the gas phase, and the liquid phase is solved using a Lagrangian approach with a secondary breakup model [5] . Reduced non-stiff kinetics with 31species-202-reactions and 26-species-182-reactions [10] are used for A2 and C1, respectively. An eddy-viscosity closure [19] is used for subgrid-scale (SGS) fluxes, with a simplified partially stirred reactor model [20] for filtered reaction rates. ...
Article
Large eddy simulation (LES) is conducted for lean blowout (LBO) prediction in the Referee rig combustor, developed within the national jet fuel combustion program (NJFCP). Starting from the stable flame solution at a stable equivalence ratio (ER) (discussed elsewhere [1]), multiple step-by-step and direct reductions in the fuel flow rate are conducted. Each reduced ER is simulated until either the flame stabilizes to a new burning state or completely blows out, which is then defined as the predicted LBO ER. In the NJFCP, two fuels are considered: a baseline fuel (A2), and an alternate fuel (C1). The predicted LBO points are within 13% and 15% of the experimental values for A2 and C1, respectively, and C1 blows out at an ER that is 5.7% higher than that of A2, consistent with experimental observations. The heat release rate (HRR) efficiency gradually reduces with ER before it suddenly drops to zero at the LBO point, whereas, the evaporation efficiency stays the same, suggesting a chemistry-dominant blow-out for this configuration. Burning mode analysis shows that the gradual reduction in HRR is correlated with the non-premixed part of the flame, whereas the premixed anchoring stays the same, until it finally extinguishes as pockets from the cold recirculation zone hit it over time. Pressure signatures show activated acoustic modes during the stable burning, but a highly irregular dynamics is observed as the LBO is approached. Fuel-sensitive differences through burning mode analysis and pressure signatures are apparent even at stable burning conditions, that point towards an eventual fuel-sensitive LBO for this configuration.
... For example, when liquid fuels are used as thermal heat sinks in hypersonic vehicles, the increase in temperature promotes thermal cracking and the formation of alkenes -specifically ethylene [52]. Also, modern approaches to develop chemical kinetic models divide reaction chemistry of jet fuels in to two stages (see Hychem model [53][54][55][56]). The first stage involves break down of large fuel species to smaller species (such as alkenes) via pyrolysis and the second stage is the oxidation of these smaller species to final products. ...
... Clearly, alkenes play a major role in affecting the laminar flame speed of n-decane/air mixtures. The importance of alkenes in jet fuel chemistry is further evidenced by a recent approach (HyChem) to chemical kinetic mechanism development that replaces the numerous and detailed pyrolysis and partial oxidation steps for complex (heavy hydrocarbon) jet fuels with a few "global" steps that produce smaller hydrocarbons considered to be important [53][54][55][56]. Essentially, the mechanism is based on the validated assumption that the conversion of the complex hydrocarbons to simple hydrocarbons occurs rapidly, much faster than the oxidation reactions that lead to the majority of the heat release. ...
Thesis
Conducting full-scale experiments as part of the design process of jet engine combustors is a costly and time-consuming process. Therefore engine developers have been increasingly using numerical modeling approaches to assess new designs or design changes. The reaction chemistry, which is dependent on the flow conditions, the fuel composition, and the oxidizer composition, plays an important role in the accuracy of these simulations. The kinetic mechanisms that describe this chemistry need to be validated. Various global combustion characteristics are used to validate mechanisms against experimental data; one of these is laminar flame speed (SL). In this work, laminar flame speeds of various fuels relevant to jet engine combustion are measured using a previously developed, modified Bunsen Flame Technique (BFT). The accuracy of the BFT is examined here, both through a comparison to experimental results from other standard approaches for a range of fuels and through a detailed analysis of the impact of flame stretch. The measured flame speeds are also used to test leading chemical kinetic mechanisms, primarily the NUI and USC models. Laminar flame speeds of n-decane, ethylene and propylene are measured at conditions relevant to jet engine main combustors and afterburners. The experimental conditions include high preheat temperatures (up to 650 K) and reduced O2 levels (down to 15% mole fraction in the oxidizer); the latter is relevant to vitiation, where there is partial pre-burning of the oxidizing flow. Furthermore, vitiation introduces combustion products such as CO2 into the reactant stream that can participate in the combustion chemistry. Therefore, flame speeds are measured using dilution with both CO2 and N2 (considered non-reactive) to study these effects. SL measurements for alkenes using BFT are within 10% of measurements from literature and chemical kinetic mechanism predictions at 300 K and atmospheric pressure. At high preheat temperatures, the mechanisms accurately predict SL for ethylene mixtures, while they over predict SL of propylene mixtures at 650 K. Vitiation studies at 650 K preheat show that for N2 dilution and ethylene, the reduction in flame speed is mostly due to thermal effects. Some chemical effects were observed when the O2 level in the oxidizer was reduced to 15% (vol.). For propylene, reducing O2 had a larger impact on flame speed than that predicted by the mechanisms. With CO2 as a diluent, the mechanisms over predicted the flame speed, and the prediction error increased with higher levels of CO2. Reactions involving the allyl (C3H5-A) radical were identified as a likely source of the propylene flame speed errors, increase in the pre-exponential rate factor of the allyl-H recombination reaction improved the predictive capability of the mechanism at high preheat temperatures. Similarly, analysis of different sources of errors with CO2 dilution suggest the third-body efficiency of CO2 is underestimated in a three-body association (such as H + O2 (+M) <=> HO2 (+M)) type of reactions.
... Measurements using a rapid compression machine (RCM) were done by Min et al. [8] and Valco et al. [9] at 625 K < T < 725 K and p = 20 bar. Recently, Wang et al. [10] proposed a physics-based model for simulating ignition characteristics of Gevo's AtJ-SPK, and validated their model against laminar flame speeds at p = 1 atm for unburned mixtures at 403 K, as well as ignition delay times at various conditions and speciation data obtained from shock tube and flow reactor facilities. ...
... A comparison of the predicted laminar flame speed of AtJ-SPK at 403 K using the LLNL mechanism from the present work to experimental data from Wang et al. [10] is given in Fig. S3 of the supplementary material. With only a slight overprediction the new mechanism reproduces these experimental data as well. ...
Article
This work presents an investigation of fundamental combustion properties, specifically laminar burning velocity and ignition delay time, of an Alcohol-to-Jet Synthetic Paraffinic Kerosene (AtJ-SPK). Used in blends, this fuel is a sustainable aviation fuel that consists mostly of two long-chained, highly branched alkanes. Laminar burning velocities were measured at a preheat temperature of 473 K and pressures of 1 and 3 bar using the cone angle method. Ignition delay times of fuel-air mixtures diluted in nitrogen (N2) were experimentally determined behind reflected shock waves at two fuel-air equivalence ratios, 1.0 and 2.0, at a pressure of 16 bar. In addition to these experiments, a modeling study was conducted using a new chemical kinetic reaction mechanism developed to describe the combustion behavior of the investigated AtJ-SPK. The simulations show that the new detailed mechanism is able to predict sufficiently the laminar flame speed at ambient pressure as well as the ignition delay time at elevated pressure. Sensitivity analyses for laminar flame speed and ignition delay time were performed as well.
... Zero-stretch laminar flame speeds were calculated with the HyChem mechanisms [29][30][31] in Cantera [32], and are reported in Table 1. Adiabatic flame temperatures agree within 5 % for the three fuels. ...
... Here, S L,b is the burned laminar flame speed evaluated at the strain rate k, and S L b0 is the unstretched burned laminar flame speed. Flame simulations were performed using Cantera [32] and the HyChem mechanisms for jet-A [30,31], C5 1 , and C1 [29]. S L b was evaluated at the location of peak heat release in the strained laminar flame, and S L b0 was determined by extrapolating S L b to zero-stretch. ...
Preprint
Large hydrocarbon fuels are used for ground and air transportation and will be for the foreseeable future. Despite their extensive use, turbulent combustion of large hydrocarbon fuels, remains relatively poorly understood and difficult to predict. A key parameter when burning these fuels is the turbulent consumption speed; the velocity at which fuel and air are consumed through a turbulent flame front. Such information can be useful as a model input parameter and for validation of modeled results. In this study, turbulent consumption speeds were measured for three jet-like fuels using a premixed turbulent Bunsen burner. The burner was used to independently control turbulence intensity, unburned temperature, and equivalence ratio. Each fuel had similar heat releases (within 2%), laminar flame speeds (within 5-15 %), and adiabatic flame temperatures. Despite this similarity, for constant Re_D and turbulence intensity, A2 (i.e., jet-A) has the highest turbulent flame speeds and remains stable (i.e., without tip quenching) at lower {\phi} than the other fuels evaluated. In contrast the C1 fuel, which contains no aromatics, has the slowest turbulent flame speeds and exhibits tip quenching at higher {\phi} then the other fuels. C1 was the most sensitive to the influence of turbulence, as evidenced by this fuel having the largest ratio of turbulent to laminar flame speeds. The C1 fuel had the highest stretch sensitivity, in general, as indicated by calculated Markstein numbers. This work shows that turbulent flame speeds and tip stability of multi-component large hydrocarbon fuels can be sensitive to the chemical class of its components. The results from the current work indicate that caution may be needed when using alternative or surrogate fuels to replicate conventional fuels, especially if the alternative fuels are missing chemical classes of fuels that influence stretch sensitivities.
... Heptane flames were simulated using a detailed kinetic model by Smallbone et al. [124] with 955 reactions and 130 species. To simulate the kerosene (A2 and C1) flame chemistry model based on a hybrid approach proposed by Ref. [125,126] was used. In the hybrid chemistry approach, the fuel's kinetics of thermal and oxidative pyrolysis are modeled using lumped kinetic parameters. ...
... In the hybrid chemistry approach, the fuel's kinetics of thermal and oxidative pyrolysis are modeled using lumped kinetic parameters. [7,127,[122][123][124][125][126] and within. The trend in S L with φ is captured well for different fuels. ...
Thesis
To achieve low emission targets, combustion technologies have increasingly implemented lean premixed flames as they facilitate low NOx and soot emissions. In most modern transport vehicles, combustion occurs under highly turbulent conditions. However, stabilizing lean premixed flames within the high Reynolds number conditions of practical devices is difficult because they are prone to blow-off, resulting in reduced efficiency or worse, engine failure. Thus, there is a need to understand the underlying physics of lean blow-off (LBO) so that accurate, yet computationally tractable models can be developed to predict its onset. In this dissertation, the lean blow-off limits and turbulent flame structure of unconfined, pre-vapourised liquid fuels stabilised on a bluff body burner were investigated at two conditions: far from blow-off (φ/φbo = 1.20) and close to blow-off (φ/φbo = 1.01). Four different fuels were considered, two of which comprised of a single component (ethanol and heptane) while the other two were multi-component kerosene blends (A2 and C1 from the National Jet Fuel Combustion Programme). The lean blow-off limit indicates that the ethanol and heptane flames are more resilient to blow-off than the kerosene fuels. To facilitate comparisons with gaseous-fueled flames, results were also obtained from methane flames. Furthermore, a correlation based on a Damköhler number (Da), which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the Da correlations based on laminar flame speed are not applicable. The flame structure and lean blow-off behaviour were studied with OH* chemiluminescence and high-speed (5 kHz) OH-PLIF imaging. Additionally, CH2O-PLIF imaging was used to assess the impact of fuel composition on the CH2O-layer thickness. As the flame approached LBO, fragmentation was observed downstream. The two sides of the flame merged at the axis, pockets of OH and CH2O were found in the recirculation zone (RZ), and eventually, the individual fragments were extinguished. The CH2O seemed to enter the RZ from downstream early in the LBO process, with reactants following suit at times closer to LBO. During LBO, the integrated OH* signal decreased slowly to zero. The duration of this transition was ~25 (d/UBO) in the methane and ethanol flames and ~60 (d/UBO) in the flames operated with heptane and the two kerosenes (where d is the bluff-body diameter and UBO the LBO velocity). This large difference could be due to the re-ignition of partially-quenched fluid inside the RZ during the LBO event. Additionally, for the same bulk velocity, the kerosene flames blow-off at higher equivalence ratios than the single-component fuelled flames, possibly due to the higher Lewis number and lower extinction strain rates of these fuels. The results suggest that the blow-off mechanism is qualitatively similar for each of the fuels; however, the complex chemistry associated with heavy hydrocarbons appears to result in a prolonged LBO event. The average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to those of methane: the flame brushes of both exhibit an M-shape when close to blow-off. In contrast, the distribution of OH* signal in the kerosene flames is primarily concentrated in regions further downstream of the bluff body. Also, whilst close to blow-off, the flame fronts on opposite sides of the bluff body in the downstream region of the recirculation zone merge to create a seemingly closed region above the bluff body for all four flames. The OH-PLIF images of ethanol at far from blow-off display a higher intensity of OH-LIF signal along the annular jets, while the OH-LIF signal was more distributed in the heptane- and kerosene-fueled flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (∑2D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region at close to blow-off. No significant variation in ∑2D, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH2O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, when LBO is approached, an increased amount of CH2O-LIF signal is observed within the recirculation zone, which is consistent with prior results obtained from methane flames. Overall, the thickness and appearance of the CH2O-layers are qualitatively similar between the single- and multi-component fuels; however, the kerosene fuels tend to exhibit wider CH2O-layers. Additionally, these fuels tend to possess more isolated pockets of CH2O-LIF signal within the recirculation zone, suggesting that a considerable amount of partially-combusted fluid enters it. High-speed particle image velocimetry was performed to measure the local velocity fields and place these flames on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. Ultimately, this effort's results highlight the influence fuel-type has on the LBO of bluff-body stabilized flames. Moreover, this work indicates that the LBO behavior of flames produced with complex hydrocarbon fuels can not be fully understood by high-temperature chemistry concepts such as laminar flame speed.
... One of the efficient approaches is the use of shock tube apparatus in combination with advanced laser absorption to measure the pyrolysis speciation. 31,47,48 However, such analytical instruments are usually species dependent and expensive. In contrast, the single-pulse shock tube (SPST) has been implemented by several groups in recent years for fuel pyrolysis studies due to the simplicity in post-shock sampling and analysis. ...
... From the formation of major products, the pyrolysis process of the DCL-derived jet fuel under the studied conditions starts above 1100 K. C 2 H 4 is the most abundant product for the studied two experimental conditions, which is the same as previously studied other jet fuels. 31,42,44,46,48 It is worth noting that the quantity of acetylene (C 2 H 2 ) increases significantly as the temperature increases mainly due to the high-temperature cracking process of C 2 H 4 , 1,3-butadiene (C 4 H 6 ), and so on. 44,57,58 CH 4 is another major product, and its yield gradually increases as the temperature increases. ...
Article
Full-text available
A basic understanding of the high-temperature pyrolysis process of jet fuels is not only valuable for the development of combustion kinetic models but also critical to the design of advanced aeroengines. The development and utilization of alternative jet fuels are of crucial importance in both military and civil aviation. A direct coal liquefaction (DCL) derived liquid fuel is an important alternative jet fuel, yet fundamental pyrolysis studies on this category of jet fuels are lacking. In the present work, high-temperature pyrolysis studies on a DCL-derived jet fuel and its blend with the traditional RP-3 jet fuel are carried out by using a single-pulse shock tube (SPST) facility. The SPST experiments are performed at averaged pressures of 5.0 and 10.0 bar in the temperature range around 900–1800 K for 0.05% fuel diluted by argon. Major intermediates are obtained and quantified using gas chromatography analysis. A flame-ionization detector and a thermal conductivity detector are used for species identification and quantification. Ethylene is the most abundant product for the two fuels in the pyrolysis process. Other important intermediates such as methane, ethane, propyne, acetylene, and 1,3-butadiene are also identified and quantified. The pyrolysis product distributions of the pure RP-3 jet fuel are also performed. Kinetic modeling is performed by using a modern detailed mechanism for the DCL-derived jet fuel and its blends with the RP-3 jet fuel. Rate-of-production analysis and sensitivity analysis are conducted to compare the differences of the chemical kinetics of the pyrolysis process of the two jet fuels. The present work is not only valuable for the validation and development of detailed combustion mechanisms for alternative jet fuels but also improves our understanding of the pyrolysis characteristics of alternative jet fuels.
... IDTs of another typical jet fuel named RP-3 widely used in China were also studied recently [15,16]. These experimental measurements were also mostly predicted by using detailed kinetic mechanisms [5,[17][18][19][20]. Three to six representative hydrocarbon molecules based on the composition of real jet fuels were selected to construct the surrogate models [5,19]. ...
... Fig. 9 shows the time evolution of major intermediates during the oxidation of the three fuels at 1200 K and 10 bar with equivalence ratio of 1.0. The high-temperature ignition properties of jet fuels are mainly controlled via small fuel molecules as demonstrated by a series of studies [18,40,41], however, the distribution of major small fuels molecules controlling the ignition of the three jet fuels are still different as shown in Fig. 9. For the three fuels, ethylene is the most abundant intermediate. ...
Article
Jet fuel from direct coal liquefaction (DCL) is an important alternative kerosene. The study of its ignition characteristics is not only critical for the development of combustion kinetic models, but also is valuable to evaluate its compatibility in current aeroengines. This work reports the first experimental study on autoignition characteristics of a jet fuel from DCL and its blends with conventional RP-3 jet fuel. Ignition delay times (IDTs) for DCL fuel, RP-3 and their blends are measured by using heated shock tube facility over a wide range of conditions. The studied pressure is 2 and 10 bar, the equivalence ratios are 0.5, 1.0 and 2.0, and the temperature range is from 920 K to 1700 K. The influence of temperature, pressure, and equivalence ratio on the IDTs is investigated. Surrogate models for the three fuels and detailed kinetic mechanism are developed to describe the combustion chemistry. Comparison between the predictions using the detailed kinetic models and the experimental data shows that the developed models exhibit acceptable performance versus the measured IDTs. Brute force sensitivity analyses are carried out to identify the key reactions controlling the ignition characteristics. The experimental data set and kinetic models extend our understanding of the ignition characteristics of alternative jet fuels.
... The main difference in chemical behaviour between the two fuels based on the selected species shown is the presence of ethylene and toluene. Both fuels pyrolyse primarily into C 2 H 4 after the semi-global pyrolysis reaction steps [51], but C5 consistently has about 10% lower peak ethylene values compared to A2. However lower peaks of ethylene in the C5 flame are more than counterbalanced by the presence of the aromatic toluene. In the low 0 flamelet in Fig. 2a, peak C5 toluene mass fraction is 67% greater than in the A2 flame, and for the high 0 flamelet the difference is even greater at 77%. ...
Article
Full-text available
Swirl-stabilized turbulent spray flames operating at atmospheric pressure, with both a conventional Jet-A kerosene (‘A2’) and a synthetic fuel blend (‘C5’) and radially-injected dilution air were investigated from the perspective of soot emissions in a model Rich-Quench-Lean (RQL) combustor. The primary and downstream dilution air flow split was varied while keeping the global equivalence ratio constant (ϕg=0.4). 2.5 kHz PIV, 5 kHz OH-PLIF, and simultaneous 10 Hz Mie scattering and LII were used. Results revealed a correlation between OH and soot distributions. With no dilution air, soot volume fractions (fv) peaked in a v-shaped cone between the OH-containing regions along the spray cone and those anchored to the bluff body. With dilution air at 20% of total air flow, fv shifted to the central axis as OH distributions widened. At 40% dilution, soot initiation began farther downstream along the spray cone, OH regions merged into one, and fv peaked farther downstream at the edge of the Central Recirculation Zone (CRZ). With no air dilution, peak time-averaged fv was 20% lower in the C5 flame compared to A2, despite C5 having a higher aromatics content. However, at 20% dilution fv in C5 increased by 75% compared to the no dilution case. fv in the C5 flame dropped by 12.5% in the high dilution case compared to 40% reductions in A2. Mie scattering showed that the C5 spray was shorter and more resistant to change from dilution air compared to A2. The different spray behaviours were due to C5’s flat boiling range, lower viscosity and density, leading to higher evaporation rates. Detailed chemistry laminar flamelet simulations of A2 and C5 showed that peak soot precursor species mass fraction values and temperatures decreased as the scalar dissipation increased and a significantly stronger presence of toluene in the C5 flame compared to A2. Toluene’s soot yielding behaviour may cause C5’s increased fv with moderate dilution compared to A2. C5 demonstrates the importance of the fuel’s chemical and physical properties when developing future sustainable aviation fuels for practical applications to maximize their soot mitigation potential.
... Such experimental measurements can provide detailed, time-resolved data to inform both traditional detailed kinetic mechanism development [e. g., 36] and novel strategies for efficiently modeling real, complex fuels, such as HyChem models [e.g., [53][54][55][56][57]. ...
Thesis
Full-text available
Developed through the combination of two preexisting methods — a shock tube acting as an impulse heater and a flame speed measurement from a spherically expanding flame — the shock-tube flame speed method brought significant promise to enable fundamental laminar flame speed measurements at previously inaccessible temperature conditions. Nevertheless, early applications of the method, as originally devised, encountered challenges associated with flame stability and structure that limited its ability to fulfill its full potential. In this dissertation, a series of efforts undertaken to characterize and optimize the shock-tube flame speed method are reported; the newly refined methods are subsequently validated and applied to demonstrate flame speed measurements at extreme unburned-gas temperatures, up to and exceeding 1,000 K, for the first time. After introducing the fundamentals of shock tubes, spherically expanding flames, and their combination in the first-generation shock-tube flame speed method, three investigations extending the original methods are described. First, the development of a technique for performing side-wall emission imaging through a small diagnostic port allowed for the identification of axial flame distortion in shock tube experiments. Then, the side-wall imaging was again leveraged in the development of the [laser-induced] flame image velocimetry ([LI]FIV) technique as a seedless, single-point velocimetry method for combustion environments, which was used in the first systematic investigation of core-gas velocities in the post-reflected-shock environment. Finally, a meta-analysis to identify conditions producing stable flames was performed on a collection of ten groups of experiments performed using variations of the first-generation method. In the resulting binary-classifier model, the unburned-gas ratio of specific heats and the ignition location were found to most strongly affect stability, guiding the optimal selection of an oxygen-argon oxidizer mixture for future experiments and motivating the need for additional experimental flexibility. Inspired by the significant insight gained through the application of side-wall imaging to shock-tube flame experiments, and seeking to realize the flexibility required to perform optimized flame speed experiments, a novel side-wall imaging flame test section (SWIFT) was designed and procured. The SWIFT features first-of-their-kind side-wall windows designed as cemented-doublet cylindrical lenses in order to provide large field-of-view, schlieren-compatible optical access through the curved side walls of the shock tube. Together with an enhanced suite of instrumentation, the implementation of the SWIFT enabled what would become the second-generation of the shock-tube flame speed method through the studies that followed. Making use of the new schlieren capabilities, the effect of the axial position on the stability of flames was reevaluated, both using static experiments to quantify the effect of asymmetric endwall confinement and through post-reflected-shock experiments performed near 650 K and 1 atm to observe the effect of the post-shock flow field, reaffirming the presence of significant axial distortion at a certain (6.4-cm) axial location. Then, based on the need for a model capable of extracting laminar flame data from experiments exhibiting aspherical flames, an area-averaged formulation of the linear-curvature extrapolation model (the AA-LC model) was derived for use in shock-tube flame experiments. Applied to the static and 650 K experiments at different ignition locations, the model was demonstrated to yield precise and repeatable measurements, even in cases in which flame distortion was observed. The SWIFT, side-wall schlieren, and AA-LC model were finally applied in laminar flame speed measurements of propane, n-heptane, and iso-octane at highest-ever-reported unburned-gas temperatures, up to and exceeding 1,000 K.
... The parameters of the lumped fuel decomposition model are inferred from speciation measurements conducted in carefully designed shock tube and flow reactor experiments. Despite its successes in modeling the high temperature (>1100 K over pressures of 1-60 atm) kinetics of a wide variety of fuels [4][5][6], the efficacy of the HyChem approach in modeling and predicting low-temperature oxidation of fuels has not been demonstrated yet. A strategy geared towards this was described by Xu et al. [4]. ...
Conference Paper
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The complexity of modeling the oxidation kinetics of conventional transportation and aviation fuels stems from the immense number of unique components that constitute them. Unsurprisingly, these components give rise to a multitude of possible chemical pathways. Identifying and modeling each of these pathways accurately is a challenge that has attracted significant interest from the chemical kinetic research community for the past few decades. The HyChem approach for modeling kinetics of real fuels was one of the major successful modeling strategies that was borne out of decades worth of research [1,2]. The HyChem approach was developed at Stanford and has been demonstrated to be remarkably successful in predicting the oxidation kinetics of distillate fuels at high temperatures [3]. The HyChem approach models the fuel, regardless of its composition, as a single molecule. Recognizing the distinct difference in the time scale of decomposition of heavier components of the fuel into smaller and simpler (foundational) hydrocarbons, and their oxidation, the modeling approach attempts to describe the overall oxidation kinetics by an amalgamation of a simple, lumped fuel pyrolysis model and a detailed foundational fuel model. In doing so, the model implicitly assumes a cause-and-effect relationship between the identity of decomposition products, which is intimately linked to the composition and structure of the fuel, and global combustion properties like ignition delay times (IDT) and flame speeds. The parameters of the lumped fuel decomposition model are inferred from speciation measurements conducted in carefully designed shock tube and flow reactor experiments. Despite its successes in modeling the high temperature (>1100 K over pressures of 1-60 atm) kinetics of a wide variety of fuels [4-6], the efficacy of the HyChem approach in modeling and predicting low-temperature oxidation of fuels has not been demonstrated yet. A strategy geared towards this was described by Xu et al. [4]. They adopted a reduced skeletal model to describe low-temperature oxidation based on the formulation developed by Bikas and Peters [7]. The model parameters were tuned to match the second stage IDTs measured in shock tubes for stoichiometric, and lean fuel/oxidizer mixtures. There are two major issues with this approach: 1) The skeletal model is based upon the state of knowledge of low-temperature oxidation of n-alkanes in early 2010s, 2) The model parameters were directly inferred from a global combustion property (IDT), which goes against the philosophy of the HyChem approach, renders the model susceptible to inaccuracies in IDT data (for example, due to inhomogeneous ignition), and incapable of predicting the timing of first stage ignition and low-temperature heat release (LTHR) accurately. The updated skeletal model for oxidation of alkanes based on these recent studies that incorporates these pathways is shown in fig. 1. Recent synchrotron, and photoionization-mass spectroscopy studies with flames have revealed the formation of highly oxygenated transient and stable species containing more than four oxygen atoms [8]. The formation of these species could only be explained by reaction pathways that would enable the addition of a third oxygen molecule to the ketohyroperoxy species (QOOH) upon their isomerization to P(OOH)2 via internal H-abstraction. It is worth noting that these additional channels could also contribute to chain branching and raise the overall reactivity of the fuel under certain conditions [9,10]. Therefore, it is imperative for modelers to account for these recent advances in the knowledge of oxidation of hydrocarbons in developing chemical kinetic models for distillate fuels. This work attempts to do so by augmenting the low-temperature HyChem model of Xu et al. by addition of four fuel dependent species and seven more reactions to the model. Modifications were also made to the reactions retained from the Xu et al. model to enable inference of model parameters from shock tube experiments. The details of these experiments are described in the following sections of the paper. These experiments have already been performed and reported [11,12] with four neat fuels, i.e., n-heptane, n-octane, n-decane, and 2-methylhexane. Additional experiments were conducted with a three-component gasoline surrogate containing n-heptane, iso-octane, and toluene (TPRF 60). The augmented HyChem model was developed for each of these fuels based, and its performance was compared against second stage IDT measurements reported in the literature. The models show good agreement with the measurements and pave the path for application of this modeling approach to real fuels.
... fuels and its blending with petroleum jet fuel[101].Fig. 15.4 shows a schematic diagram of the different strategies for process simulation and optimization. ...
Chapter
Bio-jet fuels have great potentials in decreasing the reliance on fossil-based jet fuels and decrease CO2 discharges. The International Air Transport Association (IATA) reported with the aim of bio-jet fuels created by that using sustainable sources like biomass to produce bio-jet fuels is a promising strategy to develop and industrialize an alternative aviation fuel to encounter the sustainable growth in the aviation sector. Bio-jet fuels chemical compositions have a significant impact on their performance characteristics. The main performance characteristics of bio-jet fuel performance characteristics include are thermal oxidation Stability of Thermal thermal oxidation, the bio-based jet fuels compatibility with the current system of aviation, low-temperature fluidity, combustion characteristics, fuel metering, and fuel volatility are the bio-jet fuel performance characteristics. Bio-jet fuels These characteristics have been evaluated by the ASTM standards. The conversion technologies of bio-based feed stock can be classified by alcohol to jet (ATJ) alcohol to jet, sugar to jet (STJ), oil to jet (OTJ), and gas to jet (GTJ). Hydrogenated esters and fatty acids Fatty acids (HEFA), Hydrogenated esters, and catalytic hydro- thermolysis (CH) are the common pathways for bio-jet fuel production. The impact of bio-jet fuels delivered from different feedstock, including algae, on jet engine performance was the focus of the researcher by numerical modeling and virtual simulation. Researchers found that the thermodynamic behavior, fuel consumption level of the aircraft and emissions characteristics have been improved by using Biobio-jet fuel as compared with the conventional Jet-A fuel. The mean focus of the current chapter is to summaries summarize the most available study studies of the algae-based bio-jet fuels conversion technologies, characteristics, performance, and process simulation.
... Some of the key barriers to adopting alternative jet fuels are the extensive fuel testing, approval and certification processes. Therefore, improvements on the certification process and optimisation of the existing technological pathways are important to reduce aviation carbon emissions [108]. During the production of alternative jet fuels, heterogenous catalysts are commonly used as compared with homogenous catalysts. ...
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... The shock tube facility can reach high-temperature conditions to study fuel pyrolysis, and the pyrolysis process of several jet fuels was mainly studied using the Stanford shock tube apparatuses. 18,20,21,29 However, it requires expensive laser to measure the species profiles. In contrast, the single-pulse shock tube (SPST) is a well-established reactor for post-shock sampling and analysis. ...
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A single-pulse shock tube study of the pyrolysis of two different concentrations of Chinese RP-3 jet fuel at 5 bar in the temperature range of 900–1800 K has been performed in this work. Major intermediates are obtained and quantified using gas chromatography analysis. A flame-ionization detector and a thermal conductivity detector are used for species identification and quantification. Ethylene is the most abundant product in the pyrolysis process. Other important intermediates such as methane, ethane, propyne, acetylene, butene, and benzene are also identified and quantified. Kinetic modeling is performed using several detailed, semidetailed, and lumped mechanisms. It is found that the predictions for the major species such as ethylene, propene, and methane are acceptable. However, current kinetic mechanisms still need refinement for some important species. Different kinetic mechanisms exhibit very different performance in the prediction of certain species during the pyrolysis process. The rate of production (ROP) is carried out to compare the differences among these mechanisms and to identify major reaction pathways to the formation and consumption of the important species, and the results indicate that further studies on the thermal decomposition of 1,3-butadiene are needed to optimize kinetic models. The experimental data are expected to contribute to a database for the validation of mechanisms under pyrolytic conditions for RP-3 jet fuel and should also be valuable to a better understanding of the combustion behavior of RP-3 jet fuel.
... An efficient core skeletal mechanism is the foundation in the development of kinetic models for larger fuels, specifically in the decoupling or hybrid chemistry methods. 11,35 For this purpose, the derived skeletal mechanism is validated against a fivecomponent gasoline surrogate model via a decoupling method. Specifically, the C 0 −C 3 skeletal mechanism is coupled with the lumped global mechanisms for the five components, including iso-octane, n-heptane, isohexane, 1-hexene, and toluene, with the compositions of 39.91, 6.99, 9.31, 9.28, and 34.51 mol %, respectively, to describe the high-temperature pyrolysis and oxidation at negative temperature coefficient (NTC) regions, which has been detailed previously. ...
... Additional details of the hybrid tabulation construction are available in [28]. The tabulation for the different fuels is obtained based on the HyChem family of chemistry mechanisms [30][31][32][33]. In another article, the applicability of this framework to Jet-A, C1, and C5 fuels has been validated against experimental data [6]. ...
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For safety purposes, reliable reignition of aircraft engines in the event of flame blow-out is a critical requirement. Typically, an external ignition source in the form of a spark is used to achieve a stable flame in the combustor. However, such forced turbulent ignition may not always successfully relight the combustor, mainly because the state of the combustor cannot be precisely determined. Uncertainty in the turbulent flow inside the combustor, inflow conditions, and spark discharge characteristics can lead to variability in sparking outcomes even for nominally identical operating conditions. Prior studies have shown that of all the uncertain parameters, turbulence is often dominant and can drastically alter ignition behavior. For instance, even when different fuels have similar ignition delay times, their ignition behavior in practical systems can be completely different. In practical operating conditions, it is challenging to understand why ignition fails and how much variation in outcomes can be expected. The focus of this work is to understand relight variability induced by turbulence for two different aircraft fuels, namely Jet-A and a variant named C1. A detailed, previously developed simulation approach is used to generate a large number of successful and failed ignition events. Using this data, the cause of misfire is evaluated based on a discriminant analysis that delineates the difference between turbulent initial conditions that lead to ignition or failure. From the discriminant analysis, a compressed sensing algorithm is then applied to help pinpoint the locations of relevant turbulent features. Findings from the discriminant analysis are confirmed with the time history of near kernel properties. Next, a clustering strategy is used to identify ignition and misfire modes. With this approach, it was determined that the cause of ignition failure is different for the two fuels. While it was found that Jet-A is influenced by fuel entrainment, C1 was found to be more sensitive to small scale turbulence features. A larger variability is found in the ignition modes of C1, which can be subject to extreme events induced by kernel breakdown.
Chapter
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Machine learning provides a set of new tools for the analysis, reduction and acceleration of combustion chemistry. The implementation of such tools is not new. However, with the emerging techniques of deep learning, renewed interest in implementing machine learning is fast growing. In this chapter, we illustrate applications of machine learning in understanding chemistry, learning reaction rates and reaction mechanisms and in accelerating chemistry integration.
Chapter
Focusing on a critical aspect of the future clean energy system - renewable fuels - this book will be your complete guide on how these fuels are manufactured, the considerations associated with utilising them, and their real-world applications. Written by experts across the field, the book presents many professional perspectives, providing an in-depth understanding of this crucial topic. Clearly explained and organised into four key parts, this book explores the technical aspects written in an accessible way. First, it discusses the dominant energy conversion approaches and the impact that fuel properties have on system operability. Part II outlines the chemical carrier options available for these conversion devices, including gaseous, liquid, and solid fuels. In the third part, it describes the physics and chemistry of combustion, revealing the issues associated with utilizing these fuels. Finally, Part IV presents real-world case studies, demonstrating the successful pathways towards a net-zero carbon future.
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This study evaluates the effects of cetane number (CN) on autoignition reactivity and chemical ignition delay. Recent interest in sustainable aviation fuels and single-fuel policies in the US necessitates the investigation of the effect of various fuels' properties on combustion. CN is a representative metric quantifying ignition quality across a range of fuels’ relevant properties. However, there is no concrete relationship between CN and chemical reactivity. To investigate the impact of CN on combustion characteristics, a set of test fuels have been formulated with varying CNs while maintaining other properties. Fuels were tested in the RCM and shock tube at various conditions. Typically, the reactivity is linearly proportional to the CN. However, the results showed that DCN could not be the sole predictor of ignition delay across the range of thermodynamic conditions relevant to propulsion systems. CN has been used as an indirect marker (or correlation) of chemical composition effects on ignition, and it is known that the DCN correlation breaks down for certain conditions and fuels. The structure of chemical components mainly affects combustion pathways by fuel breakdown, hydrogen abstraction, and isomerization. The results provide a guideline for practical experiments to evaluate the impact of CN on combustion characteristics.
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Emerging detonation-based combustors have motivated the study of the use of real distillate fuels for detonation devices. However, NOx emissions from real distillate fuel in a detonating environment have so far received very little attention. Therefore, it is essential to address the NOx emissions from a real distillate fuel in an extreme combustion environment as it can give us a strong set of information about the emission characteristics of detonation-based combustors. In the current study, NOx emissions were numerically studied for an alcohol-to-jet synthetic (C1) fuel for its potential application in detonation-based combustors. The study aims to enhance the qualitative understanding of the NOx emissions from a synthetic biofuel in detonation-based combustors. Also, primary methods of NOx reduction, such as the utilization of lean mixtures or dilution with inert diluents, are investigated in the present study for C1-air mixtures. For the mixtures and operating conditions featuring promising detonability, the formation of the oxides of nitrogen under detonating conditions has been studied using a detailed hybrid chemistry reaction model combined with the NOx model of Glarborg et al. The computations were carried out to cover a broad range of conditions where NOx emissions from stoichiometric C1-air detonations were studied. The computations were then extended to include the effect of the mixture's initial conditions, where the effects of varying equivalence ratios (φ), initial pressure (P0), and initial temperature (T0) on NOx emissions were studied. Additionally, computations were also carried out to study the effect of dilution on NOx emissions using inert gases such as argon and helium. The present study lays the groundwork for the optimized operation of liquid hydrocarbon-fueled detonation-based engines and enables an insight into the potential measures that can be employed for reduced NOx emissions in such devices.
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The use of conventional jet fuels in detonation-based engines is emerging as a promising possibility due to the risks associated with the use of hydrogen as a fuel for commercial aviation. The development of liquid-fueled detonation engines heavily depends on the basic understanding of the detonation chemistry and the combustion behavior of these real fuels in a detonating environment. The current work presents a systematic study of the detonating behavior of two real fuels. The fuels studied are Jet A, a conventional jet fuel used in the aviation industry, and a synthetically developed bio-derived jet fuel, C1. 1D ZND computations are used to compute the relevant detonation properties. The high-temperature chemistry of Jet A and C1 is modeled using a HyChem chemical kinetics model. The detonation chemistry of real distillate fuels was investigated in this study numerically, where relevant chemical length and time scales were calculated and compared. The critical detonation parameters were also evaluated and compared over a range of initial conditions and equivalence ratios. The detonability limits of real distillate fuels were investigated for their application in detonation-based combustors. The fuel–air–diluent mixtures were also studied in the present work, with argon and helium as inert diluents. The ZND computations show that the induction length scale for Jet A–air detonations is nearly half when compared to that of C1–air detonations which can be attributed to the detonation chemistry of the two fuels considered here. The major difference between the detonation chemistry of Jet A and C1 is a result of the composition of major pyrolysis products. The major decomposition product for Jet A is ethylene (C2H4); whereas, for C1, it is iso-butene (i-C4H8). The larger molecular weight of iso-butene leads to smaller diffusivity which results in larger detonation length and time scales for C1 when compared to Jet A at the same initial conditions. The primary objective of the present study is to show how fuel chemistry plays a crucial role in the detonation phenomenon. The study also highlights the effect of fuel composition and their pyrolysis products on the detonating behavior of real fuels for their application in detonation-based engines.
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A reactive Molecular Dynamics (MD) study of n-dodecane combustion at high temperatures under externally applied electrostatic fields is performed to investigate their effect on chemical kinetics. A local charge equilibration method is used to enable charge transfers up to the overlap of the atomic orbitals and introduce molecular polarization induced by an electric field. The atomic charges of an isolated n-dodecane molecule with and without external electrostatic fields are first compared with Density Functional Theory (DFT) computations, to assess the accuracy of the charge equilibration method and its ability to capture polarization. Then, the impact of external electrostatic fields on the reaction kinetics of fuel, oxidizer and products is studied for a range of ambient temperatures and densities. The activation energy and pre-exponential factor of Arrhenius-type reactions under various electrostatic fields are also investigated by performing Nudged Elastic Band (NEB) computations on selected reactions’ Minimum Energy Path (MEP) and by analysing the collision frequency, respectively. Results show that the atomic charge transfers due to close interactions and molecular polarisation are relatively weak in all investigated conditions, leading to the necessity of strong external electric fields to induce changes to chemical kinetics. The consumption rate of n-dodecane decreases for strong electrostatic fields, whereas for low values of the electrostatic field strength no clear trend is observed. In addition, at high temperature and density conditions, oxygen consumption increases under strong electrostatic fields, whereas the opposite trend is observed as the temperature and density decrease. NEB analysis shows alterations of the activation energy up to 2.3 kcal/mol for oxygen compound reactions with varying strength of the external electrostatic field. Furthermore, analysis of the translational, rotational and vibrational kinetic energy modes and collision frequency reveals the influence of translational motion and molecular stabilization on the reaction rates. The kinetics of oxygen molecules was found to be of primary importance to determine the reaction behaviour under external electrostatic fields, as oxygen molecules have a direct effect on the oxidation reactions and also indirectly affect n-dodecane pyrolysis when an electrostatic field is present. This study provides fundamental understanding of the interactions between heavy hydrocarbons and electrostatic fields for the development of future hybrid thermal-electrical technologies.
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This paper presents the impact of the alternative fuels properties on the parameters characterizing the combustion process in a turbojet engine, expressed in the form of a mathematical model. Laboratory tests, bench tests and a regression analysis of the obtained results were conducted. The developed and published combustion process models were briefly described. It has been demonstrated that these models were insufficient in taking into account the impact of fuel properties on the course of the combustion process. The experimental data enabled developing a mathematical model of the combustion process using statistical methods. The developed model, unlike other currently known models, takes into account the chemical composition of the fuel to a greater extent, which is characterized by its physicochemical properties. Mathematical model enables predicting engine operating parameters and the emissions characteristics, based on analysing laboratory test results, and can be used as a tool verifying the environmental impact of new fuels, through predicting the exhaust gas emissions.
Thesis
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Alternative jet fuels are being developed for use with existing jet engines, however there are still knowledge gaps concerning how unusual compositions and properties of these fuels will affect combustion performance. Physical and chemical processes leading to problematic engine stability phenomena like flame extinction and lean blow-off (LBO) are still not well-understood for conventional spray flames, but alternative fuels provide additional challenges as they have been observed to have increased variability from expected behaviour at conditions close to LBO. Evaporation is known to be the limiting factor for combustion in spray flames, and experiments have shown both gaseous and spray flames exhibit increased amounts of local extinctions as the equivalence ratio is decreased. The flame structure and transient behaviour of spray flames behave very differently compared to gaseous flames at near-blow-off conditions and during the blowoff transient. Fuel starvation has been proposed in past experiments as a significant reason for why spray flames blow off more quickly and at richer equivalence ratio compared to gaseous flames but has been explored very little in computational studies. The prediction of fuel starvation and LBO phenomena using numerical simulations with detailed chemistry are the primary focus in this work. Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) turbulence-combustion model are used, as this methodology has shown good results in simulating extinction and blow-off in both gaseous and spray flames in a lab-scale bluff body swirl spray flame configuration. The jet fuels simulated are the Dagaut Jet-A1 surrogate and the U.S. National Jet Fuels Combustion Program (NJFCP) fuels of interest: A2, C1, and C5. A2 is a conventional Jet-A used as a reference fuel, whereas C1 and C5 are synthetic kerosenes with unusual fuel chemistry or liquid property characteristics. These NFJCP fuels are represented using the Hybrid Chemistry “HyChem” lumped pyrolysis detailed kinetic mechanisms. Simulations in non-premixed laminar counterflow flamelet configurations are conducted at pressures of 1 atm and 10 atm for stable scalar dissipation value flamelets up to extinction, and during the extinction transient. Species trends in the three HyChem fuels and the Dagaut Jet-A1 surrogate are compared in detail. In comparison with experimental blow-off trends, only C5 deviates from expected behaviour and is the most robust fuel against extinction via high scalar dissipation rate. This highlights the interplay of both chemical and physical forces contributing to a real fuel’s tendency for LBO. Reignition of an extinguishing laminar flamelet using the HyChem A2 mechanism is also achieved through decrease of the scalar dissipation rate, although after a certain time the flamelet is not recoverable due to lack of chain-branching radical species. A stable condition LES-CMC simulation using the HyChem A2 (Jet-A) chemical mechanism is used as a starting point and reference for lean blow-off simulations. The computational domain is based on the Cambridge bluff body swirl burner, with a structured LES mesh and a coarse structured CMC grid. The simulation is run using an Eulerian-Lagrangian framework for multiphase flow with the Abramzon and Sirignano evaporation model. Overall flame size and shape from the LES are fairly similar to experimental OH* and OH-PLIF with Mie scattering results, however there are significant differences in location of peak heat release rate and further work is required for validation of the simulations against experiments. CH is discussed as a promising experimental marker for local extinction and location of heat release. Three fuel mass flow rates from the experimental blow-off curve for the Jet-A flame are simulated. The three simulations exhibited LBO at air flows between 5–20% greater than experimental bulk air blow-off velocities. Heat release rate decreased by at least 80% in the flame zone around the stoichiometric mixture fraction, however globally the combustor saw an increase in heat release rate due to the presence of unburnt droplets continuing to vaporise downstream. The asymmetric flame structure and duration of the blow-off transient in the simulations align very well with previous experiments with kerosene and other low-volatility fuels. The LBO transient lasted between 10–30 ms. Fuel starvation is suggested to be a driver of spray flame extinction, through decreased temperature and reduced evaporation caused by increased quantities of cold air in the system. Unburnt vaporised fuel remains in regions of temperature below 1200 K, where the fuel is no longer able to pyrolyse completely, resulting in non-flammable local mixtures. The quantity of local extinctions observed in both conditional and unconditional space is lower than expected compared to gaseous flames, and is linked to low values of the conditional scalar dissipation rate. Changing the model used to close the conditional scalar dissipation rate in the CMC equations is suggested as a potential way to improve the LBO results, as the Amplitude Mapping Closure model does not account for the very lean mixtures experienced at LBO conditions.
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Alkenes formed, during the refining of crude oil, by cracking the heavier fractions are present in transportation fuels in significant amounts, up to as much as 15–20% in gasoline. Moreover, alkenes are also the major intermediate products of the oxidation of alkanes, which play a significant role in autoignition chemistry. This review has assessed the recent progress in gas-phase detailed kinetic model development for species with C=C double bond, mostly C2–C10 alkenes and 1,3-butadiene. The compiled knowledge on alkene combustion chemistry enabled a better understanding of the influence of the number and the position of the C=C double bond on the chemical kinetics and hence combustion behavior of alkenes in engines. At first, the article gives an extensive overview of fundamental combustion experiments by considering studies of C2–C10 alkenes and 1,3-butadiene in shock tubes, rapid compression machines, laminar flames, and jet-stirred and flow reactors. The value of the data from such experiments is critically discussed. Secondly, this article highlights the important reaction classes involved in alkene oxidation over low-, intermediate- and high-temperature ranges. Combustion chemistry covering C2 to C10 alkenes, with a special emphasis on C2 to C7 isomers is discussed by presenting a large body of experimental and modeling investigations. Detailed chemistry differences between alkene isomers and also between alkenes and alkanes are also addressed. Thirdly, the article presents important reaction pathways for PAH precursor formation in different alkenes. Finally, a summary of the distinguishing features of alkene combustion chemistry and an outlook towards future research in this area are presented. This review is focused on linear and branched chain alkenes, and the chemistry of cyclo-alkenes is not included.
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Ways to improve the performance of a dual-mode scramjet engine within a generic X-51 hypersonic vehicle were computed using the MASIV reduced-order model. Two new aspects are that the finite-rate chemistry of a hydrocarbon (JP-7) fuel was added for the first time, and certain tradeoffs were quantified that arise due to the short vehicle length (4.6 m), which narrows the operating ranges of ram and scram modes. A goal was to maximize the operating range of the ram mode, which is limited by unstart, as well as the range of the scram mode, which is limited by incomplete fuel burn. Four parameters were varied: fuel–air equivalence ratio, Mach number, fuel injector diameter, and number of fuel injectors. Results indicate that achieving an acceptable design is complicated by the mixing–reaction–strain out tradeoff. A solution was achieved by directing the fuel to two sets of wall injectors having small diameters for ram mode and larger diameters for scram mode. Results demonstrate the need to include finite-rate chemistry to properly compute the heat release profile and interactions between the inlet, isolator, combustor, and thermal choking.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-0225.vid The data-driven optimized mechanism has recently been proposed for chemical kinetic modeling of the combustion of real, multi-component fuels. Experimental datasets of ignition delay times across wide temperature and equivalence ratio ranges are obtained by using a rapid compression machine (RCM) and shock tube. The HyChem (Hybrid Chemistry) and lumped NTC (Negative Temperature Coefficient) approaches have been used to model chemical reactions under high and low temperature chemistry, respectively. The reaction coefficients including pre-exponential factors, corrective coefficients, and activation energies are optimized against empirical results. This paper employs and compares three different types of heuristic optimization techniques: a micro-genetic algorithm, a Bayesian optimization, and a stochastic gradient descent (SGD). We demonstrate the approaches in HyChem-oriented chemical kinetic models for multi-component fuels, Jet A. The results show that all techniques are capable of optimizing the chemical kinetics models, but computational costs and performance vary among the different approaches.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-0819.vid The use of jet fuels in detonation-based engines is emerging as a promising possibility due to the risks associated with using hydrogen as a fuel for commercial aviation. The development of liquid-fueled detonation engines heavily depends on the basic understanding of the detonation chemistry and detonating behavior of these real fuels in the harsh environments of detonation combustion. The current work presents a systematic study of the detonating behavior of two real fuels. The fuels studied are Jet-A, a conventional jet fuel used in the aviation industry, and a synthetically developed biofuel, C1. 1D ZND computations are used to compute the relevant detonation properties. The high-temperature chemistry of Jet A and C1 is modeled using a HyChem chemical kinetics model. The detonation chemistry of real distillate fuels was investigated in this study numerically, where relevant chemical length and time scales were calculated and compared. The critical detonation parameters were also evaluated and compared over a range of initial conditions and equivalence ratios. The detonability limits of real distillate fuels were investigated for their application in detonation-based combustors. The fuel-air-diluent mixtures were also studied in the present work, with argon and helium as inert diluents. The ZND computations show that the induction length scale for Jet A-air detonation is nearly half when compared to that of C1-air detonation. The major difference between the detonation chemistry of Jet A and C1 is the major pyrolysis product. The major decomposition product for Jet-A is ethylene (C2H4), whereas, for C1, it is iso-butene (i-C4H8). The larger molecular weight of iso-butene leads to smaller diffusivity and thus larger length and time scales for C1 as compared to Jet-A. The fundamental objective of this article is to show how fuel chemistry plays a role in the detonation phenomenon and highlight the effect of foundational fuel composition on the detonating behavior of real fuels for their application in detonation-based engines
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-1255.vid Cetane number (CN) is a metric widely used to measure the performance characteristics of jet fuels. While it is a common parameter, the exact definition of cetane number is not easily correlated to typical combustion parameters that are measured through experiment such as gas phase ignition delay or atomization. In this study, ignition delay, a very common descriptor of fuel, will be related to cetane number to determine how well cetane number can be used as a predictive tool. Specifically, derived cetane number (DCN), which is a very similar but not an exact replacement for cetane number, is used in this study. A wide variety of fuels were selected for this study including standard multi-component jet fuels and non-standard sustainable jet fuels. These fuels cover a derived cetane number range from 16 to 55. The fuels used for this study were organized into the following sets: a blended fuel set, a set of fuels with constant properties such as average molecular formula, density and viscosity, and a set of fuels with the same derived cetane number. Ignition delay results were obtained using a combination of shock tube and rapid compression machine measurements. Results from the comparisons of experimental ignition delay data to DCN show that DCN does have correlation with ignition delay in the intermediate temperature or NTC region but does not show a useful correlation at low or high temperatures. Due to the variation shown in this study, the use of cetane number as a detailed descriptor of fuel may not be perfect, but with additional data and a larger variety of fuel types, a defined region may be created than provides a boundary for the possible ignition delay characteristics of a fuel with a measured CN or DCN.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-0518.vid Fuel-sensitization is one of the possible techniques that ensure the operation of a detonation-based combustor near its limits without the risk of failure or attenuation of a detonation wave. The primary effect of fuel sensitization is to alter the ignition chemistry of the fuel-air mixture without affecting the other thermodynamic and gas dynamic properties of gaseous detonation. The chemical kinetics of biofuels is relatively slow for their application in detonation-based engines. Ignition promoters like ozone and hydrogen peroxide could improve the ignition and combustion kinetics of synthetic biofuels tremendously for applications in ramjets and detonation-based engines. However, ignition promoters are strong oxidizing agents and could lead to larger NOx emissions. Therefore, the effect of fuel-sensitization on NOx emissions from a synthetic biofuel under detonation conditions is investigated in the present study. In the present study, one-dimensional ZND calculations are carried out for C1-air detonations with ozone and hydrogen peroxide as ignition promoters where C1 is an alcohol-to-jet synthetic biofuel. The effect of ignition promoters on the emission characteristics and the critical detonation parameters is studied over a range of equivalence ratios. It is observed that while ozone is a better ignition promoter and reduces the length and time scales drastically, it leads to larger NOx emissions when compared to hydrogen peroxide. Hydrogen peroxide exhibits a dual behavior. It acts both as an ignition promoter and a NOx mitigating agent. The dual nature of hydrogen peroxide can be harnessed for its practical application in detonation-based combustors. The results from the present study are promising and can be used as a starting point to efficiently utilize synthetic biofuels for detonation-based combustors.
Article
Large Eddy Simulations (LES) are performed to compute the sensitivity of a conventional (A-2) and an alternate bio-jet (C-1) fuel to Lean Blowout (LBO). A realistic aviation gas turbine engine combustor configuration is considered. Reliable experimental LBO data and OH* chemiluminescence data for the conventional and alternate jet fuel in the combustor configuration have recently become available. The present work utilizes a highly automated, on-the-fly meshing strategy, along with adaptive mesh refinement, to demonstrate the feasibility of capturing the realistic combustion processes. A Lagrangian framework, with initial conditions specified using measurements of spray statistics, is used to model the fuel spray. Newly developed compact reaction mechanisms based on fuel surrogates are validated for the A-2 and the C-1 fuels. The compact reaction mechanisms are implemented using a detailed finite rate chemistry solver. Spray statistics computed by the present LES simulations compare well with available measurements at stable flame conditions near the lean blowout limit. The computed shape of the stable flame as represented by line integrated OH concentrations compares well with the experimental OH* chemiluminescence data. Lean blowout is reached by gradually decreasing the fuel flow rate in the computations, similar to that in the experiments. The results of the LES simulations effectively capture the fuel composition effects and estimate the sensitivity of the LBO limits to the fuel type. The computed trends in LBO limits agree within engineering accuracy with the experimental results for conventional and alternative aviation fuels. The methodology for predicting the fuel composition effects on the lean blowout limits in a fully resolved realistic, complex combustor is established for the first time. Link to full paper: https://authors.elsevier.com/a/1eI2w6CY3ylqwH
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2021-3678.vid Emerging detonation-based combustors have motivated the study of the use of real distillate fuels for detonation devices. However, NOx emissions from real distillate fuel in a detonating environment have so far received very little attention. Therefore, it is essential to address the NOx emissions from a real distillate fuel in an extreme combustion environment as it can give us a strong set of information about the emission characteristics of detonation-based combustors. In the current study, NOx emissions were numerically studied for an alcohol-to-jet synthetic (C1) fuel for its potential application in detonation-based combustors. The study aims to enhance the qualitative understanding of the NOx emissions from a synthetic biofuel in detonation-based combustors. Also, primary methods of NOx reduction, such as the utilization of lean mixtures or dilution with inert diluents, are investigated in the present study for C1-air mixtures. For the mixtures and operating conditions featuring promising detonability, the formation of the oxides of nitrogen under detonating conditions have been studied using a detailed hybrid chemistry reaction model combined with the NOx model of Glarborg et al. The computations were carried out to cover a broad range of conditions where NOx emissions from stoichiometric C1-air detonations were studied. The computations were then extended to include the effect of the mixture's initial conditions, where the effects of varying equivalence ratios (φ), initial pressure (P0), and initial temperature (T0) on NOx emissions were studied. Additionally, computations were also carried out to study the effect of dilution on NOx emissions using inert gases such as argon and helium. The present study lays the groundwork for the optimized operation of liquid hydrocarbon-fuelled detonation-based engines and enables an insight into the potential measures that can be employed for reduced NOx emissions in such devices.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2021-3458.vid This research presents the result of numerically simulating 7-element swirl-venturi Lean Direct Injector (SV-LDI) lean blowout (LBO) experiments conducted at NASA Glenn Research Center in May of 2019. After simulating a cold flow case to confirm the pressure drop agrees well with the experiment, additional cases with two different fuels (an average jet fuel and a Gevo alcohol-to-jet fuel) from the National Jet Fuels Combustion Program (NJFCP) were computed to numerically determine the LBO condition. The procedure to approach the LBO follows the method used in the experiment where the air mass flow rate is gradually increased while the fuel supply is maintained. Transient history of global heat release rate as a function of air flow rate is presented, as well as temperature contours at different conditions to give a visual representation of the flame state. The Open National Combustion Code (OpenNCC) used in this research adopted reduced HyChem (Hybrid Chemistry) models along with k-LES turbulence model and a Lagrangian spray model that takes into account droplet internal temperature distribution affected by the shear force on the droplet surface. The transport equations and chemical reaction terms are integrated together to enhance conservation of chemical species that are especially important in the near LBO conditions. After showing the computed range of LBO agrees well with the experimental measurements, time averaged solutions of both fuels at their initial condition and at their limiting condition just before LBO are compared in detail to facilitate understanding of the LBO mechanism.
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Conventional transportation fuels used in aviation (jet fuel) or in ground transportation (gasoline, diesel) contain multitude of hydrocarbon components and are difficult to be modeled, if one has to consider each of the component present. A typical approach is the definition of a fuel surrogate with a limited number of fuel components. In this context, a single semi-detailed high temperature reaction kinetic mechanism is presented in this work, which contains all the important molecular classes required for the detailed surrogate modeling of a hydrocarbon fuel. The appeal of the mechanism is the suitability for a broad range of technical fuels covering gasoline, diesel and jet fuels. The reaction mechanism for hydrocarbon combustion is consisted of 238 species and 1814 reactions and is rigorously validated for 70 neat hydrocarbon components over a wide range of experimental conditions including combustion setups such as shock-tubes, laminar flames, jet-stirred and flow reactors. The purpose of this study is to provide a single reaction model that (1) includes variety of hydrocarbon molecules of varying degree of complexity and carbon numbers, (2) has capability to model a spectrum of different fuels, initially aviation fuels, and (3) is compact to apply both in simple (fundamental kinetic investigations) and complex geometries (CFD studies) of combustion system enabled through customized mechanism reductions. The ultimate goal is to resolve the fuel differences using the model predictions obtained from the reaction mechanism that will supply parameters for fuel design and optimization of fuels. Extensive supporting information is available in this work.
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In Part III of our study on alternative aviation fuels, we present a comprehensive database of modeled speciation data consisting of seven hydrocarbons of varying molecular structure and 26 alternative and conventional aviation fuels. The speciation data is obtained from the DLR atmospheric high temperature flow reactor with a coupled molecular beam mass spectrometry (MBMS) detection system (Part-I). The chemical reactivity of these real liquid fuels is investigated both experimentally and numerically. For modeling, detailed fuel surrogates (up to 14 components) are employed for characterizing the fuels. The surrogate formulation strategy is defined based on the fuels’ compositional analysis. This work employs high temperature reaction kinetic mechanism for the combustion of wide variety of hydrocarbons of varying molecular classes from n-, iso-, cy-paraffins, to five-ring aromatics presented in Part-II. The reaction mechanism is applied to model 26 aviation fuels using 21 different validated fuel surrogate components to predict the fuel intermediates and soot precursors measured in the reactor. This work aims to identify the effect of fuel surrogate components on the intermediates’ formation and its influence on the emissions. Through compilation of many fuels with composition of wide range of chemical classes, we provide a systematic evaluation of how the fuel composition can be used to extract information of specific fuel intermediates and emissions formed.
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Ensuring efficient and clean combustion performance of liquid-fueled engines requires comprehension of the influence of fuel composition and properties on flame behavior, such as flame liftoff height (LOH) and lean blowout limit (LBO). Spray flame stability is strongly affected by both the fuel reactivity and physical properties. Herein, the flame stability mechanism represented by LOH is investigated for seven jet turbine fuels, including surrogate, alternative, and conventional jet fuels, using a laboratory spray burner. Based on the experimental observations, the current work introduces a new analysis, which provides insight into the competing/complementing processes that occur in a multi-phase reacting system and highlights the key properties important in spray flame dynamics, accounting for both the fuel spray/vaporization as well as the chemical reactivity, to explain the relative differences in LOH of complex multicomponent fuels. Results show that spray flame stabilization occurs when there is a balance between the local spray burning velocity and the incoming jet velocity, which is strongly associated with laminar flame speed and the relative amount of liquid and gaseous fuel crossing the flame preheat region. Using a multicomponent droplet evaporation model, it was observed that preferential vaporization of the lighter and more reactive species of the simple surrogate fuels contribute to a shorter lifted flame as compared to fuels consisting of heavier and/or less reactive components. The LOH of real jet fuels showed strong sensitivity to droplet vaporization and mixing, which is controlled by the fuels’ volatility (i.e., boiling temperature at 50% distillation volume) and atomized droplet size, and to a lesser extent the reactivity represented by laminar flame speed. The enhanced linearity in the correlation of 50% vapor distilled compared to the other distillation cuts suggests that preferential vaporization could play an important role in defining the LOH stabilization mechanism even for more complex fuels.
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Oxidation of alcohol to jet (ATJ)/n‐heptane blends was studied over a wide range of test conditions, using single pulse shock tubes. Test conditions were designed to study the effect of pressure (4 and 50 bar), and fuel loading (∼100 to 1400 ppm) on the oxidation of the blends across a wide range of temperatures (800–1300 K). These effects were observed by measuring concentrations of intermediate species formed using in‐line gas chromatography (GC) and GCxGC TOF‐MS. Results showed that increasing the initial fuel load does not have a significant impact on the results. However, increasing the pressure shifts the mixture reactivity to a lower temperature by about 150 K and causes the fuel to be oxidized instead of decomposing pyrolytically as at lower pressures. Additional experiments for pure ATJ and pure n‐heptane were performed at conditions matching the ATJ/n‐heptane 50 bar experiments to analyze the differences between the pure and mixed fuels. Speciation measurements were compared against predictions from a detailed kinetic model. The ability of the kinetic model to capture the effects of varying experimental test conditions on the evolution of intermediate species is discussed, and kinetic analyses have been conducted to identify the important reaction pathways.
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An ultralow emission combustor concept based on “flameless oxidation” is demonstrated in this paper for aviation kerosene. Measurements of gas emissions, as well as of the size and number of nanoparticles via scanning mobility particle sizing, are carried out at the combustor outlet, revealing simultaneously soot-free and single-digit NOx levels for operation at atmospheric conditions. Such performance, achieved with direct spray injection of the fuel without any external preheating or prevaporization, is attributed to the unique mixing configuration of the combustor. The combustor consists of azimuthally arranged fuel sprays at the upstream boundary and reverse-flow air jets injected from downstream. This creates locally sequential combustion, good mixing with hot products, and a strong whirling motion that increases residence time and homogenizes the mixture. Under ideal conditions, a clean, bright-blue kerosene flame is observed, free of soot luminescence. Although soot is intermittently formed during operation around optimal conditions, high-speed imaging of the soot luminescence shows that particles are subjected to long residence times at O2-rich conditions and high temperatures, which likely promotes their oxidation. As a result, only nanoparticles in the 2–10 nm range are measured at the outlet under all tested conditions. The NOx emissions and completeness of the combustion are strongly affected by the splitting of the air flow. Numerical simulations confirm the trend observed in the experiment and provide more insight into the mixing and air dilution.
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The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH 2 O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme. OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (2 D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region as the flames approached blow-off. No significant variation in 2 D , curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH 2 O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, heptane and kerosene flames showed more pockets of CH 2 O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties.
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New laminar flame speed experiments have been collected for some kerosene-based liquid fuels: Jet-A, RP-1, and Diesel Fuel #2. Accurately understanding the combustion characteristics of these, and all kerosene-based fuels in general, is an important step in developing new chemical kinetics mechanisms that can be applied to these fuels. It is well known that the precise composition of these fuels changes from one production batch to the next, leading to significant uncertainty in the mixture average properties. For example, uncertainty in a fuel blend's molecular weight can have a noticeable effect on defining an equivalence ratio for a typical fuel-air mixture, on the order of 15%. Because of these uncertainties, fuel mole fraction, XFUEL, is shown to be a more appropriate parameter for comparison between different batches of fuel. Additionally, a strong linear correlation was detected between the burned-gas Markstein length and the equivalence ratio. This correlation is shown to be useful in determining the acceptability and accuracy of individual data points. Spherically expanding flames were measured over a range of fuel mole fractions corresponding to equivalence ratios of φ = 0.7 to φ = 1.5, at initial conditions of 1 atm and 403 K in the high-temperature, high-pressure constant volume vessel at Texas A&M University. These new results are compared with the limited set of laminar flame speed data currently available in the literature for this fuel.
Conference Paper
The autoignition characteristics of an alternative alcohol to jet (ATJ) fuel are examined by analyzing chemical ignition delay using a rapid compression machine (RCM) and shock tube. Additionally, a data-driven chemical kinetic mechanism based on the HyChem approach of ATJ is proposed for modeling the ignition process. Ignition delay times of ATJ are measured at a compressed pressure of PC = 2 MPa and at stoichiometric condition (ϕ=1) in synthetic dry air, between 667 K and 1250 K. The unique chemical structures of isoalkanes result in relatively low chemical reactivity under intermediate and low temperatures. Based on empirical results for ignition delay curves, a new lumped mechanism is optimized using a genetic algorithm. The newly introduced data-driven mechanism shows good performance not only for the high temperature regions as expected for HyChem, but over the entire negative temperature coefficient (NTC) and low temperature regimes. The experimental data and the kinetic model developed for ATJ in this study can offer understanding of the oxidation of extreme fuels in practical combustion systems which operate in NTC or low temperature conditions.
Conference Paper
Ways to improve the performance of a dual-mode scramjet engine within a generic X-51 hypersonic vehicle were computed using the MASIV reduced-order model. Two new aspects of the study are (a) the finite-rate chemistry of a hydrocarbon (JP-7) fuel was added for the first time. JP-7 burns slower than hydrogen, which was used in many previous computations. (b) The short vehicle length of the X-51 (4.6 m) narrows the operating range of both the ram mode and scram-mode. The goal was to maximize the operating range of the ram-mode, that is limited by unstart, as well as the range of the scram-mode, that is limited by incomplete fuel burn. Five parameters were varied: fuel-air equivalence ratio (ER), flight Mach number (Moo), fuel port diameter (d), number of fuel ports (N), and combustor wall divergence angle (c). Results indicate that achieving an acceptable design is complicated by the Mixing-Reaction-Strainout (MRS) tradeoff. Fuel-air mixing and chemical reaction rates become excessively large during ram-mode, causing too short of flame that causes unstart. The opposite occurs during scram-mode, when the flame can become so long that it leads to unacceptably low combustion efficiency. A solution was achieved by directing the fuel to two different sets of wall ports; a small number of small diameter ports are best for ram-mode and larger number of larger diameter ports are recommended for scram-mode. Results also demonstrate the need to include finite-rate chemistry, as is done in MASIV, in order to compute the interactions between the inlet, isolator, the combustor wall divergence angle, the thermal choking location and the flame length.
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The lean blow-off (LBO) limits and structure of turbulent premixed flames were investigated with vaporized liquid fuels stabilized by a bluff-body burner. Ethanol, heptane, and two kerosenes were used. To facilitate comparisons to gaseous-fueled flames, results were also obtained from methane flames. The measured LBO limits indicate that, for this burner, the ethanol and heptane flames are more resilient to blow-off than the kerosene fuels. Furthermore, a correlation based on a Damköhler number (Da), which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the Da correlations based on laminar flame speed are not applicable. Average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to that from methane: the flame brushes of both exhibit an M shape when close to blow-off. In contrast, the distribution of OH* signal in the kerosene flames is primarily concentrated in regions further downstream of the bluff body. Ultimately, the results of this effort highlight the influence fuel type has on the LBO of bluff-body stabilized flames. Moreover, this work indicates the LBO behavior of flames produced with complex hydrocarbon fuels cannot be fully understood via high-temperature chemistry concepts such as the laminar flame speed.
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Real fuels are complex mixtures of hundreds of molecules, which makes it challenging to unravel their combustion chemistry. Several approaches in the literature have helped to clarify fuel combustion, including multi-component surrogates, lumped fuel chemistry modeling, and functional-group based methods. This work presents an innovative advancement to the lumped fuel chemistry modeling approach, using functional groups for mechanism development (FGMech). Stoichiometric parameters of lumped fuel decomposition reactions dictate the population of the key pyrolysis products, previously obtained by fitting experimental data of real-fuel pyrolysis. In this work, a functional group-based approach is proposed, which can account for real-fuel variability and predict stoichiometric parameters without experimentation. A database of the stoichiometric parameters and/or yields of key pyrolysis products was first constructed for approximately 50 neat fuels, based on previous pyrolysis data and a lumped kinetic model we developed. The effects of functional groups on the stoichiometric parameters and/or yields of key pyrolysis products were then identified and quantified. A quantitative structure-stoichiometry relationship was developed by multiple linear regression (MLR) model, which was used to predict the stoichiometric parameters and/or yields of key pyrolysis products based on ten input features (eight functional groups, molecular weight, and branching index). Products from the pyrolysis of surrogate mixtures and real-fuels were predicted using the MLR model and validated against experimental data in the literature. Comparison with the stoichiometric parameters from the HyChem experiment-based approach (Xu et al., 2018) showed that the predicted values in this work were in reasonable agreement (generally within a factor of two). When the stoichiometric parameters in the jet fuel (POSF 10325) HyChem kinetic model were replaced with this functional-group based prediction, only minor discrepancies were observed in the predictions of key pyrolysis products and global combustion parameters (such as ignition delay times and laminar flame speeds). Sensitivity analysis on stoichiometric parameters revealed their different roles in predicting speciation and global parameters. The functional group approach for predicting stoichiometric parameters in this work was the first step towards developing FGMech for modeling real-fuel combustion chemistry. Further development of the FGMech model's thermodynamic, kinetic, and transport data will be presented in a following study.
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Flow reactors are commonly employed in investigations of the pyrolysis and oxidation chemistry of fuels. Typical flow reactors use either electrical heaters or vitiation heaters to provide the energy to preheat the reactants to the desired experimental conditions. The present study seeks to determine the impact of vitiation on flow reactor studies of fuel combustion chemistry by conducting both fuel pyrolysis and oxidation experiments in a flow reactor in which either an electrical heater or a vitiation heater is used as the energy source. Other than the heater all other aspects of the flow reactor are identical. Two fuels relevant to air-breathing propulsion systems were investigated – Jet A and JP-10. Profiles of the stable reaction products were measured using gas chromatography for a temperature of 1030 K at a pressure of 1 atmosphere, with residence times between 30 ms and 70 ms. The primary hydrocarbon products for Jet A pyrolysis and oxidation were C2H4, C3H6, CH4, C4H8, C6H6 and C7H8. For JP-10, in addition to these species, cyclopentene (C5H8) and cyclopentadiene (C5H6) were measured. Under the conditions investigated, vitiation decreases the reaction times scales for both pyrolysis and oxidation. However, the product yields at a fixed fuel conversion were nearly identical for both the vitiated and non-vitiated experiments. Current kinetic models for Jet A and JP-10 pyrolysis and oxidation adequately captured the observed effects of vitiation on the stable species profiles.
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Fast and reliable high altitude re-ignition is a critical requirement for the development of alternative jet fuels (AJFs). To achieve stable combustion, a spark kernel needs to transit in a partially or fully extinguished flow to develop a flame front. Understanding the relight characteristics of the AJFs is complicated by the chaoticity of the turbulent flow and variations in the spark properties. The focus of this study is the prediction of such characteristics by high-fidelity simulations, with a specific focus on fuel composition effect on the ignition process. For this purpose, a previously developed computational framework is applied, which utilizes high-fidelity LES simulations, a hybrid tabulation approach for modeling forced ignition and detailed quantification of uncertainty resulting from initial and boundary conditions to predict ignition probability. The method is applied to two alternative fuels (named C1 and C5) and Jet-A fuel (named A2) under gaseous conditions. Results show that the mixing of kernel and fuel–air mixture is not affected by the ignition process, but chemistry effects strongly dominate ignition probability. In particular, C1 exhibits much lower ignition probability than the other two fuels, especially at lean operating conditions. More importantly, this behavior is contradictory to ignition delay experiments which predict longer delay times for C5 compared to C1. Comparisons with experiments show that the comprehensive modeling approach captures the ignition trends. Analysis of kernel trajectories in composition space shows that the variations are caused by the relative effects of kernel mixing, response to strain, and ignition properties of the fuel.
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