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The effects of the stratifications of temperature, T , and equivalence ratio, ϕ , on the ignition characteristics of a lean homogeneous biodiesel/air mixture at high pressure and intermediate temperature are investigated using direct numerical simulations (DNSs). 2-D DNSs are performed at a constant volume with the variance of temperature and equivalence ratio ( T ′ and ϕ ′ ) together with a 2-D isotropic velocity spectrum superimposed on the initial scalar fields. In addition, three different T – ϕ correlations are investigated: (1) baseline cases with T ′ only or ϕ ′ only, (2) uncorrelated T – ϕ distribution, and (3) negatively-correlated T – ϕ distribution. It is found that the overall combustion is more advanced and the mean heat release rate is more distributed over time with increasing T ′ and/or ϕ ′ for the baseline and uncorrelated T – ϕ cases. However, the temporal advancement and distribution of the overall combustion caused by T ′ or ϕ ′ only are nearly annihilated by the negatively-correlated T – ϕ fields. The chemical explosive mode and Damköhler number analyses verify that for the baseline and uncorrelated T – ϕ cases, the deflagration mode is predominant at the reaction fronts for large T ′ and/or ϕ ′ . On the contrary, the spontaneous ignition mode prevails for cases with small T ′ or ϕ ′ , especially for cases with negative T – ϕ correlations, and hence, simultaneous auto-ignition occurs throughout the entire domain, resulting in an excessive rate of heat release. It is also found that turbulence with large intensity, u ′ , and a short time scale can effectively smooth out initial thermal and compositional fluctuations such that the overall combustion is induced primarily by spontaneous ignition. Based on the present DNS results, the generalization of the effects of T ′ , ϕ ′ , and u ′ on the HCCI combustion is made to clarify each effect. These results suggest that temperature and composition stratifications together with a well-designed T – ϕ correlation can alleviate an excessive rate of pressure rise and control the ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. [ABSTRACT FROM AUTHOR]

Abstract: A bifurcation analysis was developed to systematically detect limit flame phenomena, including ignition, extinction and changes in flame stability, and to understand the underlying physicochemical processes that control the limit phenomena. The bifurcation analysis was demonstrated with steady-state perfectly stirred reactors (PSRs) using dimethyl ether (DME) with the negative temperature coefficient (NTC) chemistry. Flame stability was first analyzed to identify ignition and extinction states based on the eigenvalues of the Jacobian of the governing equations. It was found that for DME–air mixtures, extinction may not occur at the turning points on the S-curves. A bifurcation index (BI) was then defined at each bifurcation point on the S-curves to quantify the contribution of each reaction and the mixing process to the limit flame phenomenon. Results show that extinction of the strong flames of DME–air is primarily controlled by the reactions involving small molecules, such as HCO and CO, while extinction of the cool flames is primarily controlled by the NTC chemistry involving larger molecules. To validate this method, the pre-exponential “A”-factors of the selected reactions were perturbed. It was found that the perturbations in reactions with large BI values have significant effects, while those with small BI values have minor effects, on the ignition and extinction states. The BI-based method was further compared to sensitivity analysis, and overall-consistent results were observed on the importance of the reactions at the bifurcation points, indicating that the bifurcation analysis is effective in identifying controlling reactions for limit flame phenomena. The BI values were then employed to guide the refinement of the rate constants in the DME mechanism. A skeletal model with substantially reduced reaction set and systematically tuned rate constants was obtained to accurately capture both steady-state and transient ignition and extinction behaviors of DME–air in PSR. [Copyright &y& Elsevier]

Abstract: Ignition and extinction of steady state combustion are known to be associated with the lower and upper turning points on the “S”-curves. In the present study, this concept is further investigated with eigen-analysis on the Jacobian matrix for oxidation of methane and dimethyl ether (DME), respectively, in perfectly stirred reactors (PSRs). It was found that there can be multiple ignition and extinction turning points on the “S”-curves for DME–air due to negative temperature coefficient (NTC) behaviors. Furthermore, the physical extinction points for DME–air obtained from flame stability analysis can be different from the turning points on the “S”-curves although there is no differential diffusion or heat loss in PSR. Physically unstable segments were observed on the branches of an “S”-curve for DME–air corresponding to both strong and cool flames. A rigorous definition of ignition and extinction of steady state combustion based on eigen-analysis of the Jacobian matrix is proposed for practical fuels in the present study. [Copyright &y& Elsevier]

Abstract: The effect of thermal stratification on the ignition of a lean homogeneous n-heptane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 58-species reduced kinetic mechanism developed for very lean mixtures from the detailed LLNL mechanism (H.J. Curran et al., Combust. Flame 129 (2002) 253–280). Two-dimensional DNS are performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields. The influence of variations in the initial temperature field, imposed by changing the mean and variance of temperature, and the ratio of turbulence to ignition delay timescale on multi-stage ignition of a lean n-heptane/air mixture is studied. In general, the mean heat release rate increases more slowly with increasing thermal stratification regardless of the mean initial temperature. Ignition delay decreases with increasing thermal stratification for high mean initial temperature relative to the negative temperature coefficient (NTC) regime. It is, however, increased with increasing thermal fluctuations for relatively low mean initial temperature resulting from a longer overall ignition delay of the mixture. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. Chemical explosive mode analysis (CEMA) also verifies that mixing counterbalances chemical explosion at the reaction fronts for cases with large temperature fluctuation. It is also found that if the ratio of turbulence to ignition delay timescale is short, resultant diminished scalar fluctuations cause the overall ignition to occur by spontaneous ignition. However, the overall effect of turbulence is small compared to the effect of thermal stratification. These results suggest that the critical degree of thermal stratification for smooth operation of homogeneous charge compression-ignition (HCCI) engines depends on both the mean and fluctuations in initial temperature which should be considered in controlling ignition timing and preventing an overly rapid increase in pressure in HCCI combustion. [Copyright &y& Elsevier]

Abstract: A detailed mechanism for methane–ethylene mixtures enriched with excessive amount of NO was systematically reduced for efficient numerical simulations of flames in arc-heated co-flowing air. Methane and ethylene were selected as the surrogate fuel in the present study due to their drastically different features of ignition and extinction properties and flame propagation speeds, such that the mixtures of them may be utilized to mimic practical hydrocarbon fuels with various kinetic properties in experiments. The recently released USC Mech-II for C1–C4 was grafted with the NO x sub-mechanism in GRI-Mech 3.0 with updated reaction parameters for prompt NO formation. The resulting detailed mechanism with 129 species and 900 reactions was first validated against experiments involving NO x enrichment and reasonably good agreements were observed. The detailed mechanism was then employed as the starting mechanism for the reduction. A skeletal mechanism with 44 species and 269 reactions was derived using the methods of directed relation graph (DRG) and DRG-aided sensitivity analysis (DRGASA); a 39-species reduced mechanism with 35 semi-global reaction steps was further obtained using the linearized quasi steady state approximations (LQSSA). Five species related to prompt NO were retained in the reduced mechanism because of their significant impacts on the fuel oxidation. The reduced mechanism closely agrees with the detailed mechanism for ignition and extinction of homogenous mixtures, as well as selected 1-D flames over a wide range of parameters with NO concentrations between 0% and 3%. The observed worst-case relative error of the reduction is approximately 20%. The reduced mechanism was further validated with experiments involving excessive NO x enrichment. [Copyright &y& Elsevier]

Abstract: A systematic approach was developed to derive non-stiff reduced mechanisms for direct numerical simulations (DNS) with explicit integration solvers. The stiffness reduction was achieved through on-the-fly elimination of short time-scales induced by two features of fast chemical reactivity, namely quasi-steady-state (QSS) species and partial-equilibrium (PE) reactions. The sparse algebraic equations resulting from QSS and PE approximations were utilized such that the efficiency of the dynamic stiffness reduction is high compared with general methods of time-scale reduction based on Jacobian decomposition. Using the dimension reduction strategies developed in our previous work, a reduced mechanism with 52 species was first derived from a detailed mechanism with 561 species. The reduced mechanism was validated for ignition and extinction applications over the parameter range of equivalence ratio between 0.5 and 1.5, pressure between 10 and 50atm, and initial temperature between 700 and 1600K for ignition, and worst-case errors of approximately 30% were observed. The reduced mechanism with dynamic stiffness removal was then applied in homogeneous and 1-D ignition applications, as well as a 2-D direct numerical simulation of ignition with temperature inhomogeneities at constant volume with integration time-steps of 5–10ns. The integration was numerically stable and good accuracy was achieved. [Copyright &y& Elsevier]

Abstract: A criterion based on computational singular perturbation (CSP) is proposed to effectively distinguish the quasi steady state (QSS) species from the fast species induced by reactions in partial equilibrium. Together with the method of directed relation graph (DRG), it was applied to the reduction of GRI-Mech 3.0 for methane oxidation, leading to the development of a 19-species reduced mechanism with 15 lumped steps, with the concentrations of the QSS species solved analytically for maximum computational efficiency. Compared to the 12-step and 16-species augmented reduced mechanism (ARM) previously developed by Sung, Law & Chen, three species, namely O, CH3OH, and CH2CO, are now excluded from the QSS species list. The reduced mechanism was validated with a variety of phenomena including perfectly stirred reactors, auto-ignition, and premixed and non-premixed flames, with the worst-case error being less than 10% over a wide range of parameters. This mechanism was then supplemented with the reactions involving NO formation, followed by validations in both homogeneous and diffusive systems. [Copyright &y& Elsevier]

Abstract: A 55-species reduced mechanism for n-heptane oxidation was derived from a 188-species skeletal mechanism, which was previously obtained from a detailed mechanism consisting of 561 species using a directed relation graph (DRG). This reduced mechanism was derived by first obtaining a skeletal mechanism with 78 species using DRG-aided sensitivity analysis. The unimportant reactions were eliminated by using the importance index defined in computational singular perturbation (CSP), with a newly posited restriction to treat each reversible reaction as a single reaction. An isomer lumping approach, also developed in the present study, then groups the isomers with similar thermal and diffusion properties so that the number of species transport equations is reduced. It was found that the intragroup mass fractions of the isomers can be approximated as constants in the present reduced mechanism, leading to a 68-species mechanism with 283 elementary reactions. Finally, 13 global quasi-steady-state species were identified using a CSP-based time-scale analysis, resulting in the 55-species reduced mechanism, with 283 elementary reactions lumped into 51 semiglobal steps. Validation of the reduced mechanism shows good agreement with the detailed mechanism for both ignition and extinction phenomena. The inadequacy of the detailed mechanism in predicting the experimental laminar flame speed is also demonstrated. [Copyright &y& Elsevier]

Abstract: A systematic approach was developed for reducing the size as well as the computational cost in the evaluation of diffusion coefficients for mechanisms with mixture-averaged diffusivities by bundling species with similar diffusivities into groups. The systematic reduction was formulated as an integer programming problem and solved efficiently with a greedy algorithm. Reduction error was controlled by a user-specified threshold value, and the algorithm was fully automated. The method was then applied to a 20-species reduced mechanism for ethylene and a 188-species skeletal mechanism for n-heptane. Nine bundled species groups were identified for ethylene, while reduced models with 19, 9, and 3 diffusive species groups were developed for n-heptane in ascending order of reduction errors. Validations of the reduced diffusion models obtained with about 10% reduction error in premixed and nonpremixed flames show good agreement with the detailed model, and the worst case reduction error is close to the user-specified level of 10%. Significant reduction in CPU time was observed in the evaluation of the diffusion terms, while the overall time saving is simulation-dependent due to the existence of other terms, such as the chemical source term, that are not affected by the reduction in the diffusion term. [Copyright &y& Elsevier]

Abstract: The conditions for application of the directed relation graph (DRG) method in skeletal reduction of mechanisms with vastly different time scales were systematically analyzed. It was found that the existence of quasi-steady-state species induces no additional restriction on the application of DRG. When there are partial equilibrium reactions, DRG requires reactions with fast forward or backward rates to be reversible and the backward rate to be computed through the equilibrium constant. The effect of loss of significant digits in the evaluation of species relations due to substantial cancellation between the forward and backward rates of partial equilibrium reactions was identified and a criterion for minimum accuracy in sampled reaction states for DRG reduction was identified. The method of DRG was then compared with two methods recently developed for skeletal reduction: one is based on computational singular perturbation (CSP) and another is the directed relation graph with error propagation (DRGEP). Advantages and restrictions of including fast–slow subspace separation in skeletal reduction and the validity of the geometric error propagation model in DRGEP were discussed, with examples in the existence of exhausted fast processes. [Copyright &y& Elsevier]

Abstract: The algorithm of directed relation graph recently developed for skeletal mechanism reduction was extended to overall linear time operation, thereby greatly facilitating the computational effort in mechanism reduction, particularly for those involving large mechanisms. Together with a two-stage reduction strategy and using the kinetic responses of autoignition and perfectly stirred reactor (PSR) with extensive parametric variations as the criteria in eliminating unimportant species, a detailed 561-species n-heptane mechanism and a detailed 857-species iso-octane mechanism were successfully reduced to skeletal mechanisms consisting of 188 and 233 species, respectively. These skeletal mechanisms were demonstrated to mimic well the performance of the detailed mechanisms, not only for the autoignition and PSR systems based on which the reduced mechanisms were developed but also for the independent system of jet-stirred reactor. It was further observed that the accuracy of calculated species concentrations was equivalently bounded by the user-specified error threshold value and that the reduction time for a single reaction state is only about 50 ms for the large iso-octane mechanism. [Copyright &y& Elsevier]

A new spectral method is proposed to speedup sensitivity analysis (SA) for ignition and extinction of perfectly stirred reactors (PSR). The method takes advantage of the linear dependency in the stoichiometric coefficient vectors of the elementary reactions in detailed kinetic mechanisms. Compared with the conventional SA that needs to perturb each elementary reaction, the spectral method lumps the elementary reactions into a smaller set of semi-global reactions that are basis vectors for the elementary reaction set, and perturbs each semi-global reaction instead. The sensitivities for the elementary reactions are then computed as linear combinations of the sensitivities for the semi-global reactions. As a result, the computational cost of the spectral method scales as the number of species, while that of the conventional method scales as the number of reactions. The proposed method is demonstrated for PSR extinction using a detailed H 2 mechanism. [ABSTRACT FROM AUTHOR]

A dynamic adaptive hybrid integration (AHI) scheme of second-order accuracy (AHI2) is proposed for time-integration of chemically reacting flows involving stiff chemistry. AHI2 is extended from a first-order AHI method (AHI1) developed in a previous study, which showed that when significant radical sources are present in the non-chemical source terms, splitting the chemical and the transport sub-systems may incur O (1) errors unless the splitting time steps are comparable to or smaller than that required for explicit integration. As such, the transport term needs to be carried during the integration of stiff chemistry to avoid the large splitting errors. In AHI, fast species and reactions that may induce stiffness are treated implicitly, while the non-stiff variables and source terms, including slow reactions and the mixing term, are treated explicitly. The separation of fast-slow chemistry is performed on-the-fly based on analytically evaluated timescales for species and reactions, such that the complexity of the implicit core in the governing equations is minimized at each time step and the time-integration can be performed with high efficiency. The newly developed AHI2 scheme combines the midpoint scheme and the trapezoidal rule to achieve second-order accuracy. The second-order scheme is tested with a toy problem, as well as auto-ignition and unsteady perfectly stirred reactors (PSR) with detailed chemistry. Results show that AHI2 can significantly improve accuracy compared with AHI1. It was further found that AHI2 can accurately predict extinction of unsteady PSRs while the Strang splitting scheme fails to control the error, showing the necessity not to split the chemistry and transport source terms for prediction of extinction or forced-ignition problems involving significant radical sources. Further analysis of numerical efficiency shows that for auto-ignition AHI2 reduces computational cost primarily through the reduction in the number of variables to be solved implicitly, and the time-saving increases with the mechanism size, reaching approximately 70% for the 111-species USC-Mech II compared with a fully implicit scheme. For unsteady PSR involving homogeneous mixing, AHI2 achieved speedup factors of 20 to 30 compared with the Strang splitting scheme. Furthermore, sparse matrix techniques are integrated into AHI2 (AHI2-S) to achieve high computational efficiency. It is shown that the computational cost of AHI2-S is overall linearly proportional to the mechanism size and is comparable to that of evaluating reaction rates using CHEMKIN-II subroutines. It is further shown that AHI2-S achieves a speed-up factor of around two compared with the efficient fully implicit sparse solver LSODES with analytic Jacobian. [ABSTRACT FROM AUTHOR]

A method of linearized error propagation (LEP) based on Jacobian analysis is developed for systematic skeletal mechanism reduction. Jacobian analysis is performed in LEP to estimate the reduction error in selected target species induced by the elimination of other species. Skeletal mechanisms are obtained by eliminating species that induce negligible worst-case errors to the target species. LEP is compared with the methods of directed relation graph (DRG) and DRG with error propagation (DRGEP) on the accuracy of reduction error estimation. It is shown that DRG can effectively control the worst-case error of every species retained in the skeletal mechanism by assuming that error may not decay in the worst cases along the graph-search paths, while it tends to overestimate the errors in the starting species, such that the skeletal mechanism can typically be further reduced if only the starting species are of interest. In contrast, DRGEP assumes geometric error decay along graph-search paths, and may overestimate the error decay and subsequently underestimate the reduction errors in species many steps away from the starting species, resulting in potential unsafe species-eliminations. Compared with DRG and DRGEP, LEP can overall more accurately estimate the errors in the target species such that smaller mechanisms can be obtained compared with DRG, while unsafe species eliminations are less likely compared with DRGEP. To demonstrate the performance of the LEP method, skeletal mechanisms are derived based on perfectly stirred reactor (PSR) solutions sampled along the S-curves, including both the ignition and the extinction states. A 35-species skeletal mechanism is obtained for ethylene−air based on the 111-species USC-Mech II, and a 146-species skeletal mechanism for n-heptane−air is obtained based on a 188-species skeletal mechanism previously developed using DRG. Validation of global flame behaviors, including PSR ignition and extinction residence time, auto-ignition delay, and laminar flame speed, shows that no significant errors are further induced by LEP. [ABSTRACT FROM AUTHOR]

A systematic computational study is conducted to understand the propagation of uncertain chemical kinetic parameters into turbulent premixed flame simulations. Two snapshots of the cross-section of a spherical flame kernel in homogeneous isotropic turbulence is extracted from a three dimensional direct numerical simulation (DNS) and serve as the initial conditions for a series of subsequent two-dimensional DNS that sample and quantify the impact of chemical kinetic uncertainties on the integrated heat release rates and certain species concentrations. The target configuration features real jet fuel chemistry modeled by a 31-species reduced reaction model. The importance of each reaction is first ranked, by comparing the normalized reaction flux based on a snapshot from the DNS. A two-step process is then taken where initial Monte Carlo screening is performed by perturbing 99 reaction rate constants simultaneously. Following this, a subset of 19 reactions is studied using a larger collection of 2D DNS simulations. As the major quantity of interest, the integrated heat release rate is consistently the most sensitive to reactions R8 (H + O 2 = O + OH) and R32 (CO + OH = CO 2 + H). When combined with a consideration of the reaction rate uncertainty, R71 ( CH 3 + OH = CH 2 * + H 2 O) consistently impacts the quality of flame prediction most prominently. For the case starting with a strong burning flame, the rate limiting reactions are found to be not significantly different between the turbulent and laminar conditions, although the magnitudes of sensitivity coefficients are amplified for certain reactions under turbulent conditions due to an enlarged thermochemical space. For the case initialized with a near-extinction flame, more upstream and/or competing reactions with R8 and R32 are observed. Another manifestation of the enlarged thermochemical space in turbulent flames is the wider range of influential reactions for integrated mass of CO, in comparison with the laminar condition where the sensitivity is dominated by R32. For H and C 2 H 2 , reactions R71, R42 (HCO + O 2 = CO + HO 2) and R40 (HCO + M = CO + H + M) are identified to impact the model predictions for such species as acetylene in the most significant manner. [ABSTRACT FROM AUTHOR]

Abstract The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U 0 , with different hydrogen ratio of the fuel jet, R H , and the inlet temperature, T 0. Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, H L , behavior with increasing U 0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing H L behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes. [ABSTRACT FROM AUTHOR]

We present a direct numerical simulation of a temporal jet between n -dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion. [ABSTRACT FROM AUTHOR]

The effects of temperature and composition stratifications on the ignition of a lean n -heptane/air mixture at three initial mean temperatures under elevated pressure are investigated using direct numerical simulations (DNSs) with a 58-species reduced mechanism. Two-dimensional DNSs are performed by varying several key parameters: initial mean temperature, T 0 , and the variance of temperature and equivalence ratio ( T ′ and ϕ ′) with different T − ϕ correlations. It is found that for cases with ϕ ′ only, the overall combustion occurs more quickly and the mean heat release rate (HRR) increases more slowly with increasing ϕ ′ regardless of T 0 . For cases with T ′ only, however, the overall combustion is retarded/advanced in time with increasing T ′ for low/high T 0 relative to the negative-temperature coefficient (NTC) regime resulting from a longer/shorter overall ignition delay of the mixture. For cases with uncorrelated T − ϕ fields, the mean HRR is more distributed over time compared to the corresponding cases with T ′ or ϕ ′ only. For negatively-correlated cases, however, the temporal evolution of the overall combustion exhibits quite non-monotonic behavior with increasing T ′ and ϕ ′ depending on T 0 . All of these characteristics are found to be primarily related to the 0-D ignition delays of initial mixtures, the relative timescales between 0-D ignition delay and turbulence, and the dominance of the deflagration mode during the ignition. These results suggest that an appropriate combination of T ′ and ϕ ′ together with a well-prepared T − ϕ distribution can alleviate an excessive pressure-rise rate (PRR) and control ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. In addition, critical species and reactions for the ignition of n -heptane/air mixture through the whole ignition process are estimated by comparing the temporal evolution of the mean mass fractions of important species with the overall reaction pathways of n -heptane oxidation mechanism. The chemical explosive mode analysis (CEMA) verifies the important species and reactions for the ignition at different locations and times by evaluating the explosive index (EI) of species and the participation index (PI) of reactions. [ABSTRACT FROM AUTHOR]

Abstract: Two and three dimensional direct numerical simulations (DNS) of an autoignitive premixture of air and ethanol in Homogeneous Charge Compression Ignition (HCCI) mode have been conducted. A special feature of these simulations is the use of compression heating through mass source/sink terms to emulate the compression and expansion due to piston motion. Furthermore, combustion phasing is adjusted such that peak heat release occurs after Top Dead Center (TDC) during the expansion stroke, as in a real engine. Zero dimensional simulations were first conducted to identify important parameters for the higher dimensional simulations. They showed that for ethanol, temperature and dilution are the parameters the problem is most sensitive to. One set of two dimensional simulations were conducted with a uniform mixture composition and different levels of temperature stratification, both with and without compression heating. Another set of simulations varied the mixture stratification with constant temperature stratification. Both sets showed considerable differences in ignition delay, heat release and peak temperature and peak pressure. Compression heating was also found to have a significant effect on the heat release profile. A three dimensional simulation was conducted for Spark-Assisted HCCI (SACI). It was initiated with a small spark kernel, which evolved into a premixed flame. The entire mixture eventually underwent autoignition. Distance function based analysis showed a strongly attenuating flame. Analysis of scalar mixing frequencies shows that differential diffusion and reaction induced mixing play an important role in predicting the mixing of reactive scalars. This has significant implications for mixing models for reactive flows. Chemical explosive mode analysis (CEMA) was applied to the 3D simulation and showed promise in identifying the transition from flame propagation to autoignition. [Copyright &y& Elsevier]

Abstract: Detailed chemical kinetics is an integral component for predictive simulation of turbulent flames and is important for reliable prediction of flames and emissions. Major challenges of incorporation of detailed chemistry in flame simulations are induced by the large number of chemical species and the wide range of timescales involved in detailed kinetics. In this work, dynamic adaptive chemistry (DAC) and in situ adaptive tabulation (ISAT) for efficient chemistry calculations in calculating turbulent reactive flows with detailed chemistry are studied in iso-octane/air homogeneous charge compression ignition (HCCI) and methane/air combustion in a partially-stirred reactor (PaSR). Chemistry calculations are accelerated by DAC via expediting the integration of ordinary differential equations (ODEs) governing chemical kinetics with local skeletal mechanisms obtained on-the-fly using the directed relation graph (DRG) method, and by ISAT via reducing the number of ODE integrations through tabulating and re-using the ODE solutions. It is shown that, in contrast to ISAT, the performance of DAC is mostly independent of the nature of combustion simulations, e.g., steady or unsteady, premixed or non-premixed combustion, and its efficiency increases with the size of chemical kinetic mechanisms. DAC is particularly suitable for transient combustion simulations with large mechanisms containing hundreds of species or more, such as those for gasoline or diesel fuels. A speedup factor of about 30 is achieved for HCCI combustion of iso-octane/air with good agreements in the histories of temperature and species concentrations. In contrast, ISAT performs better for simulations where chemistry calculations can be predominantly resolved by retrieving from the ISAT table, i.e., re-using the ODE solutions. It is shown that ISAT achieves speedup factors of about 100 with only about 10%, 0.1% and 0.01% incurred errors in NO, CO, and temperature, respectively, for the premixed methane/air PaSR simulations. Moreover, a coupled DAC and ISAT approach, namely ISAT–DAC, has been developed and demonstrated in this study to accelerate chemistry evaluation. It is shown that the incurred errors in temperature and species concentrations in ISAT–DAC are well controlled, and it can significantly enhance the performance of ISAT, when the fraction of direct ODE integration is significant, via accelerating the ODE integrations by DAC. [Copyright &y& Elsevier]

A reduced, 12-species reaction model (FFCMy-12) is proposed for modeling high-speed turbulent methane flames at high Karlovitz numbers. The model was derived from an early development version (FFCMy) of the 119-species Foundational Fuel Chemistry Model Version 2.0. The reduction was carried out by combining direct species pruning, quasi-steady-state assumption, and reaction lumping, targeting a minimum possible set of species that can capture methane combustion over a wide range of thermodynamic conditions. The performance of the reduced FFCMy-12 is compared to that of a 21-species skeletal reaction model (FFCM1-21) generated through conventional directed relation graph theory (DRG) and DRG-aided sensitivity analysis (DRGASA) algorithms. Model testing starts with legacy combustion properties such as homogeneous ignition delay time, laminar flame speed, and extinction/ignition residence time in a perfectly stirred reactor. More importantly, reduced model testing is extended to three-dimensional direct numerical simulations (DNS) of statistically planar, freely propagating turbulent premixed flames at Karlovitz numbers Ka = 10 , 1 0 2 , 1 0 3 , and 1 0 4 , which nominally represent conditions from corrugated flamelets to broken reaction zones. Comparisons are made between the DNS results generated by the two chemical kinetic models with respect to turbulent flame structures, turbulent flame speed, and species distributions. Overall, presented results demonstrate the potential of FFCMy-12 for efficient modeling of the methane flames under highly turbulent mixing conditions characterized by a wide range of Ka. As importantly, the one-dimensional turbulence (ODT) model, developed in the companion paper (Part I), is shown to reproduce adequately the mean values of the local thermochemical states observed in the DNS, and as such, the ODT model is a viable DNS surrogate for testing the accuracy and applicability of a reduced model. Novelty and significance We present a 12-species reduced methane oxidation reaction model for the modeling of highly turbulent reacting flows. The reduced model is validated using DNS of premixed turbulent methane-air flames over a wide range of turbulent intensities, from relatively modest corresponding to Karlovitz number Ka = 10 to ultra-high intensities at Ka = 1 0 4 . This represents virtually the entire range of turbulent intensities that could be encountered in any realistic situations. The performance of the 12-species reduced model is evaluated against a 21-species skeletal methane oxidation reaction model. The results show excellent agreement between the two models for DNS at Ka = 10 − 1 0 3 . The one-dimensional turbulence (ODT) model is also examined over the same conditions and is shown to be an effective DNS surrogate for evaluating chemical kinetic model reductions. [ABSTRACT FROM AUTHOR]

The ability of the one-dimensional turbulence (ODT) model to serve as a surrogate direct numerical simulation (DNS) is assessed for highly turbulent flames. The ODT model is applied to freely propagating premixed methane–air flames at Karlovitz numbers 10, 10 2 , 10 3 , and 10 4 , and results are compared with DNS. The ODT model solves the conservation equations for momentum, energy, and species on a one-dimensional domain, which corresponds to a streamwise line of sight spanning the DNS domain. The effects of turbulent advection are modeled via a stochastic process, in which the Kolmogorov and reactive length and time scales are explicitly resolved. Molecular transport and chemical kinetics are concurrently advanced in time. Both the ODT and DNS simulations use a 21-species skeletal chemical model for methane combustion. The accuracy of the ODT model is assessed by comparing its predictions of several key characteristics of the flames for each Karlovitz number tested, including the turbulent flame speed and width and the joint probability density functions (jPDFs) of major and selected minor species as well as the heat release rate conditioned on temperature with the results of DNS under comparable conditions. The ODT model is shown to yield qualitative and quantitative agreement with the DNS data for most of the above flame characteristics. Discrepancies are observed primarily for the jPDFs of several minor species examined. Overall, the ODT approach is shown to be an effective surrogate of DNS, potentially useful for guiding chemical reaction model reduction and for assessing the sensitivities of the flame structure and the burning rate to chemistry under highly turbulent conditions. Novelty and Significance : The direct numerical simulations (DNS) of premixed turbulent methane–air flames presented in this work span a uniquely wide range of turbulent intensities, from relatively modest corresponding to Karlovitz number Ka = 10 to ultra-high intensities at Ka = 1 0 4 . This represents virtually the entire range of turbulent intensities that could be encountered in any realistic situation. This is also the first time that such a wide range of conditions is probed for methane in high-fidelity, fully resolved simulations, which use a fully compressible set of flow equations. The one-dimensional turbulence (ODT) model utilizes the same forcing that is present in the DNS enabling a direct comparison between the ODT and DNS. The results show that ODT captures the key features of the DNS results. ODT is shown to be an effective surrogate for DNS and may be useful in guiding chemical reaction model reduction, where many simulations are required. [ABSTRACT FROM AUTHOR]

Abstract: The effects of fuel composition, thermal stratification, and turbulence on the ignition of lean homogeneous primary reference fuel (PRF)/air mixtures under the conditions of constant volume and elevated pressure are investigated by direct numerical simulations (DNSs) with a new 116-species reduced kinetic mechanism. Two-dimensional DNSs were performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields with different fuel compositions to elucidate the influence of variations in the initial temperature fluctuation and turbulence intensity on the ignition of three different lean PRF/air mixtures. In general, it was found that the mean heat release rate increases slowly and the overall combustion occurs fast with increasing thermal stratification regardless of the fuel composition under elevated pressure and temperature conditions. In addition, the effect of the fuel composition on the ignition characteristics of PRF/air mixtures was found to vanish with increasing thermal stratification. Chemical explosive mode (CEM), displacement speed, and Damköhler number analyses revealed that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, rendering the mean heat release rate more distributed over time subsequent to thermal runaway occurring at the highest temperature regions in the domain. These analyses also revealed that the vanishing of the fuel effect under the high degree of thermal stratification is caused by the nearly identical propagation characteristics of deflagrations of different PRF/air mixtures. It was also found that high intensity and short-timescale turbulence can effectively homogenize mixtures such that the overall ignition is apt to occur by spontaneous ignition. These results suggest that large thermal stratification leads to smooth operation of homogeneous charge compression-ignition (HCCI) engines regardless of the PRF composition. [Copyright &y& Elsevier]

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H 2 /air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, T c , between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although T c at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with | α | < 1 first appears in the near field of the jet, it develops into a local extinction mode with α < − 1 as local temperature decreases due to the excessive mixing of heated coflow and cold H 2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H 2 , a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit. [ABSTRACT FROM AUTHOR]

The safe operation of aeronautical engines requires an understanding of flame ignition, propagation and extinction. In this study, direct numerical simulations are performed using a 29 species reduced chemical mechanism for jet fuel surrogate Jet A to understand the flame quenching process. Initially laminar spherical flames of varying sizes and equivalence ratios are subject to an identical periodic domain of decaying and isotropic high intensity turbulence with a turbulent Reynolds number of 2400. All cases become quenched, except for the larger kernel with lower Karlovitz number. An analysis of the flame structure shows broadened preheat zone, flame shortening on the product side, differential species diffusion and partial fuel pyrolysis in the fresh mixture. Two extinction mechanisms are identified arising from flame shortening and high flame stretch. Flame shortening occurs due to turbulence-chemistry interactions that resemble the flame–flame interaction in a laminar counterflow reactant-to-reactant configuration, which contorts and breaks up the ignition kernel. Flame stretch is a local effect that attenuates the heat release rate and causes the flame to retreat towards the product mixtures, similar to what has been observed for reactant-to-product laminar counterflow flames. Chemical explosive mode analysis was also performed to quantify the flame structure and local combustion mode. The diffusion–reaction balance in pinched-off flame islands favors extinction of these smaller structures, while auto-ignition modes are observed within the flame kernel after fresh mixture is engulfed and preheated in the product kernel. Statistics of the density-weighted displacement speed conditional on local combustion mode indicates strong correlation between the local extinction mode and negative displacement speed. The local balance between diffusion and reaction ultimately determines the propensity for local extinction in both laminar and turbulent flames, the extent of which has an impact on global flame propagation. [ABSTRACT FROM AUTHOR]

The HyChem (hy brid chem istry) approach is utilized for modeling the combustion behaviors of gasoline fuels. The approach combines an experimentally constrained, lumped-step model for fuel pyrolysis under the high-temperature combustion condition and a detailed foundation fuel chemistry model to describe the subsequent oxidation of the pyrolysis products. We present results obtained for two Shell gasoline fuels as examples. The results show that with the parameters in the lumped reactions determined by matching the experiment time history data of key products of gasoline pyrolysis, the HyChem reaction models capture the ignition delay times and laminar flame speeds over a wide range of thermodynamic conditions. The HyChem approach is also extended to model the negative-temperature coefficient (NTC) behaviors for the gasoline fuels. The results show that the NTC-enabled models are capable of capturing the ignition delays under both high-temperature conditions and the conditions under which the NTC behaviors are important. The relationship between fuel composition and combustion properties is analyzed. Finally, the HyChem models are reduced to about 40 species to enable turbulent combustion modeling of gasoline fuels in practical engine simulations. [ABSTRACT FROM AUTHOR]

Several compression-ignition and low-temperature combustion strategies require a fuel where the ignition-delay-time (IDT) is highly sensitive to the fuel-to-air equivalence ratio (ϕ). Quantitative prediction of ϕ -sensitivity (i.e., the change in IDT with respect to ϕ) would enable rapid screening of the numerous possible (bio)fuel candidates for this desired high ϕ -sensitivity characteristic. We propose a new ϕ -sensitivity metric (η), which is primarily a function of only temperature (T) and pressure (P). We assess the reliability of 0-D (perfectly homogeneous) simulation and state-of-the-art reaction mechanisms for two well-studied fuels, namely, iso-octane and a primary reference fuel (PRF80). 0-D simulation results are in good agreement with experimental constant volume IDT data at low- and intermediate-temperatures, while systematic deviations are observed at higher temperatures (where full 3-D computational fluid dynamics simulations are required for accurate prediction). We also perform a traditional single-parameter sensitivity analysis to determine the key reactions that affect ϕ -sensitivity. This is followed by a more rigorous Bayesian uncertainty quantification (UQ) analysis to elucidate the possible sources for the discrepancies at high T. Due to the computational cost of UQ, we train artificial neural networks to rapidly predict η for randomly perturbed sets of low- and intermediate-temperature reaction rate parameters. The primary implications of this study are that experimental ϕ -sensitivity data can be used to refine and validate proposed reaction mechanisms, while machine learning and uncertainty quantification of 0-D simulations are essential for quantitative prediction of ϕ -sensitivity in order to rapidly screen fuel candidates. [ABSTRACT FROM AUTHOR]

Real transportation fuels are complex mixtures of a variety of hydrocarbon components. Predicting NO x formation in practical combustors burning real fuels is usually made with the assumption that the NO x submodels developed and tested for small hydrocarbon combustion are applicable to mixtures of large hydrocarbons as found in real fuels. Additionally, NO x data are scarce for flames of real fuels. The aims of the current study are (i) to provide reliable NO x data in flames of a typical jet fuel, and (ii) to test our capability to predict these data by combining a recently proposed HyChem reaction model of jet A combustion (Xu et al., 2018) with the NO x submodel of Glarborg (2018). Specifically, NO x concentrations were measured in stretch-stabilized premixed flames of methane and Jet A (POSF10325) from fuel lean to rich conditions and of ethylene at a fuel-rich equivalence ratio. This range of stoichiometries allows both thermal NO and prompt NO pathways to be tested. The results show reasonably good agreement between the experimental data and model predictions for all flames tested, although the model appears to underpredict NO x concentrations in the Jet A flames under fuel rich conditions. Sensitivity analyses were conducted to illustrate the influence of the reaction pathways and flame boundary conditions on NO x predictions. The analyses also suggest that additional prompt NO reaction pathways may play a role in flames of large hydrocarbons. [ABSTRACT FROM AUTHOR]

Structure of strongly turbulent premixed n -dodecane/air flames with high Karlovitz numbers (Ka) is studied based on three-dimensional (3D) direct numerical simulation (DNS) datasets. Heat release and fuel consumption rates in these flames are observed to be enhanced compared to what can be conventionally described as increases in flame surface area. To explain the cause for the burning rate enhancement, temperature and species mass fractions are first investigated to reveal the overall flame structure. The chemical explosive mode analysis (CEMA) is then employed to identify local combustion modes, including local assisted ignition, auto-ignition, and extinction, each of which is found to play a role in the overall burning rates. The spatial distribution of the local modes is found to be drastically different from that in comparable laminar flames where the local extinction mode is mostly absent. For the high-Ka cases (Ka = 103 and 104), the extinction mode is shown to be comparable to or more important than the auto-ignition mode for heat release and fuel consumption rates. In contrast, the auto-ignition mode plays a more important role in heat release than the extinction mode in laminar and the relatively low-Ka flames (Ka = 102). In addition, two types of mixture pockets are identified by CEMA: pockets of reactants in bulk products and pockets of hot products in bulk reactants. The dynamics of these pockets are strongly affected by the local modes of the spatially adjacent mixtures. While the pockets of reactants in bulk products are almost always consumed by auto-ignition and/or inward flame propagation, the pockets of products in bulk reactants may either grow themselves due to outward flame propagation or contract volumetrically due to local extinction. In contrast to the conventional understanding, local extinction can promote the overall burning process, as it enables mixing of the radicals and sensible energy from the product pockets into the surrounding reactants, thus facilitating their ignition. Clearly, such effects must be considered in order to closely model these flames. [ABSTRACT FROM AUTHOR]

The flame initiation process in dual-fuel spray assisted combustion is presently not fully understood. Here, diesel spray assisted combustion of premixed methane/oxidizer/EGR is explored in the post-ignition phase by scale-resolved simulations. The modified dual-fuel ECN Spray A forms the baseline configuration. An extensive local grid refinement (approaching DNS limit) around one of the first high-temperature ignition kernels is carried out in order to examine the validity of hypothesized flame initiation and deflagration. A high quality LES is used to solve the spray dynamics, while the embedded quasi-DNS (eq-DNS) region offers detailed information on the ignition kernel evolution. The finite-rate chemistry is directly integrated, utilizing 54 species and 269 reactions. Local combustion modes are investigated for the ignition kernel development toward spontaneous ignition and premixed flame propagation using various approaches, including the reaction front displacement speed, energy transport budget, and chemical explosive mode analysis. Furthermore, a new criterion based on reaction flux analysis is introduced, which is compatible with dual-fuel combustion. The spatial and temporal scales associated with the ambient methane consumption and consequent flame initiation are characterized. For the first time in dual-fuel spray assisted simulations, numerical evidence is provided on the initiation of premixed flames, and the corresponding timescale is reported. Particularly, there is a transient mixed-mode combustion phase of approximately 0.2 ms after the spray second stage ignition wherein extinction, ignition fronts, and quasi-deflagrative structures co-exist. After such a transient period, the combustion mode becomes essentially deflagrative. Finally, interactions between turbulence and premixed flame front are characterized mostly in the corrugated regime. [ABSTRACT FROM AUTHOR]

Abstract Bluff-body stabilized premixed flames have been a subject of significant technological interest in practical combustion devices. A collaborative computational-experimental effort is reported here, on simulating a series of bluff-body stabilized premixed propane flames near lean blowoff. An OpenFOAM-based large eddy simulation (LES) solver is coupled with an adaptive hybrid integration solver with sparse matrix technique, to enable efficient chemistry integration. A 31-species skeletal mechanism matching interested flame characteristics is systematically reduced from USC-Mech II, and is utilized to describe the finite-rate chemistry. A procedure to convert numerical mass fractions to pseudo Planar Laser Induced Fluorescence (PLIF) images is established to facilitate the comparison of flame topology and statistics with corresponding experiments. The study focuses on two stably burning conditions, and subsequently on a condition where global blowoff is observed. Velocity statistics, PLIF images of OH and CH 2 O, flame brush thickness, turbulent flame speed, and statistics of two dimensional strain rates are compared against experimental data, achieving reasonably good agreement under stably burning conditions. Approaching blowoff, the flame surfaces recede into the recirculation zone due to the reduced flame speed at lower equivalence ratios. Frequent flame-flame interaction in the narrowed necking region near the top boundary of the recirculation zone, asymmetric helical flame structure, and accumulated CH 2 O in the recirculation zone are observed near blowoff. Elevated levels of unburned fuel is observed in the recirculation zone, suggesting significant entrainment of fresh mixture near global blowoff. Analysis of the enstrophy transport indicates increasing strength of enstrophy approaching blowoff, which results from diminishing baroclinic torque and dilatation effects when density ratios in the anchoring region are reduced. [ABSTRACT FROM AUTHOR]

Abstract The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin. In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel. The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model. Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels. The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products. The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels. [ABSTRACT FROM AUTHOR]

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." [ABSTRACT FROM AUTHOR]

A dynamic adaptive combustion modeling framework based on chemical explosive mode analysis (CEMA) is proposed to account for different flame features such as local auto-ignition, premixed and non-premixed flamelets in diesel spray flames. The proposed modeling strategy is achieved by assigning zone-dependent combustion models on-the-fly to different flame zones segmented using a CEMA-based approach. An approximate CEMA formulation is developed to approximate the eigenvalue of the chemical explosive mode with high computational efficiency in three-dimensional (3-D) turbulent flame simulations. The utility of the CEMA-based criterion for dynamic flame segmentation is first demonstrated using CEMA-based adaptive chemistry by applying different reduced chemistry to different flame zones. The capability of the dynamic adaptive combustion modeling strategy is then demonstrated in large eddy simulations (LES) of turbulent lifted n -dodecane spray flames. Specifically, inert mixing is used for chemically inactive zones, and the well-mixed combustion model with finite rate chemistry is applied in the pre-ignition zone to capture the two-stage ignition as well as premixed reaction fronts. Adaptive mesh refinement (AMR) is further adopted near the premixed reaction fronts to capture the local flame structure and flame propagation speed. For the post-ignition zone, a recently developed tabulated flamelet model (TFM) is applied and compared with the flamelet progress variable (FPV) method. It is shown that CEMA-based adaptive chemistry induces small errors to the statistically-averaged flame structures, as CEMA is an effective and robust approach for on-the-fly flame segmentation. It is further seen that the CEMA-based adaptive modeling strategy more accurately predicts the ignition delay time and flame lift-off length compared with the low-cost flamelet models such as TFM and FPV, while the computational cost is substantially lower compared with the well-mixed combustion model using finite rate chemistry. [ABSTRACT FROM AUTHOR]

We propose and test an alternative approach to modeling high-temperature combustion chemistry of multicomponent real fuels. The hy brid chem istry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel pyrolysis products. The pyrolysis (or oxidative pyrolysis) process is modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients are derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three conventional jet fuels and two rocket fuels as examples. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all five fuels. Sensitivity analysis shows that for conventional, petroleum-derived real fuels, the uncertainties in the experimental measurements of C 2 H 4 and CH 4 impact model predictions to an extent, but the largest influence of the model predictability stems from the uncertainties of the foundational fuel chemistry model used (USC Mech II). In addition, we introduce an approach in the realm of the HyChem approach to address the need to predict the negative-temperature coefficient (NTC) behaviors of jet fuels, in which the CH 2 O speciation history is proposed to be a viable NTC-activity marker for model development. Finally, the paper shows that the HyChem model can be reduced to about 30 species in size to enable turbulent combustion modeling of real fuels with a testable chemistry model. [ABSTRACT FROM AUTHOR]

This study reports cyclopentane ignition delay measurements over a wide range of conditions. The measurements were obtained using two shock tubes and a rapid compression machine, and were used to test a detailed low- and high-temperature mechanism of cyclopentane oxidation that was presented in part I of this study (Al Rashidi et al., 2017). The ignition delay times of cyclopentane/air mixtures were measured over the temperature range of 650–1350 K at pressures of 20 and 40 atm and equivalence ratios of 0.5, 1.0 and 2.0. The ignition delay times simulated using the detailed chemical kinetic model of cyclopentane oxidation show very good agreement with the experimental measurements, as well as with the cyclopentane ignition and flame speed data available in the literature. The agreement is significantly improved compared to previous models developed and investigated at higher temperatures. Reaction path and sensitivity analyses were performed to provide insights into the ignition-controlling chemistry at low, intermediate and high temperatures. The results obtained in this study confirm that cycloalkanes are less reactive than their non-cyclic counterparts. Moreover, cyclopentane, a high octane number and high octane sensitivity fuel, exhibits minimal low-temperature chemistry and is considerably less reactive than cyclohexane. This study presents the first experimental low-temperature ignition delay data of cyclopentane, a potential fuel-blending component of particular interest due to its desirable antiknock characteristics. [ABSTRACT FROM AUTHOR]

Iso-Octane (2,2,4-trimethylpentane) is a primary reference fuel and an important component of gasoline fuels. Moreover, it is a key component used in surrogates to study the ignition and burning characteristics of gasoline fuels. This paper presents an updated chemical kinetic model for iso-octane combustion. Specifically, the thermodynamic data and reaction kinetics of iso-octane have been re-assessed based on new thermodynamic group values and recently evaluated rate coefficients from the literature. The adopted rate coefficients were either experimentally measured or determined by analogy to theoretically calculated values. Furthermore, new alternative isomerization pathways for peroxy-alkyl hydroperoxide (ȮOQOOH) radicals were added to the reaction mechanism. The updated kinetic model was compared against new ignition delay data measured in rapid compression machines (RCM) and a high-pressure shock tube. These experiments were conducted at pressures of 20 and 40 atm, at equivalence ratios of 0.4 and 1.0, and at temperatures in the range of 632–1060 K. The updated model was further compared against shock tube ignition delay times, jet-stirred reactor oxidation speciation data, premixed laminar flame speeds, counterflow diffusion flame ignition, and shock tube pyrolysis speciation data available in the literature. Finally, the updated model was used to investigate the importance of alternative isomerization pathways in the low temperature oxidation of highly branched alkanes. When compared to available models in the literature, the present model represents the current state-of-the-art in fundamental thermochemistry and reaction kinetics of iso-octane; and thus provides the best prediction of wide ranging experimental data and fundamental insights into iso-octane combustion chemistry. [ABSTRACT FROM AUTHOR]

Transported probability density function (TPDF) methods are an attractive modeling approach for turbulent flames as chemical reactions appear in closed form. However, molecular micro-mixing needs to be modeled and this modeling is considered a primary challenge for TPDF methods. In the present study, a new algebraic mixing rate model for TPDF simulations of turbulent premixed flames is proposed, which is a key ingredient in commonly used molecular mixing models. The new model aims to properly account for the transition in reactive scalar mixing rate behavior from the limit of turbulence-dominated mixing to molecular mixing behavior in flamelets. An a priori assessment of the new model is performed using direct numerical simulation (DNS) data of a lean premixed hydrogen–air jet flame. The new model accurately captures the mixing timescale behavior in the DNS and is found to be a significant improvement over the commonly used constant mechanical-to-scalar mixing timescale ratio model. An a posteriori TPDF study is then performed using the same DNS data as a numerical test bed. The DNS provides the initial conditions and time-varying input quantities, including the mean velocity, turbulent diffusion coefficient, and modeled scalar mixing rate for the TPDF simulations, thus allowing an exclusive focus on the mixing model. The new mixing timescale model is compared with the constant mechanical-to-scalar mixing timescale ratio coupled with the Euclidean Minimum Spanning Tree (EMST) mixing model, as well as a laminar flamelet closure by Pope and Anand (1984). It is found that the laminar flamelet closure is unable to properly capture the mixing behavior in the thin reaction zones regime while the constant mechanical-to-scalar mixing timescale model under-predicts the flame speed. The EMST model coupled with the new mixing timescale model provides the best prediction of the flame structure and flame propagation among the models tested, as the dynamics of reactive scalar mixing across different flame regimes are appropriately accounted for. [ABSTRACT FROM AUTHOR]

Modern spark ignition internal combustion (IC) engines rely on highly diluted fuel-air mixtures to achieve high brake thermal efficiencies. To support this, new engine designs have introduced high stroke-to-bore ratios and cylinder head designs that promote high tumble flow and turbulence intensities. However, mixture dilution through exhaust gas recirculation (EGR) is limited by combustion instabilities manifested in the form of cycle-to-cycle variability. Propane has been observed to have superior EGR dilution tolerance than gasoline, which makes it a very competitive low-carbon fuel for the new IC engines without sacrificing efficiency. Two-dimensional direct numerical simulations (DNS) are performed with detailed chemistry to study and contrast the effect of turbulence intensity and dilution on propane and iso-octane premixed flames at high pressure conditions similar to those in-cylinder. A new reduced mechanism for propane consisting of 53 transported species and 17 quasi-steady state species is developed based on a previously published mechanism and used in these simulations. Three levels of turbulence intensity and two levels of exhaust gas dilution are chosen based on conditions relevant to IC engine operation. The DNS results are analyzed based on the evolution of the flame surface area and the statistics of its driving terms, which are found to be similar for both fuels when there is no dilution but considerably different under high dilution. The analysis of the DNS data provides fundamental insights into the underlying mechanisms for improved stability under dilution. [ABSTRACT FROM AUTHOR]

A sparse stiff chemistry solver based on dynamic adaptive hybrid integration (AHI-S) is developed and demonstrated for efficient combustion simulations. In a previous study, a dynamic adaptive method for hybrid integration (AHI) was developed to speed up the time integration of chemically reacting flows with detailed chemistry. The AHI method solves the fast subcomponent of chemistry implicitly and the slow subcomponent of chemistry and transport explicitly, and it was shown that AHI is more accurate and efficient than the operator-splitting schemes when there are significant radical sources from the transport term. In the present study, the AHI method is first improved to minimize the number of nontrivial entries in the Jacobian. Sparse matrix techniques are further integrated into AHI to achieve high computational efficiency. The performance of the new AHI-S solver is investigated in constant-pressure auto-ignition systems using different mechanisms that consist of 9–2878 species. It is shown that the computational cost of the AHI-S solver is overall linearly proportional to the mechanism size and is comparable to that of evaluating reaction rates using CHEMKIN-II subroutines. The AHI-S solver achieves speed-up factors ranging from approximately 10, for the 9-species hydrogen mechanism, to approximately 3000, for the 2878-species biodiesel mechanism, compared with the fully implicit VODE solver with Jacobian evaluated through numerical perturbations and factorized with dense matrix operations. It is further found that for mechanisms with less than approximately 100 species, the time saving of AHI-S is primarily attributed to the reduced size of the implicit core of the governing equations, while for mechanisms with more than 100 species, the computational cost of VODE is dominated by the dense LU factorization, such that the time saving of AHI-S is mostly attributed to the sparse LU factorization. The AHI-S solver is then applied to unsteady perfectly stirred reactors involving extinction and re-ignition. Speed-up factors from 50 to 30,000 are achieved compared with the Strang splitting scheme with the chemistry substeps implicitly integrated with VODE, while speed-up factors of 10–100 are achieved compared with the Strang splitting scheme implemented with the sparse stiff LSODES solver. In the end, the performance of AHI-S is investigated in one-dimensional (1-D) unsteady freely propagating laminar premixed flames for a methane/air mixture, for which the time step size in AHI-S is limited by the fastest transport process. A speed-up factor of approximately 200 is achieved compared with the Strang splitting scheme for fixed time step sizes between 10 − 8 s and 10 − 6 s. [ABSTRACT FROM AUTHOR]

A systematic approach to developing compact reduced reaction models is proposed for liquid hydrocarbon fuels using n -dodecane and n -butane as the model fuels. The approach has three elements. Fast fuel cracking reactions are treated by the quasi-steady state approximation (QSSA) and lumped into semi-global reactions to yield key cracking products that are C 1 –C 4 in size. Directed relation graph (DRG) and sensitivity analysis reduce the foundational fuel chemistry model to a skeletal model describing the oxidation of the C 1 –C 4 compounds. Timescale-based reduction using, e.g., QSSA, is then employed to produce the final reduced model. For n -dodecane, a 24-species reduced model is derived from JetSurF and tested against the detailed model for auto-ignition, perfectly stirred reactors (PSR), premixed flame propagation, and extinction of premixed and non-premixed counterflow flames. It is shown that the QSSA of fuel cracking reactions is valid and robust under high-temperature conditions from laminar flames, where mixing is controlled by molecular diffusion, to perfectly stirred reactors, which correspond to the limit of fast turbulent mixing. Bifurcation analysis identifies the controlling processes of ignition and extinction and shows that these phenomena are insensitive to the details of fuel cracking. To verify the applicability of the above finding to turbulent flames, 2-D direct numerical simulation (DNS) of a lean turbulent premixed flame of n -butane/air with Karlovitz number of 250 was carried out using a reduced model developed from USC-Mech II. The results show that QSSA for fuel cracking remains valid even under intense turbulence conditions. Statistical analysis of the DNS data shows that fuel cracking is complete before the flame zone, and for the conditions tested, turbulent transport does not bring any significant fuel molecules into the flame zones, thus further substantiating the validity of the approach proposed. [ABSTRACT FROM AUTHOR]

An updated reduced gas-phase kinetic mechanism was developed and integrated with aerosol models to predict soot formation characteristics in ethylene nonpremixed and premixed flames. A primary objective is to investigate the sensitivity of the soot formation to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). The gas-phase chemical mechanism adopted the KAUST-Aramco PAH Mech 1.0, which utilized the AramcoMech 1.3 for gas-phase reactions validated for up to C2 fuels. In addition, PAH species up to coronene (C 24 H 12 or A7) were included to describe the detailed formation pathways of soot precursors. In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph with expert knowledge (DRG-X) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames at low strain rate sooting conditions were considered, for which the sensitivity of soot formation characteristics to different nucleation pathways were investigated. Premixed flame experiment data at different equivalence ratios were also used for validation. The findings show that higher PAH concentrations result in a higher soot nucleation rate, and that the total soot volume and average size of the particles are predicted in good agreement with experimental results. Subsequently, the effects of different pathways, with respect to pyrene- or coronene-based nucleation models, on the net soot formation rate were analyzed. It was found that the nucleation processes (i.e., soot inception) are sensitive to the choice of PAH precursors, and consideration of higher PAH species beyond pyrene is critical for accurate prediction of the overall soot formation. [ABSTRACT FROM AUTHOR]

The binary fuel blend of H 2 /CH 4 is one of the most promising hydrogen-enriched hydrocarbon fuels in spark-ignition (SI) engines. Yet, the undesirable phenomenon of super-knock, which can severely and instantaneously damage an SI engine, limits its widespread adoption. Moreover, there is still a lack of consensus on the precise mechanism by which this phenomenon occurs i.e. via flame acceleration or spontaneous ignition, despite numerous previous investigations. At the same time, recent studies [M. P. Burke, S. J. Klippenstein, Nat. Chem. 9 (2017) 1078 - 1082, Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522] have demonstrated a high probability of occurrence of non-thermal reactions in premixed flames of such H 2 /CH 4 fuel blends with air due to the presence of non-trivial amounts of highly reactive radicals including H, O and OH apart from O 2. The present study focuses on the evolution of an initial deflagration front to a detonation wave in H 2 /CH 4 -air mixtures under SI engine relevant conditions through fully resolved, constant volume 1D simulations with and without non-thermal reactivity. Non-thermal reactions were included in the macroscopic kinetics model as chemically termolecular reactions facilitated by the H + CH 3 and H + OH radical-radical recombination and the H + O 2 radical-molecule association reactions. The nonthermal reactions result in a corresponding decrease in the reaction fluxes of the incipient recombination/association reactions. Therefore, an additional set of simulations were performed by applying corrections to the respective incipient recombination/association rate constants using the methodology demonstrated by Tao et al. [Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. Compared to the baseline case, the onset of spontaneous ignition in the end-gas region was observed to be delayed in the presence of non-thermal termolecular reactions. Concurrently, the developing detonation was observed to be significantly stronger. In contrast, applying corrections to the recombination/association rate constants resulted in a completely different behavior. Specifically, detonation was observed to occur due to self acceleration of the primary flame in the absence of spontaneous ignition in the end-gas region. Sensitivity analysis was performed to quantify the effects of non-thermal reactions on the duration of heat release rate and thereby the mechanism of detonation formation. In addition, chemical explosive mode analysis (CEMA) was performed to identify the dominant species/reactions responsible for the observed results. [ABSTRACT FROM AUTHOR]

An n -dodecane spray in temperature and pressure conditions typical of diesel engines, known as Spray A, is modelled by the transported probability density function (TPDF) method coupled with a time-dependent Reynolds-averaged k – ∊ turbulence model and a Lagrangian discrete phase model of the liquid spray. To establish a baseline for comparisons, non-reacting cases are first studied. Good results are obtained for the vapour penetration, the mean and variance of fuel mixture fraction, and velocity profiles, with variations in ambient density and injection pressure. These comparisons are more extensive than previous studies due to new experimental data being available. Reacting cases are then investigated for a number of ambient conditions and injection parameters, employing a reduced chemical kinetic model. The chemical mechanism incorporates an OH ∗ sub-mechanism (Hall and Petersen, 2006) which enables a direct comparison with experimental measurements of the lift-off length that are based on OH ∗ chemiluminescence. To assess the importance of interactions between turbulence and chemistry, the results from the PDF model are compared to the measurements and to those from a well-mixed model that ignores turbulent fluctuations. Variations of ambient temperature, ambient oxygen concentration, ambient density, and injection pressure are considered. In all cases the PDF model with the EMST mixing model and C ϕ = 1.5 shows an excellent agreement with the experimental lift-off length and presents improved results compared with the well-mixed model. Ignition delay is however over-predicted by both the PDF method and well-mixed models. Available shock tube data suggests that this may be due to the chemical kinetic model over-predicting ignition delay at higher pressures. [ABSTRACT FROM AUTHOR]

The operator-splitting schemes for integration of stiff diffusion–reaction systems were found to fail in error control, i.e. incurring O (1) relative errors, with splitting time steps larger than that required for fully explicit integration, when significant non-chemical radical sources are present. It was shown that, by excluding the transport term from the chemistry integration, errors by orders of magnitude may occur in radical concentrations solved in the chemistry sub-step, resulting in significant errors in the major species. The failing scenario is demonstrated with a toy problem and an unsteady perfectly-stirred reactor (PSR) for hydrogen/air with significant H radical concentration at inlet. A dynamic adaptive method for hybrid integration (AHI) of stiff chemistry is then proposed as a substitute for the operator-splitting schemes in such cases. The AHI method can obtain accurate solutions by integrating the fast species and reactions implicitly and the non-stiff terms, including slow reactions and non-chemical source terms, explicitly. Specifically, fast species and reactions are identified on-the-fly based on their analytically derived timescales, the rates of slow variables are evaluated explicitly and those of fast species are evaluated partial-implicitly. As such, the number of variables to be implicitly solved at each integration time step is reduced to the number of the fast species, resulting in a smaller Jacobian matrix and consequently lower computational cost compared with the fully implicit solvers. The hybrid method is validated in auto-ignition for hydrogen/air with different equivalence ratios and initial temperatures, and compared with the Strang splitting scheme for the toy problem and the unsteady PSR. Results show significant improvement in accuracy using the AHI method. [ABSTRACT FROM AUTHOR]

The diffusion models for multicomponent mixtures are investigated in planar premixed flames, counterflow diffusion flames, and ignition of droplet flames. Discernable discrepancies were observed in the simulated flames with the mixture-averaged and multicomponent diffusion models, respectively, while the computational cost of the multicomponent model is significantly higher than that of the mixture-averaged model. A systematic strategy is proposed to reduce the cost of the multicomponent diffusion model by accurately accounting for the species whose diffusivity is important to the global responses of the combustion systems, and approximating those of less importance. The important species in the reduced model are identified with sensitivity analysis, and are found to be typically among those in high concentrations with exception of a few radicals, e.g. H and OH, that are known to participate in critical reactions. The reduced model is validated in simulating the propagation of planar premixed flames, extinction of counterflow non-premixed flames and ignition of droplet flames. The reduced model was shown to feature similar accuracy to that of the multicomponent model while the computational cost was reduced by a factor of approximately 5 for an n -heptane mechanism with 88 species. [ABSTRACT FROM AUTHOR]

Direct Numerical Simulations (DNS) of ethylene/air diffusion flame extinctions in decaying two-dimensional turbulence were performed. A Damköhler-number-based flame extinction criterion as provided by classical large activation energy asymptotic (AEA) theory is assessed for its validity in predicting flame extinction and compared to one based on Chemical Explosive Mode Analysis (CEMA) of the detailed chemistry. The DNS code solves compressible flow conservation equations using high order finite difference and explicit time integration schemes. The ethylene/air chemistry is simulated with a reduced mechanism that is generated based on the directed relation graph (DRG) based methods along with stiffness removal. The numerical configuration is an ethylene fuel strip embedded in ambient air and exposed to a prescribed decaying turbulent flow field. The emphasis of this study is on the several flame extinction events observed in contrived parametric simulations. A modified viscosity and changing pressure (MVCP) scheme was adopted in order to artificially manipulate the probability of flame extinction. Using MVCP, pressure was changed from the baseline case of 1 atm to 0.1 and 10 atm. In the high pressure MVCP case, the simulated flame is extinction-free, whereas in the low pressure MVCP case, the simulated flame features frequent extinction events and is close to global extinction. Results show that, despite its relative simplicity and provided that the global flame activation temperature is correctly calibrated, the AEA-based flame extinction criterion can accurately predict the simulated flame extinction events. It is also found that the AEA-based criterion provides predictions of flame extinction that are consistent with those provided by a CEMA-based criterion. This study supports the validity of a simple Damköhler-number-based criterion to predict flame extinction in engineering-level CFD models. [ABSTRACT FROM AUTHOR]

Abstract: Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252K, and at nominal pressures near 40 and 60bar. Jet stirred reactor experiments are reported at 5atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40bar, for three equivalence ratios, and temperatures below 800K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1–60atm, temperatures of 650–1500K, equivalence ratios of 0.25–4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions. [Copyright &y& Elsevier]

Abstract: Computational flame diagnostics (CFLDs) are systematic tools to extract important information from simulated flames, particularly when detailed chemical kinetic mechanisms are involved. The results of CFLD can be employed for various purposes, e.g. to simplify detailed chemical kinetics for more efficient flame simulations, and to explain flame behaviors associated with complex chemical kinetics. In the present study, the utility of a recently developed method of chemical explosive mode analysis (CEMA) for CFLD will be demonstrated with a variety of flames for n-heptane including auto-ignition, ignition and extinction in steady state perfectly stirred reactors (PSRs) and laminar premixed flames. CEMA was further utilized for analyses and visualization of a direct numerical simulation (DNS) dataset for a 2-D n-heptane–air flame under homogeneous charge compression ignition (HCCI) conditions. CEMA was found to be a versatile method for systematic detection of many critical flame features including ignition, extinction, premixed flame fronts, and flame stabilization mechanisms. The effects of cool flame chemistry for n-heptane on ignition, extinction and flame stability were also investigated with CEMA. [Copyright &y& Elsevier]