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Experiments and numerical simulations were conducted in this paper to study the combustion behavior of dimethyl ether in the moderate or intense low-oxygen dilution regime, in terms of thermal/chemical structure and chemical kinetics associated with nitrogen oxide and carbon monoxide emissions. Several co-flow temperatures and oxygen concentrations were involved in the experiments to investigate their impacts on the flame behavior systematically. The results show that in the moderate or intense low-oxygen dilution regime, oxygen concentrations in the flame base slightly increased because of the prolonged ignition delay time of the reactant mixture due to oxidizer dilution, which changed the local combustion process and composition considerably. The oxidation rates of hydrocarbons were significantly depressed in the moderate or intense low-oxygen dilution regime, such that a fraction of unburned hydrocarbons at the furnace outlet were recirculated into the outer annulus of the furnace, which changed the local radial profiles of carbon monoxide, methane, and hydrogen partially. Moreover, with the increment in co-flow temperature or oxygen mole fraction, flame temperature, and hydroxyl radical, carbon monoxide, and hydrogen mole fractions across the reaction zone increased gradually. For the dimethyl ether-moderate or intense low-oxygen dilution flame, temperature homogeneity was improved at higher co-flow temperature or lower oxygen mole fraction. The carbon monoxide emission depended on the levels of temperature and hydroxyl radical concentration inside the reaction zone significantly. Emission index of carbon monoxide increased at lower co-flow temperature or oxygen mole fraction; and it was more sensitive to the variation in co-flow oxygen mole fraction. Additionally, the dominant formation pathways of nitrogen oxide in the dimethyl ether-moderate or intense low-oxygen dilution flame were clarified. The contribution of the thermal pathway was fairly unimportant. Emission index of nitrogen oxide increased as co-flow temperature or oxygen mole fraction was increased. The ratio of nitrogen dioxide emission index to nitrogen oxide emission index decreased with the increment in co-flow temperature or oxygen mole fraction. [ABSTRACT FROM AUTHOR]

Methyl palmitate is a long-chain methyl ester and a major constituent of palm-oil-derived biodiesel. A detailed mechanism for its combustion was recently developed by Herbinet and coworkers. This detailed mechanism involves 4442 species and 30,425 reactions, which makes it too complex for direct use in flame structure modeling, for instance, in studies of one-dimensional laminar opposed-flow diffusion flames. We used the improved directed relation graph method to derive a skeletal biodiesel combustion mechanism that retains the key properties of the detailed mechanism including auto-ignition behaviors and extinction temperature profiles of stoichiometric methyl palmitate/air mixture at pressures of 1–100 atm. The initial temperatures for ignition were from 600 to 1600 K. This skeletal mechanism, containing only 402 species and 2503 reactions, was used to study methyl palmitate conversion rates and key species profiles in a jet-stirred reactor and an opposed-flow diffusion flame at atmospheric pressure and stoichiometric fuel/oxidizer conditions. Supplemental materials are available for this article. Go to the publisher's online edition ofCombustion Science and Technologyto view the free supplemental file. [ABSTRACT FROM PUBLISHER]

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: 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: The need and prospect of incorporating realistic fuel chemistry in large-scale simulations of combustion phenomena and combustor performance are reviewed. The review first demonstrates the intricacies of chemical kinetics in homogeneous and diffusive systems, and emphasizes the essential importance of the comprehensiveness of chemical fidelity for mechanisms at the detailed and reduced levels. A systematic approach towards developing detailed reaction mechanisms is then outlined, followed by an extensive discussion on the development of reduced mechanisms and the associated strategies towards facilitated computation. Topics covered include skeletal reduction especially through directed relation graph; time-scale reduction based on the concepts of quasi-steady species enabled through computational singular perturbation; the lumping of isomers and of species with similar diffusivities; on-the-fly stiffness removal; the relative merits of implicit versus explicit solvers; and computation cost minimization achieved through tabulation and the judicious re-sequencing of the computational steps in arithmetic evaluations. Examples are given for laminar flames and direct numerical simulations of turbulent combustion to demonstrate the utility of the integrated strategy and the component methods in incorporating realistic chemistry of practical fuels in large-scale simulations, recognizing that the detailed mechanisms of these fuels may consist of hundreds to thousands of species and thousands to tens of thousands of reactions. Directions for further research are suggested. [Copyright &y& Elsevier]

Abstract: Direct numerical simulation of a three-dimensional spatially developing turbulent slot-burner Bunsen flame has been performed with a new reduced methane–air mechanism. The mechanism, derived from sequential application of directed relation graph theory, sensitivity analysis and computational singular perturbation over the GRI-1.2 detailed mechanism is non-stiff and tailored to the lean conditions of the DNS. The simulation is performed for three flow through times, long enough to achieve statistical stationarity. The turbulence parameters have been chosen such that the combustion occurs in the thin reaction zones regime of premixed combustion. The data is analyzed to study possible influences of turbulence on the structure of the preheat and reaction zones. The results show that the mean thickness of the turbulent flame, based on progress variable gradient, is greater than the corresponding laminar flame. The effects of flow straining and flame front curvature on the mean flame thickness are quantified through conditional means of the thickness and by examining the balance equation for the evolution of the flame thickness. Finally, conditional mean reaction rate of key species compared to the laminar reaction rate profiles show that there is no significant perturbation of the heat release layer. [Copyright &y& Elsevier]

Abstract: A systematic approach for mechanism reduction was developed and demonstrated. The approach consists of the generation of skeletal mechanisms from detailed mechanism using directed relation graph with specified accuracy requirement, and the subsequent generation of reduced mechanisms from the skeletal mechanisms using computational singular perturbation based on the assumption of quasi-steady-state species. Both stages of generation are guided by the performance of PSR for high-temperature chemistry and auto-ignition delay for low- to moderately high-temperature chemistry. The demonstration was performed for a detailed ethylene oxidation mechanism consisting of 70 species and 463 elementary reactions, resulting in a specific skeletal mechanism consisting of 33 species and 205 elementary reactions, and a specific reduced mechanism consisting of 20 species and 16 global reactions. Calculations for laminar flame speeds and nonpremixed counterflow ignition using either the skeletal mechanism or the reduced mechanism show very close agreement with those obtained by using the detailed mechanism over wide parametric ranges of pressure, temperature, and equivalence ratio. [Copyright &y& Elsevier]

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]

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]

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 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]

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]

• Ignition dynamics in turbulent DME/CH 4 -air under RCCI condition is studied via DNS. • Low-temperature ignition transits to cool flame in the stratified DME/ CH 4 -air mixture. • High-temperature autoignition is accelerated by the cool flame. • The four branches of typical tetrabranchial flames coexist in the field. A study of ignition dynamics in a turbulent dimethyl ether (DME)/methane-air mixture under reactivity controlled compression ignition (RCCI) conditions was conducted using direct numerical simulation. Initially, the directly-injected DME and in-cylinder premixed methane-air mixture are partially mixed to form a mixing layer in between. A reduced DME/CH 4 oxidization mechanism, consisting of 25 species and 147 reaction steps, is developed and validated. Ignition is found to occur as a two-stage process. Low-temperature autoignition is first initiated in the fuel-rich part of the mixture and then transits to a cool flame, propagating towards the even richer mixture through a balanced reaction-diffusion mechanism. Cool flames not only develop in the mixing layer, but also in the initially stratified DME/methane-air mixture. The formation of high-temperature autoignition kernels is earlier than that in the homogeneous mixture at the same mixture fraction, which is thought to be accelerated by the cool flame. The expanding flames from high-temperature kernels are connected with the neighboring flames before they engulf the stoichiometric mixture iso-lines. The four branches of typical tetrabranchial flames, i.e. cool flame, fuel-rich premixed flame, diffusion flame, fuel-lean premixed flame coexist in the field. The fuel-lean premixed flame branch finally triggers the premixed methane-air flame. The multi-stage and multi-mode nature of the ignition process highlights the intractable challenge to model the RCCI engine combustion. [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]

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]

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]

High-fidelity simulations of ignition, extinction and oscillatory combustion processes are of practical interest in a broad range of combustion applications. Splitting schemes, widely employed in reactive flow simulations, could fail for stiff reaction–diffusion systems exhibiting near-limit flame phenomena. The present work first employs a model perfectly stirred reactor (PSR) problem with an Arrhenius reaction term and a linear mixing term to study the effects of splitting errors on the near-limit combustion phenomena. Analysis shows that the errors induced by decoupling of the fractional steps may result in unphysical extinction or ignition. The analysis is then extended to the prediction of ignition, extinction and oscillatory combustion in unsteady PSRs of various fuel/air mixtures with a 9-species detailed mechanism for hydrogen oxidation and an 88-species skeletal mechanism for n -heptane oxidation, together with a Jacobian-based analysis for the time scales. The tested schemes include the Strang splitting, the balanced splitting, and a newly developed semi-implicit midpoint method. Results show that the semi-implicit midpoint method can accurately reproduce the dynamics of the near-limit flame phenomena and it is second-order accurate over a wide range of time step size. For the extinction and ignition processes, both the balanced splitting and midpoint method can yield accurate predictions, whereas the Strang splitting can lead to significant shifts on the ignition/extinction processes or even unphysical results. With an enriched H radical source in the inflow stream, a delay of the ignition process and the deviation on the equilibrium temperature are observed for the Strang splitting. On the contrary, the midpoint method that solves reaction and diffusion together matches the fully implicit accurate solution. The balanced splitting predicts the temperature rise correctly but with an over-predicted peak. For the sustainable and decaying oscillatory combustion from cool flames, both the Strang splitting and the midpoint method can successfully capture the dynamic behavior, whereas the balanced splitting scheme results in significant errors. [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 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 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]

Abstract: The recently developed method of chemical explosive mode (CEM) analysis (CEMA) was extended and employed to identify the detailed structure and stabilization mechanism of a turbulent lifted ethylene jet flame in heated coflowing air, obtained by a 3-D direct numerical simulation (DNS). It is shown that CEM is a critical feature in ignition as well as extinction phenomena, and as such the presence of a CEM can be utilized in general as a marker of explosive, or pre-ignition, mixtures. CEMA was first demonstrated in 0-D reactors including auto-ignition and perfectly stirred reactors, which are typical homogeneous ignition and extinction applications, respectively, and in 1-D premixed laminar flames of ethylene–air. It is then employed to analyze a 2-D spanwise slice extracted from the 3-D DNS data. The flame structure was clearly visualized with CEMA, while it is more difficult to discern from conventional computational diagnostic methods using individual species concentrations or temperature. Auto-ignition is identified as the dominant stabilization mechanism for the present turbulent lifted ethylene jet flame, and the contribution of dominant chemical species and reactions to the local CEM in different flame zones is quantified. A 22-species reduced mechanism with high accuracy for ethylene–air was developed from the detailed University of Southern California (USC) mechanism for the present simulation and analysis. [Copyright &y& Elsevier]