Search

This work investigates the ignition and flame development processes of low reactivity fuel combustion under compression ignition conditions based on the large eddy simulation approach. The chemical explosive mode analysis (CEMA) is employed to characterize the local combustion features, including gas-liquid fuel zone, auto-ignition, diffusion-assisted, extinction, cool flame and post-ignition zone, among which auto-ignition and post-ignition are found to play a key role in the overall heat release process. The local flame propagation modes in gasoline compression ignition (GCI) are determined by quantifying the relative magnitude of diffusion/chemistry at a representative progress variable in the pre-ignition zone. The results show that autoignition fronts and deflagration waves exist simultaneously in the ignition and intense high temperature heat release (HTHR) stages, but autoignition fronts dominate. In addition, the chemical kinetic processes of four heat release periods are analyzed. The heat release during the ignition period is found to be dominated by the reactions CH3+ H (+M) <=> CH4 (+M) and CH3CHO + H <=> CH2CHO + H2. The reaction CH2OH + OH <=> CH2O + H2O always plays an important role in the heat releases during the other three combustion stages including intense HTHR, moderate HTHR and post-combustion. [ABSTRACT FROM AUTHOR]

In this study, one-dimensional detonations in ammonia/hydrogen-air mixtures are numerically investigated by solving the fully compressible Navier-Stokes equations with detailed chemistry. Pulsating instabilities with single-mode are observed during the detonation wave propagation, accompanied by periodic coupling and decoupling of the lead shock wave and the reactive front. The ratio between driver pressure and initial pressure determines the overdrive degree and thus the oscillatory mode of detonation for a premixture with certain composition. The effects of hydrogen dilution and mixture equivalence ratio on pulsating detonations are also examined under a constant driver pressure. The growing hydrogen fraction in fuel blends significantly increases the oscillation frequency. In addition, the pulsating detonation frequency rises with increasing equivalence ratio under fuel-lean conditions, peaks under stoichiometric conditions, and falls under fuel-rich conditions as the equivalence ratio increases further. Evolutions of reactants, main intermediate radicals, and products are analysed in both fuel-lean and fuel-rich conditions. A chemical explosive mode analysis further confirms the highly-autoignitive nature of the mixture in the induction zone between reaction front and shock front where thermal diffusion plays a negligible role. • Numerical study of one-dimensional pulsating detonation is performed. • Ammonia and hydrogen fuel blends with various compositions are considered. • The effects of driver pressure and fuel/air equivalence ratio are examined. • Different pulsating detonation frequencies are observed from parametric studies. • Chemical explosive mode analysis reveals chemistry dominates thermal diffusion at detonation front. [ABSTRACT FROM AUTHOR]

Large-eddy simulation with the thickened flame model (LES/TFM) is conducted to simulate a three-dimensional dual swirl partially premixed methane/air gas turbine model combustor. Finite-rate chemistry is described by a skeletal chemical mechanism consisting of 16 species and 41 reactions. Flame sensors based on formyl radical (HCO) and chemical explosive mode analysis (CEMA) are proposed and implemented within the TFM framework. The two sensors are designed for multi-step chemical kinetic models to avoid thickening the low-intensity heat release rate (HRR) region. One-dimensional freely-propagating laminar premixed flames are first employed to assess the two new sensors. The HCO-based sensor can successfully avoid the low-intensity HRR region for lean flames, but can fail under fuel-rich conditions. The CEMA-based sensor can robustly avoid low-intensity HRR regions under both fuel-lean and fuel-rich conditions. A second test case using a hydrogen/air reheat burner further demonstrates the robustness of the CEMA-based sensor. The two new sensors are subsequently applied to the gas turbine model combustor, and effects of different flame sensors are studied. Baseline results from the HCO-based sensor are first compared with experimental measurements to validate the LES/TFM solver. The mean and r.m.s. velocity, temperature, and mass fractions of O 2 and CO agree reasonably well with the experiment, although the mixture fraction within the inner recirculation zone (IRZ) is under-predicted. The predicted mean and r.m.s. temperature and species profiles are comparable at most locations, except near the IRZ where the CEMA-based sensor predicts the largest fluctuations by only thickening the chemically explosive regions. After optimisation, only 15% of overhead in computational cost is imposed when the CEMA sensor is employed on-the-fly. Future work includes further reduction of the computational cost of the CEMA-based sensor. [ABSTRACT FROM AUTHOR]

Numerical simulations of one-dimensional pulsating detonation in off-stoichiometric n -heptane/air mixtures are conducted by solving the reactive Navier–Stokes equations with a skeletal chemical mechanism. The effects of mixture equivalence ratio, initial pressure and temperature on pulsating detonations are studied. The results show that the pulsating instabilities in n -heptane/air mixtures are strongly affected by equivalence ratio. It is seen that pulsating instability only occurs in the fuel-lean or fuel-rich cases, whereas stable detonation is obtained for near-stoichiometric mixtures. Low-frequency pulsating detonations with single mode are observed, and decoupling / coupling of the reaction front and leading shock front occur periodically during the pulsating detonation propagation. The heat release and flame structure at the reaction front of the fuel-lean case differ from those in the fuel-rich case, and thus affects the DDT process of the reaction front. The pulsating detonation frequency is considerably influenced by equivalence ratio, initial pressure and temperature. The results of chemical explosive mode analysis and budget analysis of energy equation reveal that the mixture between the reaction front and shock front is highly explosive and thermal diffusion would promote the periodic dynamics of the reaction front and shock front. It is also found that the chemical explosion mode in the induction zone consists of two parts, i.e. the autoignition dominated reaction immediately behind the leading shock front and a following propagating reaction front. [ABSTRACT FROM AUTHOR]

Large eddy simulation is applied for the combustion process of n-heptane spray flame in methane environment. The influence of methane (CH4) concentration on the auto-ignition process and flame structures are discussed in detail. By increasing the CH4 concentration in the ambient gas, auto-ignition is delayed obviously at a low initial temperature because of the delayed first-stage ignition. While at a high temperature, the auto-ignition time is less influenced by the addition of CH4. However, after the addition of CH4, the concentration of the mixture where the high-temperature ignition is initialed becomes much richer and it is independent of the initial temperatures. In addition, the chemical explosive mode analysis (CEMA) method shows that the mixture in high-temperature regions is non-explosive without CH4 addition, and the abrupt transition between the explosive and non-explosive mixture at the tip of the reacting spray is observed by adding CH4 in the ambient gas, indicating that a flame front is developing downstream. This flame front exists for a long time at low CH4 concentration. The eigenvalue of the Jacobian matrix for the CH4/air mixture in the ambient gas is increased significantly with the addition of CH4, indicating that the combustion process is dominated by auto-ignition. Finally, results show that the development of the flame is accelerated by the addition of CH4 in the ambient gas. [ABSTRACT FROM AUTHOR]

In the present work, artificial neural networks (ANN) technique combined with flamelet generated manifolds (FGM) is proposed to mitigate the memory issue of FGM models. A set of ANN models is firstly trained using a 68-species mass fractions in mixture fraction-progress variable space. The ANN prediction accuracy is examined in large eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) simulations of spray combustion. It is shown that the present ANN models can properly replicate the FGM table for most of the species mass fractions. The network models with relative error less than 5% are considered in RANS and LES to simulate the Engine Combustion Network (ECN) Spray H flames. Validation of the method is firstly conducted in the framework of RANS. Both non-reacting and reacting cases show the present method predicts very well the trend of spray and combustion process under different ambient temperatures. The results show that FGM-ANN can replicate the ignition delay time (IDT) and lift-off length (LOL) precisely as the conventional FGM method, and the results agree very well with the experiments. With the help of ANN, it is possible to achieve high efficiency and accuracy, with a significantly reduced memory requirement of the FGM models. LES with FGM-ANN is then applied to explore the detailed spray combustion process. Chemical explosive mode analysis (CEMA) approach is used to identify the local combustion modes. It is found that before the spray flame is developed to the steady-state, the high CH2O zone is always associated with ignition mode. However, high CH2O zone together with high OH zone is dominated by the burned mode after the steady-state. The lift-off position is dominated mainly by the diffusion mode.

In the context of vapour cloud explosion, the flame acceleration process can lead to conditions promoting deflagration to detonation transition (DDT), potentially leading to increased damages in accidental scenarios. This study focuses on this phenomenon by performing simulations of detonation reinitiation for fast flames in the Chapman–Jouguet deflagration regime. It is obtained experimentally by the attenuation of an incident detonation by an array of obstacles. A primary objective of the paper is to demonstrate the ability of the numerical model to reproduce the major experimental trends, namely the variation of the reinitiation propensity for different initial pressures and blockage ratios (BRs). Chemical explosive mode analysis (CEMA) is also adapted to the context of this study, in order to identify locally the propagation regime and to provide insights on the reinitiation mechanism. An a priori validation of the CEMA methodology is first performed on relevant canonical one-dimensional configurations. Subsequently, ensembles of five realizations are computed at different initial pressures and BRs and compared to experimental data. They are shown to reproduce the major observed trends in terms of detonation reinitiation length with respect to the operating conditions, with significant variability from one realization to another. In addition, the reinitiation mechanism is also found to be consistent with experimental observations and a previous numerical study of the same configuration. The CEMA methodology adapted to this context is able to identify locally the different propagation regimes, and to track the highly reactive zones that coherently couple with transverse pressure perturbations, leading to the formation of a strongly reacting kernel which eventually triggers the detonation reinitiation. [ABSTRACT FROM AUTHOR]

An n -dodecane swirling spray flame from the Cambridge flame series is simulated using large-eddy simulation (LES) at conditions near the lean blow-out (LBO) limit. The focus of this study is to examine effects of low-temperature chemistry (LTC) and spray evaporation in turbulent spray combustion. To this end, a first simulation is performed using a finite rate chemistry model with a 55-species skeletal mechanism including LTC in a fully compressible Eulerian-Lagrangian formulation. A second simulation is performed using the same formulation, but with the LTC chemical sub-mechanism deactivated. The interactions of spray and gas-phase mixing and combustion are investigated through the consideration of mean and instantaneous LES results. Chemical explosive mode analysis (CEMA) is extended to account for droplet evaporation in the context of spray combustion, the results of which reveal that the flame is dominated by non-premixed combustion without significant auto-ignitive behavior. CEMA results further show that the effect of spray evaporation on the reaction is two-fold, namely to inhibit reaction near the injector through heat absorption, and to facilitate reaction further downstream by supplying fuel to the gas phase. Mixture fraction-conditioned analysis is then performed to evaluate the importance of LTC in the turbulent spray flame, showing its effect on heat release despite the flame not exhibiting auto-ignitive behavior. A polar mapping is proposed for analyzing the complex interplay of low and high temperature chemistry heat release. The results have consequences for numerical modeling of spray combustion systems where LTC effects are commonly neglected. [ABSTRACT FROM AUTHOR]

A new computational diagnostic method for pressure-induced compressibility is proposed by projecting its local contribution to the chemical explosive mode (CEM) in the chemical explosive mode analysis (CEMA) framework. The new method is validated for the study of detonation development during the deflagration-to-detonation transition (DDT) process. The flame characteristics are identified through the quantification of individual CEM contributions of chemical reaction, diffusion, and pressure-induced compressibility. Numerical simulations are performed to investigate the DDT processes in a stoichiometric hydrogen-air mixture. A Godunov algorithm, fifth-order in space, and third-order in time are used to solve the fully compressible Navier-Stokes equations on a dynamically adapting mesh. A single-step, calibrated chemical diffusive model (CDM) described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The new diagnostic method is first applied to one-dimensional (1D) canonical flame configurations followed by two-dimensional (2D) simulations of DDT in an obstructed channel where different detonation initiation scenarios are examined using the new CEMA projection formulation. Detailed examinations of the idealized configuration of detonation initiation through shock focusing mechanism at a flame front are also studied using the new formulation. A comparison of the currently proposed CEMA projection and the original formulation by the authors suggests that including the pressure-induced compressibility is essential for the use of CEMA in DDT process. The results also show that the new formulation of CEMA projection can successively capture the detonation initiation through either a gradient mechanism or a direct initiation mechanism, and therefore can be used as an effective local analytical tool for the computational diagnostics of detonation initiation in a DDT process. It was found that detonation development is characterized by a strong contribution of chemistry role to the CEM which is pivotal to the initiation of detonation. The role of compressibility is found enhanced at the edge of the detonation front where diffusion was found to have minimal effects on detonation development. A new computational diagnostic method for pressure-induced compressibility is proposed by projecting its local contribution to the chemical explosive mode (CEM) in the chemical explosive mode analysis (CEMA) framework. The proposed method is tested and validated for the study of detonation development during the deflagration to detonation transition (DDT) process. The new method is found to be an effective local analytical tool for the computational diagnostics of detonation initiation in a DDT process. The proposed method is versatile and can be used on various different platforms which makes this study more impactful. [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]

In this study, the novel conservative representation of chemical explosive mode analysis is augmented to analyze the key flame features in the Burrows-Kurkov flames simulated by both Reynolds-Averaged Navier-Stokes (RANS) and large eddy simulation (LES). Subtle difference are revealed in flame stabilization mechanisms resulting from the difference in modeling and spatial resolution. RANS shows that, ahead of the flame onset location, the composition diffusion and shock wave compression play dominant roles in chemical explosion indicating that the flame is stabilized by the assisted-ignition combustion mode. In contrast, LES shows that the flame is stabilized by the auto-ignition mode since the nonchemical contribution counteracts chemical reaction during the development of ignited flame kernels. For RANS, the radical pool builds up through the unphysical back diffusion near the flame stabilization front, which reveals the limitation of RANS method in the resolution and characterization of the key flame features in Burrows-Kurkov flames. • The novel CCEMA method is improved in the aspect of EI definition. • The impacts of high-speed source terms on CEMs are rigorously quantified. • CCEMA is applied to the Burrows-Kurkov flames simulated by both RANS and LES. • The subtle difference in flame features between RANS and LES results are detected. • The limitation of RANS in characterization of Burrows-Kurkov flame is revealed. [ABSTRACT FROM AUTHOR]

Abstract Numerical simulations of shock-induced combustion (SIC) with detailed H 2 /O 2 reaction mechanisms near CJ detonation velocity were investigated. The effect of grid resolution and the chemical reaction mechanisms were examined for unsteady SIC over a blunt body at Mach number 4.48 presented by Lehr. Using high resolution schemes, it is found that Jachimowski reaction mechanism, which has been used dominantly for the SIC, develops an unphysical low frequency high amplitude oscillation at high grid resolutions. These disturbances were not observed at lower grid resolutions, while UCSD reaction mechanism is less sensitive to the spatial resolutions. The cause of the low frequency oscillation was investigated with Dynamic Mode Decomposition (DMD) and Chemical Explosive Mode Analysis (CEMA) technique. The spatial structure of the decomposed modes from DMD analysis shows that the shock wave moves closer to the projectile surface and this creates a disturbance near the projectile surface with increase in spatial resolution. Using CEMA, it is found that the influence of the reaction step H 2 + OH = H 2 O + H is high in the burned section of the flowfield for Jachimowski mechanism and this creates the disturbance which slowly gets converted into the unphysical phenomena. [ABSTRACT FROM AUTHOR]

Abstract A criterion based on chemical explosive mode analysis (CEMA) is proposed to identify different local combustion modes, such as auto-ignition, diffusion-assisted ignition, and local extinction in laminar and turbulent premixed flames. The criterion is employed to distinguish between two different propagation modes of one-dimensional (1-D) freely propagating premixed flames, that is the canonical deflagration wave controlled by heat and species back-diffusion, which features a unique flame speed, and the auto-ignition controlled reaction front propagation, which may feature an arbitrary propagation speed. In the CEMA based diagnostic, the local chemistry and diffusion source terms are projected to the chemical explosive mode (CEM), such that the roles of diffusion and chemistry in the CEM can be quantified and compared for identification of the flame propagation mode. The new criterion is further applied to analyze two-dimensional (2-D) DNS datasets of homogeneous charge compression ignition (HCCI) combustion of n -heptane/air with different levels of thermal stratification. Both flame propagation modes are observed and contribute significantly to the overall flame burning in the case with a high level of thermal stratification, while the flame propagation is found dominated by the auto-ignition mode in the case with a low level of thermal stratification, which agrees with the finding in previous studies. [ABSTRACT FROM AUTHOR]

In this work, the dynamic combustion characteristics in a scramjet engine were investigated using three diagnostic data analysis methods: DMD (Dynamic Mode Decomposition), STFT (Short-Time Fourier Transform), and CEMA (Chemical Explosive Mode Analysis). The data for the analyses were obtained through a 2D numerical experiment using a DDES (Delayed Detached Eddy Simulation) turbulence model, the UCSD (University of California at San Diego) hydrogen/oxygen chemical reaction mechanism, and high-resolution schemes. The STFT was able to detect that oscillations above 50 kHz identified as dominant in FFT results were not the dominant frequencies in a channel-type combustor. In the analysis using DMD, it was confirmed that the critical point that induced a complete change of mixing characteristics existed between an injection pressure of 0.75 MPa and 1.0 MPa. A combined diagnostic analysis that included a CEMA was performed to investigate the dynamic combustion characteristics. The differences in the reaction steps forming the flame structure under each combustor condition were identified, and, through this, it was confirmed that the pressure distribution upstream of the combustor dominated the dynamic combustion characteristics of this scramjet engine. From these processes, it was confirmed that the combined analysis method used in this paper is an effective approach to diagnose the combustion characteristics of a supersonic combustor.

Abstract Explosive modes such as ignition and extinction are characterized by an eigenvalue of the chemical Jacobian matrix with positive real part, representing the transient instability of chain-branching chemistry and thermal feedback. Formation and eigen-decomposition of the Jacobian matrix are expensive operations whose cost increases cubically with chemical mechanism size. As an alternative to directly computing the eigenvalues of the Jacobian, we explore principal component analysis (PCA) along with nonlinear regression as a methodology to parameterize the eigenvalues by state variables (or linear combinations thereof). We evaluate this modeling strategy using homogeneous autoignition data on two different applications: pseudotransient continuation (Ψtc)-based ODE solvers and chemical explosive mode analysis (CEMA). Results indicate that the PCA-based parameterization of the eigenvalues appears feasible for Ψtc solvers in autoignition calculations over a range of temperatures and pressures. Our results also show that eigenvalue models are capable of tracking sharp discontinuities (such as ignition or extinction) in the eigenvalue for computational flame diagnostics such as CEMA. [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]

The formulation of reactive flow problems can be both quite challenging and important to the efficiency and robustness of solution algorithms. In this article, we focus on the choice of the thermochemical state vector as it relates to recently-developed computational techniques for complex combustion chemistry problems. We identify over-specification of the state vector as a source of both ambiguity and error in the partial derivatives used in forming analytical forms of the chemical source Jacobian matrix. We review and compare several approaches taken to increase sparsity of the Jacobian matrix, as it relates to the use of Newton–Krylov methods for implicit time integration, and identify proper techniques for achieving sparsity that do not rely on ad-hoc choice of state variables with inconsistent Jacobians. Chemical explosive mode analysis and linearly-implicit methods, such as Rosenbrock methods, are identified as areas where Jacobian accuracy may be critical. The distinction between how Jacobian exactness impacts Rosenbrock and Newton–Krylov methods is demonstrated with a simple example. We demonstrate the errors in conservation obtained from over-specification of the state vector with auto-ignition calculations for hydrogen, ethylene, and n-heptane chemistry with a high-order implicit Runge–Kutta method. [ABSTRACT FROM AUTHOR]

An investigation of the local flame extinction and re-ignition of hydrogen syngas non-premixed flame are carried out using one-dimensional turbulence (ODT) model with detailed chemistry. Based on the Lagrangian tracer method and chemical explosive mode analysis (CEMA), diagnostic tools have been defined to distinguish the different scenarios of local extinction and re-ignition. The results show that the extinction regime can be characterized by a large negative Damköhler number based on CEMA. From Lagrangian investigation, two scenarios of local extinction, i.e., severe extinction with lower temperature and moderate extinction with higher temperature, are shown. Meanwhile, two re-ignition mechanisms are distinguished by the explosion index of the process prior to re-ignition. It is found that re-ignition mechanism through premixed flame propagation corresponds to a rather long preheat period with thermal runaway and high temperature explosion index, while re-ignition mechanism via independent flamelet corresponds to a high radical explosion index and small temperature gradient. [ABSTRACT FROM AUTHOR]

The paper presents computational results of a turbulent non-premixed flame with large extinction, using the large eddy simulation probability density function methodology. The simulation captures local flame extinction at different flame locations, and major species predictions show good agreement with experimental data. A chemical explosive mode analysis is used to determine the flame structure including the explosive modes and the Damköhler number distribution. Due to the nature of the turbulence combustion model, statistical information on the sub-grid flame structure is available: such as sub-grid explosive modes. The analysis suggests that, in the present flame, sub-grid structures are only relevant close to the inlet nozzle, and that downstream extinction is governed by large-scale interactions. [ABSTRACT FROM PUBLISHER]

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]

The Multiple Mapping Conditioning / Large Eddy Simulation (MMC-LES) approach is used to simulate a supersonic lifted hydrogen jet flame, which features shock-induced autoignition, shock-flame interaction, lifted flame stabilization, and finite-rate chemistry effects. The shocks and expansion waves, shock-reaction interactions and overall flame characteristics are accurately reproduced by the model. Predictions are compared with the detailed experimental data for the mean axial velocity, mean and root-mean-square temperature, species mole fractions, and mixture fraction at various locations. The predicted and experimentally observed flame structures are compared through scatter plots of species mole fractions and temperature against mixture fraction. Unlike most past MMC-LES which has been applied to low-Mach flames, in this supersonic flame case pressure work and viscous heating are included in the stochastic FDF equations. Analysis indicates that the pressure work plays an important role in autoignition induction and flame stabilization, whereas viscous heating is only significant in shear layers (but still negligibly small compared to the pressure work). The evolutions of particle information subject to local gas dynamics are extracted through trajectory analysis on representative fuel and oxidizer particles. The particles intermittently enter the extinction region and may be deviated from the full burning or mixing lines under the effects of shocks, expansion waves and viscous heating. The chemical explosive mode analysis performed on the Lagrangian particles shows that temperature, the H and OH radicals contribute dominantly to CEM respectively in the central fuel jet, fuel-rich and fuel-lean sides. The pronounced particle Damköhler numbers first occur in the fuel jet / coflow shear layer, enhanced at the first shock intersection point and peak around the flame stabilization point. [ABSTRACT FROM AUTHOR]

Abstract: An idealized extinguishing agent with two main inhibition mechanisms was designed to test the chemistry of hydrogen flame extinguishment. The One-Dimensional Turbulence (ODT) model and chemical explosive mode (CEM) analysis (CEMA) were used to perform an in-depth analysis on the role of turbulence-chemistry interaction in diffusion hydrogen/air flame. The function route of the fire extinguishment agent in diffusion flame has been analyzed. The results show that the agent can significantly reduce the CEM. The decrease of CEM will decrease the Da number, and lead to flame extinction. The CEMA of ODT results present that the extinction of flame is controlled by the CEM, the mixing rates and the eddy events. The results also illustrated the main inhibition mechanism in which catalytic cycles lead to the destruction of major chain carriers, H. The acting areas of variables and reactions are consistent and clearly visualized with CEMA, which proves that the CEMA is a very efficient method to identify the control variable during the combustion process. [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]

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]

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]

In this work, the dynamic combustion characteristics in a scramjet engine were investigated using three diagnostic data analysis methods: DMD (Dynamic Mode Decomposition), STFT (Short-Time Fourier Transform), and CEMA (Chemical Explosive Mode Analysis). The data for the analyses were obtained through a 2D numerical experiment using a DDES (Delayed Detached Eddy Simulation) turbulence model, the UCSD (University of California at San Diego) hydrogen/oxygen chemical reaction mechanism, and high-resolution schemes. The STFT was able to detect that oscillations above 50 kHz identified as dominant in FFT results were not the dominant frequencies in a channel-type combustor. In the analysis using DMD, it was confirmed that the critical point that induced a complete change of mixing characteristics existed between an injection pressure of 0.75 MPa and 1.0 MPa. A combined diagnostic analysis that included a CEMA was performed to investigate the dynamic combustion characteristics. The differences in the reaction steps forming the flame structure under each combustor condition were identified, and, through this, it was confirmed that the pressure distribution upstream of the combustor dominated the dynamic combustion characteristics of this scramjet engine. From these processes, it was confirmed that the combined analysis method used in this paper is an effective approach to diagnose the combustion characteristics of a supersonic combustor. [ABSTRACT FROM AUTHOR]

• The auto-ignition assisted flame propagation has been analyzed. • Heat release indictors have been identified for n-heptane spray flames. • The performance of indicators has been analyzed in different combustion regimes. The quantification of the heat release process is important for engine combustion, for which species concentrations are often employed as the heat release rate indicators (HRRIs) considering the difficulty in the direct measurement of the local heat release rate (HRR). In this study, the correlation between HRR and species concentrations in auto-ignition and auto-ignition assisted flame propagation of n-heptane/air mixtures is first quantified to validate the commonly used formaldehyde-based HRRIs (i.e. [HCO] and [OH][CH 2 O]). Potential candidates for HRRIs are then proposed and further investigated in the auto-igniting n-heptane spray flame obtained with direct numerical simulations using a 44-species mechanism. It is found that [HCO] and [OH][CH 2 O], as well as the potential candidates [C 2 H 3 ], [CH 2 ], [OH][HO 2 ] and [H][HO 2 ], in general all have good linear correlations with the HRR in the spray flame, and [H][HO 2 ] has the best performance. Chemical explosive mode analysis is then performed to further analyze the performance of the HRRIs in the three combustion regimes: post-flame, auto-ignition and flame propagation. It is shown that the primary combustion mode in this spray flame is auto-ignition. Regime-dependent correlation has been observed and the above HRRIs yield the best linear correlation for the region with the Damköhler number being far more than unity in this turbulent auto-igniting n-heptane flame. [ABSTRACT FROM AUTHOR]