Flame Wrinkling Research Articles

Combustion characteristics of premixed ammonia-hydrogen/air swirl flames at elevated pressure

Ammonia (NH3)-hydrogen (H2) blends which are obtainable by NH3 cracking can be an eminent carbon-free fuel. To provide a valuable database for the practical application of cracking-based NH3-H2 fuel, the effects of pressure on the combustion characteristics of premixed NH3-H2(-nitrogen (N2))/air swirl flames are investigated in terms of H2 mole fraction in the NH3-H2 blend and fuel-equivalence ratio. Extinction mechanisms at fuel-lean and rich flames are the same for both normal and elevated pressures, though only the blowoff is observed for elevated pressure. With increasing pressure, fuel-rich limits are generally extended but fuel-lean limits are reduced, NOx emissions decrease and the condition of the maximum NOx emissions moves towards fuel-richer condition. Flame visualization exhibits the shifted main reaction zone upstream, the increased flame wrinkling and the reduced OH and NO intensities with increasing pressure. Dilution effects of N2 at elevated pressure become remarkable compared with those at normal pressure.

Delineating the effects of ignition propensity and Markstein number on deflagration-to-detonation transition

Although the process through which a deflagration transitions to a detonation (DDT) has been studied for over a century, it is not well-understood how the propensity for DDT varies among different combustible mixtures. We investigate the effect of two fundamental mixture properties, ignition delay (τign) and Markstein number (Ma), on this stochastic process. Experiments were conducted in a smooth cylindrical tube, and the critical state (characterized by the velocity of the reacting front) at which a detonation formed from an accelerating deflagration was measured. The novelty of the study lies in the selection of mixture compositions that enabled changing only one relevant combustion property at a time and hence isolating the sensitivity to that property. Experiments with mixtures with similar deflagrative properties but different τign revealed that the critical state is not measurably sensitive to ignition propensity. Mixtures studied with different thermo-diffusive properties revealed that Ma has a strong effect on the critical state. Specifically, thermo-diffusively-unstable mixtures (with low Ma) underwent DDT at significantly lower velocities. Furthermore, experiments were performed with and without a spiral to isolate the effect of flame wrinkling and turbulence. The spiral-affected flames underwent DDT at lower velocities, similar to thermo-diffusively-unstable mixtures. This observed effect of thermo-diffusive nature of the mixture and turbulence on DDT propensity is not predicted by existing models.

Investigation of the three-dimensional structure of low-swirl methane-hydrogen flames with multiple UV cameras-based OH* chemiluminescence tomography

Blending hydrogen with natural gas is useful for reducing CO2 emissions, but this alters the flame structure, creating challenges for flame stability. Numerous researches on methane-hydrogen flame structure have been carried out based on two-dimensional combustion diagnostic techniques such as flame chemiluminescence and planar laser-induced fluorescence. In this paper, a low-cost OH* chemiluminescence imaging system consisting of 8 ultraviolet (UV) cameras is proposed to reconstruct the three-dimensional (3D) structure of methane-hydrogen flames, and validated through use of the correlation coefficient. Further experimental investigations of the low-swirl methane-hydrogen flames are performed by this computed tomography of chemiluminescence (CTC) system. The flame properties are determined to evaluate the effects of various fractions of blended hydrogen on the flame structure, including the flame center of mass, flame surface density and fractal dimension. Results showed that the flame initially has bowl-shape structure. The flame height gradually decreases and the flame cross-sectional area increases as the blended hydrogen fraction increases. Meanwhile, the bottom of the flame moves toward the nozzle outlet. Nevertheless, when the blended hydrogen fraction reaches 50%, the flame transforms into a cup-shaped flame and shows a double-layer flame structure. When the flame is bowl-shaped, the flame center of mass moves away from the nozzle center axis with increasing blended hydrogen fraction, but the fractal dimension and flame surface density are not sensitive to the blended hydrogen fraction. In contrast, when the flame is cup-shaped, the flame center of mass moves towards the nozzle center axis, and the flame surface density and fractal dimension increase significantly, indicating the increase of flame wrinkling. These results reveal that blending hydrogen leads to two distinct flame structures and characteristics in low-swirl flame, also demonstrate that the proposed 3D low-cost CTC system based on multiple UV cameras is well-suited for combustion diagnostics.

Effects of liquid water injection on flame surface topology and propagation characteristics in spray flames: A direct numerical simulation analysis

The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as they approach the flame even though some droplets can survive until the burnt gas side is reached, whereas water droplets do not significantly evaporate ahead of the flame and the evaporation of water droplets starts to take place in the reaction zone and is completed within the burnt gas. However, the gaseous-phase combustion occurs predominantly in fuel–lean mode although the overall equivalence ratio remains equal to unity. The water injection has been found to suppress the fuel droplet-induced flame wrinkling of the progress variable isosurface under the laminar condition, and this effect is particularly strong for small water droplets. However, turbulence-induced flame wrinkling masks these effects, and thus, water injection does not have any significant impact on flame wrinkling for the turbulent cases considered here. The higher rate of evaporation and the associated high latent heat extraction for smaller water droplets induce stronger cooling effects, which weakens the effects of chemical reaction. This is reflected in the decrease in the mean values of density-weighted displacement speed with decreasing water droplet diameter. The weakening of flame wrinkling as a result of injection of small water droplets is explained through the curvature dependence of the density-weighted displacement speed. The combined influence of cooling induced by the latent heat extraction of water droplets and flame surface flattening leads to a decrease in volume-integrated burning rate with decreasing water droplet diameter in the laminar cases, whereas the cooling effects are primarily responsible for the drop in burning rate with decreasing water droplet diameter in the turbulent cases.

Direct pore-level simulation of hydrogen flame anchoring mechanisms in an inert porous media

This work addresses the stabilization of hydrogen flames inside porous media. Hydrogen is a peculiar fuel with a large molecular diffusivity, triggering preferential diffusion and other effects related to non-unity Lewis numbers. Consequently, the macro-scale behavior of hydrogen flames is highly influenced by the flame-wall interactions at the pore-scale. In this study, direct pore-level simulations are used for the investigation of anchoring mechanisms and burning-rate enhancement for lean hydrogen flames embedded in inert porous media. It is observed that preferential diffusion greatly enhances flame anchoring, in the wake of the struts, leading to a high increase in flame surface. Also, the local heat release rate is enhanced not only by the higher gas temperature resulting from the heat recirculation but also by the higher local equivalence ratio resulting from preferential diffusion. Furthermore, most of the heat produced by a hydrogen flame is released by the hydroperoxyl reactions, effective at low temperatures but necessitating radicals, mostly H. These radicals are produced by the shuffle reactions, effective at higher gas temperatures. The coupling between those two reaction groups is very dependent on the transport of radicals, which can be impacted by the pore scale velocity gradient, leading to hydrodynamic dispersion and flame wrinkling. Consequently, it is shown that the burning-rate enhancement observed within porous burners fueled with hydrogen is not only due to heat recirculation but also to increased transports of radicals and preferential diffusion effects, which should be accounted for in macroscopic models.

Effects of cryogenic temperature on turbulent premixed hydrogen/air flames

As a carbon-free fuel, hydrogen (H2) is of increasing importance in the development of low-emission engines. Due to a low volumetric energy density, H2 is preferably stored at cryogenic temperatures (Tu≤100 K). In this context, it is indispensable to investigate the combustion behavior of hydrogen at such low temperatures. Although some studies focused on the ignition and detonation of hydrogen, investigations about premixed H2/air flame propagation interacting with turbulence at cryogenic temperatures are rather scarce. In this work, stoichiometric turbulent premixed H2/air flames are studied at cryogenic temperature (Tu=100K) and normal temperature (Tu=300K), using three-dimensional direct numerical simulations with detailed chemistry and transport. It is found that at cryogenic temperature, dimensionless turbulent flame speed and flame surface area increase significantly due to Darrieus–Landau instability (DLI) induced by a large expansion ratio. Since the effective Lewis numbers of the two cases are close to unity, the diffusive-thermal instability (DTI) is negligible in the cases. Furthermore, it is found that there are substantial differences in the peaks of HO2 and H2O2 mass fractions between the two cases, probably due to smaller local flame curvatures at cryogenic temperature. Moreover, the results indicate that the flame response to stretch is not sensitive to the change of the initial temperature. A larger fractal inner cutoff scale is found at Tu=100K, suggesting that the flame exhibits more large-scale flame wrinkling than that at Tu=300K due to the impact of DLI. All these facts lead to the conclusion that cryogenic temperature can significantly promote large-scale flame wrinkling, increase turbulent flame speed and flame surface area, and affect intermediate species distribution. This suggests that combustion of cryogenic H2 may have a high risk of flashback.

Large-eddy simulation of the Darmstadt multi-regime turbulent flame using flamelet-generated manifolds

The Darmstadt multi-regime turbulent flame MRB26b is modelled using the Flamelet-Generated Manifold (FGM) in the context of Large Eddy Simulation (LES). Laminar free-propagating premixed flames are used to build a premixed chemical database, and laminar counter-flow flames are adopted to build a non-premixed and a partially premixed database. A detailed comparison of the LES-FGM results with the experimental data using the three databases is conducted, combining two sub-filter closure models, i.e., the PPDF (Presumed Probability Density Function) and DTF (Dynamically Thickened Flame). It is found that different databases lead to similar modelling of velocity, temperature, mixture fraction and major species (e.g., CH4, O2, CO2 and H2O). However, the intermediate minor species CO and H2 are better predicted using the two databases based on counter-flow flames due to a better inclusion of the fuel/air mixing and straining effects. Similar to previous studies, using the DTF model is observed to over-predict CO and H2. This is revealed to be a combined effect of the artificial flame front thickening and flame wrinkling loss, which is characterized by a lower variance of the progress variable. Comparatively, better minor species predictions are obtained using the PPDF without flame thickening. A correction method is proposed and validated to improve the minor species prediction for the DTF model. The combustion regimes and flame propagation directions are also examined. The results are significant for high-fidelity simulation of multi-regime turbulent flames using the flamelet-tabulated combustion model.

Self-adaptive turbulence eddy simulation of a premixed jet combustor

The self-adaptive turbulence eddy simulation (SATES) is employed to investigate the lean premixed methane/air turbulent flame in a single-nozzle model gas turbine combustor, in which the high axial momentum jet issuing from an off-center nozzle facilitates the development of a large-scale, dominant lateral recirculation zone that stabilizes the flame. For turbulence modeling, the SATES method can dynamically adjust the proportion of resolving and modeling of turbulent scales according to the local grid scale and turbulence length scales, thus depressing the gird-sensitivity in large eddy simulation (LES) calculation. For combustion modeling, the thickened flame model with a reduced chemistry mechanism is integrated into the SATES turbulence modeling framework to capture the unsteady flame dynamics. The accuracy of SATES results is assessed against experimental data, as well as the ones from LES and the detached eddy simulation (DES) of this burner with the same combustion model and grids. The predicted length of large-scale recirculation of the flow field by SATES is significantly better than that by LES and DES with low mesh resolution. Detailed comparisons show that the SATES adaptively improves the modeling degree of near-wall turbulence to improve the prediction accuracy with the low grid resolution. Similar to LES, the SATES method does not cause serious delay of shear layer instability and weakening of flame wrinkle observed in DES. The study demonstrates the suitability and accuracy of the hybrid turbulence modeling methods of SATES for complex turbulent flame simulations coupled with suitable combustion model.

Fundamental Study of Premixed Methane Air Combustion in Extreme Turbulent Conditions Using PIV and C-X CH PLIF

This paper presents the flow and flame characteristics of a highly turbulent reactive flow over a backward-facing step inside a windowed combustor. Flow and combustion experiments were performed at Re = 15,000 and Re = 30,000 using high-resolution 10 kHz PIV and 10 kHz PLIF diagnostic techniques. Grid turbulators (Grid) with two different hole diameters (HD of 1.5 mm and 3 mm) and blockage ratios (BR of 46%, 48%, 62%, and 63%) were considered for the turbulence study. Grids introduced different turbulent length scales (LT) in the flow, causing the small eddies and turbulence intensity to increase downstream. The backward-facing step increased the turbulence level in the recirculation zone. This helped to anchor the flame in that zone. The small HD grids (Grids 1 and 3) produced continuous fluid structures (small-scale), whereas the larger HD grids (Grids 2 and 4) produced large-scale fluid structures. Consequently, the velocity fluctuation was lower (~25.6 m/s) under small HD grids and higher (~27.7 m/s) under large HD grids. The flame study was performed at ɸ = 0.8, 1.0, and 1.2 using C-X CH PLIF. An Adaptive MATLAB-based flame imaging scheme has been developed for turbulent reacting flows. Grids 1 and 3 induced more wrinkles in the flame due to higher thermal instabilities, pressure fluctuation, and diffusion under those grids. The flamelet breakdown and burnout events were higher under Grids 2 and 4 due to higher thermal diffusivity and a slower diffusion rate. It was observed that the flame wrinkling and flame stretching are higher at Re = 30,000 compared to Re = 15,000. The Borghi–Peters diagram showed that the flames were within the thin reaction zone except for Grid 1 at Re =15,000, where flames fell in the corrugated zone. It was observed from PIV and PLIF analyses that Re and LT mostly controlled the flame and flow characteristics.

Effects of H2 addition on the instability characteristics of CO/air and CH4/air in a horizontal narrow channel

ABSTRACT This paper investigates the instability characteristics of CO/H2/air and CH4/H2/air mixtures in a narrow channel under standard temperature and pressure conditions. The CO/H2 mixture exhibits a greater overpressure growth rate and maximum explosion overpressure as compared to the CH4/H2 mixture at the same hydrogen content. As the hydrogen content increases, the maximum explosion overpressure (P max) value increases, while the corresponding time (θ) value gradually decreases. The maximum explosion overpressure of CH4/H2 and CO/H2 mixtures is 28.27 kPa and 25.09 kPa, respectively. The flame wrinkles to form cells due to the Darius-Landau instability, and the interaction between the vortex tube and premixed flame affects the structure of turbulent premixed combustion. The oscillation frequency of the overpressure increases with increasing hydrogen content, and the oscillation belongs to the Helmholtz oscillation. The CO/H2 mixtures exhibit greater flame thickness than CH4/H2 mixtures, resulting in lower Peclet numbers and therefore greater relative importance of heat losses. As the hydrogen content increases, the sensitivity coefficient of R99 (OH + CO <=> H + CO2) in the CO/H2 mixture gradually decreases, while the sensitivity coefficients of R38 (H + O2 <=> O + OH) and R84 (OH + H2 <=> H + H2O) increase. The elementary reaction R38 plays a leading role in increasing the laminar burning velocity of the CH4/H2 mixture. The trend of R38 variation in the two mixtures is opposite with increasing hydrogen content.

Effects of Mixture Distribution on the Structure and Propagation of Turbulent Stratified Slot-Jet Flames

The influence of mixture stratification on the development of turbulent flames in a slot-jet configuration has been analysed using Direct Numerical Simulation data. Mixture stratification was imposed at the inlet by varying the equivalence ratio between 0.6 and 1.0 with different alignments to the reaction progress variable gradient: aligned gradients (back-supported), opposed gradients (front-supported) and misaligned gradients. An additional premixed case with a global equivalence ratio of 0.8 was simulated for comparison. The flame is shortest for the front-supported case, followed by the premixed flame, with the back-supported and misaligned gradient flames being the tallest and of comparable size. This behaviour has been explained in terms of the variations of the mean equivalence ratio within the flame and the volume-integrated reaction rate in the streamwise direction. The difference in mixture composition for these cases results in significant variations in the burning rate, flame area, flame wrinkling and flame brush thickness in the streamwise direction. The globally front-supported case has the highest volume-integrated burning rate and flame area, while the back-supported case has the lowest. The misaligned scalar gradient case exhibits qualitatively similar behaviour to that of the globally back-supported case. The burning intensity is unity for a major part of the flame length but assumes values greater than unity towards the flame tip where the effects of flame curvature become strong. All cases predominantly exhibit the premixed mode of combustion within the flamelet regime, so flamelet assumption-based reaction rate closures, originally proposed for premixed combustion, were evaluated using a priori analysis. The terms which require improved closures have been identified and existing closures have been improved where necessary. It was found that the global nature of mixture stratification does not influence the performance of the mean reaction rate closures or the parameterisation of marginal probability density functions of scalars in turbulent stratified mixture combustion.

A universally applicable 0D combustion model with a novel analysis of the critical factors affecting combustion in spark-ignition engines

Notwithstanding the continuous technological development of spark-ignition engines, the physics of in-cylinder phenomena has not been explicitly clarified to date. Furthermore, the advent of diverse engine technologies and in-cylinder compositions has increased the difficulty of understanding these phenomena using existing analysis methods. Under those circumstances, the modeling based on the fundamental combustion behaviors is becoming critical for analyzing combustion. Built upon the zero-dimensional flame surface density model, therefore, the models proposed herein attempt to reproduce a vast majority of combustion behaviors revealed to date, such as differential diffusion, thermo-diffusive instability, cutoff scales, distributed reaction, and flame quenching. As a result, we successfully predicted the combustion of numerous operating points of four different SI engines, including the extremely lean operating points of gasoline/air mixture up to [Formula: see text]. Furthermore, the extensive thermodynamic ranges are explored for the validation of the models with the aid of continuously variable valve timing modules; the pressure and the calculated temperature of the unburned mixture at ignition timing are varied from 4.5 to 28.6 bar and 559–794 K, respectively. In this study, we suggest an expression for a fully-developed turbulent flame speed well-fitted with the explored ranges of chemical and thermodynamic states by inspecting the influencing factors for flame wrinkling. Additionally, we analyze combustion using the developed models and identify several distinctive features of both stoichiometric and lean-burn points. Lastly, this study reveals the importance of flame thickness (or inner cutoff), which has a comparable or more severe effect on early flame propagation compared to planar laminar flame speed.

Analysis of Pressure Effect on Three-Dimensional Flame Surface Density Estimation

In experiments, flame surface density (FSD), defined as flame area per volume, is usually approximated by its two-dimensional (2D) value, as flame length per area. However, this approximation may underestimate the flame wrinkle due to the presence of a fluctuating component outside the measurement plane. Obtaining a three-dimensional (3D) flame surface density (FSD) from experiments is challenging, but it can be estimated from low-dimensional measurements under certain assumptions. Models used to estimate 3D FSD can be significantly affected by ambient pressure, as high pressure can cause a substantial decrease in small flame front scales. In this study, a CH4/air premixed turbulent flame is stabilized on a Bunsen burner and measured using the OH-PLIF technique at pressures up to 1.0 MPa. The flame front is extracted with an in-house auto adaptive threshold binarization code. Different models estimating 3D FSD with the corresponding assumptions are summarized from the definition of FSD. The reliability of the assumptions under different pressures is investigated and analyzed. The models are compared through analyzing the assumptions, and are tested in terms of global fuel consumption. The pressure’s effect on the reliability of the models could provide an essential improvement in the context of modeling turbulent combustion.

Pressure gradient effect on flame–vortex interaction in lean premixed bluff body stabilized flames

This investigation considers the effect of axial pressure gradient on the dynamics of flame–vortex interaction for a lean premixed bluff body stabilized flame. Large eddy simulations (LESs) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff body stabilized combustor for a propane/air flame is analyzed first. The results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame–vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability is presented. The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame–vortex dynamics by suppressing flame-induced vorticity sources, baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near-lean blowoff dynamics also affect the emission characteristics, and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core, resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.

Modelling challenges of volume-averaged combustion in inert porous media

In porous burners, the flame thickness can be smaller than the pore size, resulting in sharp and locally-anchored flame fronts. The presence of such steep gradients at the pore level is a major hurdle for the derivation of volume-averaged models, particularly for the highly non-linear reaction rates. With the intent to address the difficulties associated with volume-averaging for porous media combustion, this work makes use of 3D Pore-Level Direct Numerical Simulations including conjugate heat transfer and complex chemistry in burners of finite length. These detailed 3D simulations are compared to their 1D volume-averaged counterpart, with effective properties estimated directly on the computational domains and of identical thermo-chemical scheme. Discrepancies in terms of burning rate, profiles, and a priori analysis on the reaction rates are discussed. Various pore sizes and geometries are considered. At the pore level, it is shown that preheating, wrinkling and wall quenching are the three main factors driving the global burning rate. Importantly, hydrodynamic dispersion is shown to have an indirect role on combustion processes. From the observations of combustion at pore scale, a new closure for reaction rates based on a flamelet assumption is proposed. It accounts for flame wrinkling and eliminates the unwanted effect of hydrodynamic dispersion on burning rate.

Flame Propagation Characteristics of Gas Explosions in Utility Tunnels Considering Spatial Obstacles

Because of their adverse impact on social infrastructure and economic property, gas explosions are among the most dangerous disasters that can befall utility tunnels. Flame propagation is the most intuitive manifestation during the explosion process. It can be significantly influenced by spatial obstacles. To analyze the propagation law of gas explosions in utility tunnels more realistically, an explosion model of the utility tunnel was established by considering actual spatial obstacles, including the pipeline support and reserved frame. The flame propagation characteristics, including the flame shape, temperature distribution, and propagation velocity, were analyzed systematically. Results demonstrate that spatial obstacles have a prominent influence on flame evolution laws. Flame propagation is disturbed but not completely blocked by spatial obstacles. Rather, irregular flame shapes and wrinkles of the flame front are easily developed. The explosion process is insufficient because of the existence of residual methane in the obstacle area. Flame propagation velocity is improved remarkably, whereas temperature distribution is affected slightly. In particular, the spread velocity increases with the increase of propagation distance. The maximum velocity of monitoring surfaces can be improved by 60% under conditions with obstacles.

High-Efficient Flame-Retardant Finishing of Cotton Fabrics Based on Phytic Acid.

In this study, an efficient phosphorus-containing flame retardant, PAPBTCA, was synthesized from phytic acid, pentaerythritol, and 1,2,3,4-butane tetracarboxylic acid, and its structure was characterized. PAPBTCA was finished on cotton fabrics by the pad-dry-curing process, and the flame retardancy, flame-retardant durability, and wrinkle resistance of the obtained flame-retardant fabrics were investigated. It should be noted that the heat release rate value of the flame-retardant cotton fabrics treated with 200 g/L PAPBTCA decreased by 90% and its excellent flame retardancy was maintained after 5 washing cycles. Meanwhile, the wrinkle resistance of flame-retardant cotton fabrics has been significantly improved. In addition, compared with the control, the breaking force loss of PAPBTCA-200 in the warp and weft directions was 24% and 21%, respectively. This study provides a new way to utilize natural phosphorus-based flame retardants to establish multifunctional finishing for cotton fabrics.

Uticaj parametara mreže i sagorevanja na CFD simulaciju motora sa varničkim paljenjem

In the current paper, a CFD simulation study on a pent-roof, spark ignition engine was performed. The scope was to analyze the basic parameters that influence the performances of the simulation: mesh configuration, mesh refinement, turbulence model, and flame wrinkling model parameters. It was used as the OpenFOAM solver for spark ignition engines. The solver was modified by adding the layer addition/removal mechanism for the mesh. Further, a Zeldovich extended NOx model was implemented. Four different mesh configurations were tested with different refinement levels. A set of different turbulence models and initial parameters and different flame wrinkling model parameters were tested. It was observed how all these parameters influence the rate of heat release, cylinder pressure, temperature, the level of turbulent kinetic energy, the flame wrinkling factor, and finally the resulting concentration of nitride monoxide.

Lower flammability limits of H2/NH3/CH3OH mixtures under elevated pressures, temperatures and different blending ratios

The lower flammability limits (LFLs) of H2/NH3/CH3OH mixtures and related flame morphologies are experimentally investigated under elevated pressures, elevated temperatures, and blending ratios. The results show that the flame morphologies of LFLs are mushroom-like, darkening with an increase in initial pressures and remaining unchanged with an increase in initial temperatures. Except for the flame wrinkles at 10:0:0 and 5:5:0, there is little difference in the flame morphologies with blending ratios. When NH3:CH3OH = 1:1, the LFLs of 0 % H2 decrease with elevated pressures while the rest increase, and the LFLs steadily decrease as H2 increases. When H2:CH3OH = 1:1, the LFLs of 100 % NH3 decrease with elevated pressures while the rest increase, and the LFLs increase as the NH3 increases. The LFLs of 0–40 % CH3OH rise with the increasing pressure when H2:NH3 = 1:1, while the LFLs of 60 % CH3OH are almost unchanged and the LFLs of 80–100 % CH3OH decrease. Additionally, the LFLs increase at 50–100 kPa and decrease at 150–250 kPa as CH3OH increases. The LFLs of the mixtures decrease with elevated temperatures. The prediction formula of H2/NH3/CH3OH mixtures’ LFLs under elevated pressures, elevated temperatures, and blending ratios is established. The average absolute error (AAE) and average relative error (ARE) are 0.11 % and 1.54 %.

A modified thickened flame model for simulating extinction

For large-eddy simulation of turbulent premixed reacting flows, major challenges stem from the inability to resolve the flame in a computationally affordable manner. These challenges are most evident in combustors characterized by large domains and thin flames. In these applications, the thickened flame model may be used to extend the flame artificially to a numerically resolvable size through a thickening factor. Thicker flames exhibit suppressed wrinkling in the presence of turbulence, so an efficiency factor increases the flame speed without influencing flame thickness. In contrast to the detailed considerations of unresolved turbulent flame wrinkling, recent work shows that thickened flames do not respond correctly to resolved-scale stretch. In this work, errors in stretch-induced extinction are considered. The already established effect of thickening on extinction is illustrated, and the effect of efficiency factor is characterized in detail. Significant errors in extinction stretch rate are observed analytically and numerically in twin premixed counterflow flame simulations. In general, the original thickened flame formulation does not permit control over extinction, in contrast to its control over freely-propagating-flame thickness and speed. For reactant mixtures with a Lewis number greater than 1, a novel modification of the thickened flame formulation is presented, and through Lewis number adjustments, extinction errors are significantly reduced, while key flame thickening and speed properties of the original formulation are preserved. A test case featuring a turbulent premixed bluff-body-stabilized flame demonstrates that the extinction errors of the original formulation can lead to premature blowoff dynamics and significant statistical errors, if the grid is too coarse. The modified thickened flame model applied to the same grids addresses this issue and provides reasonable flame predictions on all grids, indicating the potential for extending this combustion model to resolutions of greater engineering relevance.