Analysis of high-pressure hydrogen, methane, and heptane laminar diffusion flames: Thermal diffusion factor modeling

Direct numerical simulations are conducted for one-dimensional laminar diffusion flames over a large range of pressures (1?P₀?200 atm) employing a detailed multicomponent transport model applicable to dense fluids. Reaction kinetics mechanisms including pressure dependencies and prior validations at both low and high pressures were selected and include a detailed 24-step, 12-species hydrogen mechanism (H₂/O₂ and H₂/air), and reduced mechanisms for methane (CH₄/air: 11 steps, 15 species) and heptane (C₇H₁₆/air: 13 steps, 17 species), all including thermal NO_x chemistry. The governing equations are the fully compressible Navier-Stokes equations, coupled with the Peng-Robinson real fluid equation of state. A generalized multicomponent diffusion model derived from nonequilibrium thermodynamics and fluctuation theory is employed and includes both heat and mass transport in the presence of concentration, temperature, and pressure gradients (i.e., Dufour and Soret diffusion). Previously tested high-pressure mixture property models are employed for the viscosity, heat capacity, thermal conductivity, and mass diffusivities. Five models for high-pressure thermal diffusion coefficients related to Soret and Dufour cross-diffusion are first compared with experimental data over a wide range of pressures. Laminar flame simulations are then conducted for each of the four flames over a large range of pressures for all thermal diffusion coefficient models and results are compared with purely Fickian and Fourier diffusion simulations. The results reveal a considerable range in the influence of cross-diffusion predicted by the various models; however, the most plausible models show significant cross-diffusion effects, including reductions in the peak flame temperatures and minor species concentrations for all flames. These effects increase with pressure for both H₂ flames and for the C₇H₁₆ flames indicating the elevated importance of proper cross-diffusion modeling at large pressures. Cross-diffusion effects, while not negligible, were observed to be less significant in the CH₄ flames and to decrease with pressure. Deficiencies in the existing thermal diffusion coefficient models are discussed and future research directions suggested.     

A quasi-discrete model for heating and evaporation of complex multicomponent hydrocarbon fuel droplets

A quasi-discrete model for heating and evaporation of complex multicomponent hydrocarbon fuel droplets is suggested and tested in Diesel engine-like conditions. The model is based on the assumption that properties of components are weak functions of the number of carbon atoms in the components (n). The components with relatively close n are replaced by the quasi-components with properties calculated as average properties of the a priori defined groups of actual components. Thus the analysis of heating and evaporation of droplets consisting of many components is replaced by the analysis of heating and evaporation of droplets consisting of relatively few quasi-components. In contrast to previously suggested approaches to modelling the heating and evaporation of droplets consisting of many components, the effects of temperature gradient and quasi-component diffusion inside droplets are taken into account. The model is applied to Diesel fuel droplets, approximated as a mixture of 21 components C_nH_2n+2 for 5 ? n ? 25, which correspond to a maximum of 20 quasi-components with average properties for n = n_j and n = n_j+1, where j varies from 5 to 24. It is pointed out that droplet surface temperatures and radii, predicted by a rigorous model taking into account the effect of all 20 quasi-components, are very close to those predicted by the model, using just five quasi-components. Errors due to the assumptions that the droplet thermal conductivity and species diffusivities are infinitely large cannot be ignored in the general case.     

Numerical study of a high-hydrogen micromix model burner using flamelet-generated manifold

The flamelet-generated manifolds (FGM) method was adopted in this study to consider the preferential diffusion in a high-hydrogen micro-mixing model burner. That is, when solving the FGM flamelet, accurate diffusion rate was obtained from two methods: multicomponent formulation and constant detailed Lewis numbers assumption. Then a new method of filling the thermochemical state and the source term in the mixture fraction and the process variable space also was proposed, namely the linear triangular dissection interpolation method, to predict the position of the hydrogen-rich micro-mixing flame front. Compared with the Fluent approach to establish the diffusion FGM flamelet, the results showed that the two FGMs have similar flame predictions in high hydrogen content fuels, and both can accurately capture the location of the internal and external shear layer boundaries of the micro-mixing multi-jet flame in the steady state, while the Fluent approach based on the uniform Lewis number assumption predicts results that deviated significantly from the experimental results. However, for the internal shear layer, both methods have large predicted OH gradients compared to the experimental results due to the lack of effective Lewis number correction for the control variable transport equation. The results using linear triangular dissection interpolation maybe superior to the method with linear interpolation of the process variable quenching boundary toward zero, which leads to flashback due to overestimation of the process variable source term in the region below the diffusion FGM quenching boundary.     

Extended Shvab–Zel’dovich formulation for multicomponent-fuel diffusion flames

In this paper an extension of the Shvab–Zel’dovich formulation is presented. This extended formulation, based on the Burke–Schumann kinetic mechanism, describes the combustion of multicomponent fuels in a diffusion flame in terms of mixture fraction and the excess enthalpy. Under the condition of Burke–Schumann kinetic mechanism, the multicomponent fuel is burned in a single flame. The model is applied to a diffusion flame generated by the burning of mixtures of n-heptane and hydrogen diluted in nitrogen in a counterflow configuration. Due to the very small ratio of the hydrogen molecular weight to the n-heptane molecular weight, small quantities of hydrogen (in terms of mass) in the mixture does not change significantly the properties related to the mass, like as the total heat released per unit of mass at the flame. However, properties related to the hydrogen mole fraction does change expressively with small quantities, like as the radiative energy loss from the hot region around the flame. The results show the flame properties as a function of the reciprocal scalar dissipation and hydrogen quantity in the mixture. It is observed that, by reducing the reciprocal scalar dissipation, the radiative energy loss decreases and by increasing the presence of the hydrogen, the sensitivity of the flame properties with the reciprocal scalar dissipation reduces. It is also revealed by the results, the effects of the potentiated preferential hydrogen mass diffusion in compositions in which nitrogen and n-heptane are the majority species, and the potentiated preferential n-heptane thermal diffusion in compositions in which nitrogen and hydrogen are the majority species, on the flame properties. Although, this work do not treat the extinction problem, the fluid dynamical results will be properly handled to provide information about the reciprocal scalar dissipation and the Liñán’s parameter necessary for future flame stability analyses.     

A detailed experimental and kinetic modeling study of n-decane oxidation at elevated pressures

The oxidation of n-decane/oxygen/nitrogen is studied at stoichiometric conditions of 1000 ppm fuel in the Princeton variable pressure flow reactor at temperatures of 520–830 K and pressures of 8 and 12.5 atm. The overall oxidative reactivity of n-decane is observed in detail to show low temperature, negative temperature coefficient (NTC) and hot ignition regimes. Detailed temporal speciation studies are performed at reactor initial temperatures of 533 K and 740 K at 12.5 atm pressure and 830 K at 8 atm pressure. Significant amounts of large olefins are produced at 830 K, at conditions of transition from NTC to hot ignition behavior. The predictions using available chemical kinetic models for n-decane oxidation are compared against each other and the experiments. Only the kinetic models of Westbrook et al., Ranzi et al., and Biet et al. capture the NTC behavior exhibited by n-decane. However, each of these models yields varying disparities in the mechanistic predictions of major intermediate species, including ethylene and formaldehyde. Analyses of the Westbrook et al. model are compared with the new data. The predicted double-peaked species yield of ethylene, a behavior not found for the other models or in the experimental observations results from deficiencies in the C₂ chemistry. Mechanistic validation information about fuel oxidation chemistry is also provided by the measurement of various larger carbon number alkene isomers at 830 K and 8 atm. The modeling analysis suggests that in addition to n-alkyl beta-scission chemistry, alkyl peroxy radical chemistry contributes significantly to the formation of these alkenes. Specific reaction pathways and rate constants which affect the computation of these observations are discussed.     

Spark ignition transitions in premixed turbulent combustion

Recent discoveries and developments on the dynamic process of premixed turbulent spark ignition are reviewed. The focus here is on the variation of turbulent minimum ignition energies (MIE_T) against laminar MIE (MIE_L) over a wide range of r.m.s. turbulence fluctuation velocity (uʹ) alongside effects of the spark gap between electrodes, Lewis number, and some other parameters on MIE. Two distinguishable spark ignition transitions are discussed. (1) A monotonic MIE transition, where MIE_L sets the lower bound, marks a critical uʹ_c between linear and exponential increase in MIE_T with uʹ increased. (2) A non-monotonic MIE transition, where the lower bound is to be set by a MIE_T at some uʹ_c, stems from a great influence of Lewis number and spark gap despite turbulence. At sufficiently large Lewis number >> 1 and small spark gap (typically less than 1 mm), turbulence facilitated ignition (TFI), where MIE_T < MIE_L, occurs; then MIE_T increases rapidly at larger uʹ > uʹ_c because turbulence re-asserts its dominating role. Both phenomena are explained by the coupling effects of differential diffusion, heat losses to electrodes, and turbulence on the spark kernel. In particular, the ratio of small-scale turbulence diffusivity to reaction zone thermal diffusivity, a reaction zone Péclet number, captures the similarity of monotonic MIE transition, regardless of different ignition sources (conventional electrodes versus laser), turbulent flows, pressure, and fuel types. Furthermore, TFI does and/or does not occur when conventional spark is replaced by nanosecond-repetitively-pulsed-discharge and/or laser spark. The latter is attributed to the third lobe formation of laser kernel with some negative curvature segments that enhance reaction rate through differential diffusion, where MIE_L < MIE_T (no TFI). Finally, the implications of MIE transitions relevant to lean-burn spark ignition engines are briefly mentioned, and future studies are suggested.     

Low-temperature speciation and chemical kinetic studies of n-heptane

Although there have been many ignition studies of n-heptane—a primary reference fuel—few studies have provided detailed insights into the low-temperature chemistry of n-heptane through direct measurements of intermediate species formed during ignition. Such measurements provide understanding of reaction pathways that form toxic air pollutants and greenhouse gas emissions while also providing key metrics essential to the development of chemical kinetic mechanisms. This paper presents new ignition and speciation data taken at high pressure (9 atm), low temperatures (660–710 K), and a dilution of inert gases-to-molecular oxygen of 5.64 (mole basis). The detailed time-histories of 17 species, including large alkenes, aldehydes, carbon monoxide, and n-heptane were quantified using gas chromatography. A detailed chemical kinetic mechanism developed previously for oxidation of n-heptane reproduced experimentally observed ignition delay times reasonably well, but predicted levels of some important intermediate chemical species that were significantly different from measured values. Results from recent theoretical studies of low temperature hydrocarbon oxidation reaction rates were used to upgrade the chemical kinetic mechanism for n-heptane, leading to much better agreement between experimental and computed intermediate species concentrations. The implications of these results to many other hydrocarbon fuel oxidation mechanisms in the literature are discussed.     

A theoretical analysis of the extinction limits of a methane-air opposed-jet diffusion flame

A theoretical analysis is described for a methane-air diffusion flame stabilized in the forward stagnation region of a porous metal cylinder in a forced convective flow. The analysis includes effects of radiative heat loss from the porous metal surface and finite rate kinetics but neglects the effects of gravity. The theoretically predicted extinction limits compare well with experimentally observed extinction limits from the literature.After the predicted limits compared well with the experimental limits, a parametric study of the effect of fuel surface emissivity and Lewis number was conducted with the numerical model. It was found that the computed blowoff limit is independent of radiative heat loss for high fuel blowing velocities but is a strong function of Lewis number. At low fuel blowing velocities, the extinction limit varies with both radiative heat loss and Lewis number. It is discovered, however, that even if thermal losses from the fuel surface are absent, the flame can extinguish at the fuel surface independently of Lewis number due to excessive reaction zone thinning.     

Autoignition and structure of nonpremixed CH4/H2 flames: Detailed and reduced kinetic models

The ignition dynamics and subsequent flame evolution of hydrogen-enriched methane mixtures are investigated numerically in a reacting vortex ring configuration. The CH₄/H₂ combustion is studied using a detailed reaction mechanism (GRI-Mech v3.0) and two augmented reduced mechanisms (11-step and 12-step). The main objective of this study is to identify the extent that the current reduced mechanisms can go in replicating the dynamics of the ignition process and flame structure in an unsteady nonpremixed configuration. The parameters of the numerical simulations are adjusted such that flame ignition occurs during either the formation or the postformation of the ring. The quasi-steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times than those in the other kinetic models. For formation-phase ignition runs, the flame structure near the stoichiometric region is captured well by the 12-step model compared to GRI-Mech 3.0. For postformation ignition runs, the 12-step model predicts larger heat release rates and main species mole fractions compared to GRI-Mech 3.0. The 11-step model predicts well the ignition delay time. At later times the fuel-rich side of the flame predicted by this reduced mechanism exhibits differences from the detailed model. Counterflow diffusion flame results are used to further compare the fuel-rich chemistry for the detailed and augmented reduced kinetic models.     

Ignition characteristics of heptane-hydrogen and heptane-methane fuel blends at elevated pressures

There is significant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C₇H₁₆/H₂ and n-C₇H₁₆/CH₄ fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30 atm and 55 atm, equivalence ratios of ? = 0.5, 1.0 and 2.0, temperature range of 800-1400 K, and mole fractions of H₂ or CH₄ in the blend between 0 and 100%. For n-C₇H₁₆/air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefficient regime. Consequently, this mechanism is used to characterize the effects of H₂ or CH₄ on the ignition of n-C₇H₁₆. Results indicate that H₂ or CH₄ addition has a relatively small effect on the ignition of n-C₇H₁₆/air mixtures, while the n-C₇H₁₆ addition even in small amount modifies the ignition of H₂/air and CH₄/air mixtures significantly. The n-C₇H₁₆ addition decreases and increases the ignition delays of H₂/air mixtures at low and high temperatures, respectively, while its addition to CH₄/air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C₇H₁₆-2) and hydoxyl (OH) radicals being the two most important species.     

The auto-ignition of single n-heptane/iso-octane droplets

Numerical simulations including detailed chemical and physical models are performed to investigate the influence of different physical parameters on the auto-ignition of n-heptane/iso-octane droplets in air. Simulations are performed for isobaric conditions with an ambient pressure of 8 bar and a droplet radius of 200 μm. The ambient gas temperature ranges from 800 K to 2000 K and the droplet temperature was varied from 300 K to 400 K. Below an ambient temperature of 1000 K the ignition delay time is found to increase with an increasing volume fraction of iso-octane. Above 1000 K the ignition delay time appears to be almost independent of the mixture composition of the droplet. The local ignition conditions are also studied. It turns out that ignition occurs at points, where the mixture is lean. This trend is more significant, if the ambient temperature increases. The influence of physical properties of the mixture components, like diffusion coefficients, heat conductivity, heat of vaporization and vapor pressure, is investigated. Furthermore, the influences of simplifying assumptions such as the distillation and diffusion limit are studied.     

A shock tube and chemical kinetic modeling study of the pyrolysis and oxidation of butanols

The pyrolysis and oxidation of all four butanols (n-, sec-, iso- and tert-) have been studied at pressures from 1 to 4 atm and temperatures of 1000–1800 K behind reflected shock waves. Gas chromatographic sampling at different reaction times varying from 1.5 to 3.1 ms was used to measure reactant, intermediate and product species profiles in a single-pulse shock tube. In addition, ignition delays were determined at an average reflected shock pressure of 3.5 atm at temperatures from 1250 to 1800 K. A detailed chemical kinetic model consisting of 1892 reactions involving 284 species was constructed and tested against species profiles and ignition delays. The little-known chemistry of enols is included in this work to explain the temperature dependence of acetaldehyde produced in the thermal decomposition of isobutanol.     

Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition

The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.     

Modeling curvature effects in diffusion flames using a laminar flamelet model

The goal of this paper is to investigate the effects of curvature of mixture fraction iso-surfaces on the transport of species in diffusion flames. A general flamelet formulation is derived mathematically considering both curvature effects and differential diffusion effects. These theoretical results suggest that curvature does not play a role in the transport process irrespective of the flame curvature if species transport is described with a unity Lewis number. On the other hand, a curvature-induced term becomes explicit when differential diffusion effects are considered, and it acts as a convective term in mixture fraction space. It is found that this term needs to be taken into account when the radius of curvature is comparable or smaller than the local flame thickness. For the proper integration of the flamelet equations, the scalar dissipation rate and curvature dependences on mixture fraction are modeled by considering two basic curved one-dimensional flame configurations. The flamelet equations accounting for curvature effects are solved with various prescribed curvature values. Results indicate that the mass fraction profiles of species with very small or large Lewis numbers are shifted significantly in mixture fraction space by the inclusion of curvature. Differential diffusion effects are enhanced by negative curvature values and suppressed by positive curvature values. In cases where flame curvature is not uniform, the curvature-induced convective term generates gradients along mixture fraction iso-surfaces, which enhance tangential diffusion effects. Budget analysis is performed on an axisymmetric laminar coflow diffusion flame to highlight the importance of the curvature-induced convective term compared to other terms in the full flamelet equation. A comparison is made between full chemistry simulation results and those obtained using planar and curved flamelet-based chemistry tabulation methods.     

Modeling lithium diffusion in nickel composite graphite

A simple theoretical model is presented to simulate the galvanostatic discharge behavior of the Ni-composite graphite electrode. The discharge profiles predicted by using a constant diffusion coefficient (CDC) and by a varied diffusion coefficient (VDC) are compared in this paper. The results show that, the VDC model can be simplified to the CDC for discharge rates less than 2C for a 5 μm particle. Also, an approximate analytical solution is presented for VDC model, which is found to be valid for discharge rates up to 6C. Exchange current and diffusion coefficient for the lithium-diffusion are predicted.     

Numerical simulation of double-diffusive mixed convective flow in rectangular enclosure with insulated moving lid

The present study is concerned with the mixed convection in a rectangular lid-driven cavity under the combined buoyancy effects of thermal and mass diffusion. Double-diffusive convective flow in a rectangular enclosure with moving upper surface is studied numerically. Both upper and lower surfaces are being insulated and impermeable. Constant different temperatures and concentration are imposed along the vertical walls of the enclosure, steady state laminar regime is considered. The transport equations for continuity, momentum, energy and spices transfer are solved. The numerical results are reported for the effect of Richardson number, Lewis number, and buoyancy ratio on the iso-contours of stream line, temperature, and concentration. In addition, the predicted results for both local and average Nusselt and Sherwood numbers are presented and discussed for various parametric conditions. This study was done for 0.1 ≤ Le ≤ 50 and Prandtl number Pr = 0.7. Through out the study the Grashof number and aspect ratio are kept constant at 10⁴ and 2 respectively and ?10 ≤ N ≤ 10, while Richardson number has been varied from 0.01 to 10 to simulate forced convection dominated flow, mixed convection and natural convection dominated flow.     

A simplified model for bi-component droplet heating and evaporation

A simplified model for bi-component droplet heating and evaporation is developed and applied for the analysis of the observed average droplet temperatures in a monodisperse spray. The model takes into account all key processes, which take place during this heating and evaporation, including the distribution of temperature and diffusion of liquid species inside the droplet and the effects of the non-unity activity coefficient (ideal and non-ideal models). The effects of recirculation in the moving droplets on heat and mass diffusion within them are taken into account using the effective thermal conductivity and the effective diffusivity models. The previously obtained analytical solution of the transient heat conduction equation inside droplets is incorporated in the numerical code alongside the original analytical solution of the species diffusion equation inside droplets. The predicted time evolution of the average temperatures is shown to be reasonably close to the measured one, especially in the case of pure acetone and acetone-rich mixture droplets. It is shown that the temperatures predicted by the simplified model and the earlier reported vortex model are reasonably close. Also, the temperatures predicted by the ideal and non-ideal models differ by not more than several degrees. This can justify the application of the simplified model with the activity coefficient equal to 1 for the interpretation of the time evolution of temperatures measured with errors more than several degrees.     

An analytical and numerical investigation of hetero-/homogeneous combustion with deficient reactants having larger than unity Lewis numbers

The heterogeneous combustion and the combined hetero-/homogeneous combustion of deficient reactants with Lewis numbers (Le) larger than unity was investigated analytically and numerically in two geometrical configurations, the flat plate and the planar channel, under the condition of infinitely fast catalytic chemistry. Analytical results based on similarity solutions for the catalytic flat plate and on heat and mass transfer solutions for the channel were complemented by detailed 2-D numerical simulations. The larger than unity Lewis number led to the underadiabatic surface temperatures, which in turn gave rise to gas-phase regions with local energy excess. For the flat plate case, the maximum gas-phase energy excess was a non-monotonic function of the Lewis number. The peak occurred at Le = 6.5 with a corresponding energy excess 6.4% above the total energy of the fresh reactants. In channel-flow combustion, the maximum gas-phase energy excess was a monotonically increasing function of Lewis number, approaching asymptotically the considerably higher value of 20.8% as Le → ∞. For current catalytic combustion methodologies, which include the fuel-lean hydrocarbon/air combustion (Lewis numbers of deficient hydrocarbon fuels up to ∼3.2) and the fuel-rich hydrogen/air combustion (Lewis number of deficient oxygen ∼2.3), the energy excess in the gas was significant and could reach up to 14%. Hetero-/homogeneous combustion simulations have shown that, upon homogeneous ignition, the gas-phase energy excess manifested itself with superadiabatic flame temperatures. However, the superadiabaticity in the gas was confined to the channel core, such that the surface temperature did not exceed the adiabatic equilibrium temperature. This behavior had key implications for the reactor thermal management and catalyst stability. Moreover, the gas-phase superadiabaticity led to peak prompt NO_x values 30% higher than those achieved by a diffusionally neutral deficient reactant (Le = 1).     

The structure of diffusion flames burning pure,binary, and ternary solutions of methanol,heptane, and toluene

The structure of counterflow diffusions flames burning heptane, toluene, binary solutions of methanol and toluene, and heptane and toluene, and ternary solutions of methanol, heptane, and toluene is characterized in the vicinity of extinction. Composition profiles of stable species were measured in these flames by use of gas sampling with quartz microprobes and analyzed by use of on-line, two column gas chromatography. Temperature profiles were measured by use of coated thermocouples. A number of compounds were observed during pyrolysis of the fuel. Experimental results were analyzed by using the mixture fraction (conserved scalar) as the independent variable. Results show that for a diffusion flame burning heptane the approximation that the Lewis numbers for all species are approximately equal to unity is valid. When the composition profiles for major chemical species and temperature profiles are plotted with mixture fraction as the independent variable, the maximum value of the concentration of the major stable species and the maximum value of the temperature were found to occur on the rich side of stoichiometry. For diffusion flames burning solutions of methanol, heptane, and toluene, the compositions of the hydrocarbon fuels were observed to extrapolate to a value of zero at nearly the same location; however, methanol was observed to extrapolate to a value of zero at a different location. It is suggested that a previously developed theoretical analysis which did not allow the fuel concentrations to attain a value of zero at different locations may give slightly inaccurate results, when used to predict flame extinction.     

Ignition of non-premixed C3–C12 n-alkane flames

Ignition temperatures of non-premixed C₃ and C₅–C₁₂ n-alkane flames were determined in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 448 K, a local strain rate of 140 s^?1, and fuel mole fractions ranging from 1% to 12% in the fuel stream. The strain rate was measured on the fuel side using Laser Doppler Velocimetry. Simulations of the experiments were performed using the recently developed JetSurF 1.0 reaction model consisting of 194 species and 1459 reactions. In both experiments and simulations, the ignition temperatures of all n-alkane flames were found to decrease with increasing fuel concentration. The computed ignition temperatures are in close agreement with the experimental data for all C₅–C₁₂ n-alkanes, while for propane flames the data are slightly over-predicted. Detailed sensitivity analyses on both reaction rates and binary diffusion coefficients were performed, and the ignition temperature was determined to be sensitive both to fuel diffusion and the H₂/CO and small hydrocarbon kinetics. The dependence of the ignition temperature on the fuel molecular weight was determined to differ at low and high fuel mole fractions, due to the competition between the fuel reactivity and diffusive transport.