Variation Of Flame Speed Research Articles

Numerical analysis on the combined transport effects of H2 and CO2 on the laminar premixed flame characteristics of biogas-hydrogen blends

The combined and respective transport effects of H2 and CO2 on the flame structure, laminar flame speed and radical pool of the BG40H60 blends at different equivalence ratios are investigated quantitatively with the numerical simulation in this study. The results show that H2 transport dominates the decrease and enhancement of HRR and mole fractions of minor species at the fuel-lean and fuel-rich conditions. However, H2 or CO2 transport hardly affects concentrations of major species expect for H2 and CO2. Besides, the dominated H2 transport contributes to the decreased/increased laminar flame speed at the fuel-lean/fuel-rich condition, while the OH radical can reflect the laminar flame speed variation caused by the H2 and CO2 transport. Based on the rate-of-production (ROP) analysis of OH radical, the most sensitive reactions to H2 and CO2 transport are OH + H2H2O + H/H + O2O + OH and OH + CH2OHCO + H2O at the fuel-lean and fuel-rich conditions respectively. The major production reactions (H + O2O + OH, H + HO2 = 2OH, O + H2H + OH, 2OH = O + H2O) of OH radical are suppressed or improved more significantly with the H2 and CO2 transport at the fuel-lean or fuel-rich condition, leading to the suppressed or improved OH radical pool and the flame propagation at the fuel-lean or fuel-rich condition. Furthermore, it is demonstrated that CO2 transport suppresses the reaction of OH + H2H2O + H considerably to improve the OH radical pool at the fuel-rich condition and cannot be neglected when investigating the flame propagation of biogas-hydrogen blends.

Experimental and chemical kinetic study on the laminar flame characteristics of the blends of n-propanol and isooctane at elevated temperature and pressure

Laminar flame characteristics of n-propanol-isooctane blends were investigated at the initial temperature of 433 K, two pressures of 0.1 and 0.5 MPa, different equivalence ratios of 0.7–1.6 and various fuel blending ratios in a constant volume bomb. Laminar flame speeds and flame instability parameters were determined, and the effect of initial conditions were evaluated. It was found that laminar flame speeds of n-propanol decrease with the pressure increases, and the addition of n-propanol into isooctane results in the rise of laminar flame speed. To interpret the flame speed variation, the thermal and diffusion properties were discussed by determining the adiabatic flame temperature and Markstein length. However, the chemical kinetics is concluded as the dominant factor since the variation of adiabatic flame temperature and Markstein length exert different behaviors with laminar flame speed does. A high temperature chemical kinetic model was developed, and the model accuracy was validated with present data as well as several sets of data published previously. With this model, detailed sensitivity and reaction pathway analyses were carried out to aid the understanding of the inherent mechanism. It was found that the laminar flame speed is mainly sensitive to the small molecule reactions. The addition of n-propanol into isooctane induces limited effect on the reaction pathways of each fuel. Finally, the flame thickness and density ratio were presented with the hydrodynamic instability evaluated. Combining the schlieren pictures and the flame instability parameters, it was concluded that the thermal-diffusional mechanism dominates the flame instability variation with the addition of n-propanol.

Uncertainty quantification of fuel variability effects on high hydrogen content syngas combustion

Fuel variability effects on high hydrogen content syngas combustion physicochemical properties and NOx emission characteristics are investigated invoking polynomial chaos expansion based uncertainty quantification. The focus is put on providing an in-depth understanding of fuel variability effect under very lean conditions, i.e., near lean blowout limit. It is found that leaner combustion of syngas leads to higher flame speed fluctuation. Under 1.5% small fluctuations in species concentration of syngas fuels, a maximum of 5% fluctuation of flame speed is observed at equivalence ratio of 0.45 for H60CO30 (60%H2 + 30%CO), while the lowest fluctuation is observed near equivalence ratio of 0.8 for the cases considered. Meanwhile, maximum NO concentration fluctuation of 8.3% and NO2 concentration fluctuation of 6.5% are observed at very lean conditions. Uncertainty analyses show that on one hand, hydrogen always has the highest contribution to flame speed variation followed by carbon monoxide, carbon dioxide, and methane. On the other hand, carbon monoxide is found to have the highest contribution to variation of flame temperature, followed by hydrogen, carbon dioxide, and methane. The quantified sensitivity information reported in present study can be used to guide targeted uncertainty reduction from syngas upstream gasification process.

Investigation of flame propagation in autoignitive blends of n-heptane and methane fuel

The effects of pre-ignition chemistry on laminar flame speed in methane/n-heptane fuel blends are investigated numerically, leading to flame speed modelling accounting for these effects. The laminar flame speeds of fuel blends are important input parameters for turbulent combustion models needed to support design of dual-fuel engines. At the autoignitive conditions found in engines, pre-ignition reactions cause the speed of the reaction front to increase. Fuels that exhibit two-stage ignition behaviour, such as n-heptane, also exhibit a two-stage increase in the speed of the reaction front as the reactant residence time increases. There is a corresponding reduction in the flame thickness until the residence time approaches the ignition delay time, whereupon the deflagrative scaling of flame thickness breaks down. The analysis shows that the increase in flame speed is due to distinct contributions of heat release, reactant consumption, and enhanced reactivity ahead of the flame. Addition of methane to n-heptane–air mixtures retards and reduces the first-stage increase in flame speed, in part due to dilution of the more-reactive n-heptane fuel, and in part due to consumption of radical species by the methane chemistry. The effect of methane/n-heptane fuel blending on flame speed is described adequately by a linear mixing rule. The effect of pre-ignition chemistry can then be modelled as a linear function of the progress variable ahead of the flame – accounting for heat release, reactant consumption, and enhanced reactivity ahead of the flame. The flame speed model accurately describes the variation of flame speed across the full range of methane/n-heptane blends at engine-relevant conditions, up to the deflagration/ignition transition.

Insights into the Bending Effect in Premixed Turbulent Combustion Using the Flame Surface Density Transport

ABSTRACTThe bending effect of turbulent flame speed variation (i.e., the deviation from the linear increase of flame speed with increasing root-mean-square turbulent velocity fluctuation) has been investigated based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames propagating into forced unburned gas turbulence. The validity of Damköhler’s first hypothesis has been utilized to analyze the bending effect in terms of generalized Flame Surface Density (FSD) evolution. The volume-integrated value of the tangential strain rate term of the FSD transport equation remains positive, whereas the volume-integrated value of the curvature term assumes negative values. Under statistically stationary state, the positive value of the volume-integrated tangential strain rate term remains in equilibrium with the negative value of the volume-integrated curvature term. It has been found that the contribution of the normal strain rate to the flame surface area remains negative for small turbulence intensities, which eventually become positive for large turbulence intensities. This is a consequence of the change of collinear alignment of the reaction progress variable gradient from the most extensive principal strain rate direction to the direction of the eigenvector associated with the most compressive principal strain rate with increasing turbulence intensity. An increase in turbulence intensity increases the width of the probability density functions of flame curvature, and thereby increases the surface-averaged curvature squared values. This eventually makes the FSD curvature term due to the tangential diffusion component of displacement speed as the major contributor to the negative contribution of the volume-integrated curvature term in the FSD transport equation for large turbulence intensities. However, the negative contribution of the volume-integrated FSD curvature term does not increase indefinitely with increasing turbulence intensity and the inner cut-off scale, which also limits the maximum possible value of the volume-integrated FSD strain rate term under statistically stationary state, governs the maximum possible destruction of flame surface area. It has been argued that the upper limits of the flame surface area generation and destruction are responsible for the bending effects in the variations of turbulent flame speed and flame surface area.

Comparative study on the laminar flame speeds of methylcyclohexane-methanol and toluene-methanol blends at elevated temperatures

Laminar flame speeds of methylcyclohexane (MCH)-methanol and toluene-methanol blends were experimentally determined with spherically expanding flame method in a constant volume bomb at atmospheric pressure, initial temperatures of 393 and 433 K, covering wide equivalence ratio range. The blending ratio of methanol in liquid volume varies as 0%, 20%, 40%, 60%, 80%, 100%. Nonlinear methodology was employed to remove the stretch effect in the data processing. Experimental results show that MCH-air flame propagates faster than toluene at the same condition. The addition of methanol into MCH and toluene results in acceleration of laminar flame speed especially at the rich mixtures. Since the published model suffers difficulty in reproducing experimental data, model refinements were carried out and the refined model yields better performance. Comprehensive analyses were developed regarding thermal and chemical kinetic properties. For MCH has lower adiabatic flame temperature than toluene and methanol addition into the two cyclic fuels decreases the adiabatic flame temperature, the thermal effect on laminar flame speed difference is negligible. Therefore, the effect of chemical kinetics was specifically discussed. MCH and toluene have different ring structures. The disintegration of aromatic ring plays as the limiting step in the high-temperature oxidation of toluene, resulting in low concentration of active radical pool and overall reactivity. However, the ring opening reaction of MCH occurs easily after the initial H-abstraction reaction, which favors the production of active intermediates and the enhancement of flame propagation. For the blending fuels, the analyses show that the laminar flame speed variation of the blends are primarily caused by the methanol substitution and the disturbance of reaction pathway through affecting the generation of important intermediates.

Experimental and kinetic modeling study of laminar flame characteristics of higher mixed alcohols

Mixtures of alcohols combining strengths of lower and higher alcohols show promising alternative properties as engine fuel. To aid the extensive application of the mixed alcohols, detailed combustion investigation is necessary. In this study, laminar flame speeds and Markstein lengths were measured for simplified higher mixed alcohols (blends of methanol and n‑hexanol/n‑heptanol/n‑octanol) with spherically propagating flame at 0.1 MPa, elevated temperature of 433 K, and different mixing ratios. A comprehensive model was developed to describe the high temperature chemistry of the mixed alcohols. The accuracy of the model was validated with present data. Results reveal that the three higher alcohols exhibited close laminar flame speeds and Markstein lengths resulting from their similar thermal, diffusion, and chemical kinetic properties. With the increasing mixing ratio of the higher alcohols, the laminar flame speed decreased, especially for the rich mixtures. The maximum laminar flame speed slightly shifted to the lean side. The Markstein length increased at extremely lean mixtures and decreased at extremely rich mixtures, with the intersection occurring at the equivalence ratio between 1.2 and 1.3. The laminar flame speed variation of the mixed alcohols was dominated by chemical kinetics. Reaction pathway analyses indicated that the cracking process of each component in the mixed alcohols remains as that in the pure alcohols. Such result demonstrated the chemical kinetic effect resulting from changes in fuel components.

An investigation of fuel variability effect on bio-syngas combustion using uncertainty quantification

Fuel variability effects on physicochemical properties such as adiabatic flame temperature and laminar flame speed of premixed bio-syngas combustion are investigated via polynomial chaos expansion (PCE) based uncertainty quantification (UQ) approach at several equivalence ratios. Questions regarding confidence level of using bio-syngas with varying fuel composition are tackled from a statistical point view. Impacts of unburnt gas temperature and different chemical mechanisms (GMI-Mech 3.0 and San Diego Mechanism) on predicted uncertainties of these combustion properties are discussed. It was found that fluctuation of flame temperature at various equivalence ratios is less affected by bio-syngas fuel variabilities, while flame speed is sensitive to uncertainties in fuel composition. For instance, 1.5% fluctuation of bio-syngas constituent can lead to 14% fluctuation of flame speed for rich combustion, and 3% for lean combustion. Less than 0.8% fluctuation of flame temperature due to variability of bio-syngas fuel composition was observed. UQ of bio-syngas combustion showed that hydrogen variability plays a significant role (70–80% at lean condition) in flame speed variation, while methane variability, although thought to be important, has a negligible impact except for fuel-rich combustion. Overall, the current study has provided a fundamental understanding of the effects of fuel variability on physicochemical properties of bio-syngas combustion. Dominating compositions to variations of bio-syngas combustion are provided quantitatively to guide targeted uncertainty reduction from the upstream gasification process.

Direct numerical simulations of a high Karlovitz number laboratory premixed jet flame – an analysis of flame stretch and flame thickening

This article reports an analysis of the first detailed chemistry direct numerical simulation (DNS) of a high Karlovitz number laboratory premixed flame. The DNS results are first compared with those from laser-based diagnostics with good agreement. The subsequent analysis focuses on a detailed investigation of the flame area, its local thickness and their rates of change in isosurface following reference frames, quantities that are intimately connected. The net flame stretch is demonstrated to be a small residual of large competing terms: the positive tangential strain term and the negative curvature stretch term. The latter is found to be driven by flame speed–curvature correlations and dominated in net by low probability highly curved regions. Flame thickening is demonstrated to be substantial on average, while local regions of flame thinning are also observed. The rate of change of the flame thickness (as measured by the scalar gradient magnitude) is demonstrated, analogously to flame stretch, to be a competition between straining tending to increase gradients and flame speed variations in the normal direction tending to decrease them. The flame stretch and flame thickness analyses are connected by the observation that high positive tangential strain rate regions generally correspond with low curvature regions; these regions tend to be positively stretched in net and are relatively thinner compared with other regions. High curvature magnitude regions (both positive and negative) generally correspond with lower tangential strain; these regions are in net negatively stretched and thickened substantially.

Analysis of turbulent flame propagation in equivalence ratio-stratified flow

Equivalence ratio-stratified combustion is an important technology for achieving stable low-emission operation in internal combustion engines and gas turbines. This study examines how equivalence ratio stratification affects the physics of turbulent flame propagation using Direct Numerical Simulation. Three-dimensional simulations of a turbulent slot-Bunsen flame configuration are performed with accurate multi-step kinetic modelling for methane–air combustion. We compare one perfectly-premixed and three equivalence ratio-stratified cases with the mean equivalence ratio gradient aligned with, tangential to or opposed to the mean flame brush. The simulation results are analysed in terms of flame surface area and the burning intensity. The local effects of stratification are then investigated further by examining statistics of the displacement speed conditioned on the flame-normal equivalence ratio gradient. The local burning intensity is found to depend on the orientation of the stratification with respect to the flame front, so that burning intensity is enhanced when the flame speed in the products is faster than in the reactants. This effect of alignment between equivalence ratio gradients and flame fronts has been observed previously in laminar flames and it is found here that it also affects the global behaviour of turbulent flames. The flame surface area is also influenced by equivalence ratio stratification and this may be explained by differences in the surface-averaged consumption rate and differential propagation effects due to flame speed variations associated with equivalence ratio fluctuations.

Theoretical and computational studies of wall friction flame acceleration mechanism with linear variation of planar flame speed.

Majority of theories, associated with the variety of flame acceleration scenarios, are based on the "geometrical formulation”: namely, the wrinkled to planar flame speeds ratio, Sw / SL , is evaluated as the scaled increase in the flame surface area, while the entire combustionchemistry is immersed inSL , which is assumed to be constant. However, in the practical reality,SL may experience spatial and temporal variations; at least, due to the associated pressure andtemperature distribution within a combustor, and their evolution during burning. In the present work, we initiate the systematic study of a much more intriguing situation – when SL - variations are externally imposed in a manner being a free functional of the formulation. This is relevant, in particular, to multi-phase combustion in dusty environment, with a non-uniform distribution of combustible and/or inert dust; as well as to the event of spatial variations of the equivalenceratio. First, linear variation of spatialSL - distribution is incorporated into the Bychkov theories offlame acceleration due to wall friction [Physical Review E 72 (2005) 046307] and solved analytically. Second, we implement linear variation of flame speed into computational platform. A backbone for the platform is a fully compressible, finite-volume Navier-Stokes code solving for the hydrodynamics and combustion equations in a homogenously and non-homogeneously - gaseous, laminar environment. It is investigated how linear variation of flame speed can affect the flame acceleration mechanism. Theory also validated by means of computational simulations. This work is supported by the Alpha Foundation for the Improvement of Mine Safety and Health as well as West Virginia University’s Program to Stimulate Competitive Research (PSCoR) and West Virginia University’s Senate Award for Research and Scholarship.

Laminar Flame Characteristics of C1–C5 Primary Alcohol-Isooctane Blends at Elevated Temperature

The laminar combustion characteristics of blends of isooctane and C1–C5 primary alcohols (i.e., methanol, ethanol, n-propanol, n-butanol and n-pentanol) were investigated using the spherical expanding flame methodology in a constant volume chamber at various equivalence ratios and volume fractions of alcohol. The stretch effect was removed using the nonlinear methodology. The results indicate that the laminar flame speeds of alcohol-isooctane blends increase monotonously with the increasing volume fraction of alcohol. Among the five alcohols, the addition of methanol is identified to be the most effective in enhancing laminar flame speed. The addition of ethanol results in an approximately equivalent laminar flame speed enhancement rate as those of n-propanol, n-butanol and n-pentanol at ratios of 0.8 and 1.5, and a higher rate at 1.0 and 1.2. An empirical correlation is provided to describe the laminar flame speed variation with the volume fraction of alcohol. Meanwhile, the laminar flame speed increases with the mass content of oxygen in the fuel blends. At the equivalence ratio of 0.8 and fixed oxygen content, similar laminar flame speeds are observed with different alcohols blended into isooctane. Nevertheless, with the increase of equivalence ratio, heavier alcohol-isooctane blends tend to exhibit higher values. Markstein lengths of alcohol-isooctane blends decrease with the addition of alcohol into isooctane at 0.8, 1.0 and 1.2, however they increase at 1.5. This is consistent with the behavior deduced from the Schlieren images.

Numerical investigation of pressure and temperature influence on flame speed in CH4[sbnd]H2 premixed combustion

In this study, effect of high pressure and temperature was investigated on flame speed (SL) in CH4H2 premixed combustion. In the numerical investigation; H2 molar fraction, equivalence ratio, pressure and temperature were varied in range of 0–1.0, 0.4–1.1, 4–12 atm, 300–700 K respectively. Heat of reaction was calculated for important reactions and their rates were compared at different mixture conditions. Variation in flame speed was correlated with rate of important reactions. The flame speed tends to increase with hydrogen addition. However, the slope of the relation depends upon the equivalence ratio. Pressure highly influences the chemistry and with high pressure radical concentration seems to decrease. Moreover, temperature increases the reaction rate of almost all the reactions.

Combustion characteristics of the effect of hydrogen addition on LPG–air mixtures

The present study reports the effect of hydrogen addition on combustion characteristics of LPG–air mixtures for different mixture compositions, temperatures and pressures. Numerical simulation has been carried out using USC Mech II reaction mechanism, consisting of 111 species and 784 reactions. It is found that, variation of the flame speeds were relatively small for various compositions of LPG–air mixtures used. Thus 50% propane – 50% butane mixture is considered for the present work. The effect of volumetric H2 addition on laminar flame speed and ignition delay of selected LPG (50% propane + 50% butane) air mixtures is then studied for hydrogen addition ratio varying from 0 < RH < 0.5 and over a wide range of mixture equivalence ratio. Also, the investigation is carried out for mixture temperature up to 450 K and pressure ranging from 1 to 10 bar. A parabolic variation of laminar flame speed is observed with equivalence ratios giving peak value for slightly rich mixture. A linear correlation has been observed for the flame speed as a function of hydrogen addition, RH at all the conditions studied. Whereas, power law correlation has been proposed for the hydrogen added LPG–air mixture, to understand the dependency of laminar flame speed on temperature and pressure. A generic correlation has been proposed to study the combined effect of temperature and pressure on laminar flame speed.

The influence of reactant temperature on the dynamics of bluff body stabilized premixed flames

The impact of increased reactant temperature on the dynamics of bluff-body stabilized premixed flows is investigated using numerical simulation. A two-dimensional triangular bluff body is considered. Flow compressibility is assumed to exist at the low Mach number limit and combustion is fast and robust such that a flamesheet representation is assumed to apply. In this formulation, reactant temperature variations are represented via corresponding temperature ratio and flame speed variations. The Lagrangian, Transport Element Method is used to provide the numerical solutions. Results indicate that as reactant temperature increases, the fluid dynamics transition from a low amplitude, broadband, coarsely symmetric (about the bluff-body centerline) behavior, to a high amplitude, tonal and asymmetric one that bears similarities to the corresponding non-reacting flow. The reasons for this are that as the reactant temperature increases, (i) the temperature ratio across the flame is reduced, thus reducing combustion exothermicity, and (ii) the flame speed increases causing the flame to propagate away from the bluff-body wake. In both cases the ability of the two main combustion-driven fluid dynamical processes, namely volumetric expansion and baroclinic generation to impact the bluff body generated vorticity is reduced. Reduction in baroclinic generation enables to wake to survive futher downsteam and makes the flow susceptible to the wake instability. As reactant temperature is increased the location of the onset of the instability moves upstream. At very high reactant temperatures even the near field symmetrizing effect of volumetric expansion is overwhelmed and asymmetric vortex shedding is witnessed at the bluff body. Even in this regime, the flow differs from the non-reacting flow in that it is susceptible to bifurcations in vortex shedding behavior that are linked to local flame-vortex interactions. Results also show that in the general case, knowledge of the fluid dynamics alone is not sufficient to characterize the flame dynamics, as the flame position in relation to the vorticity field is critical to the unsteady flame response. Specifically, the flame exhibits a similar transition from a broadband to a tonal response but the amplitude is not monotonically increasing. Rather it experiences a regime of decreasing response for intermediate reactant temperatures where the flame propagation away from the wake appears to dominate the increase in fluid dynamical induced oscillations due to the enhancement of the asymmetric mode.

BURNING RATES AND FLAME OSCILLATIONS IN GLOBALLY HOMOGENEOUS TWO-PHASE MIXTURES (FLAME SPEED OSCILLATIONS IN DROPLET CLOUD FLAMES)

The study of spray combustion in industrial equipment is difficult due to the multiplicity of interdependent variables. Therefore, a combustion vessel was used for fundamental studies of aerosol combustion, under strictly controlled conditions of pressure, temperature and equivalence ratio. The droplet clouds were characterized using laser diagnostics, variables such as Sauter mean diameter, number density, gas and liquid equivalence ratios were determined. Nearly mono-sized droplet clouds were obtained in this vessel being the droplet diameter a function of time. The phenomenon of flame oscillation, the cyclic variation of flame speed during flame development, has been addressed in the past using several approaches but there is no yet a unique accepted explanation. The combustion studies in the present work revealed the differences between the speed of droplets and the gas velocity near the flame front. These resulted in variations in local equivalence ratio, which in turn manifested in flame speed oscillations.

Analysis of premixed-flame propagation in media with periodic flame-speed variations using the level-set equation

Propagation of premixed flames in a nonuniform concentration field is investigated using the level-set approach. In order to eliminate interactions with the fluid dynamic effects, the flow is assumed to be quiescent and of constant density, so that the level set will be moved only by flame propagation. The effects of a nonuniform propagation medium are taken into account by introducing a variable laminar-flame speed, corresponding to nonuniform concentration, which is assumed to be periodic in the transverse direction. Numerical simulations with zero Markstein number show that the level set achieves a steady propagation typically after the characteristic passage time of concentration inhomogeneity. In the steady propagation stage, the level-set distribution exhibits three types of characteristic point. These characteristic points are the leading head at the maximum laminar flame speed, the inflection point at the other extrema of laminar flame speed and the trailing edge with a cusp structure towards the burnt side to ensure the continuity of the level set. Asymptotic analysis is carried out for a distinguished limit where the Markstein length is much smaller than the wavelength but much greater than the flame thickness. Under this condition, centred around the cusp at the trailing edge exists an inner layer with a characteristic length scale of the Markstein length, in order to smooth out the singularity. In the asymptotic analysis, the two-term outer solution and two-term inner solution are obtained to construct a composite expansion of the level-set profile. In addition, the three-term expansion of the overall flame propagation speed, which includes corrections up to the order of the square of the Markstein number, is also obtained to give the overall flame propagation speed as a function of the Markstein number and the degree of concentration inhomogeneity.

Local scalar flame properties of freely propagating premixed turbulent flames at various Lewis numbers

Local scalar flame properties of freely propagating turbulent premixed flames including the flame curvature h, and local displacement speed relative to the fresh gases S d u , have been measured simultaneously for methane, propane, and hydrogen/air flames at Lewis numbers varying from 0.33 to 1.4 and ratios of rms turbulent velocity to unstretched laminar burning velocity u′/ S L,0 u varying from 0 to 3.1. Three different mixtures were separately spark-ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field-imaging technique based on high-speed laser tomography measured the temporal evolution of local flame properties. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (pdfs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame speed variations. The temporal evolution of the mean flamelet radius of curvature converges towards half of the integral length scale measured in the cold flow. Flame response in terms of displacement speed to curvature is found to be statistically dependent, and a linear relationship is observed. For propane/air flames, the displacement speed can be assumed to be independent of local flame curvature at each stage of flame propagation, whereas a very strong increase of displacement speed with positive curvatures can be observed along the wrinkled flame contour for hydrogen/air flames.

Effects of flow transients on the burning velocity of laminar hydrogen/air premixed flames

The effects of unsteady strain rate on the burning velocity of hydrogen-air premixed flames are studied in an opposed nozzle configuration. The numerical method employs adaptive time integration of a system of differential-algebraic equations. Detailed hydrogen-air kinetic mechanism and transport properties are considered. The equivalence ratio is varied from lean to rich premixtures in order to change the effective Lewis number. Steady Markstein numbers for small strain rate are computed and compared with experiment. Different definitions of flame burning velocity are examined under steady and unsteady flow conditions. It is found that, as the unsteady frequency increases, large deviations between different flame speeds are noted depending on the location of the flame speed evaluation. Unsteady flame response is investigated in terms of the Markstein transfer function which depends on the frequency of oscillation. In most cases, the flame speed variation attenuates at higher frequencies, as the unsteady frequency becomes comparable to the inverse of the characteristic flame time. Furthermore, unique resonance-like behavior is observed for a range of rich mixture conditions, consistent with previous studies with linearized theory.

Effects of stretch on the local structure of preely propagating premixed low-turbulent flames with various lewis numbers

An experimental investigation of the flame response to strain rate in the case of unsteady premixed low-turbulent flames is presented. In order to point out the fundamental aspects of the mutual interaction between combustion and turbulence, measurements of local flame properties (curvature, displacement speed) and tangential strain rate were performed under varying conditions of Lewis number and turbulence. Three different mixtures (methane/air, propane/air, and hydrogen/air) were successively spark ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field imaging technique coupling high-speed laser tomography and cross-correlation particle image velocimetry (PIV) was used to measure the temporal evolution of local flame stretch exerted by the turbulent cold flow. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (PDFs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame-speed variations. Tangential strain rate was determined by using the velocity field in the neighborhood of the flame front and appears to be independent of the Lewis numbers. A strong correlation between local flame curvature and tangential strain rate was demonstrated, underlining the cold flow effects on the local flame structure. The influences of turbulence and Lewis number were evaluated and compared with numerical simulations. Then, local flame stretch distributions were determined versus time, indicating that a significant proportion of the flame was under compression.