Study Of Laminar Flames Research Articles

Effects of Soret diffusion on the exergy losses in hydrogen laminar premixed flames

The analysis of the exergy loss is an effective tool for evaluating second-law irreversibility in laminar flames. However, despite numerous studies underlining the importance of taking into account thermal diffusion in laminar flame studies, especially in hydrogen/air flames, this phenomenon is usually neglected in the exergy analysis of these flames. Therefore, this work investigates the effect of Soret diffusion on the exergy loss in laminar premixed flames for hydrogen/air mixtures using a detailed reaction mechanism and the multicomponent transport model. The study starts from conditions in which the importance of the Soret effect is well established in the scientific literature. It is found that, while the exergy losses directly due to the Soret effect are negligible, the Soret effect can appreciably affect the other exergy loss contributions and hence the total exergy loss. Hence, the Soret effect, unlike what has usually been assumed, is not negligible in flame calculations at least when this effect is known to affect laminar flame speed.

Soot particle size distributions in turbulent opposed jet flames with premixed propene–air reactants

Emissions of soot are strongly dependent on turbulence-chemistry interactions due to the relatively slow formation and oxidation processes. Studies of laminar flames have shown that both flow conditions and the chemistry of the parent fuel have a significant impact on measured particle size distributions (PSDs). The current study determines the impact of flow on the development of PSDs in premixed turbulent flames through a variation in the total rate of strain using an opposed jet configuration with fractal grid generated turbulence. The impact of fuel chemistry is investigated under such conditions through the use of propene–air flames with results compared to the corresponding ethene–air flames quantified in earlier studies. Samples were extracted using a quartz probe featuring aerodynamic quenching and dual port dilution at the probe tip and in the transfer-line. Spatially resolved PSD data is obtained along the stagnation point streamline using a scanning mobility particle sizer equipped with nano- and long-DMA columns to show the evolution through the turbulent flame brush. Results confirm that PSDs of soot in premixed turbulent flames are exceptionally sensitive to both the chemistry of the parent fuel and the flow field. The reduced residence times in the current turbulent flows lead to maximum median and mean mobility diameters below 10 nm with higher rates of strain promoting unimodal PSD shapes. It is further shown that the chemistry of the parent fuel has a strong influence on PSDs with propene causing a two order of magnitude increase in smaller particles compared to the corresponding ethene flame.

Partial premixing and stratification in turbulent flames

This paper reviews recent advances in understanding the structure of turbulent partially premixed and stratified flames. The term “partially premixed” refers here to compositionally inhomogeneous mixtures that include flammable and non-flammable fluid while “stratified” combustion refers to a reacting front propagating through a range of compositions within the flammable limits. An overview of relevant laminar flame concepts is first introduced. In laminar partially premixed flames, the interaction between rich and lean mixtures is significant leading to improvement in the flame’s resistance to extinction by straining. In lean back-supported laminar stratified flames, the flux of excess heat and radicals into the lean fluid results in higher flame speeds, broader reaction zones, and extended flammability limits compared to homogeneous counterparts. Rich stratified flames are more complex due to the combined fluxes of heat as well as reactive species such as H2 and CO.Recent research in turbulent partially premixed as well as stratified flames is reviewed. Detailed measurements in burners representative of those found in gas turbine combustors show that partial premixing at the lifted flame base increases with instability. Well-characterised laboratory burners where different fuel concentration gradients may be imposed at the jet exit plane show improved flame stability due to mixed-mode combustion. Maximum stability is reached at some optimum level of compositional inhomogeneity. Highly resolved measurements in turbulent stratified flames show that the mass fractions of CO and H2 increase with stratification; a result that is consistent with laminar flame studies. Such experiments are, however, very difficult and require multi-level conditioning of the data. The paper concludes with a brief review of potential numerical approaches employed in the calculations of turbulent flames with inhomogeneous inlet conditions. A key challenge here is to reproduce the effects of increasing levels of stratification and/or inhomogeneity on the compositional structure of turbulent flames.

A study of laminar flames from three pine needle species

A study of laminar flames from three pine needle species

(2-10) Towards Evaluation of Turbulent Flame Speed((SI-4)S. I. Engine Combustion 4-Flame Propagation)

During the last decade, a new approach to multi-dimensional computations of premixed turbulent combustion has been developed. It consists of the use of an analytical expression for the local turbulent flame speed in order to close the averaged balance equations describing the combustion process. Several models have been suggested utilizing this approach and a number of successful applications to 3D computations of combustion in S. I. engines was reported at COMODIA 98. Although a submodel of turbulent flame speed, S_t, is at the core of this approach, a consensus has not been reached on this issue. The choice of the most reliable submodel is strongly impeded by the wide scatter of the published experimental data on S_t. Moreover, this scatter questions the physical correctness and practical usefulness of the concept of turbulent flame speed. The main goal of this work is to support the concept by discussing certain physical sources of the scatter and by outlining methods of reducing it. Three physical mechanisms are discussed in the paper. First, numerous experimental data indicate that typical premixed turbulent flames are developing flames, rather than fully developed ones, i. e., the mean turbulent flame brush thickness, δ_t, is not constant but grows permanently either with time in expanding flames or with the distance from the flame-holder in stationary flames. Due to the growth of δ_t, the speeds associated with different reference surfaces inside the flame brush can be substantially different, e. g., the difference can be as large as u'. A new method of unambigouosly defining the reference surface, the speed of which straightforwardly characterizes the mass burning rate, is developed by theoretically analyzing self-similar (i. e., spatial profiles of the mean progress variable, measured at different flame development times and presented in the dimensionless form by using δ_t(t), are collapsed to a universal curve) premixed turbulent flames in the statistically planar, one-dimensional case. This reference surface is characterized by the following reference value of the Favre-averaged progress variable [numerical formula] ; [numerical formula] ; [numerical formula], where ρ(ε)≡ρ^^-((x-x_f)/δ_t)/p_u is a universal dimensionless profile of the mean density (ρ(-∞)=1) and x_f(t) s the flame position. Numerical simulations indicate that the speed of this reference surface is equal to the mass burning rate divided by p_u and, thus, support the proposed method. Second, the mass burning rate in expanding spherical flames is well known to develop as the flame kernel grows, because, in particular, it experiences a wider range of the turbulence spectrum. Simple estimates discussed in the paper show that such transient effects can change the dependence of flame speed on turbulence characteristics not only quantitatively but even qualitatively. To suppress these effects, flame development time should be sufficiently large (e. g., larger than the turbulence time scale). Third, the speed of a statistically spherical flame differs from the speed of the planar one, other things being equal, due to (1) the mean flame curvature effects, and (2) the reduction in the maximum gas flow velocity induced by hot combustion product expansion, due to a wide flame thickness. In laminar flames, such effects are well known but weak. In turbulent flames, the effects can be substantial, because δ_t, is much larger than the laminar flame thickness. By analogy with the laminar combustion, the following equation [numerical formula], is proposed to be used in order to evaluate a fully developed turbulent flame speed, S^o_t, by processing the dependencies of the mean flame radius, r_f, on time, t, measured in expanding spherical flames. Here, Ma_t is a turbulent analog of the Markstein number used widely in laminar flame studies. This processing has been applied to the published experimental data. The

Effect of turbulence on extinction of counterflow diffusion flame

A laminar counterflow diffucion flame established in the forward stagnation region of a porous cylinderhas been used widely for the laminar flame studies. In the present study, this flame was applied to investigating the extinction of a diffusion flame due to air stream turbulence. Propane or methane was ejected from a 30-mm porous cylinder. The turbulence was given to the counterflowing air stream by perforated plates. The results of this study show that the air stream turbulence causes large-scale distortions with small amplitude on the apparently laminar diffusion flame, and the time-averaged thickness of this flame is three times as large as the purely laminar flame. Even if the air stream becomes turbulent, there exists a critical stagnation velocity gradient beyond which the flame can never be stabilized, however large the fuel ejection velocity is. Local extinction near the stagnation region always leads to global extinction of the whole flame. As expected, the critical velocity gradient decreases as the turbulence intensity increases. This flame is subjected to the sum of the bulk stretch rate exerted by the mean flow and the turbulent stretch rate exerted by small eddies of Kolmogorov scale. The critical total stretch rate at which the extinction occurs is nearly constant for each fuel for all turbulence conditions tested in the present study and coincides with the critical stagnation velocity gradient of the laminar diffusion flame, that is, the total stretch rate without turbulence. This fact suggests that large-scale eddies are not so effective for the local extinction and that the chemical reaction that occurs in molecular scale is not affected by small eddies of the Kolmogorov scale.

Laminar diffusion flamelet models in non-premixed turbulent combustion

The laminar flamelet concept views a turbulent diffusion flame as an ensemble of laminar diffusion flamelets. Work relevant to the flamelet concept is spread over various fields in the literature: laminar flame studies, asymptotic analysis, theory of turbulence and percolation theory. This review tries to gather and integrate this material in order to derive a self-consistent formulation. Under the assumption of equal diffusivities a coordinate-free formulation of the flamelet structure is given. This assumption is relaxed and flow dependent effects are considered. It is shown that the steady laminar counterflow diffusion flame exhibits a very similar scalar structure as unsteady distorted mixing layers in a turbulent flow field. Therefore the counterflow geometry is proposed to be the most representative steady flow field to study chemistry models and molecular transport effects in laminar flamelets. The conserved scalar model is interpreted as the most basic flamelet structure. Non-equilibrium calculations are reviewed. The coupling between non-equilibrium chemistry and turbulence is achieved by the statistical description of two parameters: the mixture fraction and the instantaneous scalar dissipation rate. The hypothesis of statistical independence of these two parameters is discussed. Calculation methods for the marginal distributions are reviewed. It is shown how local quenching of diffusion flamelets leads to a reduction of burnable flamelets. However, there are burnable flamelets in a turbulent flame which are not reached by an ignition source. This phenomenon is described by percolation theory. Complementary approaches related to local quenching effects and connectedness are combined to derive criteria for the stabilization of lifted flames and to blow out. Further applications of the flamelet concept are reviewed and work to be done is discussed.