Effects of equivalence ratio variation on lean, stratified methane–air laminar counterflow flames

Abstract

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methaneair combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. 'Back supported' lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain. Nomenclature Englishproperties at location of peak heat release rate m value midway between boundary values Subscripts fu fuel st stoichiometric max maximum R component due to chemical reaction D component due to molecular diffusion CR component due to cross-dissipation IntroductionProgress in combustion technology depends heavily on the ability of engineers to combine demands for higher efficiency with those for lower emissions from stable combustion devices. Stratified combustion is becoming an increasingly widespread strategy for internal combustion engines and gas turbine engines to meet these combined requirements. Stratified combustion occurs when an inhomogeneous fuel-air mixture is either entirely lean or entirely rich, precluding the occurrence of edge flames. Nevertheless this mode of combustion has in the past received relatively little research attention, in comparison with the dominant premixed and diffusion flame accounts of combustion. Current capabilities for the measurement [1-5] and simulation [6-8] of turbulent stratified flames have begun to allow some interrogation of turbulent stratified flame structure and propagation. In

Analysis of turbulent flame propagation in equivalence ratio-stratified flow

“…the instantaneous flame front resides at approximately φ=0.7 in all cases) the difference in burning intensity is attributed to differing mean equivalence ratio gradients within the flame brush. Since the local mixture fraction-progress variable alignment necessarily correlates with the mean mixture fraction-progress variable alignment, the observation that back-supported turbulent flames have higher burning intensity than front-supported turbulent flames can be explained by modified transport of heat and radicals from the products into the reaction zone due to the local flamenormal mixture fraction gradient, as observed in previous laminar flame studies [9].…”

“…However the displacement speed shows an asymmetrical response to flame normal equivalence ratio gradients in both the laminar and turbulent cases, with back supported flames propagating faster compared to front-supported flames at a given tangential strain rate. The similar dependence of flame speed on flame-normal mixture fraction gradient in both the turbulent and laminar cases suggests that the phenomenon is due to the effect of molecular transport from the products that has been identified previously in laminar flame studies [9]. Figure 6 shows the mixture fraction-progress variable cross-dissipation rate = 2 ∇ξ∇c conditionally averaged on the sample-space variable for mixture fraction, .…”

“…It has been found that flame-normal equivalence ratio gradients affect the propagation speed of laminar flames, due to the effect of equivalence ratio gradients on the molecular transport of radical species and hot products into the reaction zone [9]. It has been observed that backsupported flames yield higher propagation speed than flames in homogenous mixture, and that flames in homogeneous mixture yield faster propagation speed than front-supported flames.…”

The Response of a Conical Laminar Premixed Flame to Equivalence Ratio Oscillations in Rich Conditions

“…Later, the back support effect introduced in Ref. (3) was found to influence the flame dynamics (6), (8)- (9), (13)- (15) .…”

Dynamic response of wall-stagnating lean methane-air premixed flame to equivalence ratio oscillation

“…They showed that if the mean equivalence ratio of a mixture subjected to concentration fluctuations is higher than that of the lean flammability limit for a steady flame (a static flame), the lean flammability limit for a dynamic flame is lower than that for a static flame, and that as the mean equivalence ratio of the mixture approaches the lean flammability limit for the static flame, the lean flammability limit for the dynamic flame approaches that for the static flame. Numerical studies similar to those described above have been conducted to determine in detail the effect of the non-uniformity of the concentration on the flame propagation velocity (Richardson et al, 2010;Rahman et al, 2012;Zhou and Hochgreb, 2013;Miyamae et al, 2014;Shi et al, 2016;Zhang and Abraham, 2016).…”