Effects of stratification on premixed cool flame propagation and modeling

Low-temperature cool flames play a critical role for the performance of stratified-charge internal combustion engines. Accurate modeling of stratified combustion in engines requires a clear understanding of cool flame propagation in a compositionally stratified mixture. There are many studies on hot flame propagation in fuel-stratified mixtures. However, comparatively little effort has been made to explore stratification effects on cool flame propagation and modeling. To this end, a series of one-dimensional numerical simulations is performed in this work to investigate unsteady stratified dimethyl ether (DME)/air cool flames propagating in six stratification configurations (i.e., lean-to-stoichiometric (L2S), stoichiometricto-lean (S2L), stoichiometric-to-rich (S2R), rich-to-stoichiometric (R2S), lean-to-rich (L2R), and rich-to lean (R2L) stratified mixtures). To better understand stratification effects, the above stratified cool flames are compared with compositionally homogeneous cool flames. Furthermore, a well-established premixed flame modeling approach - Artificially Thickened Flame (ATF)/Flamelet-Generated Manifolds (FGM) - is evaluated with respect to its suitability for modeling stratified cool flames. The comparisons between stratified cool flames and homogeneous cool flames demonstrate that the stratification with an increase (e.g., Cases L2S, S2R and L2R) or decrease (e.g., Cases S2L, R2S and R2L) in equivalence ratio can promote or suppress cool flame propagation. Both the positive and negative effects would be augmented when decreasing stratification layer thickness. Moreover, sensitivity analysis and reaction path flux analysis of OH consumption are conducted to identify a cause and effect chain that elucidates how stratification affects cool flame propagation. It is found that stratification can significantly change the competition between reactions CH2O + OH reversible arrow HCO + H2O and CH3OCH3 + OH reversible arrow CH3OCH2 + H2O and hence affect the fuel consumption rate. Furthermore, a scaling law is proposed based on a stratification Damkohler number, in order to comprehensively characterize stratification effects on cool flame propagation. In addition, the evaluation of ATF/FGM in the laminar context suggests that the FGM method is a good candidate for describing low-temperature chemistry, while the ATF approach may need further consideration for accurately capturing cool flame propagation speeds. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The study investigates the propagation of unsteady stratified dimethyl ether (DME)/air cool flames in six stratification configurations and compares them with compositionally homogeneous cool flames, finding that stratification can promote or suppress cool flame propagation based on changes in equivalence ratio. Moreover, the study reveals that stratification significantly affects the competition between specific reactions and thus impacts the fuel consumption rate. The evaluation of the ATF/FGM approach suggests that the FGM method is suitable for low-temperature chemistry, while the ATF approach may require further refinement for accurately capturing cool flame propagation speeds.