A computational analysis of strained laminar flame propagation in a stratified CH4/H2/air mixture

Propagation of a H-2-added strained laminar CH4/air flame in a rich-to-lean stratified mixture is numerically studied. The back-support effect, which is known to enhance the consumption speed of a flame propagating into a leaner mixture compared to that into a homogeneous mixture, is evaluated. A new method is devised to characterize unsteady reactant-to-reactant counterflow flames under transiently decreasing equivalence ratio, in order to elucidate the influence of flow strain on the back-support effect. In contrast to the conventional reactant-to-product configurations, the current configuration is more relevant to unsteady stratified flames back-supported by their own combustion products. Moreover, since H-2 distribution downstream of the flame is known to play a crucial role in back-supported CH4/air flames, the influence of H-2 addition in the upstream mixture is examined. The results suggest that a larger strain rate leads to a larger equivalence ratio gradient at the reaction zone through increased flow divergence, which amplifies the back-support. Meanwhile, since H-2 addition in the upstream mixture does not affect the downstream H-2 content, the relative increase in the consumption speed, i.e. the back-support, is suppressed with larger H-2 addition. Especially, when the upstream H-2 content decreases with the equivalence ratio, the H-2 preferentially diffuses toward the unburned gas, which mitigates H-2 accumulation in the preheat zone and further weakens the back-support. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Numerical study of propagation of a strained laminar CH4/air flame with H-2 addition in a rich-to-lean stratified mixture and evaluation of back-support effect. A new method devised to characterize unsteady reactant-to-reactant counterflow flames under transiently decreasing equivalence ratio, elucidating the influence of flow strain on the back-support effect. The study suggests that a larger strain rate amplifies back-support by increasing equivalence ratio gradient at the reaction zone, while H-2 addition in the upstream mixture suppresses the relative increase in consumption speed.