Response of counterflow premixed flames to oscillating strain rates

We study the response of premixed counterflow flames to an imposed oscillating strain rate which has potential application to both turbulent combustion and acoustic instabilities. We exploit the limit of large activation energy to resolve the reaction zone structure, and conduct a linear perturbation analysis for small amplitude of oscillation. Reaction- sheet fluctuations and the net heat release rate are investigated for both the symmetric twin-flame configuration and a single flame residing in a counterflow system consisting of a fresh-mixture stream impinging on a cold inert stream. The Lewis number is found to play an important role in the flame response, especially for flames near extinction. For those flames that exhibit turning point behavior, near extinction the effect of finite-rate chemistry overtakes fluid-dynamic effects such that increasing strain rate can lead to a phase reversal of the heat release response with the imposed flow oscillations. Results of the present study suggest the possibility of a wide spectrum of unsteady flame characteristics depending on Lewis number. Our results also demonstrate that extinction can be delayed when the strain rate oscillates about the static extinction point. Thus the laminar flamelet regime of turbulent combustion may be broader than predicted by steady analyses.