Analysis of Lewis number effects on dynamic response of laminar premixed flames

The dynamic response of premixed flames with different fuel Lewis numbers is crucial to understand for the design of safe combustion chambers with wide operating range. Due to preferential diffusion effects associated with fuel Lewis number for lean mixtures, a premixed flame responds differently to flame stretch. These responses could manifest in the flame's response to small perturbations. In order to investigate this effect, in this study, lean premixed hydrogen, methane and propane flames at the same adiabatic unstretched flame speed are studied using numerical simulations. Steady unperturbed flames show significant differences in flame stand-off distances, flame height, flame stabilization limits, burner temperature and recirculation zone length. Flames are perturbed using a step function excitation and noticeable changes in the gain and phase of the flame transfer function are found even when the flame burning velocity and the inlet flow speed are kept the same. Maximum gain of the response is found to increase with the mixture Lewis number and is shown to be directly dependent on the flame tip burning strong or weak. Phase difference at low frequencies is found to depend on the average of the time scales of the recirculation vortex which in turn depends on the flame burning stronger or weaker under the influence of positive stretch at the flame base, axial convective wave and perturbation travelling along the flame front. Time for the flame to settle to a new position after the step excitation is found to correlate well with the sum of time scales of vortex, flame tangential speed and flame tip speed. Overall, it is found that hydrogen flame responds the fastest to the perturbations and exhibits smaller increase in gain compared to methane and propane.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.