Cellular instability in Le < 1 turbulent expanding flames

Flow turbulence and intrinsic flamefront cellular instability can each wrinkle a flamefront and thereby increase its surface area and the corresponding global flame speed, with the characteristics of wrinkling depending on the separate and coupled spectra of their respective length scales. Extending our previous study on the interaction between turbulence and the hydrodynamic, Darrieus-Landau cellular instability, and using the same expanding, globally spherical flame as the vehicle of investigation, we report herein experimental results on the interaction between turbulence and diffusional-thermal cellular instability relevant for mixtures with sub-unity Lewis numbers. Results show that the flame acceleration is primarily controlled by cellular instability in the wrinkled flamelet regime and by turbulence in the thickened flamelet regime, respectively, while both mechanisms influence the propagation in the corrugated flamelet regime. It is also noted that while the Lewis number and thus diffusional-thermal instability does not affect the global flame acceleration, it does enhance the total burning rate. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.