On transition to self-similar acceleration of spherically expanding flames with cellular instabilities

摘要

The evolution of the self-acceleration of spherically expanding flames subjected to cellular instabilities was experimentally studied with H-2/O-2/N-2 mixtures over a wide range of Lewis numbers, flame thicknesses, and density ratios. Results show that the flames manifest a self-similar acceleration behavior, and the acceleration exponent, alpha, continuously increases and follows a power-law relation with the normalized Peclet number beyond a transition acceleration stage, which is different from the usually suggested self-similar propagation characterized by a constant alpha after the short transition acceleration period. The self-similar acceleration with reduced growth rate is found to be the results of the continuous extension of cellular spectrum on the long-wavelength side but the saturation of cellular spectrum on the short-wavelength side. The critical Peclet numbers associated with onset (Pe(cr)) and transition (Pe(ct)) of self-acceleration have a similar non-linear relationship with the Marksetin number, caused by the nonequal differential-thermal effects on the development of Darrieus-Landau (DL) cells for sub-unity (1) and super-unity (1) Lewis numbers. Based on the experimental Pe(cr) and Pe(ct), a regime diagram is presented for the laminar premixed hydrogen flames with cellular instabilities, which consists smooth propagation, transition acceleration, and self-similar acceleration regimes. In addition, an empirical power-law correlation for the alpha is suggested to describe the whole self-acceleration process. This correlation is able to predict the present alpha as well as the experimental results of the previous work. The measured alpha in the range of present interpellation is smaller than 1.5, the suggested value for the flame self-turbulization. However, this value could possibly be attained at an extremely large Peclet number (or flame radius) based on the extrapolation of the present correlation. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.