Characterization of symmetric to non-symmetric flamefront transition in slender microchannels

Abstract

The purpose of the present work is to analyze propagating two-dimensional flames confined in slender semi open channels, where the combustion process takes place towards the closed end. The study focuses on the calculation of the growth rate of the transition from symmetric to non-symmetric flames propagation identified by Jiménez et al. [1].The combustion cell is initially filled with a stoichiometric mixture of fuel and air at standard conditions. Ignition is induced close to the open end of the channel under planar and gaussian profiles in temperature and species mass fractions which activate a sustained combustion process. The gases inside the chamber, initially stagnant, are accelerated due to the heat generated in the chemical reactions, leading to the development of lateral boundary layers, so that the hot gases exit the channel following a Poiseuille velocity profile. This transverse flow velocity differences are accommodated by means of a symmetric tulip shape formed after a short initial transient.Acoustic waves generated in the ignition process, keep travelling along the channel, bouncing at the walls and interacting with the flame during all the combustion process. Additionally, the flame structure, curved by Darrieus-Landau instability, interacts with the pressure waves triggering small amplitude oscillations (primary oscillation mode), which under certain conditions can transition to higher amplitude oscillations (secondary mode).This transition is observed to be highly dependent not only on the cell geometry, but also in the initial conditions generated by the ignition procedure.The aim of this work is to improve the understanding of this process, complementing the work of Jiménez et al. [1], and to characterize the effect of the channel width in this transition.