Oscillatory dynamics of discrete reaction-zones for hydrogen–oxygen diffusion flames within a narrow channel

Abstract

A diffusion flame in the fuel-oxidizer mixing layer within a narrow channel can be split into several stationary, discrete flame segments at some particular conditions. This unique flame structure has deeply been analyzed in previous literature. In this paper, by performing numerical simulations using a transient, low-Mach number, reacting flow solver in OpenFOAM, unsteady dynamics of separated reaction zones for low Lewis number, hydrogen–oxygen diffusion flames in a 30 × 5 × 0.75 mm3 microchannel are newly discovered. For a variety of inflow rates (H2/O2 flow rate of 300/150, 200/100, and 100/50 sccm) and wall temperatures (Tw = 900, 800, 700, 600, and 500 K) investigated, stable and continuous flame sheets can only be witnessed at large Reynolds numbers or high wall temperatures. Flames at other conditions are found in a discrete form, presenting an unstable and repetitive feature. Specifically, in the wake of the leading, anchored diffusion flame, edge flames with tribrachial structures are consecutively established and then propagate downstream, while eventually end by moving out of the channel, or being extinguished halfway, or behaving in a mixed mode (the two events occur alternately for two successively generated edge flames). Effects of Lewis numbers on flame behaviors are also studied via helium addition to both the fuel and oxidizer stream. Isolated reaction zones at a steady state can be witnessed for a relatively large Lewis number under a high level of addition.