Flammability and critical parameters at low oxygen extinction limit of stagnation-point diffusion flames with varied stretch rates

Abstract

Recently, to simulate microgravity flames in a terrestrial environment, low stretched stagnation-point diffusion flames have aroused wide interest, while the extinction behaviors of varied stretched flames at reduced oxygen concentration were scarcely investigated. This issue was accomplished by an investigation in this paper. Firstly, local and total extinction modes with ‘U’-shape boundaries were found to enlarge the unstable area in the flammability map. Furthermore, gas-phase radiation inversely proportional to stretch rate contributed to decreased critical flame temperature with dropped stretch rate. Two different power laws of critical flame standoff distance versus stretch rate existed at quenching and blowoff due to the coupling effects of boundary layer and critical oxygen concentration. Surface energy balance to evaluate the mechanisms of quenching and blowoff demonstrated that, both solid-phase conduction and surface radiation dominated at quenching, while they can be ignored at blowoff. Finally, critical burning rate was obtained from the surface energy balance and found to decrease with declined stretch rate. The similar changing tendencies of flammability and burning characteristics versus stretch rate in this work to the microgravity results implied a low-cost earth-based research method to simulate microgravity flames, and some new findings can provide instructions for fire protection in space.