Dimensional effects on the transition from ignition to flame spread in microgravity

Abstract

Three-dimensional (3-D) and two-dimensional (2-D) simulations of the transition from radiative ignition on a solid fuel to flame spread in an imposed wind were performed in microgravity. Two-dimensional flames were found to quench (due to poor oxygen supply) more easily (i.e., at larger wind speeds) than 3-D flames. Results from the 2- and 3-D simulations were compared during the transition phase at wind speeds that ultimately lead to quenching of the 2-D flame but survival of the 3-D flame. In all locations near and in the flame, oxygen mass flux was larger in the 3-D flames and dominated by diffusion (as opposed to convection). Oxygen was supplied to the core of the 3-D flame due to diffusion from the sides of the flame (in a cross-wind direction). Diffusion in the 2-D flame was limited to directions parallel to the wind. This cross-wind diffusion was most significant at early times during transition when the flame was small and had a relatively large curvature. The 3-D flame, therefore, required less oxygen supply from an external wind to undergo transition to flame spread. Once flame spread was established there was little difference between the 3-D flames (in the centerline plane) and 2-D flames, due to the decreased curvature of the three-dimensional flame relative to the curvature during ignition and transition.