Effects of slow wind on localized radiative ignition and transition to flame spread in microgravity

An experimental and numerical investigation of ignition and the subsequent transition to flame spread over a thermally thin cellulosic sample is described. The experiments were conducted using a lamp as an external radian; source in a 50% oxygen atmosphere at three different wind velocities of 0, 2, and 5 cm/s in a 10 s drop tower. The results show that there are no significant effects of the slow wind on the ignition-delay time. Photographic sequences of both the experiments and the calculations show that the wind increases the flame propagation speed in the upwind direction, while decreasing it in the downstream direction. The downstream flame fails the transition to flame spread and becomes a tail of the upstream flame. The downstream char front propagates much slower than that for the upstream direction. Three-dimensional, time-dependent numerical solutions to the Navier-Stokes equations are used to simulate the experiments. Three global degradation reactions describe the pyrolysis of the sample paper, and one gasphase reaction describes the combustion of the fuel gases. The model results reflect the qualitative features of the experiments and also are in reasonable quantitative agreement, given the uncertainty of the gasphase reaction mechanism.