Microgravity Ignition Delay of Solid Fuels in Low-Velocity Flows

Abstract

Experiments have been performed in microgravity and normal gravity to determine the effects of low-velocity airflows on the piloted ignition delay of solid fuels. Natural convection prevents material testing at the low oxidizer velocities encountered in space facilities (∼0.1 m/s); thus, it is necessary to conduct these tests in reduced gravity. Tests have been conducted with two types of fuels, polymethylmethacrylate (PMMA) and a polypropylene/glass fiber composite, aboard the NASA KC-135 aircraft, under air velocities below those induced by natural convection. The short reduced gravity period (∼25 s) provided by the aircraft limits the testing to high external fluxes (∼30 kW/m 2 ) so that the ignition delay times are shorter than the microgravity time. In normal gravity, the ignition delay and critical heat flux for ignition decrease as the forced-flow velocity decreases, until they reach minimum values that are limited by natural convection. The microgravity data indicate that ignition delay is further reduced as the air velocity is lowered. A theoretical model is used to predict the ignition delay for PMMA at low flow velocities in microgravity. The model predicts that the critical heat flux for ignition at the flow conditions expected in space facilities could be as much as half the value measured in normal gravity. The results are important because they imply that, in space facilities, ignition may occur more easily than in normal gravity. If the results are confirmed by long-term microgravity testing, they may have important implications for the fire safety design of space facilities.