Effects of Ambient Parameters and Sample Width on Upward Flame Spread over Thermally Thin Solids

Abstract

Upward flame spread is the most rapid and hazardous of the different modes of flame spread. Moreover, studies on the effects of ambient parameters on flame spread are very important for fire safety on spacecraft systems. Therefore, the effects of pressure (0 kPa to 70 kPa), oxygen mole fraction (0.30 to 0.90) and sample width (0.3 cm to 2 cm) on upward flame spread over thin papers were studied. A flame break-off phenomenon over papers was firstly observed, which occurred more likely at high oxygen and high pressure conditions. Upward flame spread reached a steady state by varying the ambient parameters and using narrow samples. The smaller sample width narrowed the flammability limits. Differences in upward and downward flame spread were discussed. We found that a power law relationship between flame spread rate and pressure remained valid near the extinction limit for upward spread, while the power law broke down in the extinction region for downward spread. The controlling mechanism of upward spread was revealed by a pressure modeling method showing that the upward flame spread rate was proportional to the Grashof number raised to an exponent of 0.257. For upward flame spread over narrow solids, the gas phase kinetics and radiation effects are less important compared to the heat convection in the flame spread process, which is quite different from the downward spread. This study contributes to the fire safety of space vehicles and enhances the understanding of the controlling mechanism of flame spread.