Analytical and Numerical Study on Irradiate Ignition of Solid Combustibles in Sub-Atmospheric Pressure

Abstract

The aim of the present study is to understand the basics of spontaneous ignition of irradiated solid combustibles in sub-atmospheric pressure via analytical and numerical approaches. Final goal of this work is to provide the universal description to the recently-observed ignition characters in sub-atmospheric pressure. An irradiated ignition of horizontally-placed, thin cellulose paper in sub-atmospheric pressure is considered here; the sample is heated at the bottom to lead ignition below the surface in gravimetric environment. Two analytical solutions based on classical ignition theories are referred and 3-D numerical simulation is employed for further discussions. Analytical solutions suggest that the observed ignition trend by authors recently is similar to what would be attained in pure diffusion field, not in the stagnation-point flow field which includes the flow-associated heat loss, except near the lowest pressure range to be ignited. According to the 3-D numerical simulation, it is revealed that either pure diffusion or stagnation-point flow field appears depending on the imposed conditions. When longer ignition delay time is expected, an transport process comes to play a role and ignition is close to what would be attained in stagnation-point flow field, rather than the pure diffusion field. Classical ignition theories are useful to understand the insight of ignition processes in low pressure.