SPATIAL EFFECTS IN THE THERMAL RUNAWAY OF COMBUSTIBLE FLUIDS IN INSULATION MATERIALS

The thermal runaway and eventual ignition of flammable fluids in laggi ng materials has occurred with sufficient frequency for there to be co nsiderable awareness of this problem in the process engineering indust ry. Earlier (mainly spatially invariant) models are here modified to i nclude the important effect of diffusion. The aim is to identify bound aries in parameter space where ignition will or will not occur. In thi s paper, a model of the three main contributing quantities (temperatur e, fuel and oxygen concentration) is considered which includes exother mic oxidation and endothermic evaporation/desorption processes and pri marily centres on the energy balance equation. The shape of the igniti on boundary is found to depend crucially on a key value of the ratio o f thermal diffusion to mass diffusion of oxidant (inverse Lewis number ). The critical value of this ratio is in fact size-dependent and beyo nd a certain value, ignition will be suppressed.