TIME-DEPENDENT MIXTURE FORMATION AND HEAT TRANSFER OVER IRRADIATED SOLID COMBUSTIBLES IN SUB-ATMOSPHERIC PRESSURE ENCLOSURE

摘要

The aim of the present study is to elucidate the time-depended processes of combustible mixture formation, heat transfer from the heated solid surface and chemical reaction over irradiated solid combustibles in sub-atmospheric pressure enclosure. Final goal of this work is to explain observed ignition trend in sub-atmospheric pressure which is recently derived experimentally by author's group, that is, required partial pressure of oxygen is "reduced" in sub-atmospheric range. Solid combustible is placed horizontally and irradiated from the bottom to induce an ignition spontaneously below the surface. Theoretical study based on available ignition theories and 3-D numerical simulation is performed. Classical ignition theory suggests that the observed pressure dependency of ignition is not what is given in the stagnation point flow field including the flow-associated heat loss, but in the pure diffusion field neglect any flow-induced heat loss. According to the 3-D numerical simulation, it is revealed that either pure diffusion situation or stagnation-point flow situation would be attained depending on the imposed condition. A reduction of irradiated energy could make longer ignition delay and physical process comes to play a role on ignition behavior. Under such condition, it is suspected that ignition mode would be close to what is observed in stagnation-point flow field, rather than the pure diffusion field. Feasibility study is needed to clarify this speculation.