Experimental Study about the Influence of Heat Tightness of an Enclosure Fire on Ignition Risk of Unburnt Gases in a Connected Exhaust System
Résumé
This experimental study was performed from a reduced scale enclosure with a length/height and width of 2 m. In this work, we investigate whether heat tightness of enclosure can change the compartment fire dynamics and pose ignition risk in exhaust system. An external ventilation system provides an air supply rate ranging from 24 to 40 m3/h, corresponding respectively to 3 and 5 Air Change Per Hour (ACPH). A circular dodecane pan with a diameter of 40 cm is placed in the middle of the enclosure. Two situations with and without insulation of the enclosure are compared for several ACPH. The results show that heat tightness of enclosure leads to faster fire growth implying more important peak in heat release rate, and thus more dangerous fire with regarding the ignition risk. Heat tightness of enclosure makes the depression level of the compartment decrease, and consequently the air inlet supply rate decrease too, but the mass loss rate of liquid fuel increase. With heat tightness of enclosure, the fire becomes very-under-ventilated, and the vaporized fuel does not completely contribute to the heat release due to a significant formation of the unburnt gases as hydrocarbons, CO and H2. It is found that ignition of unburnt volatiles near the exhaust system occurs more easily when the compartment is more heat-tight. Oscillating flames in under-ventilated conditions were observed experimentally due to the ignition/extinction of the liquid fuel pan in a vitiated air enclosure. After the fire extinction, the remaining flames as ghosting flame in the vicinity of the enclosure ceiling were not observed under lowered oxygen vitiation with a mass fraction of around 7%. With a time delay of about 15 min in the current situation, the energy released per mass of oxygen consumed allows to raise the temperature of fuel-air mixture to about 300°C high enough for ignition of the unburnt gases. When the molar fraction of unburnt fuels is above the low flammability limit, any return of air from dilution duct after the fire extinction in a vitiated air enclosure could lead to spontaneous ignition near the extraction duct without needing a pilot flame. Note that heat-tightness of enclosure contributes to a reduction in the ignition delay time at entrance of the connected extraction duct due to a quick accumulation of unburnt fuel gases there. The experimental results from such reduced scale enclosure can be extended to a full scale one by imposing a preservation of the ratio (${\dot{{\rm{Q}}}}^{2}$/L5) between heat release rate ($\dot{{\rm{Q}}}$) and characteristic length (L). This relationship is derived from dimensionless variable of the energy equation.