Influence of heat tightness of an enclosure fire on ignition risk of unburnt gases in a connected exhaust system - An experimental study

An experimental study was performed from a reduced scale enclosure with a length/height and width of 2 m. A dodecane pan of 40 cm in diameter is placed at the center of the enclosure floor. An external ventilation system provides an air supply rate with an Air Change Per Hour (ACPH) ranging from 3 to 5. Influence of the intermediate levels of thermal insulation of enclosure on ignition risk in a connected exhaust system is experimentally evaluated. The results show that thermal insulation of an enclosure leads to faster fire growth, implying more important peak in heat release rate, and thus more dangerous fire in regards to the ignition risk. Heat tightness of enclosure enhances the mass loss rate of liquid fuel, but reduces the air supply rate from the admission duct due to decrease of the depression level in the compartment. As a consequence, the fire becomes quickly very-underventilated, and a large vaporized fuel is converted into the unburnt gases such as hydrocarbons, CO and H-2. In the early stages of a fire, a hotter unburnt fuel layer with a concentration above the Lower Flammability Limit (LFL) is formed in the extraction duct connected to a mechanically ventilated enclosure fire. With a long time delay in a range of 16-21 min in the current study, the energy released per mass of oxygen consumed allows to raise the smoke temperature above 350 degrees C. Occurrence of flame extinction in vitiated air enclosure makes a sudden increase of the depression level inside enclosure due to cooling effects. This results in a sudden supply of fresh air from dilution duct, providing a sufficient amount of oxygen to trigger ignition of a fuel rich mixture near the extraction duct. It is found that ignition of unburnt volatiles at entrance of the extraction duct occurs more easily when the compartment is more heat-tight with a reduction by about 30% in the ignition delay.