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The effects of air intake position on the burning behaviour of liquid pool inside a mechanically ventilated compartment are analysed by using both an experiment and a physics-based model. A series of compartment fire experiments has been undertaken from a reduced scale enclosure with a length/height and width of 2 m. An external ventilation system consists of an air admission duct placed in low or high position inside the compartment, providing an Air Change Per Hour (ACPH) in a range from 1 to 4. Lowering the inlet duct enhances fuel-air mixing in the compartment. The experimental and numerical studies highlight a faster fire growth for a low air intake with a rise of ACPH, implying more dangerous fire with regarding the more important peak in heat release rate and heat feedback to liquid fuel surface. For air intake in high position, the direction of air jet inside enclosure is found to be orthogonal to the direction of the buoyancy-induced flow. This results in an air entrainment restriction towards the fire base and a decrease of the heat feedback to liquid fuel surface due to cooling effect on the hot smoke layer near the ceiling. Thus a high inlet contributes to a reduction by a factor of 40% in HRR, and the fire growth power is practically independent to ACPH. Moreover, for a high inlet, fire exhaust occurs more easily because both the heat feedback and air entrainment are weaker than these for a low inlet. As a consequence, changing air intake position from low to high leads to a change in fire regime from an under-ventilated fire to an over-ventilated one.

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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-under-ventilated, and a large vaporized fuel is converted into the unburnt gases such as hydrocarbons, CO and H2. 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 to 21 min in the current study, the energy released per mass of oxygen consumed allows to raise the smoke temperature above 350°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.

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