Fully Coupled CFD Simulation of the Ignition Risk of Unburnt Gases in an Exhaust System Connected to a Mechanically Ventilated Enclosure Fire
Résumé
Predictive simulations of liquid pool fires are based on the previous experimental study from a reduced mechanically ventilated enclosure with a length/height and width of 2 m. An external ventilation system provides an air supply rate ranging from 24 m3/h to 40 m3/h, corresponding to 3 and 5 Air Change Per Hour, respectively. A circular heptane or dodecane pan with a diameter varying from 23 cm to 40 cm is placed in the middle of the enclosure. The proposed liquid evaporation model in combustion environments is fully coupled with CFD simulation. The results show that the fuel pan above 30 cm leads to faster fire growth implying more important peak up to 200 kW in heat release rate and thus more dangerous fire. In the early stages of a fire, a stratified hotter unburnt fuels layer with a concentration beyond the Lower Flammability Limit is formed in the extraction duct connected to a mechanically ventilated enclosure fire. With a long time delay (about 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 with an oxygen concentration below 5% makes a sudden decrease of the pressure level inside enclosure due to cooling effects. This induces a sudden supply of fresh air from dilution duct, providing a sufficient oxygen concentration with a molar fraction of about 10% for triggering the ignition of a hotter fuel–air mixture near the extraction duct. Such auto-ignition, determined experimentally by visual identification of fire, is likely a random phenomenon with a probability of 50% due to the heat leakage through the walls of the experimental facility. When the compartment is more heat-tight by using an insulating material, the auto-ignition occurs for each fire tests. Globally, the phenomena with ignition near the extraction duct can be identified by a rapid decrease of unburnt fuel concentration towards a stoichiometric fuel–air mixture and a sharp increase of temperature with a peak reaching a typical flame temperature of 550°C there.
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