Influence of air intake position on heat feedback to the fuel surface in mechanically ventilated compartment - An experimental study
Résumé
The objective of this work is to study the effects of air intake position on the burning behaviour of liquid pool inside a compartment mechanically ventilated thanks to an exhaust fan. We analysed notably the key issue relevant to the vaporization process at the fuel surface which depends on the external heat feedback and controls the heat release rate. Results indicated that this external flux, which is the sum of radiant heat fluxes from hot gases and compartment internal surfaces, can have a significant contribution to the radiant heat transmitted to the fuel surface. This contribution will depend on air intake position, fuel parameters and on the characteristic length scale of the enclosure. With an air intake duct in high position, the heat flux from walls seems to be blocked by the smoke present in the compartment, whereas with air intake in low position, the pool fire is subject to nonnegligible radiation from both the compartment surfaces and hot gases, even if oxygen concentration decreases close to the pool and reaches a low value representative of underventilated fires. These observations were made for heptane fires inside a reduced scale enclosure with a length/height and width of 2 m, and with different pan diameters and Air Change Per Hour (ACPH). These observations were allowed thanks to measurements of radiant heat fluxes, fuel Mass Loss Rate (MLR), oxygen concentration and temperature. It is also shown that when the air inlet duct is located in the upper part, poor air circulation is observed in the room and the oxygen level in the vicinity of the fire location is insufficient. This results in fire extinction due to a lack of oxygen. On the other hand, a low inlet duct enhances air circulation in the compartment, facilitating air supply towards the burning liquid pool; in this case, fire is extinguished by lack of fuel after a well established steady state phase. Results show that the MLR is always greater when the air enters by the lower part of the room, regardless of the oxygen concentration around the flame. Therefore, air intake position influences burning behaviour by influencing heat feedback to the liquid fuel surface and availability of sufficient oxygen. Furthermore, a rise of ACPH logically results in an increase of MLR.
Domaines
Milieux fluides et réactifs
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