Numerical study of under-ventilated fire in medium-scale enclosure

Abstract

In an enclosure, as all the air inflow is consumed in burning with the excess fuel, the internal fire enters the decay phase, and such process is said flame exhaust. The complicated multistage process from an initial fire growth up to a flame exhaust followed by an external burning is investigated by means of a Large-Eddy-Simulation (LES). Turbulent combustion process is modelled by an Eddy Break-Up concept by using two sequential, semi-global steps for CO prediction. The numerical model solves three dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The critical fuel supply rate needed for flame to exhaust and the time period from the fuel ignition to the appearance of an external flaming in medium-scale facilities are previously obtained experimentally by Chamchine AV, Graham TL, Makhviladze GM, et al. [Experimental studies of under-ventilated combustion in small and medium-scale enclosures. In: Proceedings of the fourth international seminar on fire and explosion hazards; 2003. p. 97–107.], and the general trends predicted by the numerical model follow closely their experimental observation. This model is capable of adequately describing the essential simultaneous phenomena (flame height, soot generation, CO production, convection and radiation) occurring in a room fire. The distinct transient stages of fire development prior to flame exhaust and scenarios of the exhaust are analysed. An external burning is followed after the flame exhaust inside enclosure, and the flame height, H f, past the ceiling is approximately in an order of the opening height. Even though the flame exhaust takes place under the critical conditions, the heat transferred from the hotter gases and the external fire source poses significant threat to people inside enclosure, and potentially induces an ignition of fuel package exposed near the opening of an enclosure.