Filling of smoke due to fire in a room with roof ventilation

Abstract

Displacement ventilation is one of the ventilation techniques employed in maintaining the interior thermal environment of low-energy buildings. Warm air rising from occupants in a room is removed through a vent in the ceiling, while cool fresh air flows in through the floor level openings. The entire flow is driven by buoyancy forces due to temperature difference of air. In the event of fire in such a room, the smoke from the fire rises to the ceiling and begins filling it from the top, while also draining through the ceiling vent. In this article we investigate filling of smoke due to fire and examine how it is affected by heat loss through walls and ceilings, along with other parameters such as the room's geometry, ceiling vent area, and fire's power. We validate our model using experimental results of Harrak et al., 2019, International Journal of Thermal Sciences. We show that heat loss reduces smoke-layer's steady state depth, and the said depth is unique in the limit of large heat losses. For small aspect-ratio rooms, steady state properties of the smoke-layer can be significantly different for the insulated and conducting wall cases, even for small conductivity in the latter case. We derive formulas for bounds on the smoke-layer's steady state depth and density (and hence temperature), as well as obtain a useful estimate for the time taken for the smoke-layer to reach its steady state depth.