Transient natural ventilation of a space with localised heating

The natural ventilation of a well mixed, pre-heated room with a point source of heating, and openings at the base and roof is investigated. The transient draining associated with the room being warmer than the exterior combined with the convective flow produced by the point source of heat leads to a fascinating series of transient flow regimes as the system evolves to the two-layer steady-state regime described by Linden, Lane-Serff and Smeed [1]. As the room begins to ventilate, a turbulent plume rises from the point source of heat to the ceiling, and typically forms a deepening layer of hot air. However, with a weak heat source, then at some point the ascending plume will intrude beneath the layer of original fluid. Otherwise, the ascending plume always reaches the top of the room as the system evolves to a steady state. We develop a simplified model of the transient evolution and test this with some new laboratory experiments. We conclude with a discussion of the implications of our results for real buildings.