On the natural ventilation of two independently heated spaces connected by a low-level opening

We study the buoyancy-driven natural ventilation of two spaces which are connected to one another by a low-level opening, and each of which is connected to the exterior through a high-level vent. Each space is heated uniformly by an independent source, which provides buoyancy driving the ventilation. Using laboratory experiments, we show that these conditions lead to each space becoming well mixed at steady state. In this regime, a net flow from one space to the other is driven by the buoyancy created in the downstream space. Although it is possible in theory for the flow to develop in either direction, our new experiments and theoretical model show that, in reality, if the vents of the two spaces are at the same height, then the actual flow regime will depend primarily on the relative strength of the heat loads. If the two heat loads are sufficiently different, only the flow from the weakly heated space to the strongly heated one is stable. If the two heat loads are comparable, both modes are stable, leading to multiple flow regimes. The problem is generalised to show that, if the heights of the vents are equal, then the flow regime will depend on the relative height of the vents, as well as the relative strength of the heat loads. There is a range of combinations of vent heights and heat loads that still allow multiple flow regimes. We identify the limits of each regime and outline principles for control.