The influence of stacks on flow patterns and stratification associated with natural ventilation

We investigate the steady state natural ventilation of an enclosed space in which vent AA, located at height hAhA above the floor, is connected to a vertical stack with a termination at height H  , while the second vent, BB, at height hBhB above the floor, connects directly to the exterior. We first examine the flow regimes which develop with a distributed source of heating at the base of the space. If hBhB>hAH>hB>hA, then two different flow regimes may develop. Either (i) there is inflow through vent BB and outflow through vent AA, or (ii) the flow reverses, with inflow down the stack into vent AA and outflow through vent BB. With inflow through vent AA, the internal temperature and ventilation rate depend on the relative height of the two vents, AA and BB, while with inflow through vent BB, they depend on the height of vent BB relative to the height of the termination of the stack HH. With a point source of heating, a similar transition occurs, with a unique flow regime when vent BB is lower than vent AA, and two possible regimes with vent BB higher than vent AA. In general, with a point source of buoyancy, each steady state is characterised by a two-layer density stratification. Depending on the relative heights of the two vents, in the case of outflow through vent AA connected to the stack, the interface between these layers may lie above, at the same level as or below vent AA, leading to discharge of either pure upper layer, a mixture of upper and lower layer, or pure lower layer fluid. In the case of inflow through vent AA connected to the stack, the interface always lies below the outflow vent BB. Also, in this case, if the inflow vent AA lies above the interface, then the lower layer becomes of intermediate density between the upper layer and the external fluid, whereas if the interface lies above the inflow vent AA, then the lower layer is composed purely of external fluid. We develop expressions to predict the transitions between these flow regimes, in terms of the heights and areas of the two vents and the stack, and we successfully test these with new laboratory experiments. We conclude with a discussion of the implications of our results for real buildings.