The response of natural displacement ventilation to time-varying heat sources

We examine transients caused by sudden changes in heat load in a naturally ventilated chamber. The space we consider has an isolated heat source, modeled as an ideal plume, and is connected to the exterior via openings at the top and bottom. Pressure differences between the exterior and interior that arise due to the buoyancy in the space drive a natural ventilation flow through the space that generates a two-layer system with buoyant (warm) fluid in the upper layer and ambient fluid in the lower layer. We develop two mathematical models, one assuming perfect mixing of each layer, the other accounting for stratification. We compare both models to small scale laboratory experiments. Neither model is significantly better than the other, and both give good agreement with the experiments.Using these models we identify many appealing features of the type of natural ventilation studied here. These include the fact that a well designed system is self-controlling. The manner in which it controls itself is very robust, because the larger the change in heat load the faster the system will readjust. Also there is a resistance against the interface penetrating deeply into the occupied zone, even for very large changes in heat load.