Pollutant flushing with natural displacement ventilation

We examine the time taken to flush pollutants from a naturally ventilated room. A simple theoretical model is developed to predict the time taken for neutrally-buoyant pollutants to be removed from a room by a flow driven by localised heat inputs; both line and point heat sources are considered. We show that the rate of flushing is a function of the room volume, vent areas (A*A*) and the distribution, number (n) and strength (B  ) of the heat sources. We also show that the entire problem can be reduced to a single parameter (μμ) that is a measure of the vent areas, and a dimensionless time (ττ) that is a function of B, V   and μμ. Small-scale salt-bath experiments were conducted to measure the flushing rates in order to validate our modelling assumptions and predictions. The predicted flushing times show good agreement with the experiments over a wide range of μμ. We apply our model to a typical open plan office and lecture theatre and discuss some of the implications of our results.