Natural Ventilation with Pre-cooling

We investigate two different natural ventilation regimes of an auditorium or other occupied open-plan space which is equipped with pre-cooling. The room has a low-level vent and a high level vent. The occupants act as a source of heat, which if without pre-cooling, can lead to the room becoming too warm for comfort. In the first ventilation regime, a chiller is fitted to the low-level vent, and the occupants provide positive buoyancy which drives the ventilation upwards and draws fresh air through the pre-cooling system. The pre-cooled air then displaces warm air in the room upwards and out through the top vent. This regime maintains a steady upflow in which the interior is warmer than the outside, and so is suitable for autumn, spring or early summer, where the exterior temperature is mild. In the second regime, a chiller is fitted instead to the high-level vent. Pre-cooled air coming through the top vent produces negative buoyancy which drives the flow downwards and out through the bottom vent. This regime maintains a steady downflow in which the interior is cooler than the outside, and so is suitable for high summer, where the exterior temperature is extreme. A quantitative model is developed to describe these two ventilation regimes and is successfully tested with analogue laboratory experiments. Our model shows that, in the upflow regime, an increase in the cooling can lead to either an increase or a decrease in the room temperature, depending on the geometry of the room and the ratio of cooling compared to heating. In the downflow regime, there is an optimum amount of cooling which is required to maintain the flow in the downward direction while keeping the interior at a desired temperature below the exterior.