Stratification and oscillations produced by pre-cooling during transient natural ventilation

Abstract

This paper investigates the transient natural ventilation of a warm room which vents to a cool exterior through a stack connected to the roof of the room, and into which air is drawn through a lower vent connected to a cooling unit. The temperature differences between the interior and exterior create positive buoyancy driving the ventilation. This results in fresh exterior air being drawn into the room through the cooler connected to the lower vent. After being cooled, this fresh air enters the room and displaces the existing warm air upwards and out through the stack. A sharp interface then develops between a lower layer of cooled invading air and an upper layer of warm original air. This interface ascends as the room continues to drain. However, the pre-cooled air is negatively buoyant, and so the flow rate and the rate of ascent of the interface gradually fall as the cold lower layer deepens. As a result, the temperature of the pre-cooled air progressively decreases towards that of the chiller, and the lower layer becomes stably stratified. Eventually, the ventilation ceases and the interface stops ascending when the positive buoyancy associated with warm air in the upper layer and stack and the negative buoyancy associated with cooled air in the lower layer are in balance. However, this equilibrium is unstable to downward motion. Colder, less buoyant air from the exterior in time displaces warm buoyant air in the stack, reducing the positive buoyancy of the upper layer and causing it to be outweighed by the negative buoyancy of the lower layer. A downward draining flow then commences. As the exterior air descends through the stack, it mixes with warm air in the upper layer, leading to the temperature of the upper layer evolving towards that of the ambient. As a result, the flow rate and the rate of descent of the interface gradually fall as the upper layer deepens and the colder lower layer drains from the room through the lower vent. Eventually, the flow ceases again when the positive buoyancy of the upper layer balances the negative buoyancy of the lower layer, but now the interface is arrested closer to the base of the room. This equilibrium is unstable to upward motion, and in time warm air from the upper layer displaces the exterior air in the stack. This causes the positive buoyancy to increase, and consequently the room drains upwards once more. In this way the pre-cooling makes the system oscillate between the upward and downward draining regimes, which ensue until the room is completely drained. The ventilation eventually stops altogether when the interface reaches the base of the room and the upper layer attains the temperature of the ambient. A simple model is developed to describe this transient oscillatory process and is compared with small-scale analogue experiments. This paper discusses the application of the model, and shows how pre-cooled draining may be employed appropriately to provide thermal comfort in an intermittently occupied room, when the exterior air is uncomfortably warm so that conventional flushing without pre-cooling may not be used effectively.