Passive ventilation of buildings at night forms an essential part of a low-energy cooling strategy, enabling excess heat that has accumulated during the day to self-purge and be replaced with cooler night air. Instrumental to the success of a purge are the locations and areas of ventilation openings, and openings positioned at low and at high levels are a common choice as there is then the expectation that a buoyancy-driven displacement flow will establish and persist. Desirable for their efficiency, displacement flows guide excess heat out through high-level openings and cooler air in through low-level openings. Herein we show that displacement flow cannot be maintained for the full duration of a purge. Instead, the flow must transition to an ‘unbalanced exchange flow’, whereby the cool inflow of air at low level is maintained but there is now a warm outflow and a cool inflow occurring simultaneously at the high-level opening. The internal redistribution of heat caused by this exchange alters the rate at which heat is self-purged and the time thought necessary to complete a purge. We develop a theoretical model that captures and predicts these behaviours. Our approach is distinct from all others which assume that a displacement flow will persist throughout the purge. Based on this enhanced understanding, and specifically that the transition to unbalanced exchange flow changes the rate of cooling and resultant emptying times, we anticipate that practitioners will be better placed to design passive systems that meet their target specifications for cooling.

晚上，建筑物的被动通风是低能耗制冷策略的重要组成部分，它可使白天积聚的多余热量自行清除，并由凉爽的夜间空气代替。通气孔的位置和面积是吹扫成功的关键，而低位和高位的开孔是常见的选择，因为人们期望浮力驱动的置换流将会建立并持续存在。由于它们的效率理想，排量流将多余的热量从高位开口引出，而较冷的空气则从低位开口引出。本文中，我们显示了在吹扫的整个过程中都无法保持置换流量。相反，流量必须转换为“不平衡的交换流量”，从而保持了低水平的冷空气流入，但是现在在高水平的开口处同时发生热流出和冷流入。由这种交换引起的热量的内部重新分布会改变热量自我清除的速率以及完成清除所需的时间。我们建立了一个理论模型来捕获和预测这些行为。我们的方法与所有其他方法都不同，其他方法都假定在整个吹扫过程中都会持续存在驱替流。基于这种加深的理解，特别是过渡到不平衡的交换流量会改变冷却速率和随之而来的排空时间，我们预计从业人员将可以更好地设计出满足其冷却目标指标的无源系统。