Transients in natural ventilation — A time-periodically-varying source

Abstract

We examine the transient behaviour of a naturally ventilated space with a time-varying heat source. The flow is modelled using the `emptying filling box' model developed by Linden et al.1 with an isolated heat source, modelled as a plume. Scaled laboratory experiments were conducted in a water bath using a brine solution to generate density differences. The room is connected to the exterior via high and low level openings. A natural ventilation flow leads to a two-layer flow with warm fluid in the upper layer and ambient fluid in the lower `occupied' layer. During the flow's transient evolution, the upper layer has a non-uniform stratification within it, which we study using a simple numerical model. The time-varying stratification leads to time-varying flow rates through the space. We identify four important flow rates and illustrate which of these dominate based on the strength and oscillation period of the thermal source. Practical applications: Most of the current models for natural ventilation assume a steady state and constant heat loads. Many real heat sources are not constant in time and vary in some quasi-periodic manner. In this work, we show that there are four characteristic flow rates associated with such heat sources. Depending on the strength and period of the source these can vary significantly. In this paper, we show how to calculate these flow rates and select which one should be chosen from a design perspective.