Natural ventilation of multiple storey buildings: The use of stacks for secondary ventilation

Abstract

The natural ventilation of buildings may be enhanced by the use of stacks. As well as increasing the buoyancy pressure available to drive a flow, the stacks may also be used to drive ventilation in floors where there is little heat load. This is achieved by connecting the floor with a relatively low heat load to a floor with a higher heat load through a common stack. The warm air expelled from the warmer space into the stack thereby drives a flow through the floor with no heat load. This principle of ventilation has been adopted in the basement archive library of the new SSEES building at UCL. In this paper a series of laboratory experiments and supporting quantitative models are used to investigate such secondary ventilation of a low level floor driven by a heat source in a higher level floor. The magnitude of the secondary ventilation within the lower floor is shown to increase with the ratio of the size of the openings on the lower to the upper floor and also the height of the stack. The results also indicate that the secondary ventilation leads to a reduction in the magnitude of the ventilation through the upper floor, especially if the lower floor has a large inlet area.

Introduction

There is increasing awareness of the high energy consumption in buildings. Many buildings use mechanical air conditioning to regulate the internal environment, but even with energy efficient designs, they typically use around $230\mathrm{KWh}/{\mathrm{m}}^2$ of energy [1]. However, in a number of buildings, alternative low energy systems use natural ventilation to significantly reduce the energy consumption. Research has developed a good understanding of the basic principles of natural ventilation [2], [3], [4] within simple building structures. One of the key challenges now, is concerned with understanding the subtleties of such flows within more complex multiple storey buildings.

A particular challenge associated with naturally ventilating large office spaces is the provision of ventilation for areas in which there is insufficient buoyancy to drive a flow. A possible solution for this is through the use of stacks to couple floors with large heat loads to those without. In this manner, the warm air expelled into a stack from a space with a large heat load may be used to drive a flow on a different floor which otherwise would have insufficient buoyancy to drive a ventilation flow (Fig. 1). In this work, the impact of a stack on the upwards buoyancy driven displacement flow of a room with a heated floor is reviewed and referred to as the primary ventilation flow. These principles are then used to investigate how secondary ventilation flows can be induced on a floor located beneath the primary heated floor through the use of common stacks.

This type of ventilation may be of use in an office or industrial environment in which there is a low occupancy zone at low level. Indeed such a scheme is being implemented for the ventilation of the basement library in the new SSEES building at UCL [5]. In a different implementation of the concept, a warehouse could be ventilated through the use of offices located at an elevated height within the space. As well as ventilating lower level floors of minimal heat load, the scheme could also be used to enhance night or evening cooling of thermal mass in an undercroft, in for example, theatres. In this work, attention is restricted to the case of upward displacement ventilation, although it is noted that where multiple stacks are employed it is possible for some of the stacks to witness downward flow [6].

The paper is organised as follows. In Section 2, a steady state model is presented for a single room connected to high level stacks. The focus here is on the pressure losses associated with different inflow designs to the stacks and also the frictional losses within the stacks. In Section 3, a small scale analogue laboratory experiment is described which is used to validate the model. The principles developed for a single room are then applied in Section 4 to describe the coupled flow on two different floors which are connected by common stacks. Analogue experiments are conducted to test and validate the model of the flow in a two storey model building. In Section 6 the results are applied to a typical building geometry to provide simple guidelines for the designers of naturally ventilated buildings. Note all physical properties and dimensionless numbers are defined in Appendix A.3, the variables used in the single floor analysis are given in Appendix A.4 and those for the two floor analysis in Appendix A.5.