The influence of stacks on flow patterns and stratification associated with natural ventilation

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• 作者：Shaun D. Fitzgerald ; Andrew W. Woods
• 关键词：Natural ventilation ; Stacks ; Stratification ; Flow regimes
• 刊名：Building and Environment
• 出版年：2008
• 期刊代码：21_03601323
• 类别：et
• 出版时间：October 2008
• 卷：43
• 期：10
• 页码：1719-1733
• 文件大小：649 K

摘要

We investigate the steady state natural ventilation of an enclosed space in which vent A, located at height h_A above the floor, is connected to a vertical stack with a termination at height H, while the second vent, B, at height h_B above the floor, connects directly to the exterior. We first examine the flow regimes which develop with a distributed source of heating at the base of the space. If h_B<h_A, then the unique flow solution involves inflow through vent B and outflow through vent A up the stack. If H>h_B>h_A, then two different flow regimes may develop. Either (i) there is inflow through vent B and outflow through vent A, or (ii) the flow reverses, with inflow down the stack into vent A and outflow through vent B. With inflow through vent A, the internal temperature and ventilation rate depend on the relative height of the two vents, 53c5e2047ca88e70e27d" title="Click to view the MathML source" alt="Click to view the MathML source">A and B, while with inflow through vent B, they depend on the height of vent 53c6e8e8f9120b94f" title="Click to view the MathML source" alt="Click to view the MathML source">B relative to the height of the termination of the stack H. With a point source of heating, a similar transition occurs, with a unique flow regime when vent B is lower than vent A, and two possible regimes with vent B higher than vent A. In general, with a point source of buoyancy, each steady state is characterised by a two-layer density stratification. Depending on the relative heights of the two vents, in the case of outflow through vent c29b2f750b7bf5bdfd0" title="Click to view the MathML source" alt="Click to view the MathML source">A connected to the stack, the interface between these layers may lie above, at the same level as or below vent A, leading to discharge of either pure upper layer, a mixture of upper and lower layer, or pure lower layer fluid. In the case of inflow through vent A connected to the stack, the interface always lies below the outflow vent B. Also, in this case, if the inflow vent A lies above the interface, then the lower layer becomes of intermediate density between the upper layer and the external fluid, whereas if the interface lies above the inflow vent A, then the lower layer is composed purely of external fluid. We develop expressions to predict the transitions between these flow regimes, in terms of the heights and areas of the two vents and the stack, and we successfully test these with new laboratory experiments. We conclude with a discussion of the implications of our results for real buildings.