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Transient ventilation dynamics following a change in strength of a point source of heat

Bower, D. J. and Caulfield, C. P. and Fitzgerald, S. D. and Woods, A. W. (2008) Transient ventilation dynamics following a change in strength of a point source of heat. Journal of Fluid Mechanics, 614 . pp. 15-37. ISSN 0022-1120. http://resolver.caltech.edu/CaltechAUTHORS:BOWjfm08

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Abstract

We investigate the transient ventilation flow within a confined ventilated space, with high- and low-level openings, when the strength of a low-level point source of heat is changed instantaneously. The steady-flow regime in the space involves a turbulent buoyant plume, which rises from the point source to a well-mixed warm upper layer. The steady-state height of the interface between this layer and the lower layer of exterior fluid is independent of the heat flux, but the upper layer becomes progressively warmer with heat flux. New analogue laboratory experiments of the transient adjustment between steady states identify that if the heat flux is increased, the continuing plume propagates to the top of the room forming a new, warmer layer. This layer gradually deepens, and as the turbulent plume entrains fluid from the original warm layer, the original layer is gradually depleted and disappears, and a new steady state is established. In contrast, if the source buoyancy flux is decreased, the continuing plume is cooler than the original plume, so that on reaching the interface it is of intermediate density between the original warm layer and the external fluid. The plume supplies a new intermediate layer, which gradually deepens with the continuing flow. In turn, the original upper layer becomes depleted, both as a result of being vented through the upper opening of the space, but also due to some penetrative entrainment of this layer by the plume, as the plume overshoots the interface before falling back to supply the new intermediate layer. We develop quantitative models which are in good accord with our experimental data, by combining classical plume theory with models of the penetrative entrainment for the case of a decrease in heating. Typically, we find that the effect of penetrative entrainment on the density of the intruding layer is relatively weak, provided the change in source strength is sufficiently large. However, penetrative entrainment measurably increases the rate at which the depth of the draining layer decreases. We conclude with a discussion of the importance of these results for the control of naturally ventilated spaces.


Item Type:Article
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http://dx.doi.org/10.1017/S0022112008003479DOIUNSPECIFIED
Additional Information:Copyright © Cambridge University Press 2008. Reprinted with permission. (Received 11 May 2007, and in revised form 26 June 2008) The authors would like to thank an anonymous referee, whose queries and suggestions greatly improved our understanding of the dominant physics, and the content of the manuscript.
Record Number:CaltechAUTHORS:BOWjfm08
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:BOWjfm08
Official Citation:D. J. BOWER, C. P. CAULFIELD, S. D. FITZGERALD and A. W. WOODS (2008). Transient ventilation dynamics following a change in strength of a point source of heat. Journal of Fluid Mechanics, 614, pp 15-37 doi:10.1017/S0022112008003479
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:12545
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:12 Dec 2008 21:27
Last Modified:26 Dec 2012 10:34

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