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Transient ventilation dynamics induced by heat sources of unequal strength

Published online by Cambridge University Press:  02 December 2013

Ajay B. Shrinivas  and
Gary R. Hunt
Ajay B. Shrinivas
Affiliation:
Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK
Gary R. Hunt*
Affiliation:
Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK
*
†Email address for correspondence: gary.hunt@eng.cam.ac.uk
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Abstract

We examine theoretically the transient displacement flow and density stratification that develops within a ventilated box after two localized floor-level heat sources of unequal strengths are activated. The heat input is represented by two non-interacting turbulent axisymmetric plumes of constant buoyancy fluxes ${B}_{1} $ and ${B}_{2} \gt {B}_{1} $. The box connects to an unbounded quiescent external environment of uniform density via openings at the top and base. A theoretical model is developed to predict the time evolution of the dimensionless depths ${\lambda }_{j} $ and mean buoyancies ${\delta }_{j} $ of the ‘intermediate’ $(j= 1)$ and ‘top’ $(j= 2)$ layers leading to steady state. The flow behaviour is classified in terms of a stratification parameter , a dimensionless measure of the relative forcing strengths of the two buoyant layers that drive the flow. We find that $\mathrm{d} {\delta }_{1} / \mathrm{d} \tau \propto 1/ {\lambda }_{1} $ and $\mathrm{d} {\delta }_{2} / \mathrm{d} \tau \propto 1/ {\lambda }_{2} $, where $\tau $ is a dimensionless time. When $\hspace{0.167em} \hspace{0.167em} \ll \hspace{0.167em} \hspace{0.167em} $1, the intermediate layer is shallow (small ${\lambda }_{1} $), whereas the top layer is relatively deep (large ${\lambda }_{2} $) and, in this limit, ${\delta }_{1} $ and ${\delta }_{2} $ evolve on two characteristically different time scales. This produces a time lag and gives rise to a ‘thermal overshoot’, during which ${\delta }_{1} $ exceeds its steady value and attains a maximum during the transients; a flow feature we refer to, in the context of a ventilated room, as ‘localized overheating’. For a given source strength ratio $\psi = {B}_{1} / {B}_{2} $, we show that thermal overshoots are realized for dimensionless opening areas $A\lt {A}_{oh} $ and are strongly dependent on the time history of the flow. We establish the region of $\{ A, \psi \} $ space where rapid development of ${\delta }_{1} $ results in ${\delta }_{1} \gt {\delta }_{2} $, giving rise to a bulk overturning of the buoyant layers. Finally, some implications of these results, specifically to the ventilation of a room, are discussed.

JFM classification

Convection: Convection Convection: Plumes/thermals Geophysical and Geological Flows: Stratified flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 738 , 10 January 2014 , pp. 34 - 64
Copyright
©2013 Cambridge University Press 

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