Hostname: page-component-78c5997874-g7gxr Total loading time: 0 Render date: 2024-11-06T10:42:58.817Z Has data issue: false hasContentIssue false

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

Published online by Cambridge University Press:  16 October 2008

D. J. BOWER
Affiliation:
BP Institute, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, [email protected], [email protected], [email protected], [email protected]
C. P. CAULFIELD
Affiliation:
BP Institute, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, [email protected], [email protected], [email protected], [email protected] Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge, CB3 0WA, UK
S. D. FITZGERALD
Affiliation:
BP Institute, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, [email protected], [email protected], [email protected], [email protected]
A. W. WOODS
Affiliation:
BP Institute, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, [email protected], [email protected], [email protected], [email protected]

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.

Type
Papers
Copyright
Copyright © Cambridge University Press 2008

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

REFERENCES

Baines, W. D. 1975 Entrainment by a plume or jet at a density interface. J. Fluid Mech. 68, 309320.CrossRefGoogle Scholar
Baines, W. D. & Turner, J. S. 1969 Turbulent buoyant convection from a source in a confined region. J. Fluid Mech. 37, 5180.CrossRefGoogle Scholar
Baines, W. D., Turner, J. S. & Campbell, I. M. 1990 Turbulent fountains in an open chamber. J. Fluid Mech. 212, 557592.CrossRefGoogle Scholar
Bolster, D. & Caulfield, C. P. 2008 Transients in natural ventilation - A time-periodically varying source. Building Serv. Engng Res. Technol. 29, 119135.CrossRefGoogle Scholar
Bloomfield, L. J. & Kerr, R. C. 2000 A theoretical model of a turbulent fountain. J. Fluid Mech. 424, 197216.CrossRefGoogle Scholar
Caulfield, C.-C. P. & Woods, A. W. 1995 Plumes with nonmonotonic mixing behaviour. Geophys. Astrophys. Fluid Dyn. 79, 173199.CrossRefGoogle Scholar
Caulfield, C. P. & Woods, A. W. 2002 The mixing in a room by a localized finite-mass-flux source of buoyancy. J. Fluid Mech. 471, 3350.CrossRefGoogle Scholar
Flynn, M. R. & Caulfield, C. P. 2006 Natural ventilation in interconnected chambers. J. Fluid Mech. 564, 139158.CrossRefGoogle Scholar
Gladstone, C. & Woods, A. W. 2001 On buoyancy-driven natural ventilation of a room with a heated floor. J. Fluid Mech. 441, 293314.CrossRefGoogle Scholar
Hunt, G. R. & Kaye, N. G. 2001 Virtual origin correction of lazy turbulent plumes. J. Fluid Mech. 435, 377396.CrossRefGoogle Scholar
Hunt, G. R. & Linden, P. F. 2001 Steady-state flows in an enclosure ventilated by buoyancy forces assisted by wind. J. Fluid Mech. 426, 355386.CrossRefGoogle Scholar
Kaye, N. B. & Hunt, G. R. 2004 Time-dependent flows in an emptying filling box. J. Fluid Mech. 520, 135156.CrossRefGoogle Scholar
Kumagai, M. 1984 Turbulent buoyant convection from a source in a confined two-layered region. J. Fluid Mech. 147, 105131.CrossRefGoogle Scholar
Lin, Y. J. P. & Linden, P. F. 2005 The entrainment due to a turbulent fountain at a density interface. J. Fluid Mech. 542, 2552.CrossRefGoogle Scholar
Linden, P. F. 1973 The interaction of a vortex ring with a sharp density interface: a model for turbulent entrainment. J. Fluid Mech. 60, 467480.CrossRefGoogle Scholar
Linden, P. F. 1999 The fluid mechanics of natural ventilation. Annu. Rev. Fluid Mech. 31, 201238.CrossRefGoogle Scholar
Linden, P. F., Lane-Serff, G. F. & Smeed, D. A. 1990 Emptying filling boxes: the fluid mechanics of natural ventilation. J. Fluid Mech. 212, 309335.CrossRefGoogle Scholar
McDougall, T. J. 1981 Negatively buoyant jets. Tellus 33, 313320.CrossRefGoogle Scholar
Morton, B. R. 1959 Forced plumes. J. Fluid Mech. 5, 151163.CrossRefGoogle Scholar
Morton, B. R., Taylor, G. I. & Turner, J. S. 1956 Turbulent gravitational convection from maintained and instantaneous sources. Proc. R. Soc. Lond. A 234, 123.Google Scholar
Settles, G. S. 2001 Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media. Springer.CrossRefGoogle Scholar
Woods, A. W., Caulfield, C. P. & Phillips, J. C. 2003 Blocked natural ventilation: the effect of a source mass flux. J. Fluid Mech. 495, 119133.CrossRefGoogle Scholar