Thermal stratification produced by plumes and jets in enclosed spaces

The airflow and thermal stratification produced by a localised heat source located at floor level in a closed room is of considerable practical intere st and is commonly referred to as a 'filling box'. In rooms with low aspect ratios H/R less than or similar to 1 (room height H to characteristic hori zontal dimension R) the thermal plume spreads laterally on reaching the cei ling and a descending horizontal 'front' forms separating a stably stratifi ed, warm upper region from cooler air below. The stratification is well pre dicted for H/R less than or similar to1 by the original filling box model o f Baines and Turner (J. Fluid. Mech. 37 (1968) 51). This model represents a somewhat idealised situation of a plume rising from a point source of buoy ancy alone-in particular the momentum flux at the source is zero. In practi cal situations, real sources of heating and cooling in a ventilation system often include initial fluxes of both buoyancy and momentum, e.g. where a h eating system vents warm air into a space. This paper describes laboratory experiments to determine the dependence of the 'front' formation and strati fication on the source momentum and buoyancy fluxes of a single source, and on the location and relative strengths of two sources from which momentum and buoyancy fluxes were supplied separately. For a single source with a non-zero input of momentum, the rate of descent of the front is more rapid than for the case of zero source momentum flux a nd increases with increasing momentum input. Increasing the source momentum flux effectively increases the height of the enclosure, and leads to enhan ced overturning motions and finally to complete mixing for highly momentum- driven flows. Stratified flows may be maintained by reducing the aspect rat io of the enclosure. At these low aspect ratios different long-time behavio ur is observed depending on the nature of the heat input. A constant heat f lux always produces a stratified interior at large times. On the other hand : a constant temperature supply ultimately produces a well-mixed space at t he supply temperature. For separate sources of momentum and buoyancy, the developing stratificatio n is shown to be strongly dependent on the separation of the sources and th eir relative strengths. Even at small separation distances the stratificati on initially exhibits horizontal inhomogeneity with localised regions of wa rm fluid (from the buoyancy source) and cool fluid. This inhomogeneity is l ess pronounced as the strength of one source is increased relative to the o ther. Regardless of the strengths of the sources, a constant buoyancy flux source dominates after sufficiently large times, although the strength of t he momentum source determines whether the enclosure is initially well mixed (strong momentum source) or stably stratified tweak momentum source). (C) 2001 Elsevier Science Ltd. All rights reserved.