On ventilation of a heated room through a single doorway

Abstract

Laboratory experiments are presented to examine the transient filling of a room of buoyant fluid when a doorway connects the room to a large reservoir of dense fluid. These experiments confirm that the filling time of the room scales as ( A/ wH)( H/ g′) 1/2 where H is the doorway height, w the doorway width, A the floor area of the room, and g′ the buoyancy contrast between the interior and exterior of the room. A model of the transient exchange flow is presented and shown to be in accordance with the experimental results. The more complex regime in which a source of buoyancy is also supplied to the room is then examined. It is found that a steady two-layer stratification becomes established, with the height of the interface, h i, dependent only on the aspect ratio of the doorway and the door height, h i= Hf 1( w/ H), while the buoyancy contrast between the two layers, g′, depends on both the buoyancy flux supplied to the room and the doorway aspect ratio g′= B 2/3 H −5/3 f 2( w/ H). The results imply that, in typical buildings, exchange flows require 5– 100 min to fill a room once a door has been opened. Also, when a heated room is connected to a cold exterior by a doorway, a layer of air 5–15°C warmer than the exterior may be maintained above a cold layer near the floor. The depth of this cold layer depends on the aspect ratio of the doorway, but may typically extend over one-half the height of the door. In steady state, all the heat supplied to the room is lost through the doorway.