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ISSN Online: 2377-424X

ISBN CD: 1-56700-226-9

ISBN Online: 1-56700-225-0

International Heat Transfer Conference 13
August, 13-18, 2006, Sydney, Australia

EFFECTS OF BUOYANCY ON CONTAMINANT TRANSPORT IN ROOM AIR FLOWS

Get access (open in a dialog) DOI: 10.1615/IHTC13.p6.270
12 pages

Abstract

The transport of hazardous gases in room environments is complex and the prediction of contaminant concentration distributions depends on many factors (e.g., laminar vs. turbulent flow, geometry of the interior space, inlet and outlet locations, contaminant properties and method of contaminant introduction, etc.). The large molecular weights of many hazardous gases leads to important buoyancy contributions in determining their transport and effect on the airflow; an effect that has received virtually no attention. In this two-dimensional numerical study of contaminant transport a gaseous contaminant, phosgene, is introduced into a room through an inlet air stream as either a continuous source or a discrete “puff”. Several flow configurations and room geometries are studied. The flow field, analyzed as a turbulent flow, is validated by comparing predicted profiles of velocity at several locations in a model room with the measurements of Nielsen et al. Room air flow rates are varied from 1 to 5 air changes per hour (ACH); 1667 ≤ Reh ≤ 8333. The effects of buoyancy on the flow field and on the distribution of contaminant within the room are shown to be important. Special attention is given to the time dependence of the contaminant concentration at the outlet where the results, normalized by the room-averaged (time dependent) contaminant concentration, are presented for 0.013 ≤ Rih ≤ 1.1 and for several inlet and outlet configurations in room geometries. For room aspect ratio, AR = L/H = 3, the increase of contaminant concentration at the outlet is predicted to lag the fully mixed result initially but then to overshoot it at later times when buoyancy effects are small (Rih ≤ 0.1). When buoyancy effects are significant (Rih ≥ 0.1), the outlet contaminant concentration shows a reduced overshoot compared with the fully mixed result, and for some room configurations the overshoot is eliminated. These findings are dependent on the room geometry and inlet and outlet configuration. All of the results studied for AR = 3 show that for times greater than approximately two room air changes, the outlet contaminant concentration asymptotically approaches the fully mixed concentration. For the smaller aspect ratio AR = 1, considerably more time is required for the outlet concentration to approach the fully mixed condition. When both inlet and outlet are located in the ceiling, the time required for the outlet contaminant concentration to reach a fully mixed condition is significantly less when the inflow is directed away from the outlet than when it is directed toward the outlet. This non-intuitive result is related to the recirculation regions set up within the room and the transport of contaminant in these complex flows. The results indicate that the best location for hazardous gas sensors is in the outlet duct of a room; anywhere else could lead to a delayed response due to the complex flow patterns and uncertainty with respect to the source of the hazardous gas.