Abstract

The reingestion of toxic or odorous gases exhausted from rooftop stacks may be a cause of indoor air quality problems of a building. In order to avoid such problems, certain precautions should be taken during the design period. One of the current engineering practices is to evaluate the exhaust dispersion using ASHRAE dispersion models. As another alternative, a wind tunnel study can be carried out to predict peak concentrations at critical locations. However, neither ASHRAE models nor wind tunnel modeling have been thoroughly validated with full-scale data. For the purpose of evaluating empirical formulas and wind tunnel modeling techniques, four field tests on the roof of the Hall Building of Concordia University and a wind tunnel study have been carried out. The influence of various parameters on the dispersion process were investigated. These included the exhaust momentum ratio, stack location, stack height and building shape. Flow visualization was performed as well to determine the flow pattern around buildings. Several results were drawn from the current study: (1) the ASHRAE models generally provide reasonable prediction of minimum dilution (D$\rm\sb{min}$) with some notable exceptions. (2) In general, wind tunnel results compared well with the full-scale data; however, for receptors close to the stack, the wind tunnel overestimates D$\rm\sb{min}$. (3) Both wind tunnel and field results indicate that D$\rm\sb{min}$ is significantly affected by the exhaust momentum ratio (M); wind tunnel tests with the isolated building do not show this dependence on M. (4) Roof level concentration can be reduced by increasing the stack height. (5) Building shape has a considerable influence on rooftop flow patterns.

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