Dilution of exhaust from a rooftop stack on a cubical building in an urban environment

The dispersion of a tracer gas emitted from a short stack on a cubical building was investigated using field and wind tunnel experiments. Air samples were obtained on the roof and the leeward side of the building. Dilution data were compared with estimates of minimum dilution (Dmin) obtained with design formulas of Wilson/Chui/Lamb and Halitsky. The Halitsky model produced conservative predictions of Dmin. Estimates of Dmin obtained with the Wilson/Chui/Lamb model were in reasonable agreement with the field data. Wind tunnel dilution values were usually within a factor of two of the field data. The accuracy of the wind tunnel data improved as distance from the source increased. Wind tunnel dilution was found to be strongly dependent on the ratio of exhaust speed to wind speed, M, particularly for M values in the range of 2–4. This dependence is believed to be associated with the wind tunnel modelling of the stack exhaust, and is thus probably not a feature of the full-scale situation.