Dispersion of exhaust gases from roof-level stacks and vents on a laboratory building

Abstract A field study of six roof exhaust locations was carried out to test and refine a minimum dilution model using 44 sequential samplers at roof- and ground-level locations within a group of buildings located on a university campus. An existing minimum dilution theory was refined to develop a new theoretically based expression for initial dilution in the bent-over momentum jet from a stack, and to express the distance-dependent dispersion directly in terms of turbulence intensity. Predictions from the model were in good agreement with measured maximum concentrations (i.e. minimum dilution) over the 5–270 m range of exhaust to receptor distances. The field measurements confirmed wind-tunnel observations that hourly averaged dilution is strongly influenced by upwind-generated turbulence as well as building-generated turbulence. The distance-dependent dilution increased as crosswind turbulence in the approach flow increased, indicating that hourly average concentrations on the roof and at ground level are sensitive to upwind turbulence. Constants of proportionality averaged over the nine test periods were within 10% of distance-dilution constants determined from previous wind tunnel simulations of generic building shapes.