Effects of Plume Merging on Model Prediction
Plume rise from a multi-flued stack configuration is conventionally modelled as an equivalent single point source emission with the same momentum and buoyancy of the combined flues. Recent studies have shown that plumes in line with the overall flow in the flue quickly mix and rise higher than expected. The mechanism is that the first plume shields the second from the flow, so that the latter bends less and rises into the first in a way that allows the internal vortices to mingle without destroying overall plume integrity. In contrast, plumes from stacks aligned across the flow do not merge, and rise more slowly due to mixing being hindered because the vortices at the edges of each plume tend to oppose each other.
Computational Fluid Dynamics (CFD) was employed to better understand the development and interaction of adjacent plumes emitted from a multi-flued stack for three primary wind directions over a range of wind speeds.
The plume behaviour under certain wind speed and direction regimes, as ascertained from the CFD modelling, was incorporated into the dispersion modelling by assigning a plume behaviour category to each hourly wind direction class. A corresponding theoretical stack configuration and diameter was then assigned to each hour.
Dispersion model performance with plume behaviour as ascertained by CFD modelling, shows better validation against measured values of NO2 and SO2 at both a near- and far-field receptor than modelling using default plume merging.