On plume rise within a convective boundary layer

Abstract

Photographic observations of buoyant emissions from a model stack within a laboratory convectively mixed layer provide the data base from which plume envelopes and centerline heights are determined. Our measurements suggest that where X = (w ∗/U)x/h exceeds a few tenths, the parameter F ∗ = F/(Uw 2 ∗h) is an appropriate dimensionless measure of the plume buoyancy. Here, πF is the flux of buoyancy from the stack, U is the mean wind speed, w ∗ is the ambient convective velocity scale, h is the height of the mixed layer and x is the downwind distance from the stack. F s* must exceed a value near 0.02 for effects of plume buoyancy to become appreciable. The two-thirds power law plume rise formula is found to progressively overestimate the plume centerline rise rate except close to the source. This discrepancy is attributed to plume dilution due to ambient turbulence. The downwind distance at which ambient turbulence leads to plume ‘breakdown’ is estimated by noting the location of a sharp increase in plume diameter growth rate. The larger the value off, the greater the value of X at which the mean plume lower edge first touches the ground. For F ∗∼- 0.1 a significant portion of the effluent is temporarily trapped near the top of the mixed layer, resulting in a highly skewed vertical distribution of pollutant concentration for X exceeding about 1. The Briggs ‘touchdown’ equation adequately describes the ground-level touchdown distance for 0.01< F ∗ < 0.1 when w d w ∗ assumes a value of about 0.6, where w d is the average convective downdraft speed.