Experimental and Theoretical Investigations into Chimney Emissions

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Abstract
The simple ‘conical’ model of plume dispersion from an elevated source of effective height H (m) which indicates that the maximum ground-level concentration Cm (units m-3) is proportional to Q (rate of emission, units s-1) (σZ/ày)/ U (wind speed, ms-1) X H2 assumes that the vertical (σz) and cross-wind (σ y) spreads of plume material are similar functions of distance downwind For time average values of Cm of duration about 1 h, the length scale of the turbulence responsible for the cross-wind spread is, in general, much greater than that responsible for the vertical spread. This length *** l is restricted either by the depth h of the boundary layer or the height above the ground. In this case (σz /σy) in the expression for Cm must be replaced by (Some representative vertical turbulent velocity * l)/(Some representative cross-wind turbulent velocity X H) ’*** In conditions of strong thermal convection and light winds the turbulent vertical velocities are effectively independent of the wind speed and so the form of the first expression for Cm will change both with wind speed, atmospheric stability and the height of the plume in relation to the top of the boundary layer. Simple boundary-layer models for ‘convective’ and ‘windy’ conditions are shown to lead to equations for predicting Cm which are similar to those previously shown by the author to give a good representation of the ground-level concentrations in all categories of wind speed and stability observed on 2500 separate occasions in the Tilbury-Northfleet plume rise and dispersion experiment. The application of these expressions to other locations and sizes of plant is discussed.

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