Aerodynamic effects of large natural-draught cooling towers on the atmospheric dispersion from a stack

Abstract

The aerodynamic effects of natural-draught cooling towers on the local low-level concentration of pollutants emitted from nearby stacks were investigated in a wind tunnel. These investigations were limited to neutral atmospheric conditions. The problem results from the large capacity of modern power plants which are equipped with natural-draught cooling towers. For an effective natural vertical draught, these types of cooling tower are of a height of at least 160 m. The flow field within the atmospheric boundary layer generated by the effective surface roughness of the surrounding terrain is heavily disturbed by aerodynamic interference in the vicinity of the cooling towers. Therefore, diffusion of effluents from stacks located in the wake of such cooling towers cannot be calculated by use of the Gaussian plume model of dispersion. Information on the local field of concentration was obtained from experiments in a wind tunnel. The experiments showed that the ground-level concentrations increased heavily in the vicinity of the cooling towers, compared with those of an isolated stack of the same height and shape. Systematic parametric investigations with variations of efflux velocity and stack height were carried out to derive practical solutions for the limitation of air pollution. In a separate test, the validity of the results could be shown for an isolated stack. From this test, a new empirical formula could be found for the dynamic plume rise.