A numerical investigation of the interactions between adjacent cooling tower plumes

Abstract

Cooling tower plume rise, dilution and dispersion is investigated using a numerical model. Both single and double sources are considered. The main aim of the investigation is concerned with comparison of the computational results to existing wind tunnel experimental data as well as simple empirical rise height formula. Analysis of the interaction of adjacent sources, and subsequent rise augmentation compared to that of a single source, is a central theme of the work. A full-scale hybrid mechanical cooling tower is modelled as a surface mounted cuboid block 20 m high with an internal development duct of 10 m diameter. Both jet and moderately buoyant plume type sources are studied. Two exit velocity ratios are also considered. An oncoming atmospheric boundary is modelled with an associated logarithmic velocity profile and profiles of turbulence kinetic energy and length scale. Two double source orientations, tandem and side-by-side with respect to the oncoming cross wind, are studied. Physical symmetry is utilised and so only half of the domain is modelled. Both the small-scale (wind tunnel) and full-scale were modelled. The small-scale work used combinations of a low Reynolds number k-e turbulence model and both hybrid and QUICK discretisation schemes. The high Reynolds numbers encountered in the fullscale allowed the use of a number of different turbulence models, namely the standard k-e model, the RNG k-e model and a differential flux model, combined again with the hybrid and QUICK discretisation schemes. The results of a number of sensitivity tests showed that plume rise in this case was not sensitive to the turbulence model constant C3 or to source turbulence levels. A decrease in the turbulent Prandtl number led to a marked increase in the turbulent diffusion of the thermal plume. Horizontal plume spreading was underpredicted in both small and full-scales compared to the experimental data. Plume rise and dilution was, in the majority of cases, predicted accurately compared to both the experimental data and also to rise heights given by simple empirical relationships. Generally, the choice of discretisation scheme was a more important factor than choice of turbulence model. Interaction of side-by-side plumes was dominated by the interaction of the rotating vortex pairs within the plumes. A tandem source arrangement led to early merging and efficient rise enhancement. Merging into a single type plume occurred sooner with an decrease in exit velocity ratio, R.