Parametric CFD analysis of a natural draft wet-cooling tower with isotropic fill

In the design of a modern natural draught wet-cooling tower, structural and performance characteristics must be considered. Air flow distortions and resistances must be minimised to achieve optimal cooling which requires that the cooling towers must be modelled two- dimensionally and ultimately three-dimensionally to be optimised. CFD models in literature are limited to counterflow cooling towers packed with film fills, which are porous in one direction only and generally have a high pressure drop, as well as purely crossflow cooling towers packed with splash fill, which simplifies the analysis considerably. Many counterflow cooling towers are however packed with trickle and splash fills which have anisotropic flow resistances, which means the fills are porous in all flow directions and thus air flow can be oblique through the fill, especially near the cooling tower air inlet. This provides a challenge since available fill test facilities and subsequently fill performance characteristics are limited to purely counter- and crossflow configuration.

In this paper, a parametric study is done on a specific natural draught wet-cooling tower (NDWCT) using a CFD model developed to predict the performance of NDWCTs with any type of fill configuration. The fill performance characteristics for oblique air flow are determined by linear interpolation between counter- and crossflow fill characteristics in terms of the air flow angle. The CFD results are validated by means of corresponding one-dimensional computational model data. The objective is to investigate the effects of different air inlet and outlet geometries, air inlet heights, cooling tower inlet and outlet diameters, variations in radial water loading and fill depth, fill configurations, and rain zone drop size distributions on cooling tower performance for optimisation purposes. The results show that NDWCT performance can be enhanced significantly.