Evaluation of drop break-up after impingement on horizontal slat grids and the effect of drop size of cooling tower rain zone performance

[摘要] ENGLISH ABSTRACT: Natural draught wet-cooling tower rain zone performance can be significantlyenhanced by reducing the mean drop size in the rain zone with the installation ofspecially designed grids below the cooling tower fill. Drops enter the rain zone in theform of a polydisperse drop distribution, dripping from below the cooling tower fill,comprising relatively large drops. In order to design and optimize a grid for breakingup these drops, the mechanisms of drop break-up after impingement on the gridsurface, referred to as splashing, straddling and dripping, need to be clearlyunderstood. Two of these mechanisms, splashing and straddling, are thereforeinvestigated experimentally using high speed video cameras to measure initial dropsizes, mass fractions and drop size distributions after impingement on differenthorizontal slats covered with a thin layer of water. The following parameters arevaried independently for these experiments: drop fall distance, initial drop size, slatwidth and the water film thickness on the slats. Dripping from below the grid, isinvestigated theoretically. The effect of drop interaction on the drop size distributionin the rain zone is also investigated experimentally by measuring the drop distributionsat the top and bottom of rain zones with a height of approximately 7.05 m to 7.65 mfor different inlet distributions. The experimental drop break-up data, numericallyobtained splash drop trajectory data and drop interaction data found in literature areused to develop a theoretical model of a purely counter flow cooling tower rain zonewith and without installed grids. The model is compared to experimental data andtheoretical data from literature and the predicted thermal and dynamic behaviour of therain zone are generally found to be in good agreement with these results. Ultimately,this model is used for the optimization of the grid layout in terms of variables such asdistance between the grid and the fill, slat width, slat spacing and slat height. It isfound that the best drop break-up is achieved for grids comprising narrower slats withlower grid porosities as opposed to grids comprising wider slats. For the determinedoptimal grid layout it is found that a significant improvement in cooling towerperformance can be achieved.

[发布机构] Stellenbosch University