Performance evaluation of natural draught cooling towers with anisotropic fills

["Wet-cooling tower", "Anisotropic fill", "Natural draught", "Computational fluid dynamics", "Dissertations -- Mechanical engineering", "Theses -- Mechanical engineering", "Cooling towers"]

Reuter, Hanno Carl Rudolf ; Kroger, D. G.

Stellenbosch University

Others  :  https://scholar.sun.ac.za:443/bitstream/10019.1/5440/2/reuter_performance_2010.pdf

瑞士|英语

来源: Stellenbosch University

【 摘 要 】

ENGLISH ABSTRACT: In the design of a modern natural draught wet-cooling tower (NDWCT), structuraland performance characteristics must be considered. Air flow distortions andresistances must be minimised to achieve optimal cooling which requires that thecooling towers must be modelled two-dimensionally and ultimately threedimensionallyto be optimised. CFD models in literature are found to be limited tocounterflow cooling towers packed with film fill, which is porous in one directiononly and generally has a high pressure drop, as well as purely crossflow coolingtowers packed with splash fill. This simplifies the analysis considerably as theeffects of flow separation at the air inlet are minimised and fill performance isdetermined using the method of analysis originally employed to determine the fillperformance characteristics from test data. Many counterflow cooling towers are,however, packed with trickle and splash fills which have anisotropic flowresistances, which means the fills are porous in all flow directions and thus airflow can be oblique through the fill, particularly near the cooling tower air inlet.This provides a challenge since available fill test facilities and subsequently fillperformance characteristics are limited to purely counter- and crossflowconfiguration.In this thesis, a CFD model is developed to predict the performance of NDWCTswith any type of spray, fill and rain zone configuration, using the commercialcode FLUENT®. This model can be used to investigate the effects of different:atmospheric temperature and humidity profiles, air inlet and outlet geometries, airinlet heights, rain zone drop size distributions, spray zone performancecharacteristics, variations in radial water loading and fill depth, and fillconfigurations or combinations on cooling tower performance, for optimisationpurposes. Furthermore the effects of damage or removal of fill in annular sectionsand boiler flue gas discharge in the centre of the tower can be investigated.The CFD modelling of NDWCTs presents various options and challenges, whichneeded to be understood and evaluated systematically prior to the development ofa CFD model for a complete cooling tower. The main areas that were investigatedare: spray and rain zone performance modelling by means of an Euler-Lagrangianmodel; modelling of air flow patterns and flow losses; modelling of fillperformance for oblique air flow; modelling of air pressure and temperatureprofiles outside and inside the cooling tower.The final CFD results for the NDWCT are validated by means of correspondingone-dimensional computational model data and it is found that the performance oftypical NDWCTs can be enhanced significantly by including protruding platformsor roundings at the air inlet, reducing the mean drop size in the rain zone, radiallyvarying the fill depth and reducing the air inlet height.