Numerical Calculation Research On Cooling Tower Rain Zone Performance In Power Plants

Natural draft counter-flow wet cooling tower is a kind of stable and efficient cold end equipment which is widely used in power plants. It’s of great importance to do precise research and calculation for cooling tower because its performance has great influence on the safety and economy of the power plant. Computational Fluid Dynamics (CFD) is an important tool of cooling tower researches which can provide prediction and guidance for design and experiment and has high eonomy and accuracy.In the rain zone of a cooling tower, large amounts of water droplets fall in the air flow, the phenomena during which are very complex. Besides, the droplets have a significant proportion of heat and mass transfer and airflow resistance in the whole tower. It will greatly improve the rationality and reliability of numerical simulation results to build accurate model for rain zone. However, rain zone model tended to be overly simplified in past researches due to its complexity or the limitations of computational methods, which would certainly produce errors. With the process of science and technology, there are more accurate formulas in water droplet movement studies, and the development of CFD also provides adequate support for the combination of these new achievements and rain zone simulation.In order to further improve the rain zone computational model, three-dimensional numerical model of wet cooling towers is built with CFD software FLUENT, in which the spray zone and rain zone are built with Discrete Phase Model (DPM) while the fill zone is expressed by means of User Defined Scalars (UDS) and source terms. The data of the three zones is connected and transferred using User Defined Functions (UDF), thus completely expressing the actual physical processes of air-water two-phase flow in cooling towers. The accuracy of the numerical model is confirmed by the measured data of real towers.The two common assumptions in past rain zone researches, namely regarding the droplets as spheres and the diameter as monodisperse, are discussed and revised. Several different drag laws are introduced into simulation for comparison, and results show that considering droplets deformation will make a big difference. Then the influence of the droplet equivalent diameter on the results is analysed, and it’s found that using monodisperse distribution can not make all indicators ideal. On this basis, two probability distribution functions supported by FLUNET are introduced into simulation by means of UDF, and it shows that using polydisperse distribution can obtain more reasonable results. According to the above conclusions, suggestions about cooling tower numerical simulation are made which have certain guiding significance for future researches.