| Natural draft wet cooling tower (NDWCT) has been widely used in electrical thermal power stations, and its performance affected by fill, cooling water, meteorological conditions and cooling tower structure. The air flow field in the NDWCT changed when crosswind exists, therefore, affects the heat and mass transfer in the tower as well as outlet water temperature. Now, some scholars make research about NDWCT under crosswind and put forward a scheme that optimizing the cooling tower performance under crosswind by installing windbreak wall at the air inlet of the tower. However, the analyses of influence mechanism of crosswind on cooling tower performance and optimization of different number of windbreak wall on cooling tower performance are insufficient.A three dimensional numerical simulation model based on heat and mass transfer theory and Computational Fluid Dynamics (CFD) software FLUENT has been developed. Discrete phase model (DPM) has been adopted in the spray and rain zones, however the fill zone has been divided into10sub-zones which loaded with mass, momentum, energy and species source terms calculated from Poppe model by User Defined Functions (UDFs). The model has utilized the standard k-ε turbulence model as the turbulence closure. The model is used to investigate air flow field, outlet water temperature, heat and mass transfer of sub-fill zones of NDWCT under different crosswinds, therefore, proposing the influence mechanism of crosswind on cooling tower performance. The best equipped scheme of windbreak wall is determined by studying the air flow field and performance of cooling tower with different windbreak walls. The paper investigates the effects of water inlet temperature, water mass flow rate, air temperature, air relative humidity and fill depth on air flow rate and outlet water temperature, and determines the effect degree of the factors, which lay foundation for optimizing cooling tower performance by variety ways.The results show as following:(1) There is a low-pressure zone in the rain zone and an axial vortex behind the air inlet when crosswind is0m/s. When crosswind increasing, the low-pressure zone develops into a radial vortex and the impact range of the vortex increases, and the vortex shifts outside of the tower when crosswind is13m/s. The axial vortex impact range increase with the increasing of crosswind, which compresses the axial ventilation area and hinders the axial air flow. On the other hand, bigger inertia force caused by higher crosswind (≥13m/s) makes transverse wind in the rain zone and enhances the heat and mass transfer of it. Moreover, the air speed and cooling capacity of sub-fill zones verify the reliability of the theory.(2) The air flow rate decreases and outlet water temperature increases with crosswind increasing from0to7m/s. Compared with no crosswind, the air flow rate of7m/s decreases25%, however the outlet water temperature has an increase of1.284K. When crosswind surpasses7m/s, cooling tower performance improves, even the outlet water temperature of crosswind equal to13m/s lower than no crosswind.(3) Installing60windbreak wall at the inlet of the tower is the best optimization scheme. At crosswind7m/s, compared with no windbreak wall, a reduction of1.066K of the outlet water temperature achieved when windbreak wall is60.(4) Under the general operating conditions, the air temperature has the greatest effect on the performance of cooling tower, and then are the water temperature and the water mass flow rate, the relative humidity has a lower influence. As the main heat and mass transfer zone, the depth of fill zone has a higher influence on cooling tower performance.The paper illustrates the influence mechanism of crosswind on the performance of NDWCT and determines the optimization installation scheme of windbreak wall. Finally, by analyzing the influence of different factors on the performance of cooling tower, the paper lays foundation for optimizing cooling tower performance by variety ways. |
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