Crosswind Impacts On The Aerodynamic Field And Thermal Characteristics Of High-level Water Collecting Cooling Tower And Relevant Optimization Mechanisms

Usual natural draft wet cooling tower(UNDWCT),as a direct heat and mass transfer equipment with high efficiency,has been widely adopted in large-scale power plants.While,High-level water collecting natural draft wet cooling tower(HNDWCT)is popular in China since 2010,because of its inherent energy saving and noise reduction merits.In order to improve the efficiency of unit generating electricity,HND WCT is required to discharge a large amount of waste heat in a short time,so even small improvements on its cooling performance are deserved and valued.The adverse effect of ambient crosswind has always been a huge obstacle for the efficient operation of cooling towers,while the aerodynamic field in the HNDWCT is closely related to the crosswind conditions.Therefore,this paper focuses on the study of the aerodynamic field in HNDWCT and its influence on the heat and mass transfer process of tower under crosswind conditions.Considering the advantages and disadvantages of each research method,the numerical simulation has been adopted in this paper.The results demonstrate that the cooling performance of HNDWCT decreases with the increase of crosswind speed,especially at high crosswind speed.Furthermore,the cooling performance decreases with the increase of the crosswind incidence angle at relative low crosswind speed,while the cooling performance increases with the increase of incidence angle at relative high crosswind speed.It has been found that the variation of the incidence angle and speed of crosswind causes some air flow phenomena,which are adverse to the heat and mass transfer of the HNDWCT,such as:the cross ventilation below the high-level water collecting devices(HWCDs),the high-speed transverse vortex behind the water collecting gutter,the longitudinal vortex around the critical heat and mass transfer zones at the windward side and the uneven distribution of airflow on the windward side and the leeward side inside the tower.Then,the longitudinal vortex at the inner periphery of HNDWCT has been proved a critical factor to reduce its cooling efficiency,whose impacts on the tower aerodynamic fields and heat and mass transfer processes have been analyzed under various crosswind conditions.It has been observed that the longitudinal vortex around the critical heat and mass transfer zones of HNDWCT causes the inner hot humid air recirculation(IHHAR),resulting in the degradation of the temperature difference and evaporation rate there.To describe the relative scale of longitudinal vortex,the concept and formula of IHHAR rate have been put forward.At the same time,the expression of IHHAR rate based on the air temperature has been deduced,which is convenient for the field test.Moreover,four optimization guidelines have been presented to cope with the adverse impacts of the vortex.Then,a new natural draft wet cooling tower(NNDWCT)with different water collecting modes at its periphery and center has been proposed to verify the feasibility of these guidelines.The results show that the longitudinal vortex in the NNDWCT is relatively small and the exit water temperature in its peripheral zone is significantly reduced,so that the cooling performance of the NNDWCT is relatively high.To mitigate the adverse effect of crosswind on tower cooling performance,cross wall has been investigated as a means to improve tower air flow structure.It has been found that the cross wall with appropriate porosity is beneficial to increase the tower efficiency.While,the optimum porosity is negatively correlated with the crosswind speed and positively correlated with the crosswind incidence angle,which ranges from approximately 0.33 to 0.53.Furthermore,the structure of heat and mass transfer zones of HNDWCT,especially the rain zone,are different with that of UNDWCT,which causes the different effect of impermeable cross wall.In this paper,the effects of crosswind direction and IHHAR on the cooling performance of HNDWCT have been systematically studied for the first time,which enrich the basic research on the cooling characteristics of HNDWCT.In addition,the formation and optimization mechanism of a variety of undesirable phenomena in the tower have been studied and some optimization guidelines and specific measures have been put forward,which provide reference for the optimization of the cooling performance of HNDWCT under crosswind impacts.