| The thermal performance of cooling tower,which is one of the most important equipments in generation station,greatly affects the safety and economy of the power plant.The natural-draft wet counterflow cooling tower(NDWCT) is the most widely used form in China,and its thermal performance is greatly influenced by environmental conditions,especially the crosswinds which may induce the air stream to flow unevenly into the tower around the circumference,may reduce the air ventilation and diminish the cooling efficiency of NDWCT.However,the adverse effect of crosswinds is not taken into consideration in most design and calculation process of cooling tower.At present,the researches about NDWCT focus mostly on the heat and mass transfer process within the tower and seldom involve into the ventilation resistance of the tower,whereas the ventilation resistance is a significant thermal parameter for the design and evaluation of cooling tower.It is found that the air deflectors installed around the air inlet of the tower could counteract the disadvantages of crosswinds,but there are few theoretical literatures and systematic investigations about the impact of air deflectors on the thermal performance of NDWCT.Hence,the ventilation resistance calculation and the thermal performance optimization of NDWCT installed with air deflectors under crosswinds conditions are investigated,by means of the model tests,orthogonal analysis,the mathematical model of the ventilation resistance,numerical simulation and the heat and mass transfer mechanism within the tower.The main researches are as follows.1.The cold state model tower that conforms to the kinematic and dynamic similarity criterions was established and tested in the wind tunnel laboratory in Shandong University.There were about 370 test conditions,and the test procedure was designed to conform to the orthogonal theory.The concept of the uniformity coefficient of the inlet air Cu was put forward to evaluate the uniformity of the inlet air flow field around the circumference,then the relationship between the deflectors installation modes(including the setting angle,quantity,shape and size of the deflectors) and Cu and the air ventilation G was studied. 2.The hot state model tower was modified,and several different windbreak models were installed to study the optimization program about the thermal performance of NDWCT.Then the relationship between the deflector's installation mode and the thermal parameters of NDWCT was investigated by means of over 300 test cases.Besides,the concept of the equivalent ventilation,equivalent heat and mass transfer coefficients were put forward to counteract the disadvantage and inaccuracy of the traditional methods caused by environmental temperature.The quasi-level method was used to perform range and variance analysis to the hot state test data,and the influencing extent of each setting parameters for deflectors was researched.3.The three dimensional mathematical model about the flow resistance of inlet air of NDWCT with deflectors subjected to crosswinds was established.Then it is combined with the three dimensional mathematical model about heat and mass transfer process within the tower and the contact resistance between air and water to numerically solve the aerodynamic field of NDWCT subjected to crosswinds. After that the numerical simulation and the related field tests were executed to a real tower of a power plant.4.The optimum installation mode for the deflectors of real tower was investigated by means of numerical simulation.Then the impact of deflector quantity N and setting angleαon ventilation G and the outlet water temperature two of real tower was discussed.The interior aerodynamic field and the temperature field of the outlet water in basin were calculated.5.The action mechanisms of some key factors to affect the tower were analysed. The concept of nondimensional contact time between water and air Tl was put forward on the basis of the research about heat transfer process within the tower. It quantifies the effective resistance time of air within the tower.Then the relationship between Tl andαis studied.6.The variation of the optimum quantity for deflectors with the tower size was investigated.The Levenberg-Marquardt method was used for the regression analysis of Nopt,and the optimum circumferential spacing for deflectors as a function of the tower size was acquired. 7.The LCS-DTL gas-diode solid laser was used to create laser sheet and visualize the flow field of the leeward air stream around the deflectors through tracer test. The flow fields of the inlet air stream around the air deflectors were investigated. After these investigations,the following conclusions are made:(1) There is an optimum quantity Nopt for the deflectors to get the maximum G and Cu.The visualization tracer test shows that in this condition,the environmental air stream could flow smoothly into the tower with a high fullness extent in the flow passages formed by two adjacent deflectors.The deviation of N from Nopt would increase the flow resistance of inlet air and reduce G and Cu.When N＜Nopt,the quantity of flow passage formed by two adjacent deflectors is Small, and G and Cu rises with the increment of N.While when N＞Nopt,small spiral vortices come into being in the flow passages and their disturbance extent intensifies as N increases,this weakens the fullness extent of the air in the flow passages and causes the flow resistance to rise.(2) The smallerαis,the higher the tangential velocity of the inlet air is.So the account of the axial velocity decreases,which intensifies the spiral vortex on the transverse section of the tower and prolongs the contact time between water and air.But the flow resistance of inlet air rises subsequently.α=70°could make G match the contact time between water and air,and get the maximum cooling efficiencyη,the equivalent heat transfer coefficient k'h and other thermal parameters.Besides,the deflectors with airfoil shape could improve not onlyηbut alsoΔQc/ΔQ,the ratio of contact heat to the gross heat rejection,and intensifies the heat transfer process.(3) The two-parameter F test indicates that the experimental errors of cold and hot state model test are small,so the test data are reliable.It lays the foundation for the research of influencing mechanism of deflectors and the calculation of air ventilation and flow resistance.(4) Both the test data and the theoretical analysis show that the effect of N on G and two is greater than that ofα.(5) The computational errors of the outlet water and air temperature are no more than 0.3℃and 0.5℃respectively,and the mean error of the radial velocity of inlet air is less than 0.3m/s.Hence,the mathematical model could reflect the distribution regularities of the inlet air and be used to simulate real tower.(6) The spiral vortex would be formed over the deflectors around the leeward air inlet if the deflectors are lower than the air inlet,which may reduce the inlet air velocity and cause two to rise.Therefore,the higher and longer the deflectors are, the betterηis.The established mathematical model lays the foundation for the design and resistance calculation of cooling tower.The concepts of equivalent ventilation and nondimensional contact time between water and air offer a new method for the evaluation of tower performance.Besides,the test results and the formula of optimum circumferential spacing for deflectors present practical instructions for the engineering modification of the tower.Hence,these conclusions are useful from the theoretical and engineering point of view.
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