Optimization And Reconstruction Of The Aerodynamic Field In Wet Cooling Tower Based On Wind Control And Guide Mechanism

At present, natural draft counterflow wet cooling tower (NDWCT) is the most widely used type of cooling towers in the electric power industry of China. The NDWCT performance has a direct and significant impact on the safe and economic operation of the power station unit. The NDWCT performance is influenced by a number of factors such as tower structure, environmental parameters, and circulating water parameters. Increasingly attention has been paid to crosswinds effect on NDWCT cooling performance. Many wind control measures have been proposed to eliminate the adverse effect of crosswinds, such as crosswalls and air deflectors. But so far, there hasn’t been systematic and theoretical research on impacting mechanism of crosswinds and countermeasures. In addition, the scale of NDWCTs is increasing quickly day by day, so the air and water parameters distribution inside NDWCTs becomes rather nonuniform even under windless conditions. And it’s helpless to NDWCT performance by adjusting the arrangement of circulating water and fill thickness distribution. Thus, it is necessary to study the relation between air and water distribution and cooling performance, and put forward reasonable and effective optimization measures.In view of this, crosswinds effect on NDWCT thermal performance under variable conditions, action mechanism of crosswalls and air deflectors, and optimization and reconstruction of aerodynamic field in NDWCTs were investigated by means of hot model experiment, orthogonal analysis,3D numerical simulation and aerodynamic field optimization theory analysis. The main research efforts are as follows.(1) Improvement of the NDWCT hot model test bed. The test bed was isolated into a small confined space with double hollow noise insulation glass. In addition, air conditioning and humidification devices were installed in the little room Uniting the existing crosswind simulation fans, the original test bed transformed into a full-featured hot model bench with variable circulating water parameters, crosswind velocity and air thermalphysical properties. This way, an environment simulation system was established with controllable air temperature, humidity and crosswind velocity. It can reduce the experimental error caused by environmental parameter fluctuations, enhance vertical and horizontal comparability of the experimental results, and can also be used to study the impact of environmental parameters changes on NDWCT performance.(2) First design of the directly measuring system for NDWCT air flowrate in the hot model test. Ventilation ducts were installed above the model tower outlet, to lead wet air directly to the outdoor. Precise pitot tube was installed in the ventilation ducts to measure the air velocity, and then the air flowrate was obtained. An optimization design calculation was performed to minimize the resistance of the ventilation ducts. An induced draft fan was installed at ventilation duct outlet to compensate for the pumping power to offset the additional resistance caused by the installation of ventilation ducts. And the compensation value was determined by the pressure feedback at the model tower outlet. The system can directly measure the actual air flowrate of the cooling tower, and the cross wind effect on air intake performance of NDWCTs.(3) In order to systematize the work done by the previous studies and this paper, an NDWCT optimal aerodynamic field theory was proposed as follows. Under a certain air flowrate conditions, when the air velocity, temperature and humidity field is the same everywhere within every effective heat transfer cross section in NDWCTs, the heat and mass transfer driving force between air and water will be the same everywhere, the heat exchange intensity between air and water will be uniform, thus, the NDWCT will obtain the optimum performance. In this case, the air velocity, temperature and moisture field inside the cooling tower is defined as optimal aerodynamic field of NDWCTs. Any measures approaching optimal aerodynamic field can improve NDWCT performance.(4) Based on the NDWCT hot model test bed, crosswinds effect on NDWCT thermal performance was investigated under different ambient temperature, circulating water flowrate and inlet water temperature. The conclusions are as follows. There exists a critical crosswind vebcity vcr, which results in the minimum NDWCT performance under crosswinds. Thus, when vc is lower than vcr, the cooling performance decreases gradually. When vc is higher than vcr, the cooling performance will gradually improve. Crosswind Froude number was defined as Frc=vc/(?), and this experiment meet the Frc similarity. To keep the same NDWCT operating state, the variation of vc should be proportional to (?). When the ambient temperature decreases, the circulating water flowrate and the inlet water temperature increases, the cooling tower draft increases, and critical crosswind velocity increases correspondingly. Broadly speaking, the environmental parameters, circulating water parameters and structural parameters of the cooling tower all can lead to critical the crosswind speed changes. Nondimensionalize vc into the ratio of vc and the airflow velocity vf at the fill cross section, denoted by the dimensionless crosswind velocity Vc. The experimental results show that Vcr=4, that is, if Vf is known, vcr would be obtained.(5) Crosswinds effect on aerodynamic field in NDWCTs was investigated by means of testing NDWCT circumferential intake air velocity, analyzing inlet air uniformity and comparative analysis between the measured and calculated air flowrate. The conclusions are as follows. The inlet uniformity coefficient Cu,v was defined to describe the circumferential air intake uniformity of the cooling tower quantitatively, and evaluate the merits of circumferential distribution ofNDWCT aerodynamic field. Under a certain air flowrate, the larger Cu,v is, the more balanced the tower aerodynamic field is, the more uniform heat and mass transfer intensity is, the better the overall cooling performance is. On the contrary, the smaller Cu,v is, the greater difference the tower aerodynamic field has, and the lower cooling efficiency is. Comparing Ge with G, it is found that when vc<vcr, the centrosymmetric aerodynamic field is destroyed, the inlet air uniformity deteriorates, Cu,v decreases, at this moment, there is no air outflow at tower air inlet, and little difference between Ge with G, both of which decreases; when vc≥vcr, Cu,v has a further reduction, cross ventilation appears at the leeward side, Ge continues decreasing, but as cross ventilation has beneficial cooling effect on the water of rain zone, it is translated into part of G, such that G has some recovery, which indicates that cross ventilation can improve NDWCT performance under high crosswinds. By comparison, the relative variation of η/η0is generally less than that of GIG0. This shows that crosswinds not only reduce the air flowrate, but also destroy the centrosymmetry aerodynamic field inside NDWCTs, which jointly result in the deterioration of the cooling performance.(6) Action mechanism of crosswalls and air deflectors effect on NDWCT thermal performance is investigated, and the conclusions are as follows. Installing crosswalls in the rain zone can optimize NDWCT aerodynamic field, increase the air flowrate, and thereby improve NDWCT thermal performance effectively, which depends on crosswind velocity, crosswall shape and setting angle. Under low crosswinds, solid and porous crosswalls can both enhance cooling performance, while porous crosswalls have better effect at high crosswind velocities. The cooling performance at α=0°is better than that at α=45°. What’s more, α=45°crosswalls can even reduce NDWCT performance under higher crosswinds. Installing air deflectors at tower inlet circumferentially can enhance the inlet air uniformity and air flowrate effectively. Cu,v increases along with air deflector quantity, and the cooling performance is improved correspondingly. But when the air deflector quantity increases to a certain number, the air flowing resistance increases sharply, which results in a reduction of air flowrate. Thus, there is an optimum air deflector quantity Nb,opt=36, which leads to a greater air flowrate, better inlet air uniformity and the optimum cooling performance. Installing both crosswalls and air deflectors can effectively improve the air flowrate and enhance the inlet air uniformity, to further enhance the cooling performance.(7) Concerning the nonuniformity of air velocity, temperature and humidity field inside NDWCTs, the idea of reasonable air distribution was put forward, and air ducts were suggested installing in the rain zone to reconstruct aerodynamic field and improve cooling performance. Action mechanism of air ducts effect on NDWCT thermal performance was investigated through hot model test. The conclusions are as follows. The air temperature uniformity coefficient Cu,θ is defined to describe the air temperature uniformity inside NDWCTs quantitatively, and to be an evaluation index of the tower aerodynamic field. Under a certain air flowrate, the larger Cu,θ is, the better cooling performance is. Installing air ducts can reduce air temperature in the central zone, elevate air temperature in the peripheral zone, enhance radial uniformity of air temperature, increase Cu,θ and improve cooling efficiency. The relative variation of cooling efficiency is generally larger than that of air flowrate before and after installing air ducts. This indicates that the improvement of cooling performance is the result of joint action by air flowrate and aerodynamic field. From great to little, the effect of improving cooling performance is guide duct (GG), circular duct (YG) and square duct (FG) respectively. The improving effect of air ducts depends on combined action of the heat transfer enhancement in the central zone and the cooling capacity of the rain zone occupied by air ducts. Under certain Ag, Lg and Ng, when the air and water parameter distributions approach to perfect uniformity, and the aerodynamic field reaches the optimum state, the cooling tower has the best performance, corresponding to the optimal layout GG_A45_L180_N8. In addition, other optimized layouts are also investigated corresponding to the maximum value of the cooling performance, including maximum air duct sectional area Ag,opt, length,opt and number Ag,opt under certain other conditions. When the other two parameters of the air duct increase, the corresponding Ag,opt, Lg,opt and Ng,opt all have a decreasing trend. Nondimensionalize the air duct size parameters, and the experimental results can be applied to the real tower. Air deflectors can improve the circumferential air intake uniformity of the cooling tower, and air ducts can balance radial distribution of air velocity, temperature and humidity inside NDWCTs. They can optimize the circumferential and radial distribution of tower aerodynamic field and reconstruct tower optimal aerodynamic field, to improve the overall cooling performance.(8) The hot model test data of air ducts was orthogonalized, and range analysis and variance analysis was performed to obtain the effect of Ag, Lg, Ng and vc on NDWCT performance parameters quantitatively. The results are as follows. From great to little, the influence on Cu,θ, G and η is vc, Ng, Ag and Lg respectively. The F significance test results show that vc, and Ag have very significant effect on η, Ng also have a certain effect on η, and Lg has no significant effect, but concerning that the error variance contains the interaction of various factors, so the experimental measurement error effect on the experimental results fluctuations is very little and the results are accurate.(9) The3D numerical model for analyzing heat and mass transfer and flow field in NDWCTs with air ducts and deflectors was established. And a numerical simulation was performed based on actual operating data of a power plant cooling tower. The conclusions are as follows. The grid system with762318grids refined in heat and mass transfer zone can eliminate the effect of grid quantity on calculation results and obtain grid-independent solutions. The maximum deviation between calculated and measured outlet water temperature is0.15℃, which is1.58%of the corresponding measured water temperature drop, indicating that the proposed model can accurately simulate and predict the actual operation of the tower. Under windless conditions, there is great difference between the air-water ratio, air temperature and humidity distribution of the central zone and the peripheral zone, and the aerodynamic field is very uneven, resulting in smaller air temperature uniformity coefficient and higher outlet water temperature. Installing air ducts can increase the air-water ratio of the central zone, and reduce the air temperature and humidity, resulting in higher air flowrate and temperature uniformity coefficient and lower outlet water temperature. The optimal arrangement of air ducts and the maximum parameters Ag,opt, Lg, opt and Ng,opt was obtained, and the numerical results are consistent with hot model test data after nondimensionalization. Under crosswind conditions, installing air deflectors at tower inlet can eliminate the vortex generated in the air duct at tower lateral side, further equalize air and water parameters distribution in the tower, to optimize the tower aerodynamic field, improve air flowrate and temperature uniformity coefficient, finally reduce outlet water temperature.This paper presents the NDWCT optimal aerodynamic field theory, and defines inlet air uniformity coefficient and temperature uniformity coefficient as evaluation indices for aerodynamic field inside NDWCTs. Besides, cooling tower aerodynamic field reconstruction theory and methods are put forward, and optimal layouts of air deflector s and air ducts are obtained to provide practical guidance for the cooling tower performance optimization, and lay a theoretical foundation for further study. The conclusions have a strong theoretical significance and engineering value.