Mechanism Research Of Crosswind Effect On Heat And Mass Transfer In Natural Draft Counterflow Wet Cooling Tower

As an effective cooling equipment, natural draft wet cooling tower (NDWCT) is used widely to cool circulating water for power plant thermodynamic system, and has a direct and important impact on the safety, economy and effectiveness of power plant cold end system. The complex air-water heat and mass transfer is subject to ambience factors such as ambient dry air temperature, relative humidity and ambient crosswind. Crosswind can destroy the axisymmetrical aerodynamic field in NDWCT at windless ambience, and would cause the asymmetric circumferential distribution of air velocity around tower air inlet, so as to deteriorate NDWCT heat and mass transfer, and cause unfavorable impact on the high economical run of power plant thermodynamic system. The heat and mass transfer zone of NDWCT is composed of spray zone, fill zone and rain zone. The impact mechanism of crosswind on each zone air-water heat and mass transfer is different from each other for differences in location and structure. To clarify the effect mechanism of crosswind on NDWCT air-water heat and mass transfer and improve NDWCT cooling performance effectively, it is very necessary to probe into the impact mechanism of crosswind on the heat and mass transfer in each zone systematically.The three-dimensional (3D) numerical computation model for NDWCT was established here to carry out the 3D numerical analyses for NDWCT based on CFD code FLUENT. The aerodynamic field and the intensity of air-water heat and mass transfer in NDWCT were analyzed numerically under crosswind so as to investigate the effect mechanism of crosswind on the heat and mass transfer in each zone and the total cooling performance of NDWCT. The heat and mass transfer mechanism in each zone was also studied when wind-control scheme was adopted, such as air-leading plate, cross-wall, etc. The thermal-state model experiment and field experiment were carried out for NDWCT to validate the established 3D numerical computation model, and also provide field experiment data for the establishment of NDWCT cooling performance evaluation method under crosswind conditions. Then the main research efforts are as follow:(1) Establishment of the perfect air-water heat and mass transfer mathematical model for NDWCT. Based on the fundamentals of air-water heat and mass transfer, the similarity between air-water convective heat transfer and convective mass transfer was analyzed, and the shortcomings of traditional heat and mass transfer mathematical model for NDWCT were also discussed. Both the traditional one-dimensional (1D) model and two-dimensional (2D) model can not consider the impact of ambience crosswind. The traditional 3D model substituted water droplet for water film in fill zone, and then debase the accuracy of computation results under different crosswind conditions. This paper corrected the computation method of moist air and water physical parameters for NDWCT, considered fully the effect of the 3D distribution of air and water parameters such as air temperature, air velocity, air moisture, water temperature and water drop velocity, introduced the Lewis factor Le_f to realize the 3D correlation between air-water convective heat transfer coefficient and convective mass transfer coefficient in fill zone, computed the convective heat transfer coefficient and mass transfer coefficient between moist air and water droplet in spray zone and rain zone, and then established the perfect 3D mathematical model for NDWCT air-water heat and mass transfer under crosswind conditions, which provided theoretical fundamentals for the mechanism research of crosswind impact on NDWCT air-water heat and mass transfer.(2) Establishment of the 3D numerical computation model for NDWCT heat and mass transfer analyses under crosswind conditions. Based on the above 3D air-water heat and mass transfer mathematical model under crosswind conditions, the 3D numerical computation model for NDWCT was established. The standard k-e turbulence model and wall function were adopted to enclose the 3D turbulence governing equations and treat the wall boundary conditions for air flow, respectively. Referring to field experiment data, the validity of the established 3D numerical computation model for NDWCT was validated. To make relevant analyses about crosswind impact have universal significance, the grid-independence check for numerical computation result was carried out to get mesh-independent solution, and the effect of key parameters on numerical computation result were also investigated such as crosswind profile index, water droplet equivalent. The investigation shows that the established 3D numerical computation model for NDWCT heat and mass transfer analyses can study the impact mechanism of ambient crosswind on NDWCT heat and mass transfer.(3) Research of the impact mechanism of ambient crosswind on NDWCT air-water heat and mass transfer. Referring to field experiment data, the impact mechanism of crosswind on NDWCT air dynamic field was investigated through numerical computation method. The introduction of some new concepts such as transverse air mass flowrate, longitudinal air mass flowrate and total air mass flowrate established theoretical analysis conditions for investigation about crosswind impact on heat and mass transfer in NDWCT each zone. Through the above analyses, it can be seen that under low speed crosswind, fill zone is the main heat and mass transfer zone of NDWCT, and the longitudinal air mass flowrate decreases rapidly with the increase of crosswind speed and reduces the heat and mass transfer intensity in fill zone. High speed crosswind increases transverse air mass flowrate and total air mass flowrate greatly so as to enhance the intensity of rain zone heat and mass transfer and make rain zone contribute greatly to the total cooling performance. The heat and mass transfer in spray zone is less sensitive to ambient crosswind than fill zone and rain zone for the synthesized effect of heat and mass transfer coefficients and drive forces. The impact of ambient crosswind on NDWCT total cooling performance is the synthesized impact of crosswind on each heat and mass transfer zone. From computation analyses, it can be known that ambient crosswind causes unfavorable effect on NDWCT total cooling performance. With the increase of crosswind speed, the total water temperature drop decreases firstly and then increases. Under different running conditions, the impact mechanism of crosswind on cooling tower cooling performance was investigated further. The results show that the impact trends of crosswind on NDWCT total cooling performance are similar under different running conditions.(4) Research of the impact mechanism of crosswind on the aerodynamic field at NDWCT air inlet. From the crosswind impact on the circumferential distribution of radial pressure gradient and radial air velocity at NDWCT air inlet, the reason why longitudinal air mass flowrate reduces and the transverse air mass flowrate increase increases was studied. Based on the crosswind impact on the circumferential distribution of air radial inflow velocity at air inlet, the concept of air inlet air inflow relative deviation degree was presented to measure the impact of crosswind on the aerodynamic field at NDWCT air inlet and on the relevant air mass flowrate quantitatively. From relevant numerical computation analyses, it can be known that under crosswind impact, the larger air inflow relative deviation degree at NDWCT air inlet make the circumferential of air radial inflow velocity more asymmetric at NDWCT air inlet, and then make the transverse air mass flowrate larger, and result in the reduction of the longitudinal air mass flowrate. Thus the impact mechanism of crosswind on the heat and mass transfer in NDWCT fill zone and spray zone was revealed fundamentally from the above relevant analyses.(5) Research of the effect mechanism of air-leading plate scheme on NDWCT air inlet aerodynamic field and its cooling performance under crosswind conditions. Referring to field experiment data, The numerical computation method was adopted to investigate the effect of air-leading plate scheme on NDWCT longitudinal air mass flowrate, transverse air mass flowrate, total air mass flowrate, air inlet air inflow relative deviation degree, each zone heat and mass transfer characteristics and total water temperature drop etc. The above analyses point out that air-leading plates can improve the aerodynamic field around NDWCT air inlet, decrease the air inflow relative deviation degree, reduce the transverse air mass flowrate, enhance the longitudinal air mass flowrate, intensify the heat and mass transfer in fill zone under crosswind conditions, so as to improve the total cooling performance of NDWCT effectively under crosswind conditions.(6) Research of the effect mechanism of cross-wall in rain zone on NDWCT cooling performance. Referring to field experiment data, the numerical computation method was adopted to investigate the effect mechanism of different type cross-wall, at different installation position, on NDWCT cooling performance under crosswind conditions. The optimum cross-wall installation position and the optimum cross-wall form were given through the above investigation. The above analyses denotes that under low speed crosswind conditions, cross-wall can intensify each zone heat and mass transfer and improve NDWCT cooling performance. Under high speed crosswind, cross-wall causes unfavorable effect on rain zone heat and mass transfer. When the angle between crosswind direction and the windward first wall plate of cross-wallθ_cw is 0°, the cross-wall can improve the total cooling performance of NDWCT under high speed crosswind. Whenθ_cw is 45°, the total cooling performance of NDWCT decreases under high speed crosswind, but the gab-cross-wall can alleviate the unfavorable effect of no-gab-cross-wall. So at area with high year-average ambient crosswind speed, the gab-cross-wall should be adopted to alleviate the unfavorable effect of no-gab-cross-wall for variable crosswind directions, and the windward first wall plate of cross wall should be parallel to the year-average ambient crosswind direction as much as possible.(7) Research of the synthesized effect of air-leading plates and cross-wall on NDWCT cooling performance under crosswind conditions. Based on field experiment data, the synthesized effect of air-leading plates and cross-wall was analyzed from numerical computation results. For NDWCT with gab-cross-wall, air-leading plates can intensify the total cooling performance of NDWCT. For NDWCT with air-leading plates, the effect of gab-cross-wall on the total cooling performance of NDWCT is similar to the situations for NDWCT without air-leading plates.(8) Thermal state model experiment research and field experiment research for NDWCT under crosswind conditions. The impact trends of crosswind on the cooling performance of thermal state model tower under different conditions are compared with relevant trends of real cooling tower numerical computation analyses to verify the validity of relevant numerical computation analyses of cooling tower under crosswind conditions. Through field experiment and numerical analyses for NDWCT with air-leading plates, the validity of the above 3D numerical computation model NDWCT is validated further. Compared with field experiment for cooling tower without air-leading plates under same ambience condition, the experiment for air inflow relevant deviation degree shows that the aerodynamic field of cooling tower with air-leading plates is optimized by air-leading plates.(9) Establishment of NDWCT cooling performance evaluation theoretical model under crosswind conditions. The conventional performance evaluation methods for NDWCT were analyzed, which could not evaluate NDWCT cooling performance correctly under crosswind conditions. The definition methods for NDWCT cooling number, cooling characteristic number, equivalent air mass flowrate and total resistance coefficient were investigated under crosswind conditions. Based on the concept of form resistance, the resistance coefficient of NDWCT was analyzed for each part separately, and the flow field structure resistance coefficient of NDWCT was presented which was relevant to the aerodynamic field of NDWCT under crosswind condition. And then the cooling performance evaluation theoretical model for NDWCT under crosswind conditions was established based on the above analyses, and the performance evaluation index was also presented to realize the NDWCT performance evaluation under same ambience conditions. Referring to field measured data and DCS measured data for two neighbor NDWCTs, the cooling performance evaluation theoretical model for NDWCT was used and examined under different conditions.These conclusions clarify the impact mechanism of ambient crosswind on the heat and mass transfer performance in spray zone, fill zone and rain zone of large-scale NDWCT, may provide theoretical foundation for further intensifying NDWCT air-water heat and mass transfer and improving NDWCT cooling performance under ambient crosswind conditions, and also present a new method for NDWCT scientific cooling performance evaluation under ambient crosswind conditions.