【作者】 赵元宾；

【导师】 孙奉仲；

【作者基本信息】 山东大学 ， 热能工程， 2009， 博士

【摘要】 自然通风逆流湿式冷却塔作为有效的冷却设备,广泛用于电站热力系统循环水的冷却,其冷却性能直接影响电站冷端系统的安全经济高效运行。冷却塔内气-水两相间复杂的传热传质极易受环境空气温度、湿度及侧风的影响。环境侧风可破坏无侧风时塔内轴对称的空气动力场,引起冷却塔进风口风速周向分布不均,弱化冷却塔传热传质,对电站热力系统的高效节能运行产生不利影响。冷却塔传热传质区由配水区、填料区和雨区组成,因各区在塔内的位置及其结构的不同,侧风对各区传热传质有着不同的影响机理。为明确环境侧风对冷却塔传热传质的影响机制,以更为有效地提高冷却塔冷却性能,有必要针对环境侧风对塔内各区传热传质的影响机理进行深入地系统研究。本文建立了侧风条件下自然通风逆流湿式冷却塔传热传质分析的三维数值计算模型,对侧风条件下冷却塔空气动力场及塔内气.水两相间的传热传质进行了三维数值计算,研究了侧风对冷却塔各区传热传质及其冷却性能的影响机理,以及侧风条件下导风板、十字隔墙等控风方案的作用机理及其耦合。并基于冷却塔的热态模型试验和现场运行试验,对所建三维数值计算模型进行了验证,为侧风下冷却塔冷却性能评价理论模型的建立奠定了基础。主要研究工作如下:(1)侧风条件下,冷却塔气-水两相间传热传质数学模型的完善。基于塔内气-水两相间传热传质的基本原理,分析了气-水间对流传热和对流传质的类似性,指出了冷却塔传热传质传统数学模型的不足,其中一维模型和二维模型无法考虑环境侧风的影响,已有三维模型采用水滴代替水膜近似模拟填料区传热传质,降低了侧风下冷却塔传热传质性能计算分析的准确性。本文修正了冷却塔内空气和水物性参数的计算方法,充分考虑了侧风条件下塔内空气和水两相温度、速度、含湿量等参数的三维分布对局部传热传质的影响,在侧风条件下引入了修正的国Le_f因子关联式实现了填料区空气和水膜间对流传热系数和对流传质系数计算的三维关联,参考离散相模型计算了空气和水滴两相间的对流传热系数和对流传质系数,在侧风条件下建立了完善的塔内气-水两相间传热传质的三维数学模型,为研究侧风对冷却塔传热传质的影响机制奠定了理论基础。(2)侧风条件下,自然通风逆流湿式冷却塔传热传质分析三维数值计算模型的建立。结合侧风条件下塔内气-水两相间传热传质的三维数学模型,采用标准κ-ε湍流模式和标准壁面函数法分别对空气运动控制方程进行湍流封闭和近壁区处理,建立了侧风条件下冷却塔传热传质分析的三维数值计算模型。结合实测侧风工况,验证了所建三维数值计算模型的正确性,对计算结果进行了网格独立性的考核,得到了与网格无关的数值解,研究了侧风廓线指数和雨滴当量直径等关键参数对计算结果的影响,结果表明所建三维数值计算模型可准确分析研究环境侧风对自然通风逆流湿式冷却塔传热传质的影响机制。(3)环境侧风对冷却塔传热传质影响机理的研究。结合实测侧风工况,从数值计算的角度研究了侧风对冷却塔雨区横截面空气动力场、纵剖面空气动力场的影响,引入了横向通风量、纵向通风量和总体进风量等概念,为分析环境侧风对冷却塔各区传热传质的影响机理创造了理论条件。(4)环境侧风对于冷却塔进风口空气动力场影响机理的研究。由环境侧风对进风口径向压力梯度和径向进风风速周向分布的影响,分析研究了横向通风量出现的原因和纵向通风量降低的机理。(5)侧风条件下,导风板控风方案对冷却塔进风口空气动力场及塔内各区传热传质影响机理的研究。结合实验数据,从数值计算的角度研究了导风板对冷却塔各区平均传热系数和平均传质系数、各区冷却水温降及冷却水总温降等性能参数的影响。指出导风板可有效改善冷却塔进风口空气动力场,实现侧风下填料区传热传质的强化,从而可在侧风条件下有效提高冷却塔总体冷却性能。(6)侧风条件下,雨区十字隔墙对冷却塔各区传热传质影响机理的研究。结合实验数据,从数值计算的角度研究了不同形式的十字隔墙,在不同风速的外界侧风作用下,在不同的安装位置时,对于塔内各区传热传质的影响机理,获得了最佳的十字隔墙安装位置与最佳的十字隔墙形状。(7)侧风条件下,导风板与十字隔墙两种控风方案对冷却塔冷却性能耦合作用的研究。结合实验数据,从数值计算角度,分析了导风板和十字隔墙对冷却塔冷却性能的耦合作用:对于带十字隔墙的冷却塔,导风板可实现冷却塔总体冷却性能的强化;对于已安装导风板的湿式冷却塔,间隙十字隔墙对冷却塔总体冷却性能的影响与无导风板时的情况基本近似。(8)侧风作用下湿式冷却塔的热态模型实验研究和现场运行实验研究。通过实验室冷却塔的热态模型试验,在无导风板、有导风板、无十字隔墙、有十字隔墙、导风板与十字隔墙并存、十字隔墙布置方式和形状变化、进塔水量变化和进塔水温变化等不同条件下,分析了侧风对模型塔冷却性能的影响规律,并与所建三维数值计算模型针对实型塔的相关数值计算分析进行了对比,验证了侧风条件下冷却塔冷却性能相关数值计算分析的正确性。通过已采用导风板冷却塔的现场试验和数值计算分析,进一步验证了所建湿式冷却塔三维数值计算模型的正确性。对比相同气象条件下运行的无导风板冷却塔现场试验,进风口进风相对偏离度的实验数据表明侧风条件下导风板控风方案可实现冷却塔进风口空气动力场的优化。(9)侧风条件下,自然通风逆流湿式冷却塔冷却性能评价理论模型的建立。分析了冷却塔常规设计和性能评价方法,指出常规方法未考虑环境侧风影响。研究了侧风条件下冷却塔冷却数、冷却特性数、当量通风量以及总体阻力系数等的确定方法,并基于形体阻力的概念,分析了冷却塔各部分阻力系数,提出了与侧风影响下冷却塔空气动力场结构有关的冷却塔流场结构阻力系数,建立侧风条件下冷却塔冷却性能评价的理论模型,给出了性能评价指标,实现了相同气象条件下运行的冷却塔冷却性能的横向比较,并结合工程实例进行了性能评价。本文的研究结论,阐明了环境侧风对大型自然通风逆流湿式冷却塔配水区、填料区和雨区等各区传热传质的作用机理,为进一步在侧风条件下强化冷却塔内气-水两相间的传热传质、提高自然通风逆流湿式冷却塔的冷却性能奠定了理论基础,并为侧风条件下自然通风逆流湿式冷却塔冷却性能的科学评价提供了一个新方法。更多还原

【Abstract】 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.更多还原