【作者】 王凯；

【导师】 孙奉仲；

【作者基本信息】 山东大学 ， 热能工程， 2009， 博士

【摘要】 冷却塔是大型发电厂中重要的热力设备之一,其运行性能对电厂的安全性和经济性都有很大影响。自然通风湿式逆流冷却塔是目前国内应用最为广泛的一种冷却塔形式,其热力性能受环境因素影响较大,尤其是环境侧风的作用使塔周向进风极不均匀,减小了塔内通风量,严重降低了其冷却效率。然而在冷却塔的设计和计算中没有充分重视侧风的影响。目前关于冷却塔的研究多集中于塔内传热传质,较少涉及冷却塔的通风阻力,但通风量和进风阻力是湿式冷却塔的设计、评价过程中极为重要的热力参数。在冷却塔进风口周围安装导风板可减小侧风带来的不利影响,但尚未有关于侧风下导风板对冷却塔热力性能影响规律的系统理论研究和报道。因此,本文从模型试验、正交理论分析、阻力模型推导、数值计算及塔内传热传质机理分析等几个方面入手,研究了侧风下带有导风板的自然通风湿式逆流冷却塔的进风阻力计算及其性能优化问题,进行的主要工作如下:1.基于运动相似和动力相似准则理论,建立了冷却塔冷态试验模型。利用山东大学的风洞试验平台对该模型进行了冷态吹风试验,试验工况共计约370个,试验过程以及实验阶段的设计均遵循正交理论。研究了导风板对塔进风均匀性及通风量的影响规律,并提出了进风均匀系数Cu的概念用于量化冷却塔周向进风的均匀程度,得到了各导风板布置参数（包括板的安装角度、安装数量、板的形状及尺寸）对Cu和通风量G的影响规律。2.在现有的热态模型试验台的基础上进行了完善,增设了进风口的几个不同防风方案模型,研究了侧风下冷却塔热力性能优化的措施,通过300多个热态试验工况,找到了冷却塔各热力性能参数随导风板各布置参数的变化规律。提出了当量通风量、当量传热系数与当量传质系数的概念,减小了传统热力参数中因环境温度变化而导致评价结果不准确的影响。采用拟水平法对各个热力参数进行了极差分析和方差分析,得到了各导风板布置参数对冷却塔热力性能影响的主次程度。3.建立了侧风下、安装有导风板的湿式冷却塔进风口区域的阻力源三维计算模型,并结合冷却塔内气水两相间传热传质的三维数学模型和气水接触阻力的计算公式,给出了侧风下、具有防风措施的湿式冷却塔内外空气动力场的数值计算方法。对某工程实例冷却塔进行了数值模拟和相关的现场试验。4.数值计算研究了工程实型冷却塔导风板布置的最优化方案。以工程实型冷却塔为研究对象讨论了导风板安装数量N和安装角α对塔实际通风量G和出塔水温t_wo的影响,得到了不同工况下塔内的气流速度场分布和集水池表面温度场分布。5.分析了关键因素对冷却塔传热传质性能的影响机理。通过对冷却塔内气水换热过程的研究,提出了无量纲气水接触时间T_l的概念,量化了空气流在塔内的有效停留时间。讨论了T_l与板安装角α的关系。6.研究了最佳导风板安装数量N_opt随冷却塔体尺寸变化的规律。利用Levenberg-Marquardt算法对N_opt进行了回归分析,得到了最佳导风板周向间距的计算公式。7.利用LCS-DTL型充气二极管式固体激光器所产生的片光对冷却塔背风侧导风板间的空气流动情况进行了示踪试验,得到了侧风下具有导风板的冷却塔风口空气流态的变化规律。通过以上研究,本文得出了以下主要结论:（1）存在一个最佳的导风板安装数量N_opt使通风量G最大、并保证塔周向进风最为均匀,可视化试验表明此时环境空气平稳的流入塔内,而且在相邻导风板之间所形成流道的充满度较高。若板安装数量N偏离N_opt则会增大塔进风阻力并使G减小,进风均匀系数G_u减小。当板安装数量N＜N_opt时,相邻板间形成的进气流道数较少,C_u和G都随N的增加而增大。当N＞N_opt时,由于相邻板间的间隙太小导致空气流开始形成小的漩涡,其扰动程度随N的增加而不断增大,流道中空气的充满度逐渐变差,进塔阻力逐渐增加。（2）板安装角α越小则环境气流在塔周切向的速度分量越大,轴向速度分量所占比例相应减小,使得塔内空气流在横向截面上形成强度更大的漩涡,增大了气水的有效接触时间,但此时的进风阻力也随之增大。采用α=70°的导风板安装角可以使塔的实际通风量G与气水接触时间的配合效果最好,使冷却效率η、当量传热系数k_h等参数达最大值。采用翼型板不仅大幅提高了塔的冷却效率,还使得塔内接触散热占总散热量的比例△Q_c/△Q较高,充分强化了塔内的传热过程。（3）双参数的F检验表明冷态试验和热态试验的误差影响较小,试验结果比较可靠,为进一步理论研究导风板各因素对塔热力性能的影响机制、侧风下塔的通风量和阻力计算等问题奠定了基础。（4）试验结果和理论分析都表明板安装数量N对通风量G、出塔水温t_wo的影响要比板安装角α的影响大。（5）模拟的出塔水温计算误差不超过0.3℃,出塔气温误差不超过0.5℃,环境气流的径向进风速度平均误差小于0.3m/s,因此所建立的数学模型反映出了冷却塔进风流场的分布规律,可用于工程实例的计算。（6）若导风板高度明显低于塔进风口高度,会在塔背风侧导风板之上的区域产生空气漩涡回流,使该处的进风速度明显偏低,导致背风侧t_wo偏高,因此导风板越高越长,会使塔冷却效率越高。本文所建立的数学模型为冷却塔的设计和阻力计算提供了理论基础,提出的当量通风量和无量纲的汽水接触时间等概念为冷却塔的评价提供了新思路、新方法,试验结果和推导出的最佳导风板周向间距的计算公式为火电厂中的冷却塔现场改造提供了实际指导和帮助,所得结论具有一定的理论价值和工程应用价值。更多还原

【Abstract】 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 C_u 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 C_u 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 t_wo 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 N_opt,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 N_opt for the deflectors to get the maximum G and C_u.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 N_opt would increase the flow resistance of inlet air and reduce G and C_u.When N＜N_opt,the quantity of flow passage formed by two adjacent deflectors is Small, and G and C_u rises with the increment of N.While when N＞N_opt,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 alsoΔQ_c/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 t_wo 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 t_wo 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.更多还原