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1、鼓风式机械通风冷却塔空气动力特性数值模拟研究赵顺安、李红莉、毋飞翔（中国水利水电科学研究院，北京 100038）Numerical research on aerodynamic characteristics of the forced draft mechanical cooling tower Zhao Shunan、Li Hongli、Wu Feixiang(China Institute of Water Resource and Hydropower Research, Beijing 100038 )摘要：鼓风式机械通风冷却塔常用于核电厂的重要厂用水系统，但相关设计规范并没有给出
2、冷却塔的空气动力特性计算公式。本文采用Fluent软件对鼓风式机械通风冷却塔的空气动力进行了数值模拟计算，对冷却塔的设计布置进行了优化，分析总结给出了冷却塔阻力计算公式。结果表明，填料安装位置对鼓风式机械通风冷却塔整塔阻力影响不大，但会影响填料断面风速分布均匀性，填料安装高度越低，风速分布越均匀；出口收缩段的高度越高，整塔阻力越小，风速分布越均匀；出口收缩段与水平的夹角越大，整塔阻力系数越小，但变化趋势不明显，收缩角基本不影响填料断面风速分布均匀性。关键词：鼓风式冷却塔；塔型；阻力系数；风速均匀性Abstract: The forced draft mechanical cooling tow
3、er is always used in a nuclear power plant, while the relevant design specifications have not formula about the aerodynamic characteristics of cooling tower. This paper uses FLUENT software to simulate and study the aerodynamic characteristics of the forced draft mechanical cooling tower, and optimi
4、ze the design of the cooling tower, and analysis to summarize the cooling tower resistance calculative formula. The results show that the height of the fill has little effects on the whole tower resistance coefficient, but it influences the wind velocity distribution uniformity of the fill section,
5、the lower the position is, the moreuniform the wind velocity distribution is; the convergent section height is higher, the whole tower resistance is smaller and the wind velocity distribution is more uniform. The angle between convergent section and horizon is bigger, the whole tower resistance is s
6、maller, while this trend is not obvious, it does not affect the wind velocity distribution uniformity on the fill section.Keywords: the forced draft mechanical cooling tower, tower shape, resistance coefficient, wind velocity distribution uniformity1研究背景内陆核电厂的重要厂用水的水量不大，但却影响核电厂的安全。鼓风式机械通风冷却塔能较好地适应核电
7、对安全性和抗震性能的要求而常被内陆核电厂采用。鼓风式机械通风冷却塔不仅在通风方式上有别于常规的抽风式机械通风冷却塔，在塔型结构布置上也有明显差异。我国的相关设计规范和资料对鼓风式机械通风冷却塔没有明确的设计计算方法15。为了解塔内气流特性并对塔型进行优化，需要通过相关的研究来确定其空气动力特性。通过物理模型试验来研究冷却塔空气动力特性是一个十分有效的手段，但是由于鼓风式机械通风冷却塔模型本身的复杂性及系统试验的塔型的变化，使模型试验研究工作量和投资都很大。本文利用Fluent软件建立鼓风式机械通风冷却塔空气动力计算的数学模型，经过与试验结果对比验证，确定模型参数和网格数量。研究了不同塔型条件下
8、塔内气流分布及阻力特性，最终分析总结出了鼓风式机械通风冷却塔的阻力计算公式以及塔型与配风均匀性的关系。阻力系数计算公式与试验结果相比偏差小于5%，可为设计提供参考。1research backgroundThe water quantity of important water system of inland nuclear power plant is not big, but it affects the security of nuclear power plant. The forced draft mechanical cooling tower can satisfy the r
9、equirements of equipment security and earthquake resistance, so it will be used more and more in inland nuclear power plant.The forced draft mechanical cooling tower is not only different from the conventional induced draft mechanical cooling tower in ventilation way, but also has distinct differenc
10、e in tower shape and structure layout. Chinas relevant design specifications and information on the forced draft mechanical cooling tower have no clear design method. For understanding the airflow characteristics of the tower and optimizing the tower shape, its necessary to do some relevant research
11、 to realize the aerodynamic characteristics. Its a very effective way to establish a physical model to study the aerodynamic characteristics of the cooling tower, however, due to the forced draft mechanical cooling tower models complexity and variability, the workload of experiment and investment is
12、 very big.This paper uses FLUENT software to build a mathematical model of the forced draft mechanical cooling tower to study the tower aerodynamic characteristics, and after comparing with the experimental results to determine the model parameters and grid number. It studies the airflow distributio
13、n and resistance characteristics in the conditions of different tower shapes, and analysis to summarize the cooling tower resistance calculative formula and the relationship between tower shape and airflow distribution uniformity. The difference of computational resistance coefficient and the experi
14、mental results is less than 5%, it can provide a reference for design.2数学模型及计算方法2.1 空气流场控制方程塔内外流场为等温、不可压流动，其控制方程包括连续方程、动量方程，并选用双方程湍流模式对方程进行封闭，各方程可写为统一形式： （1）式中：为空气密度，kg/m3；为空气流速，m/s。各控制方程的变量、扩散系数项与源项如下表1。表1 控制方程中各变量代表参数控制方程连续方程100动量方程（流速），湍能方程耗散方程其中生成项；为空气分子粘性系数；为压力；为紊流粘性系数，由动能和紊动耗散率求出：，为经验常数；和分别为和的
15、紊流普朗特数。2 Mathematical models and calculative methods2.1 Air flow governing equationsThe tower flow field is isothermal and incompressible. Its governing equations include continuity equation, momentum equation, which can be closed with two-equation turbulence model, these equations can be written as
16、 a unified form: (1)Where: is air density, kg/m3; is air velocity, m/s. All governing equations variable 、diffusion coefficient term and source term are shown as Table 1 below.Table 1 , and of every governing equationGoverning equationsContinuity equation100Momentum equation（Velocity of flow），Turbul
17、ent energy equation Dissipation equation Generated item , is viscosity coefficient of the air molecules; is pressure, Pa; is the turbulent viscosity coefficient, which is can be calculated by the turbulent kinetic energy and dissipation rate : , is an empirical constant; and are turbulent Prandtl nu
18、mber of and .2.2 边界条件底部为固壁无滑移边界条件，四周及顶部采用压力出口边界条件，塔壳采用固壁边界条件。进风口及塔的出口都设置成内部边界；填料区域设置成多孔介质边界条件，并根据实测填料阻力系数设置各方向阻力系数；风机采用Fluent风扇边界条件，也可采用第一类边界条件。2.2 Boundary conditions The bottom of the computational domain is solid wall boundary condition with no-slip, all around and top is pressure outlet boundary
19、 conditions, the tower shell is solid wall boundary condition. The boundaries of the air inlet and outlet are defined as interior; the porous model is used to simulate the fill and according to the measured resistance coefficient to set the fill resistance coefficient in each direction; the FLUENT f
20、an model is used to simulate the fan of the tower, first boundary condition can also be used.2.3 冷却塔阻力系数及风速分布均匀性计算鼓风式机械通风冷却塔，气流经由风机鼓入塔内，依次经过塔进风口，雨区、填料等，并经由出口排入到大气中，气流经过各部分的阻力为该区域前后断面的全压差，一般表示为阻力系数与填料断面平均气流速度头之积： （2）式中为气流经过某区域前后断面的全压差（Pa）；为空气密度（kg/m3）；为填料断面平均风速（m/s）。填料断面处风速分布状况影响冷却塔的热力特性，一般将填料断面风速分布均
21、匀性作为一个设计指标，用风速分布均布系数表示: （3） 式中为填料断面风速分布均布系数；为填料断面各点风速（m/s）；n为风速统计点的个数。2.3 Computational methods of the cooling tower resistance coefficient and wind velocity uniformity For the forced draft mechanical cooling tower, airflow is blown into the tower by the fan, sequentially through the tower inlet, ra
22、in zone, fill etc, and is discharged into the atmosphere through the outlet finally. The resistance of each part is the pressure loss of the region, which is generally expressed as the resistance coefficient multiply the average flow velocity head: (2)Where is the pressure loss of the region（Pa）; is
23、 air density（kg/m3）; is the average wind velocity of the fill section（m/s）.Distribution of wind velocity at the fill section affects the thermodynamic characteristics of the cooling tower, generally put the wind velocity distribution uniformity of the fill section as a design index, it can be expres
24、sed with a velocity distribution uniformity coefficient: （3）Where is the velocity distribution uniformity coefficient; is the velocity at the measure point in the fill section（m/s）; n is the velocity statistical points number.2.4 模型的验证对已具有试验结果的某抽风式机械通风冷却塔的空气动力特性模型试验6作对比验证计算，冷却塔如图1示，首先对冷却塔进行网格的敏感性分析，
25、然后再将计算结果进行对比分析。图1 抽风式机械通风冷却塔模型试验布置示意图不同填料阻力条件下模型试验实测与计算结果对比如图2所示，图中横坐标L0/L为距其中一侧塔壁的相对距离， V/为相对风速，V为测点风速，为测点风速的平均值。进风口气流流态作对比如图3所示，从图中可以看出，试验结果与数值计算结果规律较为一致，吻合良好。图2 试验与计算填料断面风速分布对比（a）模型试验结果 （b）数值计算结果图3 试验与计算进风口上沿气流流态分布对比进风口区域冷却塔阻力系数试验与计算结果对比见表2，二者相差不大于5%，吻合较好。表2 模型试验与数值计算进风口区域阻力系数对比结果填料阻力系数进风口区域阻力系数相
26、差(%)试验结果计算结果1016.116.1-0.002026.127.34.323036.138.04.922.4 Model validationTo do validation with the experimental results of aerodynamic characteristics of an induced draft mechanical cooling tower model, the layout drawing of the cooling tower is shown as Figure 1, Firstly, analysis the grid sensit
27、ivity, then compare and analyze the results.Fig. 1 Layout drawing of the induced draft mechanical cooling tower model In the conditions of different fill resistance coefficients, the results of the comparison between experimental and computational are shown in Figure 2, Abscissa L0 / L is the relati
28、ve distance from one side to the wall, V/is relative wind velocity, V is the velocity at the measure point, is the average measure points wind velocity. The results of the comparison between experimental and numerical inlet air flow state are shown in figure 3, as can be seen from Fig.3, experimenta
29、l results is consistent with the results of numerical calculation.Fig. 2 Comparison between experimental and computational fill section wind velocity distribution（a）Experimental results （b）Numerical resultsFig. 3 Comparison between experimental and Numerical inlet air flow distributionComparison bet
30、ween experimental and Numerical cooling tower air inlet area resistance coefficient are shown in table 2，the difference is not greater than 5%，the results tally well.Table 2 Comparison between experimental and computational cooling tower air inlet area resistance coefficientFill resistance coefficie
31、ntInlet resistance coefficientDifference (%)Experimental resultsNumerical results1016.116.1-0.002026.127.34.323036.138.04.923 计算结果及分析鼓风式机械通风冷却塔不同的塔型尺寸，如填料的安装高度、塔出口收缩段的高度、角度等，都会影响塔内气流阻力特性及风速分布，本文分别研究了不同塔型对冷却塔气流特性的影响。鼓风式机械通风冷却塔立面布置如图4所示，塔的平面尺寸为9.0m9.0m，风机直径为6.0m。HCHF图4 鼓风式机械通风冷却塔立面布置图3 Results and ana
32、lysisDifferent tower shapes for the forced draft mechanical cooling tower, such as installation height of the fill、the convergent section height and angle, will affect the tower airflow resistance characteristics and wind velocity distribution. This paper studies the influence of different tower sha
33、pes on the air flow characteristics. The forced draft mechanical cooling tower elevation is shown as Fig.4, tower plane size is 9.0m9.0m, fan diameter is 6.0m.Fig.4 The forced draft mechanical cooling tower elevation3.1 计算模型的建立及网格划分流体仿真计算域范围的选取影响计算的速度和精度，根据经验，当计算域到达一定的大小时，塔内的流场就不再受计算域大小的限制。假定塔高为H，宽为
34、W，进风口高为H1，经过试算分析，计算域进风口上下游宽度取为3H1、宽度取为4W、高度取为2H时再增大计算域范围对计算影响不大。数值模拟计算与计算网格的划分密切相关，本文进行了网格相关性分析计算，结果如图56所示。当网格数量达到50万时，塔内气流特性受网格数量的影响已经很小，计算区域网格图如图7所示。图5 网格数量对冷却塔阻力系数影响图6 网格数量对填料断面风速分布影响图7 塔内及计算域网格示意图3.1 Establishment of calculative model and mesh generationThe scale of fluid computational domain af
35、fects the calculative velocity and accuracy, based on experience, when computational domain reaches to a certain scale, flow field in the tower is no longer limited by computational domain scale. Assume that the tower height is H, width is W, air inlet height is H1, according to the results of the t
36、rial computation, it makes little difference to increase the computational domain when the length of upstream and downstream of air inlet is 3H1, the width of the whole computational domain is 4W and the height is 2H.Numerical simulation is closely related to grid partition, this paper analysis grid
37、 correlation, the results are shown in Figure 5 and 6. It is known according to the two figures that the grid number has little effect on air flow characteristics in the tower when the grid number reaching 500000, computational domain grid is shown as Fig.7.Fig 5 The influence of grid number on the
38、cooling tower resistance coefficient Fig 6 The influence of grid number on the fill section velocity distributionFig 7 The tower and computational domain grid schematic diagram3.2 填料安装高度对冷却塔气流特性影响不同的淋水填料安装高度时，冷却塔的阻力系数与填料断面风速分布计算结果如图8和图9所示，图中横坐标HF/L为填料底至进风口上沿距离与塔宽之比，结果表明，填料安装高度对整塔阻力系数影响不大，但填料安装高度离塔进风
39、口远时，填料阻力较小者风速分布均匀性变差。图8填料安装高度对整塔阻力系数的影响图9填料安装高度对填料断面风速分布均匀性的影响3.2 The influence of the fill installation height on the cooling tower aerodynamic characteristicsIn the conditions of different fill installation height, the computational results of cooling tower resistance coefficient and fill section
40、wind velocity distribution are shown in figure 8 and figure 9, abscissa HF/L is the distance from fill bottom to top of the air inlet divides tower width, it turns out that the bottom height of the fill haslittle effectonthe whole tower resistance coefficient, but when fill installation height is hi
41、gher, the smaller the fill resistance coefficient is ,the worse the wind velocity distribution uniformity is.Fig.8 The influence of fill installation height on the cooling tower resistance coefficientFig.9 The influence of fill installation height on the fill section velocity distribution3.3冷却塔出口收缩高
42、度对冷却塔气流特性的影响调整冷却塔出口收缩高度，冷却塔的阻力系数与填料断面风速分布计算结果如图10和11所示，图中横坐标HC/L为收缩段至进风口上沿距离与塔宽之比。由图可以看出，随着塔出口收缩高度的增加，冷却塔阻力系数降低，当HC/L达到0.75后，阻力系数变化减小，大于0.90后基本不再变化，填料断面风速分布均布系数亦有相似的规律。图10 收缩高度对整塔阻力系数的影响图11 收缩高度对填料断面风速分布均匀性的影响3.3 The influence of the outlet convergent section height on the cooling tower aerodynamic
43、characteristicsAdjusting the cooling tower outlet convergent height, the computational results of cooling tower resistance coefficient and fill section wind velocity distribution are shown in figure 10 and figure 11, Abscissa HC/L is the distance from the convergent section to the top of the air inl
44、et divides tower width. As can be seen from the two figures, with the increase of the tower outlet convergent height, the whole cooling tower resistance coefficient decrease, when HC/L reaches 0.75, the resistance coefficient change becomes slowly, when HC/L is greater than 0.90,its no change, fill
45、section wind velocity distribution uniformity coefficient also has the similar laws.Fig.10 The influence of convergent height on the cooling tower resistance coefficientFig.11 The influence of convergent height on the fill section wind velocity distribution3.4冷却塔出口收缩角度对冷却塔气流特性的影响调整冷却塔出口收缩角度，冷却塔的阻力系数
46、与填料断面风速分布计算结果如图12和13所示, 图中横坐标为收缩段与水平的夹角。随着塔出口收缩角度的增加，冷却塔阻力系数降低，但趋势不明显。填料断面风速分布均布系数基本不受塔出口收缩角度的影响。图12 收缩角度对整塔阻力系数的影响图13 收缩角度对填料断面风速分布均匀性的影响3.4 The influence of convergent angle on the cooling tower airflow characteristicsAdjusting the cooling tower outlet convergent angle, the computational results o
47、f cooling tower resistance coefficient and fill section wind velocity distribution are shown in figure 12 and figure 13, Abscissa is the angle between convergent section and horizon. With the increase of tower outlet convergent angle, the cooling tower resistance coefficient decrease, but this trend is not obvious. Fill section wind velocity distribution uniformity coefficient is not affected by tower outlet conver
