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基于空气动力场的逆流湿式冷却塔填料优化布置
Optimized Layout of Fill in Natural Draft Counter-flow Wet Cooling Tower Base on Air Dynamic Field
【作者】 高福东；
【导师】 孙奉仲；
【作者基本信息】 山东大学 ， 热能工程， 2008， 硕士
【摘要】 冷却水的冷却过程是电力生产中一个十分重要的过程,研究冷却水的温降措施具有很重要的意义。在电厂中,凝汽器的低温状态是由冷却水循环来保证的。冷却塔性能的好坏在很大程度上影响电厂的经济性和稳定性。淋水填料作为冷却塔传热传质的“心脏部位”,其性能的好坏直接影响着冷却塔效率的高低。本文以双曲型逆流式湿式自然通风冷却塔为典型的研究对象,在三维坐标空间的基础上对冷却塔内外空气动力场,塔内淋水运动,塔内气、水之间的热质交换,以及塔内气、水阻力建立控制方程,并采用κ-ε湍流模型对控制方程进行湍流封闭,采用了“热态模型”。数值计算运用了流体动力计算软件(CFD),对计算区域采用六面体网格,用有限体积法构造差分格式,并运用SIMPLE方法对冷却塔内流场和温度场进行数值模拟。计算结果显示,本文所建立的逆流湿式自然通风冷却塔塔内空气流动和热质传递的数学物理模型和计算方法是可行的、可靠的。通过对计算结果的分析发现,塔内水平布置的填料出口的大部分区域,空气的焓值、含湿量跟以水温为准的饱和值相比,还有明显的差值,也就是说,在填料径向的很大区域内,填料的冷却能力并没有充分的发挥。所以本文提出填料布置空间优化的概念,依此概念为基础,对冷却塔填料进行改造。基于数值计算的结果,提出两种填料空间优化布置的方案:4#填料和5#填料。填料空间优化布置后的冷却塔总温降有所增加,计算结果显示:4#填料冷却塔的总温降比改造前增加了0.639K,5#填料冷却塔的总温降比改造前增加了0.455K。而且改造后,填料区温降在总温降中所占的比重也有所增加。通过数值计算分析侧风对冷却塔运行的影响,侧风使冷却塔的冷却效果变差得主要原因是:第一,侧风破坏了冷却塔进风口处的进风均匀性,在塔的背风面处催生气流漩涡,从而减少了背风面处的进风,而且风速过大时还会出现穿堂风;第二,由于侧风的影响,填料底部进风面的空气流结构会出现变化,产生多个气流高温区,这样就使得填料下表面的空气温度增加,造成填料区热质传递的减少,而最终使水滴温度升高。改造后的填料在侧风的影响下,传热性能也会有所降低,其变化趋势与水平填料相似。通过室内模拟塔试验台,本文对数值模拟的结果进行试验验证,用密度傅式数相等作为动力相似准则。试验结果表明,改造后的填料的冷却效果要好于原填料,这与数值计算的结果一致。而且,随着淋水量的增大,冷却塔的温差逐渐变小。改造后的填料和水平填料在冷却效果方面,受侧风的影响是类似的,这说明改造后的填料并不会因为侧风而改变其有效性。本课题的研究成果对冷却塔的全面优化设计和改善冷却塔的冷却性能提供了理论依据。
【Abstract】 It is important to study the cooling tower in power plant. The process of cooling water is a very important part in the production of power plant. The low temperature of condenser is the result of circulating water. So the performance of cooling tower influence the economic and stability deeply. The fill is the "heart" of cooling tower and it is the most important part in the heat-mass transfer. The property of fill is the key factor that influence the efficiency of cooling tower.The thermal performance of natural draft wet cooling towers (NDWCT) has been investigated numerically. The governing equations are utilized to describe the air dynamic field of inside and outside of cooling tower, the movement of water in cooling tower, the heat-mass transfer between air and water, and the resistance of air and water in cooling tower. The three dimensional model has utilized the standard k-εturbulence model as the turbulence closure. At last, the "hot model" is established.The computational fluid dynamic (CFD) software is utilized for numerical calculation. The hexahedral mesh is used in the calculation region. The finite volume method is used to structure the difference scheme. The SIMPLE method is used in numerical simulation of flow fields and temperature field in cooling tower.The validation process of the current model has been conducted against the design conditions of the NDWCT and the result is well. As a consequence, the current code is utilized to investigate effect of different conditions on the thermal performance of natural draft wet cooling towers.The result of the model shows that the enthalpy and the humidity content of the fill-up face are lower than the value that relay on the water temperature. It is meaning that the capability of the fill can be used better. As a result, the concept of "optimization space" is proposed firstly in the paper. The reform of cooling tower that based on the concept contains tow schemes: 4#fill and 5#fill. The result of the reform is that, 4#fill and 5#fill make the temperature dropping of cooling tower increase 0.639K and 0.455K respectively, and the temperature dropping in the fill increased obviously. The crosswind of environment decreases the efficiency of cooling tower. There are tow main reasons. Firstly, the uniformity of the air that flow into the tower is destroyed by the crosswind, and the vortex is raised in the back of the tower which decreased the air input, on the other hand ,the cross ventilation will be generated when the velocity of crosswind is big enough. Secondly, the structure of the airflow under the fill is changed by the crosswind and several high temperature regions are generated which make the air temperature raise there. These two factors decrease the heat and mass transfer in the fill and raise the water temperature at last. The reformed fills have the same property in the crosswind with the previously in the crosswind.The model test of cooling tower is also carried out. And the result shows that the reformed fills work more effective than the previous which fit the calculation well.The result also shows that the temperature of outlet water will be raised when the spray water content increased. The reformed fills will not change the advantage in the crosswind.The result of this study provides a basis for the optimal design and performance improvement of cooling tower.