摘要
冷却塔作为一种有效的冷却设备,在工农业生产特别是需要大量冷却水的火电站和核电站热力系统中被广泛使用。随着电力工业的发展和水资源的日益稀缺,人们对冷却塔冷却效果和冷却效率的要求也逐渐提高,并不断地对冷却塔的性能进行研究。根据最新的《核电中长期发展规划(2011-2020年)》,在强调核电安全的前提下,我国正稳步有序地推进核电建设工作,千万千瓦级的第三代核电技术——AP1000因其具有容量大、技术先进和安全性高等特点,被各地区核电项目所采用。作为AP1000核电机组的重要组成部分,自然通风冷却塔热力性能直接影响核电站的稳定运行。AP1000核电机组冷却塔塔筒结构尺寸巨大,其规模将远超以往冷却塔,塔内的流动及传热传质过程较为复杂,超过了目前国内规范涉及的范围,常规一维、二维计算软件及方法已不能精确地计算冷却塔的各种参数及冷却塔内部复杂结构等三维因素对冷却塔效率的影响,而模型试验很难满足其中的两相流耦合情况下的相似条件且成本太高。因此开发适用于超大型冷却塔性能计算的三维数值计算模型,研究塔内两相流动、传热传质特性及各种因素对冷却塔性能的影响是十分必要的。
本文建立了超大型冷却塔的三维数值计算模型,在根据实测数据对所建三维数值计算模型进行验证的基础上,对多种工况条件下冷却塔内两相流动及传热传质进行了三维数值计算,着重对填料布置不均、分区配水、淋水密度、进塔水温和环境湿度等因素对冷却塔热力特性的影响展开了分析研究;同时,分析了侧风下各种防御措施的效果,还研究了侧风下冷却塔出口形状对塔内流场的影响以及冷却塔出口雾羽扩散现象,为超大型冷却塔优化设计和性能评价提供了参考依据。主要研究工作如下:
(1)冷却塔内流动及传热传质三维数值计算模型的建立。本文考虑了不同填料形式对填料区传热传质及阻力特性的影响,采用了由实验得出的填料特性公式,对刘易斯因子Le进行了修正,建立了较为准确的超大型冷却塔填料区流动及传热传质模型;同时也建立了相关的喷淋区和雨区的三维数值计算模型。最后基于Fluent软件平台,采用自定义函数添加源项方法,开发了适用于冷却塔特性研究的三维数值模拟程序,为冷却塔内流动和传热传质的研究提供了基础。
(2)冷却塔三维数值计算模型的验证分析。采用开发的冷却塔三维数值计算模型对两个在用冷却塔进行了数值计算,计算结果与设计计算及实测数据进行了验证分析。结果表明计算结果与实测数据吻合度较高,验证了三维数值模型的准确性和可靠性,表明所开发的三维数值计算模型可以满足冷却塔特性计算的要求。
(3)冷却塔内结构及运行参数对冷却塔性能影响的研究。根据各种条件下的塔内速度、温度、湿度的分布情况,分析了各种填料布置方式和配水方式对冷却塔性能的影响,得出了较好的布置和分配方式。同时,还分析了淋水密度、进塔水温、环境湿度等变化对冷却塔性能的影响。
(4)侧风下冷却塔防风措施的研究。研究了侧风下塔内流场的分布,并分析了多种侧风防御措施的效果。指出了各种防御措施的优缺点,并给出了不同气象条件下的侧风防御措施建议。
(5)侧风下冷却塔出口结构影响的研究。分析不同出口形状、出口面积及扩散角对冷却塔内流动的影响,得出了各种出口结构的侧风防御能力,并给出了不同侧风条件下采用的出口结构的建议。
(6)冷却塔出口雾羽扩散的研究。结合实测数据,对单塔雾羽扩散以及侧风情况下双塔出口、漂滴和温度的扩散及分布情况进行了研究,给出了不同工况下雾羽的扩散特性。希望为合理评估冷却塔出口对环境的影响以及相应防范措施的采取提供支持。
本文建立了超大型冷却塔的三维数值计算模型,并分析了各种塔内外结构及运行参数对冷却塔性能的影响以及冷却塔对周围环境的影响,为冷却塔的设计及优化提供了参考依据,同时,也为冷却塔性能的科学评价提供了一种较好的方法。
As a cooling equipment of high efficiency, cooling tower is broadly used in industry and agriculture, especially in thermodynamic system of power stations and unclear plants. With the development of power industry and capacity of unit growing up in recent years, the requirement for cooling efficiency of cooling tower is increased gradually, and the research of cooling tower have developed and deepened. Based on National Nuclear Long-and-medium Term Development Planning (2011-2020), nuclear power construction is developing orderly, steadily and smoothly in premise of assuring the safety. The third-generation technology AP1000has been adopted by many nuclear power projects for its advantage of great capacity, advanced technology and high security. Cooling tower is one of the important components of AP1000and its thermal performance would directly influence steady operation of the unclear power station. The construction dimension of the cooling tower in AP1000unit is very large and much higher than the past cooling tower. In the super large cooling tower, the gas-liquid flow, heat transfer and mass transfer process are complex and have gone beyond the available domestic standard specifications, so the conventional one-dimensional and two-dimensional calculation method can not accurately calculate its performance. Moreover, model test can not satisfy the similar conditions in case of two-phase coupling and its cost is too high. In order to study the gas-liquid flow, heat transfer and mass transfer process and various influence factors, it is very necessary to develop a three-dimensional numerical computation model for thermal performance calculation of super large cooling tower.
The three-dimensional numerical computation model for super large cooling tower was established here. The gas-liquid flow, heat transfer and mass transfer process were analyzed numerically under various conditions in super large cooling tower. The influences on thermal performance of cooling tower by the factors like fill distribution, water distribution, spray water rate, inlet water temperature and environment humidity were investigated. Further more, the effect of wind-control schemes under crosswind conditions were analyzed. The influence on flow field by the shape of tower outlet, and vapor diffusion around tower outlet were studied. These research results may provide forceful support to optimization design and scientific cooling performance evaluation of super large cooling tower. The major research works are as follow:
(1) Establishment of the three-dimensional numerical computation model for cooling tower. Considering influence on heat and mass transfer and resistance characteristics by different fill distributions, the relationships of filler property by actual measurement was introduced, Lef factor was corrected, the models of gas-liquid flow, heat transfer and mass transfer in fill zone, spray zone and rain zone were established. Based on the Fluent software, a three-dimensional numerical simulation platform composed by lots of user defined functions (UDF) was developed to study the thermal performance in super large cooling tower.
(2) Validation analysis to the three-dimensional numerical computation model. Two operating cooling towers were studied by this three-dimensional mathematical model. Through comparing with field experiment and simulated results, the accuracy and reliability of the above three-dimensional numerical computation model was tested and verified. It was found that the above3D numerical computation model can meet the requirement of the performance computation in super large cooling tower.
(3) Research of influence on performance of cooling tower by different structure factors and operation parameters. Based on velocity, temperature and humidity distributions under different conditions, the influences of different fill distribution and water distribution were analyzed to select optimum distribution scheme. Moreover, the influences on tower performance by the factors like spray water rate, inlet water temperature and environment humidity were investigated.
(4) Research of wind-control schemes under crosswind conditions. Flow distribution under crosswind condition was analyzed. The effects of crosswind-control schemes were analyzed and suggestions for crosswind-control schemes selecting under different weather conditions were proposed.
(5) Research of the structure of tower outlet. The flow field with different shape of tower outlet, outlet area and diffusion angle were studied. The crosswind-control abilities of different tower outlet structure were analyzed, and then the selection scheme of tower outlet structure was obtained.
(6) Research of vapor diffusion around tower outlet. Combining with the actual measurement data, the vapor diffusion and temperature diffusion in single tower and twin towers conditions were investigated. The vapor diffusion property under crosswind condition was analyzed, which may provide support to environmental affection appraisal and crosswind-control schemes selecting of the tower.
Based on establishing and applying a three-dimensional numerical computation model for cooling tower, the influences of various cooling tower structures and operation parameters on cooling performance were analyzed. These conclusions may offer forceful support to optimization design, and also provide a good scientific cooling performance evaluation method of super large cooling tower.
引文
毛献忠,陈允文.逆流式自然通风冷却塔流场及热质交换的数值模拟[J].计算物理,1994,11(4):385-392.
黄东涛,杜成琪.逆流式冷却塔填料及淋水分布的数值优化设计[J].应用力学学报,2000,17(1):102-109.
赵元宾,孙奉仲,王凯,等.侧风对湿式冷却塔空气动力场影响的数值分析[J].核动力工程,2008,29(6):35-40.
赵元宾,孙奉仲,王凯,等.十字隔墙湿式冷却塔冷却特性的数值研究[J].中国电机工程学报,2009(8):6-13.
赵元宾,孙奉仲,王凯,等.自然通风湿式冷却塔传热传质的三维数值分析[J].山东大学学报(工学版),2008,38(5):36-41.
王凯,孙奉仲,赵元宾,等.自然通风冷却塔进风口流场模型的建立及计算[J].山东大学学报:工学版,2008,38(1):13-17.
周兰欣,蒋波.横向风对湿式冷却塔热力特性影响数值研究[J].汽轮机技术,2009(3):165-168.
周兰欣,蒋波,叶云飞.湿式冷却塔热力性能数值分析[J].华北电力大学学报,2009,36(1):53-58.
周兰欣,蒋波,陈素敏.自然通风湿式冷却塔热力特性数值模拟[J].水利学报,2009,40(2):208-212.
蒋波,周兰欣.自然通风冷却塔加装十字挡风墙数值研究[J].汽轮机技术,2010,52(3):165-168,172.
周兰欣,金童,尹建兴,等.火电机组湿式冷却塔加装导流板的数值研究[J].汽轮机技术,2010,52(1):13-16.
蒋波.自然通风湿式冷却塔数值模拟与结构优化[D].华北电力大学(保定)华北电力大学热能工程,2008.
张郑磊,金童.大型电厂冷却塔塔群的数值研究[J].应用能源技术,2011,3:13-17.
游江,周亚素,赵敬德,等.逆流密闭式冷却塔热力特性数值模拟[J].建筑热能通风空调,2010,29(3):13-16.
李永华,李燕芳,魏杰儒,等.自然通风湿式冷却塔防冻数值研究[J].应用能源技术,2012,6:38-42.
唐磊,刘自力,易超.电厂逆流式自然通风冷却塔热力性能研究进展[J].现代电力,2010,27(6):35-40.
赵顺安,廖内平,徐铭.逆流式自然通风冷却塔二维数值模拟优化设计[J].水利学报,2003,10:26-31.
刘东兴,周亚素,李未,等.逆流密闭式冷却塔中淋水填料热工性能模拟[J].建筑热能通风空调,2009,4:14-17.
史佑吉.冷却塔运行与试验[M].北京:水利电力出版社,1990.
赵振国,魏庆鼎.自然风对空冷塔的不利影响及其改善措施[J].应用科学学报,1998,16(1):112-120.
翟志强,朱克勤.横向风对自然通风干式冷却塔空气流场影响的模型实验研究[J].实验力学,1997,12(2):306-311.
翟志强,唐革风.横向风对自然通风干式冷却塔运行性能影响的实验与数值研究[J].热力发电,1997,3:3-7.
Mohiuddin A K M, Kant K. Knowledge base for the systematic design of wet cooling towers. Part I:Selection and tower characteristics[J]. International Journal of Refrigeration, 1996,19(1):43-51.
Goshayshi H R, Missenden J F, Tozer R. Cooling tower—an energy conservation resource[J]. Applied Thermal Engineering,1999,19(11):1223-1235.
Fisenko S P, Petruchik A I, Solodukhin A D. Evaporative cooling of water in a natural draft cooling tower[J]. International Journal of Heat and Mass Transfer,2002,45(23):4683-4694.
Fisenko S P, Brin A A, Petruchik A I. Evaporative cooling of water in a mechanical draft cooling tower[J]. International Journal of Heat and Mass Transfer,2004,47(1):165-177.
Simpson W M, Sherwood T K. Performance of small mechanical draft cooling towers[J]. Ameriean Soeiety of Refrigerating Engineering,1946,52:535-543,574-576.
Barile R G, Dengler J L, Hertwig T A.Performance and design of a turbulent bed cooling tower[J]. AIChE Symposium Series,1974,70:154-162.
Neil Wikelly.A Blueprint for the preparation of crossflow cooling tower characteristic curves [A]. In:Winter Annual Meeting of the Cooling Tower Institute[C]. Houston:1976.
Albright M A. Packed tower distributors tested[J]. Hydrocarbon Processing,1984,63:173-177.
Krane S C. Radial Profiles of deposition nozzles for high uniformity[J]. APPlied Mathematical Modeling,1986,10:429-432.
Mohiuddin A K M, Kant K, Sangal R. ESTOWER:An expert system for the thermal design of wet cooling towers[J]. Engineering Applications of Artificial Intelligence, 1996,9(2):185-194.
Naphon P, Wongwises S. A study of the heat transfer characteristics of a compact spiral coil heat exchanger under wet-surface conditions[J]. Experimental Thermal and Fluid Science, 2005,29(4):511-521.
Kloppers J C, Kroger D G. A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers[J]. International Journal of Heat and Mass Transfer, 2005,48(3-4):765-777.
Qi X, Liu Z, Li D. Numerical simulation of shower cooling tower based on artificial neural network[J]. Energy Conversion and Management,2008,49(4):724-732.
Qi X, Liu Z, Li D. Prediction of the performance of a shower cooling tower based on projection pursuit regression[J]. Applied Thermal Engineering,2008,28(8-9):1031-1038.
Lemouari M, Boumaza M, Kaabi A. Experimental analysis of heat and mass transfer phenomena in a direct contact evaporative cooling tower[J]. Energy Conversion and Management, 2009,50(6):1610-1617.
赵振国.冷却塔[M].北京:中国水利水电出版社,1997.
Marehot P, Toye D, Pelsser A M, et al. Liquid distribution images on structured packing by X-ray computed tomography[J]. AIChE Journal,2001,47:1471-1476.
Hawlader M N A, Liu B M. Numerical study of the thermal-hydraulic performance of evaporative natural draft cooling towers[J]. Applied Thermal Engineering, 2002,22(1):41-59.
Kranc S C. Optimal spray patterns for counterflow cooling towers with structured packing[J]. Applied Mathematical Modeling,2007,31(4):676-686.
Datta A B, Majumdar A K. A calculation procedure for two phase flow distribution in manifolds" with and without heat transfer[J]. International Journal of Heat and Mass Transfer, 1983,26(9):1321-1328.
Dessouky H EL.Thermal and hydraulic performance of a three phase fluidized bed cooling tower[J]. Experimental Thermal and Fluid Science,1993,6:417-426.
Bedekar S V, Nithiarasu P, Seethatamu K N. Experimental investigation of the performance of a counter flow packed bed mechanical cooling tower[J]. Energy,1998,23:943-947.
Gan G, B. R A S. Numerical simulation of closed wet cooling towers for chilled ceiling systems[J]. Applied Thermal Engineering,1999(19):1279-1296.
Gan G, B. R A S. Application of CFD to closed-wet cooling towers[J]. Applied Thermal Engineering 2001,21:79-92.
Rafat A W, Masud B. The performance of natural draft dry cooling towers under crosswind:CFD study[J]. International journal of energy reseach 2004,28:147-161.
Rafat A W, Masud B. CFD simulation of wet cooling towers[J]. Applied Thermal Engineering 2006,26:382-395.
Nenad M, Pertti H. A comprehensive approach to cooling tower design[J]. Applied Thermal Engineering 2001,21 899-915.
Williamson N, Behnia M, Armfield S. Numerical Simulation of Heat and Mass Transfer in a Natural Draft Wet Cooling Tower[C].15th Australasian Fluid Mechanics Conference 2004:13-17.
Williamson N, Behnia M, Armfield S. Comparison of a 2D axisymmetric CFD model of a natural draft wet cooling tower and a 1D model[J]. International Journal of Heat and Mass Transfer,2008,51(9-10):2227-2236.
Williamson N, Armfield S, Behnia M. Numerical simulation of flow in a natural draft wet cooling tower:The effect of radial thermal fluid fields [J]. Applied Thermal Engineering 2008,28:178-189.
Goshayshi H R, Missenden J F. The investigation of cooling tower packing in various arrangements[J]. Applied Thermal Engineering,2000,20:69-80.
Kloppers J C, Kroger D G. Loss coefficient correlation for wet-cooling tower fills [J]. Applied Thermal Engineering,2003,23:2201-2211.
Kloppers J C. A critical evaluation and refinement of the performance prediction of wet-cooling towers[D]. Brunel University towers Department Mechanical Engineering, University of Stellenbosch,2003.
Kloppers J C, Kroger D G, KrOger. A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers[J]. International Journal of Thermal Sciences, 2005,48:765-777.
Colburn A P. A method of correlating forced convection heat transfer data and comparison with fluid friction[J]. Transactions of the AIChE,1993,29:174-180.
赵元宾.侧风对于自然通风逆流湿式冷却塔传热传质影响机制的研究[D].山东,山东大学热能工程,2009.
金台,张力,唐磊,等.自然通风湿式冷却塔配水优化的三维数值研究[J].中国电机工程学报,2012,32(2):9-15.
Klimanek A, Bialecki R A. Solution of heat and mass transfer in counterflow wet-cooling tower fills[J]. International Communications in Heat and Mass Transfer,2009,36(6):547-553.
上官志洪,张启明,陶云良.内陆核电厂冷却塔的环境影响预测计算[J].辐射防护,2009,4:211-218.
Du Preez A F, Kroger D G. The effect of the heat exchanger arrangement and wind-break walls on the performance of natural draft dry-cooling towers subjected to cross-winds[J]. Journal of Wind Engineering and Industrial Aerodynamics,1995,58(3):293-303.
Khan J R, Qureshi B A, Zubair S M. A comprehensive design and performance evaluation study of counter flow wet cooling towers [J]. International Journal of Refrigeration,2004,27:914-923.
Gharagheizi F, Hayati R, Fatemi S. Experimental study on the performance of mechanical cooling tower with tow types of film packing[J]. Energy Conversion and Management, 2007,48:277-280.