双流程凝汽器热力及水力特性分析

电站汽轮机配备的凝汽器大多采用双流程，冷却水的流径有上进下出和下进上出之分。作者从凝汽器的基本和和水力特性分析着手，根据有关的特性参数在凝汽器内的分布规律，研究了不路径对凝汽器性能的影响。结果表明，采用上进下出的冷却水流径，可以有效地减小凝汽器内的汽阻和凝结水的过冷度，改善冷却水侧的压力分布，明显降低所需的循环水泵扬程和冷却水漏入汽侧的可能性。     

冷却水流程数对凝汽器热力性能的影响

电站凝汽器可以设计成冷却水双流程和单流程。利用自行开发的凝汽器工作特性数值模拟软件PPOC3．0模拟和分析了冷却水流程数对电站凝汽器的热力性能的影响,结果表明单流程凝汽器比双流程凝汽器有更高的热负荷和更小的汽侧阻力。     

冷却水流径对于凝汽器性能影响的数值模拟分析

文中应用STAR-CD软件和用户自定义子程序,针对3种类型凝汽器进行数值模拟。对于每种凝汽器,分别计算上进下出和下进上出两种冷却水进水方式时凝汽器壳侧的流场分布,分析评估冷却水流径对于凝汽器运行性能的影响。     

双流程不等距布管式单背压凝汽器

以清华大学研制的性能优良的仿生双连树形管束式凝汽器为范例,介绍了第一流程与第二流程冷却管等距布置型凝汽器的结构,并分析其流场和换热特性,继而提出第一流程冷却管大间距、第二流程冷却管小间距的疏密管束优化布置方案,并进一步优化循环水流程为上进下出,两方面提高凝汽器换热效果,推荐工程设计上优先选用循环水为上进下出的双流程不等距布管式凝汽器。     

考虑物性变化的凝汽器水侧流场数值研究

在管束区域采用多孔介质模型的基础上,通过考虑冷却水吸热升温后物性的变化,对凝汽器水侧流动特性的数值模拟进行一定的改进。对某电厂双流程凝汽器水侧流场数值模拟的结果对比表明:冷却水温度变化对凝汽器水侧流动特性存在一定的影响,主要表现在冷却水密度的变化引起进水室和下流程的流动阻力的变化。此外,分析表明考虑冷却水温升与否对管束区速度分布均匀性的模拟影响不大。     

电站凝汽器非设计工况汽相流动与换热特性的数值预测

采用数值模拟方法计算与比较了一台300MW汽轮机对分式凝汽器在半负荷工况时冷却水管全部工作和一半冷却水管投入工作两种运行方式下的汽相流动与传热特性。计算结果表明，两种运行方式下的汽相流动与传热特性有明显差别。在一半冷却水管运行方式下，虽然管束传热系数较高，但由于冷却面积减半，而且汽阻显著增大，因而凝汽器总体传热效果较冷却水管全部运行时的差，使得抽气口处未凝结蒸汽量上升，空气泵负荷增大。     

600MW级双压凝汽器结构分析

多压凝汽器是现代大型电站凝汽器发展的一个重要方向，多压凝汽器比相同冷却面积和冷却水流量的单压凝汽器具有更低的平均凝汽压力，因而可以提高机组的效率和出力，近年来在我国建造的600MW及以上机组中得到广泛应用。     

双流程电站凝汽器水室的流动分析与改造

为能够准确预测凝汽器水侧流动特性,应用计算流体力学商用软件,管束区域采用多孔介质模型,对某电厂双流程凝汽器水室流场进行了数值分析。结果表明:该电站双流程凝汽器入口水室结构存在一定的缺陷,导致冷却水在局部产生了较大的阻力,形成了明显漩涡。针对凝汽器水室中漩涡产生的原因,对凝汽器水室结构进行了优化改造,并且在此基础上提出了更多合理的建议。     

基于喉部出口流场的电站凝汽器管束汽流激振评估的数值研究

管束汽流激振现象的避免可以提高电站机组凝汽器的安全保障。采用数值模拟方法,对凝汽器喉部的蒸汽流动进行模拟,在基于喉部出口流场分布的基础上,选用判别汽流激振的工程经验公式——Peake C.C.公式完成了对管束汽流激振的评估研究。研究结果表明,不同工况下喉部出口的流场分布有所不同,在喉部出口二维截面上评估出的可能发生汽流激振的位置也不同,但部分区域在不同工况下均存在较大的汽流激振可能性,建议在该区域采取防范措施。     

330MW机组凝汽器改造的数值模拟及性能分析

潍坊电厂330MW机组的凝汽器改造前存在着真空低和水质变差导致铜管泄漏趋势加重的现象,严重地影响了机组的经济性、安全性和带负荷能力。采用TEPEE两山峰形管束布置方式对凝汽器进行改造,利用数值模拟的方法计算了改造后凝汽器壳侧的汽相流动与传热特性,并用改造前后的性能试验结果进行了验证。模拟结果表明:改造后的TEPEE两山峰形管束布置方式有利于凝汽器壳侧蒸汽的流动与传热。性能试验结果表明:在设计冷却水进口温度、流量、凝汽器热负荷相同的条件下,改造后的凝汽器真空比改造前提高1.743kPa,端差下降4.63℃;凝汽器改造后,试验端差和修正后端差、修正后的凝汽器压力、循环水温升都达到设计指标。     

Effect of Cooling Water Flow Path on the Flow and Heat Transfer in a 660 MW Power Plant Condenser

The effect of the cooling water flow path on the flow and heat transfer in a double tube-pass condenser for a 660 MW power plant unit was numerically investigated based on a porous medium model. The results were used to analyze the streamline, velocity, air mass fraction and heat transfer coefficient distributions. The simulations indicate that the cooling water flow path is important in large condensers. For the original tube arrangement, the heat transfer with the lower-upper cooling water flow path is better than that with the upper-lower cooling water flow path. The reason is that the steam cannot flow into the internal of upper tube bundle and the air fractions are higher in the upper tube bundle with the upper-lower cooling water flow path. An improvement tube arrangement was developed for the upper-lower cooling water flow path which reduced the back pressure by 0.47 kPa compared to the original scheme. Thus, the results show that the tube arrangements should differ for different cooling water flow paths and the condenser heat transfer can be improved for the upper-lower cooling water flow path by modifying the tube arrangement.     

不锈钢螺纹管换热机理的探讨

从理论上对不锈钢螺纹管强化传热进行了分析,并以N-2200型和N-3500型凝汽器设计为基础,从水阻、泵耗、换热系数及真空值等方面,对不锈钢螺纹管与不锈钢光管、铜光管进行了计算比较,显示出：在以污染的淡水为冷却水的凝汽器冷却管材选择上,不锈钢螺纹管具有极好的推广价值。     

Performance Characteristics of a Closed-Circuit Cooling Tower With Multiple Paths

The performance of a closed-circuit wet cooling tower (CWCT) with multiple paths having a rated capacity of 9 kW has been studied experimentally. When the CWCT has to operate with a partial load, the required quantity of cooling water reduces and thereby the velocity of the process fluid inside the tubes decreases. The velocity of the process fluid can be increased by installing blocking tubes in the heat exchanger. The test section in this experiment has multiple paths that have been used as the inlet for cooling water that flows from the top part of the heat exchanger. The heat exchanger consists of eight rows and 12 columns and the tubes are in a staggered arrangement. Heat and mass transfer coefficients and temperature drops were calculated with several variations including multiple paths. The results obtained from this study were compared with those reported and found to conform well. The investigation indicates that a CWCT operating with two paths has higher heat and mass transfer coefficients than with one path.     

R1234ze(E)与R134a在水平双侧强化管外的凝结换热对比实验

搭建了水平单管降膜蒸发试验台,以第四代制冷剂R1234ze (E)和第三代制冷剂R134a作为工质,在新型水平双侧强化管管外分别进行了改变管内水速、热流密度和冷凝温度条件的凝结换热实验.使用Wilson-Gnielinski图解法计算得到管内表面传热系数hi,进一步采用热阻分离法分离出两种制冷剂的管外表面传热系数,并分...     

Optimization of cold-end system of thermal power plants based on entropy generation minimization

Cold-end systems are heat sinks of thermal power cycles, which have an essential effect on the overall performance of thermal power plants. To enhance the efficiency of thermal power plants, multi-pressure condensers have been applied in some large-capacity thermal power plants. However, little attention has been paid to the optimization of the cold-end system with multi-pressure condensers which have multiple parameters to be identified. Therefore, the design optimization methods of cold-end systems with single- and multi-pressure condensers are developed based on the entropy generation rate, and the genetic algorithm (GA) is used to optimize multiple parameters. Multiple parameters, including heat transfer area of multi-pressure condensers, steam distribution in condensers, and cooling water mass flow rate, are optimized while considering detailed entropy generation rate of the cold-end systems. The results show that the entropy generation rate of the multi-pressure cold-end system is less than that of the single-pressure cold-end system when the total condenser area is constant. Moreover, the economic performance can be improved with the adoption of the multi-pressure cold-end system. When compared with the single-pressure cold-end system, the excess revenues gained by using dual- and quadruple-pressure cold-end systems are 575 and 580 k$/a, respectively.     

Heat transfer performance of a double-loop separate-type heat pipe: Measurement results

An experimental study is presented for the heat transfer performance of a rectangular double-loop natural circulation system, in which the condensers are made of double tubes with water-steam as the working fluid. Detailed temperature measurements of the core fluid and the wall are made, from which overall heat transfer coefficients for the evaporator, condensers, and entire system are obtained. Parametric studies of the liquid charge level, fluid properties, and heating or cooling conditions on the heat transfer performance are presented and correlation equations are given. The results show that the overall heat transfer coefficients for the evaporator, condensers, and entire loop are all increasing with decreasing liquid charge level. Overhead phenomena at low liquid charge level and thermal oscillation at some situations are also observed and discussed.     

双排管外空气流动和传热性能的数值研究

矩形翅片椭圆管,即双排管是直接空冷凝汽器的一种基本换热元件.研究了双排管外空气侧的流动和传热性能.在不同迎面风速下,对双排管空气侧进行了三维数值模拟,并对速度场、温度场进行了分析.拟合出双排管阻力和平均传热系数随迎面风速变化的计算公式.     

凝汽器水侧流动的三维数值模拟

建立了某一凝汽器实际管束的流动计算模型,运用计算流体力学的方法,对该凝汽器水侧流场进行了三维数值模拟。采用分区对称计算方法,大大降低网格数量,从而详细预测了凝汽器水侧进出口水室以及其连接管和冷却水管束内的流动特性。计算结果可以清楚地表明:该凝汽器进口水室存在大量漩涡,使流动阻力增加,流动恶化;水室速度分布不均匀,进口水室管板中心区域流体流速较高而边缘区域较低,结构上存在一定问题;而出口水室的结构较为合理。这与采用多孔介质模型模拟的结论一致,进一步验证了采用多孔介质模型对凝汽器进行计算是正确可行的。计算结果同时表明,冷却水管束内流量和流速的分布是不均匀的,位于中心位置的冷却水管流量较大,而周边区域较小,最大流量差别可达到38%,且相邻管路的流量减小幅度与冷却水管布置有关。冷却水管内冷却水流量和流速的差异将会影响换热器的换热性能。计算结果可为分析研究管排流动不均而引起的换热效率问题提供条件,也可为凝汽器设计和结构优化提供依据。     

不可凝气体对大扁管空冷凝汽器性能的影响分析

为了研究不可凝气体(non-condensable gases, NCG)对火电与光热发电机组上广泛使用的大扁管空冷凝汽器性能的影响,以工程机组凝汽器上普遍应用的通流面积220 mm×20 mm的大扁管为研究对象,针对汽轮机典型工况下的实际蒸汽流量,基于Lee相变方程、VOF方法以及组分扩散模型,对蒸汽与NCG混合气体管内两相流凝结换热进行数学建模与数值计算。结果表明：由于大扁管的狭窄通流几何结构与高蒸汽流量,NCG对管内蒸汽凝结的抑制效果要远低于预期;当入口空气质量分数按2%增加时,凝结管凝结换热系数仅下降2%左右,这与NCG导致低流量圆管凝结性能急剧下降的结论不同;空气正常泄漏不会导致空冷凝汽器性能下降而影响发电机组效率。