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.     

两相闭式热虹吸管冷凝换热规律的实验研究

本文通过可视化热虹吸管实验,拍摄了冷凝段内部冷凝液的各种基本流型和特殊流型,设计了试验用多节风冷热虹吸管以及相应的风道系统。采用自行组装的高精度微机控制的测试系统测量温度,对不同工质(水、乙醇、丙酮、氟里昂-11)热虹吸管进行了不同工况的实验。获得了各种工程实际工况下的冷凝换热系数,还获得了冷凝换热系数随热流密度、蒸汽压力变化的规律以及沿2.5米长冷凝面长度上的变化规律。实验结果表明,热虹吸管的冷凝换热系数在很大范围内偏离努塞尔特理论解。本文结合可视化实验,对其进行了分析。     

机械制冷/回路热管一体式机房空调系统研究

降低数据中心冷却系统能耗,已成为节能减排的迫切要求。热管自然冷却是降低这一能耗的有效方法之一。为解决热管自然冷却在炎热季节需要机械制冷辅助的问题,进一步提升热管自然冷却的节能潜力,本文提出一种新型机械制冷/回路热管一体式机房空调系统。利用三介质换热器将机械制冷回路和回路热管耦合起来,实现了二者的同时或单独工作,避免了现有系统依赖电磁阀切换带来的可靠性隐患。利用焓差实验台对系统性能进行了实验研究。实验结果表明,系统最佳充液率为100%左右。三个工作模式均具备良好的制冷能力,热管模式EER值在20℃温差下达20.8。以济南地区的数据中心为例进行了全年能耗计算,计算结果表明这一新型系统与传统空调系统相比节能40%,有着良好的应用前景。     

IDC机房用热管换热器节能特性试验研究

机房空调能耗约占机房总能耗的40%。在IDC机房节能方案中,使用热管换热器利用自然冷源散热,能够减少空调工作时间,同时可避免室内外空气接触,满足湿度、洁净度的要求。参考北京某IDC机房,搭建试验模型,分析IDC机房内热管换热器和空调各自的散热负荷和能耗特性,研究围护结构的散热特性,比较设定温度和室外温度对空调能耗的影响。结果表明,北京冬季工况下,仅依靠围护结构的散热,无法满足IDC机房的散热需求;围护结构散热量约占IDC机房总散热量的19.5%;空调平均每天耗能3.5~4 kWh;使用热管换热器室内外温差不超过20℃,能耗仅为空调能耗的41%,全年节能约40%;与室内设定温度相比,室外气温对空调能耗的影响较大;室外温度提高1℃,空调能耗平均增加5%~6%。     

两相热虹吸循环蒸发侧传热模型比较

两相热虹吸换热器在工业领域应用广泛,其制冷剂内侧的传热过程是热虹吸管设计的基础。然而由于沸腾传热和两相流动的复杂性,基于实验的经验传热模型各自差异很大且适用范围有限,给热虹吸管建模时传热模型选取带来了一定的困难。建立了稳态两相热虹吸循环分布式参数模型,利用5篇文献中4种工质共424个热虹吸管传热实验数据点,对热虹吸管中常用的7种传热模型进行比较评价,为模型选取提供参考。结果发现:Kandlikar流动沸腾模型和Rahmatollah拟合的热虹吸管传热模型模拟精度最好,推荐使用;Cooper池沸腾也有较好的精度,说明核态沸腾在热虹吸管传热中占主导地位; Gungor and Winterton、Liu and Winterton等常用的流动沸腾模型模拟精度差别不大,误差尚可接受;Imura池沸腾模型不适用于热虹吸管传热模拟。     

电子器件冷却用重力型热管散热器的实验研究

用发热铜块模拟电子器件 ,油泵回路控制风温 ,毕托管和倾斜式微压计测量风速等方法 ,建立了热管型散热器性能测试系统。对所设计的重力型热管电子器件散热器 ,通过改变散热功率、风速、风温等因素来测试电子器件表面温度的变化。实验结果表明 :重力型热管散热器具有良好的散热性能 ,可满足较高热流密度 (小于 8.5 6× 1 0 4 w/m2 )电子器件的冷却要求。性能测试系统具有良好的精度和可靠性 ,可以作为改进散热器设计的重要手段     

切向漩流半开式热管稳态传热性能的实验研究

为了强化半开式热管的传热效果,提出了一种在半开式热管凝结段的侧端切向对称开孔的方法.通
过改变侧端切向开口的位置和侧端切向开口孔径的大小,研究了切向漩流半开式热管的沸腾换热和凝结换
热特性,以及这种结构的变化对切向漩流半开式热管总体传热性能的影响.实验结果表明,沸腾段的换热
系数随着开孔的相对位置增大而降低,随着侧端开口直径的增大而增大;开孔相对位置在30％～50％时,
凝结换热系数较低;凝结换热系数随侧端开口直径的增大而增大,侧端开孔相对位置越小,侧端开口直径
越大,总体传热性能越好.     

槽式太阳能集热器中萘热虹吸管启动特性的研究

对槽式太阳能集热器中萘热虹吸管的启动特性进行了理论分析,并对萘热虹吸管在不同受热方式、不同倾角和不同充液率下的启动特性进行了试验研究,同时对4°倾角、半周向受热方式下的启动特性进行了试验与理论计算的对比.结果表明:萘热虹吸管在较小工作倾角、半周向受热方式下能正常启动和稳定运行;受热方式对萘热虹吸管启动特性的影响不大;充液率对冷凝段开始启动的时间有一定的影响,充液率越小,开始启动所需的时间越短;冷凝段转折温度的试验值稍低于理论计算值,启动过程与理论分析过程基本一致.     

Investigation on the effect of filling ratio on the steady-state heat transfer performance of a vertical two-phase closed thermosyphon

Filling ratio of the working fluid has a predominant effect on the heat transfer characteristics of a two-phase closed thermosyphon (TPCT). A comprehensive model is developed to investigate the effect of filling ratio on the steady-state heat transfer performance of a vertical TPCT. Three types of flow pattern and two types of transition, according to the distribution of liquid film and liquid pool, are considered in this model, while other models generally focus on only one or two types of them. The total heat transfer rate of liquid pool, including those of natural convection and nucleate boiling, is calculated by combination of their effective areas and heat transfer coefficients. New correlations of the effective area are proposed based on the experimental results from other study. Two different geometries of the TPCT with nitrogen as working fluid are performed experimentally, and the evaporator temperatures accord well with the theoretical calculation. And the calculated results are compared with those by other empirical heat transfer correlations for liquid pool. The range of filling ratio, which can keep a TPCT steady and effective, is proposed based on analysis and comparison. The effects of heat input, operating pressure and geometries of the TPCT on the range are also discussed.     

Cooling load reduction of building by seasonal nocturnal cooling water from thermosyphon heat pipe radiator

In this study, a concept of using thermosyphon heat pipe to extract heat from water in a storage tank to generate cooling water was proposed. Heat pipe condenser was attached with an aluminum plate and acted as a thermal radiator while its evaporator was dipped in the water storage tank. Cooling water in the tank could be produced during the nighttime and used to serve the cooling load in a room during the daytime. A heat transfer model to calculate the water temperature and the room temperature during both the nighttime and daytime was developed. The input data were ambient temperature, dew point temperature, area of the radiator, volume of cooling water and room cooling load. The experiment was setup to verify the heat transfer model. A 9.0 m² tested room with six cooling coils, each of 0.87 m² was installed at the ceiling, was constructed along with the 1.0 m³ water storage tank. A 500–2000 W adjustable heater was taken as an artificial load inside the room. A 6.36 m² radiator is installed on a 45° tilting roof of the tested room. The simulated results agreed very well with those of the experimental data. With the developed model, a simulation to find the sizing of the radiator area and the volume of cooling water for cooling water production during winter of Chiang Mai, Thailand was carried out. The cooling water was used for cooling during summer in an air‐conditioned room with different cooling loads. The parameters in terms of room temperature, radiator area, volume of cooling water, cooling load and UA of cooling coil were considered to carry out the percent of cooling load reduction. Copyright © 2009 John Wiley & Sons, Ltd.