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。     

CONDENSING HEAT TRANSFER IN AN ADVANCED TWO-PHASE CLOSED THERMOSYPHON

1 INTRODUCTIONClosed thermosyphon has been developed to enhance heat transfer and recover wasteheat in various process industries [1,2].Stimulated by this success,a new type oftwo-phase closed thermosyphon was designed by inserting respectively two inner tubesinto the thermosyphon,one in the boiling section and the other in the condensing sec-tion.The two-phase flow boiling heat transfer coefficient was calculated successfully onthe basis of Chen's dual-mechanism [3].A boiling heat transfer model for thetwo-phase closed thermosyphon with an inner tube in the boiling section was pro-     

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%。     

DAM两相闭式热虹吸回路冷却系统实验研究

两相热虹吸回路由于较高的散热性能在高功率电子设备冷却领域有较好的应用前景。为了解决四通道数字阵列模块的冷却问题,本文设计了一套两相闭式热虹吸回路冷却系统样机并对系统启动特性、充液量和工作倾角对系统散热性能的影响进行了实验研究。研究结果表明,样机系统结构及散热性能满足指标要求,启动性能和工作性能良好。此外,该系统对热耗1600 W、局部热流密度接近100 W/cm~2的组件的冷却效果良好。     

同轴型热虹吸系统轴向传热特性实验研究

对一种用于冷中子源系统的自然循环进行了轴向传热特性研究,建立了以氟里昂为工质的同轴型热虹吸管传热实验系统。研究表明:同轴型热虹吸系统可强化轴向热传输能力,其传热工况有过冷沸腾流动振荡工况、稳定工况和烧干工况3类。在低功率区,过冷沸腾和携带机制造成启动过程的两相流振荡;进入稳定工况区后,系统具有自调适能力,能够维持恒定的循环质量流率;在高功率区,蒸发器烧干产生传热极限。本研究显示出同轴型热虹吸系统的动力特性与普通自然循环系统有明显区别,其循环机制与普通热虹吸系统有本质区别。     

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。     

多年冻土区宽幅公路路基降温效果研究——一种L型热管–块碎石护坡复合路基

为维护多年冻土区宽幅高等级公路的热稳定性及保护下伏冻土,提出了一种 L 型热管–块碎石护坡保温板复合路基。为研究这种新型路基的降温效果,进行了室内模型试验,试验结果初步证明了这种路基的有效降温能力。为研究这种新型路基的长期效果,采用相关理论,结合室内模型试验的结果,依据青藏高原多年冻土区宽幅高等级公路的气温和地质条件,利用数值方法对有无 L 型热管的块碎石护坡保温板复合路基的温度特征进行了分析和比较。结果表明：在年平均气温为 - 4.0 ℃的青藏高原多年冻土区,考虑未来 50 a 气温上升 2.6 ℃的条件下,新的复合路基能够从整体上有效降低路基下土体的温度,确保高温多年冻土区宽幅高等级公路路基的热稳定性。     

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.     

A waste heat recovery system development and analysis using ORC for the energy efficiency improvement in aluminium electrolysis cells

The present work investigates an active waste heat recovery system for the side walls of the aluminium electrolysis cells, enabling utilization of the extracted heat in power generation. This will potentially lead to energy efficiency improvement in the primary aluminium production industry and an enhanced aluminium production rate. An experimentally validated loop thermosyphon heat pipe model was used for heat extraction from the cell side wall. Boosting system thermal efficiency through waste heat recovery, by means of a heat utilization system, and increasing the level of control, as well as thermal equilibrium, stand as the main addressed objectives of the current study, which consequently result in an increased aluminium production rate. An organic Rankine cycle is incorporated into the system, and its performance is evaluated, taking into consideration the operating situations in terms of available temperature and thermal power range.     

Experimental study of complex two-phase instabilities during the start-up of a vertical thermosyphon reboiler operating under vacuum

This paper reports experimental studies of the start-up at reduced pressures (0.105–0.171 bar) and low heat fluxes (1.23–1.55 kW m⁻²) of the thermosyphon reboiler research facility in the Morton Laboratory at the University of Manchester. This is a full scale replica of an industrial sized natural circulation thermosyphon reboiler comprising 50 vertically mounted 25 mm OD tubes of 3 m length with water as the process fluid in the tubes and with steam condensing in the shell side of the reboiler. Unstable behaviour is obtained at start-up with flow-induced instabilities (geysering) when the level of the liquid within the tubes of the bundle is up to the top tubesheet (100% submergence) and also at 83% submergence of the tubes irrespective of the process pressure. At 100% submergence of the tubes, intermittent reversed flows in the entire loop with substantial vibrations are obtained. Chaotic behaviour is obtained at the lower submergence level of 83%. However, start-up at process pressures of 0.22 and 1.04 bar, and low heat fluxes with the entire thermosyphon loop flooded with liquid completely alleviated the flow-induced instabilities observed at the lower submergence levels within the tubes. The flow in the closed loop of liquid is induced very soon after the steam is introduced into the reboiler due to the buoyancy forces emanating from differences in density around the loop. The start-up is smooth and relatively quick, but further instrumentation and control are required for the introduction of the feed to the thermosyphon system once vapour is generated. A control strategy and procedure is proposed which will avoid the instabilities that inherently occur with start-up procedures currently practised in industry.