长距离环路热管传热性能实验研究

基于航空航天等领域对环路热管长距离传热的需求,设计制造了一套传热距离8.1m的圆柱型蒸发器环路热管,试验了不同加热功率、不同冷凝温度下该环路热管的启动和变工况运行性能,并对其热阻及最大传热能力进行了分析。研究结果表明：当其他条件一致、初始气液分布相同和不同时,加热功率由100W增大至160W后,本研究中的环路热管启动时间和启动温升均发生一定程度的下降;加热功率100W时,冷凝温度由10℃降低至-10℃使得环路热管启动时间增加,加热功率160W时,冷凝温度由10℃降至-10℃对环路热管的启动时间影响不大。在冷凝温度0℃下,该环路热管在100~500W范围内均能稳定运行,且200W时环路热管传热效率最高,传热温差最小,稳定运行温度最低;另外,由于系统传输距离较长,每个工况达到稳定所需要的时间也较长,分布于1000至3500S内。随着加热功率的增大,环路热管热阻先减小后逐渐增大,该环路热管传热热阻最大不超过0.09℃/W,最小为0.024℃/W;随着传热距离的增大,管路的热损失增加,总压降和热阻也变大。当传热距离基本相同时,蒸发器容积的大小、冷凝器的冷凝能力及气液管线的布置形状均在一定程度上影响环路热管的最大传热能力。     

基于中子成像技术环路热管的可视化试验研究

基于中子成像(NR)技术对铜-丙烯环路热管(LHP)在不同加热功率(0、5和10 W)下的运行进行了可视化研究。结果表明:在加热功率为5 W时蒸发器内液体工质在减少,冷凝器能够充分冷凝,液体管线充满液体工质;在加热功率10 W时热管内部冷凝的液体工质量在减少,蒸发器开始出现烧干现象;环路热管能够成功启动并稳定运行,且随加热功率增加启动时间减少;LHP在5 W时内部的气液分布使得传热性能最佳,在10 W时风扇的强制对流并不能达到充分冷凝的效果,导致热管热阻增大,性能变差;LHP运行过程中气体管线部分存在残留液体工质,这会减少LHP的工质实际循环量,降低其传热性能。     

甲醇乙醇混合工质振荡热管传热性能研究

混合工质可为振荡热管带来独特的传热性能．比较甲醇、乙醇纯工质以及甲醇-乙醇混合工质振荡热管在不同充液率时的热阻随加热功率的变化情况,结果发现：在小充液率（45％）时甲醇-乙醇混合工质和乙醇振荡热管开始烧干时的加热功率高于甲醇工质振荡热管;在加热功率不是很大（低于65W）和大充液率（62％～90％）时,甲醇以及甲醇-乙醇混合工质振荡热管的传热性能优于乙醇振荡热管;在大加热功率（高于65W）和大充液率（62％～90％）时甲醇以及甲醇-乙醇混合工质振荡热管的热阻十分接近,均低于乙醇工质振荡热管的热阻,且热阻随着充液率的增加曲线变化越来越平缓．     

氨工质平板式环路热管的传热性能研究

为了解决平板式蒸发器环路热管面对高集成电子元器件在较长传热距离下的散热能力不足等问题,本文采用更大面积的毛细芯和蒸发器,研制了一个传热距离达到1.6m的氨-不锈钢环路热管系统并进行了大量实验.实验过程中,加热壁面温度控制在70℃以下.实验结果表明,在热沉温度为-10℃的情况下,该环路热管可以在2.5W到180 W的热负荷范围内稳定运行,相应的热流密度是0.15 W/cm2和10.8W/cm2.蒸发器热阻的最小值为0.096℃/W,系统热阻的最小值为0.252℃/W.系统存在变热导率和定热导率两种工作特性,实验过程中没有观察到明显的温度过冲和温度波动现象.在启动测试和变热负荷测试中,系统均能够快速响应,展现出优良的可靠性.研究表明,与一般的平板式蒸发器环路热管相比,该系统更好地兼顾了传热距离和传热能力,拓展了平板式蒸发器的应用场景.     

不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

双层毛细芯对环路热管传热性能的实验分析

环路热管作为一种高效的相变传热装置,其性能与位于蒸发器和储液槽之间的毛细芯结构密切相关。为了更深入研究双层毛细芯对环路热管传热性能的影响,利用不同颗粒直径铜粉制备双层毛细芯,在毛细芯总厚度为5 mm的条件下,通过调整大粒径和小粒径层的相对厚度来改变毛细芯厚度比,对平板型蒸发器环路热管启动和变工况运行进行实验测试。实验结果表明:在同一工况下,不同厚度比的双层毛细芯启动特性存在显著差异,启动过程中出现小粒径层蒸发效率低引起的温度过冲和环路热管中气液两相流变化导致的温度振荡;同时存在一个较优的双铜层毛细芯厚度比,大粒径(180～280μm)铜层厚度为3 mm可提高蒸发效率,小粒径(56～71μm)铜层厚度为2 mm可提供足够毛细抽吸力保证环路热管稳定运行。搭载该厚度毛细芯的环路热管不仅启动速度快(125 s),而且总热阻和蒸发器壁面温度均最低,最大加热功率达到120 W(21.10 W/cm~2),对应热阻为0.17 K/W。     

脉动热管水平及小倾角条件下的传热性能

为了研究脉动热管放置方式对其传热性能的影响,以超纯水作为工质,对水平及倾角为30°放置的脉动热管的传热性能进行研究,用壁面温度振荡性能和传热热阻来描述其传热能力。在不同的放置条件下,着重分析不同加热功率和充液率(35%,50%,70%)对其传热性能的影响。研究表明：水平放置时,充液率为35%和50%时脉动热管不能启动,充液率70%时可以启动运行;脉动热管在运行时存在临界热量输入值,倾角为30°时,临界值为60 W,但水平放置条件下临界值为90 W;水平放置下的脉动热管传热热阻在不同加热功率下,显著高于倾角为30°的情况;倾角为30°,充液率为35%时的脉动热管适合在低加热功率范围运行,此时传热热阻要低于充液率为50%的情况,但传热范围很窄,传热极限低;30°倾角时,与充液率35%和50%相比,高充液率70%的脉动热管整体传热性能最优。     

平板蒸发器环路热管启动及充灌率特性研究

结合环路热管系统的优点与太阳能的广泛利用前景,设计并搭建了以太阳能利用为背景的新型毛细芯平板蒸发器环路热管系统,采用泡沫镍为毛细芯、乙醇为工质,实验研究了系统的启动运行特性,以及不同的工质充灌率对环路热管系统性能的影响。结果显示,在实验条件下,环路热管热源功率在300~1 600 W时具有良好的启动运行特性,55%的充灌率为最佳充灌率,具有更短的启动时间,相对更低的蒸发器温度与热阻。     

一种纵向供液式环路热管传热性能研究

为解决高热流电子元器件的小空间散热问题,本文研制了一种纵向供液式甲醇-铜环路热管系统,并搭建实验测试平台。对该系统在水平状态与重力辅助状态下进行了启动与变负荷性能测试。实验结果表明:加热壁面温度不超过100℃,水平状态下系统能在热负荷20～60 W成功启动运行,最大热流密度8.3 W/cm~2,最小热阻0.139℃/W。重力辅助状态下系统能在热负荷10～150 W成功启动运行,最大热流密度20.8 W/cm~2,最小热阻0.123℃/W。研究表明纵向供液结构可以极大减小环路热管系统厚度,但也带来了供液能力不足和漏热加剧的问题,对蒸发器结构进行优化设计可以提升纵向供液LHP系统性能。     

石墨烯/丙酮纳米流体振荡热管传热特性研究

通过实验研究了不同质量浓度的石墨烯/丙酮纳米流体振荡热管不同充液率下的传热性能。结果表明,小充液率(45%)下,石墨烯/丙酮纳米流体振荡热管的热阻均小于纯工质丙酮,但烧干现象并没有得到明显改善;中等充液率(62%~70%)下,石墨烯/丙酮纳米流体振荡热管较纯工质丙酮来说不再发生烧干现象,纳米流体振荡热管的热阻随着加热功率的增加而明显降低,浓度为0.01%时具有较为明显的传热优势;大充液率(90%)下,石墨烯/丙酮纳米流体振荡热管的传热性能则普遍优于纯工质,且随着加热功率的增加,传热性能的优势更加明显。     

An Experimental Study on the Performance of a Stainless Steel-Water Loop Heat Pipe under Natural Cooling Condition简

Aiming to improve the thermal characteristics of modern electronics, we experimentally study the performance of a stainless steel/water loop heat pipe （LHP） under natural cooling condition. The LHP heat transfer performance, including start-up performance, temperature oscillation and total thermal resistance at different heat loads and with different incline angles have been investigated systematically. Experimental results show that at an optimal heat load （i.e. 60 W） and with the LHP being inclined 60~ to the horizontal plane, the total thermal resistance is lowered to be -0.24 K/W, and the temperature of evaporator could be controlled steadily at around 90~C.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

低温环路热管综述

环路热管是以多孔毛细芯抽吸力为动力的相变传热设备,可根据实际应用改变结构形式,能在远距离传热的同时保持良好的均温性,并且可在微重力环境下运行。环路热管工作温区较广,按照其工作温区一般可分为高温环路热管(350 K以上)、常温环路热管(200～350 K)和低温环路热管(200 K以下)。为了满足深空探测的需要,低温环路热管广泛应用于航天设备温控系统中并表现出优异的性能。按照孔隙特征和结构形式将用于环路热管的毛细芯分为四种,简要阐述每种毛细芯制备和特点;综合分析了近年来低温环路热管技术主要理论和实验研究成果,将目前低温环路热管常见的工作温区分成五个部分,分析影响低温环路热管传热性能的因素,包括工质充装量、反重力高度、次蒸发器功率等。最后,提出优化措施以满足未来深空以及地面应用的需求。     

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.     

Flat Loop Heat Pipe with Bi-Transport Loops for Graphics Card Cooling

A flat loop heat pipe (FLHP) with bi-transport loops is developed for the cooling of graphics card with high heat flux up to 80W/cm². The evaporator and the pipes are made of copper and ultrapure water (electronic resistivity > 18 MΩ-cm) as the working fluid. To give the loop heat pipe (LHP) better performance, the evaporator is made in a flat shape to reduce the contact resistance between the evaporator and the chip. The advances of the LHP with bi-transport loops are discussed. The heat transfer performance is tested with different filling rate in different orientations. The test results show that the LHP can start up easily and can transport large amount of heat stably. The orientation of the condenser above the evaporator gives a better performance, and filling with 13 g of water gives a better performance. Limited by the evaporator temperature lower than 90°C, the LHP can transport 320 W when the evaporator is above the condenser and 380 W when the condenser is above the evaporator.     

Numerical simulation of transient heat and mass transfer in a cylindrical evaporator of a loop heat pipe

The paper investigates the transient processes of heat and mass transfer in a cylindrical evaporator of a loop heat pipe (LHP) during the device start-up. One of the most “arduous” prestart situations, which is characterized by the absence of a liquid in the evaporator central core and filled vapor removal channels, has been considered. With such liquid distribution a successful start-up of an LHP becomes possible only after formation of the vapor phase in the vapor removal channels and their liberation from the liquid. The aim of the investigations is to determine conditions that ensure the boiling-up of a working fluid in vapor removal channels. The problem was solved by a numerical method. Simulation of start-up regimes has been performed for different heat loads and different structural materials of the evaporator. Copper, titanium and nickel wick have been examined. Calculations have been made for three different working fluids; water, ammonia and acetone. Account has been taken of the conditions of heat exchange between the compensation chamber and surrounding medium.     

Evaporation Heat Transfer Coefficient in a Capillary Pumped Loop and Loop Heat Pipe for Different Working Fluids

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive two-phase heat transport devices. They have been gaining importance as a part of the thermal control system of spacecraft. The evaporation heat transfer coefficient at the tooth–wick interface of an LHP or CPL has a significant impact on the evaporator temperature. It is also the main parameter in sizing of a CPL or LHP. Experimentally determined evaporation heat transfer coefficients from a three-port CPL with tubular axially grooved (TAG) evaporator and a TAG LHP with acetone, R-134A, and ammonia as working fluids are presented in this paper. The influences of working fluid, hydrodynamic blocks in the core, evaporator configuration (LHP or CPL), and adverse elevation (evaporator above condenser) on the heat transfer coefficient are presented.     

Modeling and analysis of startup of a loop heat pipe

The startup behavior of a loop heat pipe (LHP) is one of the key aspects in evaluating its working performance. A mathematical model of the startup process of a LHP is established based on the node network method in this work. A parametric analysis on the startup characteristics of the LHP is conducted, where the effects of startup heat load, thermal capacity of the evaporator and compensation chamber, heat sink temperature, ambient temperature, heat leak from the evaporator to the compensation chamber and cooling on the compensation chamber on the startup characteristics of the LHP are evaluated, which contributes to the improvement of the LHP startup performance.     

Design of miniature loop heat pipe

In the present study, the loop heat pipe (LHP) was miniaturized for application to electronic cooling. According to the capillary limitation, the wick structure parameters that would affect the heat transfer capacity were analyzed theoretically. Among the various wick parameters, this study especially investigated the effect of wick thickness, which has rarely been mentioned in the literature. Here, various thicknesses were analyzed theoretically and then tested experimentally. The results showed that the temperature on the evaporator wall dropped with decreasing wick thickness. This effect would lead to the raising of heat transfer capacity and the descending of thermal resistance. According to the analysis and the practical demand for electronic cooling, a miniature LHP was fabricated with the evaporator outer diameter of 13 mm and the evaporator length of 50 mm. The testing results showed that, at the allowable working temperature of 80 °C, the maximum heat transfer capacity was up to 200 W and the thermal resistance was 0.17 °C/W. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 42–52, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10133