水-甲醇混合工质振荡热管温度振荡及传热特性研究

通过实验研究了水-甲醇混合工质振荡热管充液率分别为55%、62%、70%、90%下,体积比分别为13∶1、4∶1、1∶1、1∶4、1∶13时的温度振荡以及热阻特性,并与水、甲醇纯工质振荡热管进行了对比。研究表明,混合工质振荡热管的振幅较纯工质大且较为均匀;较大充液率时(62%及以上),大多数混合工质振荡热管热阻大于两种纯工质;中等充液率时,部分振荡热管出现烧干现象,受黏度影响,大多数混合工质振荡热管出现烧干现象时的功率较小,热阻较纯工质大。但是,在水中加入少量甲醇(13:1)做工质的振荡热管在充液率为55%下不易发生烧干,且热阻较其他工质低。振荡热管的热阻特性受工质黏度、气液相平衡等因素的共同作用,在不同工况下,各因素作用效果不同。     

用于脉动热管的二元混合工质热力特性探讨

脉动热管是一种新型传热元件。工质种类的选取影响脉动热管的运行与传热,因此应根据脉动热管的用途及相应的温度范围选择适当的工质。提出选取二元混合工作介质(简称混合工质)作为脉动热管的工作介质。分析二元混合工质的露点温度或泡点温度与工质的种类、配比、压力之间的关系,以选取适合脉动热管传热特性的二元混合工质热力参数,得到选取混合工质的相应方法及注意事项。     

混合工质热力参数特性与脉动热管适应性研究

以PR状态方程、逸度系数方程、混合法则、相平衡方程组为基础,求解出混合工质特定压力下的泡点温度和露点温度,从而绘制出二元混合工质在特定压力下的气液相平衡图。分析了二元混合工质的露点温度或泡点温度与工质的种类、配比、压力之间的关系。以选取适合脉动热管传热特性的二元混合工质热力参数为例,得到选取混合工质的相应方法及注意事项。     

闭式回路型脉动热管的运行状况与传热性能试验研究

为深入了解闭式回路型脉动热管的运行状况与传热性能,建立紫铜制脉动热管试验平台,分析在风冷条件下加热功率、倾斜角度、充液率和工质等因素对脉动热管运行状况及传热性能的影响,并对各点温度波动原因进行分析。研究结果表明,温度波动幅度可以反应脉动热管内工质流动状态的转变;脉动热管传热性能与其倾斜角度紧密相关;当工质为蒸馏水时,充液率在40%附近时,脉动热管的综合传热性能最优;脉动热管的启动状况与工质的热物性共同决定了工质对脉动热管传热性能的影响。     

单面波浪平板脉动热管的传热性能

对一种单面波浪平板脉动热管的传热性能进行了实验研究,分析在空气强制对流冷却条件下充液率、加热功率、倾角等因素对其传热性能的影响。研究结果表明,除0°倾角外,脉动热管的最佳充液率为20%～30%,倾斜角度对脉动热管传热性能的影响很小,但90°时相对最好。脉动热管在0°放置时其传热性能较差,在低充液率的情况下甚至丧失脉动效果,主要是工质回流不畅的原因,与平板脉动热管的槽道设计有很大关系。此外低加热功率时热管传热性能存在波动,有时甚至不能启动。     

乙烷环路热管传热性能的实验研究

为研究加热功率及充液率对环路热管传热性能的影响,建立了低温环路热管性能测试实验台,测试并记录了在不同充液率下,乙烷环路热管的启动特性及其在各工况下稳定运行时的温度分布情况。研究结果显示:该环路热管在充液率为50%和60%时,启动较迅速,运行较为稳定,其稳定运行时的温差及热阻都较小,而在充液率过大(大于70%)时出现启动困难的情况,其稳定运行的温差及热阻也会增大。该环路热管的最佳充液率约为60%。除此之外,在一定范围内对环路热管增大加热功率,其产生的蒸气会增多,使得环路热管内压差变大,工质流动加快,从而可以减小其稳定运行的温差及热阻。     

蠕动泵驱动的毛细管环路传热系统的性能研究

液态工质在满足一定内径要求的毛细管内会形成稳定的液柱——气塞系统。本文提出利用蠕动泵驱动毛细管内气液相间隔的工质单向循环流动,形成一个具有脉动热管效应的新型两相传热系统。搭建了系统实验装置,以去离子水为工质,当毛细管内径为3 mm,管内工质流速恒定为7 m/min时,实验研究了不同充液率的两相传热系统在加热功率分别为60 W、80 W和100 W时的传热特性。结果表明:该传热系统的最佳充液率为20%,在加热功率为60、80、100 W时,充液率为20%的传热系统达到稳态时的热阻分别为0. 52、0. 38、0. 30℃/W,相比于传统的水冷传热系统其热阻分别降低了32. 5%、45. 7%、50%。     

板式单环路脉动热管的流动和传热特性

以制冷剂R245fa为工质对常规以及带连通通道的板式单环路脉动热管流动特性进行了可视化研究,并对传热性能进行了测试,考察了充液率和加热功率的影响规律。结果表明:对于常规环路,充液率一定时,随加热功率增加,工作方式依次经历振荡流、有方向改变的循环流、定向循环流,且充液率越高,出现定向循环的加热功率越低;连通通道提供了流动的额外通路,工作方式更具振荡特性,定向循环在高充液率、高功率下才能出现。在合适的充液率下,连通通道不但能降低热阻,而且能提高传热极限。     

突出气相压头的环路热管启动特性实验与可视化研究

本文提出并研究了一种突出气相压头的环路热管(LHP)。该环路热管将加热面与吸液芯分离,形成一个蒸发腔。工质在蒸发腔内发生相变,热管的整体结构设计突出了工质的气相压头。实验研究了不同的灌充率对启动特性的影响,结果表明:加热功率相同时(30 W),较高的灌充率会导致较高的运行温度,当灌充率分别为55%、60%和70%时,运行温度分别为101. 9℃、102. 4℃和107. 9℃,且当灌充率高于60%时会发生明显的温度振荡。在灌充率为55%时进行了蒸发器的可视化实验,观察到该类型热管产生相变的主要特征,即:液相工质的滴落蒸发,气液相界面沿着吸液芯表面向下移动,最后液滴落到加热底板上,气液两相界面的移动规律说明该环路热管独特的运行特点。将液体工质从开始滴落经蒸发到下一次开始滴落的时间定义为一个周期,实验测定了在加热功率为55 W时滴落蒸发过程的循环周期为120 s。     

液氮温区脉动热管流动及传热特性研究

脉动热管是一种新型传热元件,具有结构简单、传热性能突出的优点。为了研究低温脉动热管管内工质流动及传热特性,采用液氮为工质,运用多相流VOF方法建立闭式环路结构的低温脉动热管的三维数值模型,并对模型进行了数值模拟。文章对低温脉动热管管内工质的流型变化和传热性能的影响因素进行了研究。结果显示,低温脉动热管从启动阶段到稳定运行阶段管内工质存在多种流型。得出低温脉动热管的倾角、充液率和内径会对低温脉动热管的传热性能产生一定的影响,并分析了倾角、充液率和内径对低温脉动热管传热性能的影响特点。     

表面活性剂对脉动热管传热特性的影响

本文试验研究了表面活性剂脉动热管的运行性能。采用浓度为0.125%的十六烷基三甲基溴化铵(CTAB)水溶液作为工质。试验结果表明,表面活性剂脉动热管的启动时间和启动温度随输入功率的增加而降低。脉动热管的传热特性与输入功率有关,且随着输入功率的增加而增强。与纯水脉动热管相比,充液率为50%的CTAB脉动热管在加热功率100 W时热阻降低52%。     

两种不同工质在微通道内沸腾换热特性的实验研究

为研究流体物性、流动和换热过程的状态参量对微通道内沸腾换热特性的影响规律,本文采用去离子水和无水乙醇在当量直径为0.293 mm的矩形微通道进行了不同质量流量和热流密度条件下的沸腾换热实验研究,通过对实验数据的计算和处理,分析总结了流体的热物性、质量流量、热流密度、干度和Bo数等参量对沸腾换热系数的影响规律。结果表明:沸腾换热系数随着热流密度、干度和Bo数的增大而降低,核态沸腾占主导地位;相同的质量流量和热流密度条件下,去离子水的沸腾换热系数明显高于无水乙醇的沸腾换热系数,并且前者的换热系数随质量流量的增大而增大,而后者变化不明显。根据考虑了通道尺寸效应及流体物性参量总结出的换热系数关联式进行了计算,计算结果对去离子水和无水乙醇的平均绝对误差分别为14.2%和16.6%,可认为该关联式适用于微通道内沸腾换热系数的预测。     

脉动热管实验与理论研究进展

脉动热管(PHP/OHP)是一种新型的高效传热元件,在航天领域、电子器件冷却以及节能技术方面极具应用潜力。这里首先介绍了脉动热管的特点和工作原理,然后分别从实验研究、理论研究和实际应用等方面介绍了目前该领域的研究现状。实验研究方面着重介绍了流动可视化应用以及纳米流体和功能流体通过强化换热提高脉动热管性能等相关研究热点。同时指出,目前脉动热管的理论分析受限于两相流理论的发展,主要研究重点在于非线性分析;数值模拟方面,特别是同纳米流体以及功能流体应用相结合将会成为下一个研究热点。     

Experimental hydrothermal characteristics of minichannel heat sink using various types of hybrid nanofluids

The hydrothermal characteristics of minichannel heat sink are analyzed experimentally by using deionized (DI) water based different nanoparticles mixture dispersed hybrid nanofluids. Al₂O₃, MgO, SiC, AlN, MWCNT and Cu nanoparticles are considered for this study. Different nanoparticles combinations (oxide-oxide, oxide-carbide, oxide-nitride, oxide-carbon nanotube and oxide-metal) in 50/50 vol ratio with base fluid (DI water) have been taken as coolants for volume concentration of 0.01%. Effects of volume flow rate (0.1–0.5LPM), fluid inlet temperature (20–40 °C) and Reynolds number (50–500) are studied for heat flux of 50 W/cm². Convective heat transfer coefficient and pressure drop are increased by about 42.24% and 22% for Al₂O₃ + MWCNT hybrid nanofluid. The maximum reduction of 21.36% in thermal resistance is obtained for Al₂O₃ + MWCNT hybrid nanofluid in comparison to DI water. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids which conclude that hybrid nanofluid is better option for electronics cooling over DI water. Al₂O₃ + MWCNT hybrid nanofluid is better in terms of heat transfer effectiveness; whereas, Al₂O₃ + AlN hybrid nanofluid (oxide-nitrite mixture) has maximum heat transfer coefficient to pressure drop ratio and coefficient of performance.     

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.     

Numerical investigation of nanofluid heat transfer in helically coiled tubes using the four-equation model

Helical coils and nanofluids are among efficient methods for heat transfer augmentation. The present study numerically investigates convective heat transfer with nanofluids in helically coiled tubes. Two boundary conditions are applied to the coil walls; constant temperature and constant heat flux. Heat transfer in nanofluids are mainly investigated using either the homogeneous model or the two-phase model. However, in the present numerical solution, the four-equation model is applied, using slip mechanisms for the base fluid and nanoparticles. Considering that the proposed model is simplified compared to the two-phase model, it can be regarded as an efficient model for numerical solution of heat transfer in nanofluids. Governing equations are solved in the non-dimensional form using the projection algorithm of finite difference method. Water/CuO with a 0.2% volume fraction and water/Ag with a 0.03% volume fraction are examined for validation of numerical results in case of constant temperature and constant heat flux boundary conditions, respectively. The obtained results show a better agreement of this model with respect to experimental data, compared to the homogeneous model.     

Investigation of heat, mass transfer and fluid flow characteristics in evaporative condensers

Heat and mass transfer and fluid flow characteristics in evaporative condensers is discussed. A complex pattern of water temperature and air enthalpy change was detected which depended upon elevation above the sump level. The results have indicated that the spray-filled space beneath the coil has a substantial effect of heat rejection in this type of apparatus. On the other hand, the effect of the upper spray nozzle zone is insignificant. It is possible to optimize the combination of various heat and mass transfer spaces, as well as the effect of extended surfaces and spray-filled spaces. Specifically, it will help to design better evaporative condensers with closely spaced staggered tubes, and to optimize the heat transfer and energy efficiency characteristics of such units.     

Role of working fluids on the cooling of discrete heated modules: a numerical approach

The study has focused on the role of working fluids (air, water and FC-72) on the cooling of discrete heated modules under free, forced and mixed convection medium. Three non-identical protruding discrete heat sources are arranged at different positions on a substrate board following golden mean ratio (GMR). Numerical simulations for these heat sources are carried out using a commercial software (ANSYS-Icepak R-15) to simulate their flow and temperature fields under three different modes of heat transfer. Results suggest that the temperature of the heat sources is a strong function of their size, position on the substrate board, the velocity of the fluid and type of working fluid used. A correlation has been proposed for the temperature of these heat sources keeping in mind their strong dependence on the afore-mentioned parameters. It has been found that water can be used for better heat removal from the heat sources due to its high boiling point. The whole idea gives a clear insight to the electronic cooling engineers regarding the selection of working fluids and modes of heat transfer for the cooling of electronic components.