薄层多孔层内池沸腾时回液特性

多孔介质可以强化相变传热,被广泛应用到电子器件散热中。热管依靠毛细芯孔隙内沸腾和凝结形成热质快速迁移的驱动,实现高密度和高效传热。薄层多孔层内沸腾时液体回流特性研究对提高热管传热效率、热流密度及寿命意义重大。通过不同多孔介质在不同液位下的池沸腾实验,获得了薄层多孔表面在较高热流密度下沸腾时的气泡特性和沸腾曲线,并结合毛细理论分析多孔表面的回液特性。实验结果表明,高热流密度下毛细回流占主导作用,较小的有效毛细半径和较大的渗透率有利于液体回流。     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响，使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面，并使用光滑硅表面进行对比实验研究。实验操作中，将驱气前后的工作液体应用于两种表面的池沸腾实验，换热表面过热度控制在0℃-35℃，工作液体过冷度分为40℃和50℃。实验结果表明，液体中含气量的变化对MWCNT纳米结构表面影响较小，而对光滑硅表面的影响较大；对比硅表面，MWCNT纳米结构表面能够有效提升沸腾传热效果，对于驱气后的工作液体提升效果更为明显。     

梯度孔密度金属泡沫的池沸腾传热性能研究

实验研究了梯度孔密度通孔金属泡沫的池沸腾传热性能。工质为去离子水,梯度孔密度金属泡沫材质为铜和镍, 孔隙率为0.98,泡沫厚度为4-14 mm。实验结果表明：相比于单层泡沫,梯度孔密度金属泡沫显著的增强了沸腾传热能力,但增强程度受孔密度变化梯度、泡沫厚度和材料的影响;梯度孔密度泡沫的池沸腾传热性能随着表面活性剂SDS浓度的增大而减小,而且SDS降低了梯度孔密度金属泡沫的临界热流密度; 添加Al2O3纳米颗粒严重的削弱了梯度孔密度铜泡沫的池沸腾传热能力。     

表面润湿性影响池沸腾传热的研究进展

沸腾传热主要受汽泡产生、成长和脱离的控制,而这些汽泡行为与表面润湿性密切相关,通过改变加热面的润湿性,可在一定程度上调节汽泡在加热面上的发展过程,从而影响沸腾传热。介绍了不同润湿性表面及其对沸腾传热的强化机理和气泡动力学,综述了近年来通过表面润湿改性影响沸腾传热的研究进展,并指明表面润湿性与多尺度结构相结合的沸腾传热的研究新动向,为未来利用润湿性强化沸腾传热提供了一个较为清晰、全面的理论基础。     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理，对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定，采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力，采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中，单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%，实验压力为一个标准大气压，热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入，使制冷剂R141b的核态池沸腾换热得到强化；当ω=0.2%时，换热系数提高比例出现峰值，为57.7%。伴随ω的增加，石墨烯纳米制冷剂的热导率增大、表面张力减小，沸腾表面润湿性增强且微腔数先增后减，综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

不凝结气体对多壁碳纳米管传热表面池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管(multi-walled carbon nanotube,MWCNT)纳米结构表面核态池沸腾过程的影响,使用气体沉积法(chemical vapor deposition,CVD)在硅表面制作MWCNT纳米结构表面,并使用光滑硅表面进行对比实验研究。实验操作中,将驱气前后的工作液体应用于两种表面的池沸腾实验,传热表面过热度控制在0.0~35.0℃,工作液体过冷度分为40.0和50.0℃。实验结果表明,液体中含气量的变化对MWCNT纳米结构表面影响较小,而对光滑硅表面的影响较大;对比硅表面,MWCNT纳米结构表面能够有效提升沸腾传热效果,对于驱气后的工作液体提升效果更为明显。     

CO_2单管池沸腾换热实验研究

在CO2跨临界水-水热泵试验台添加池沸腾实验段,进行CO2单管池沸腾换热实验,用威尔逊分析法对实验数据进行处理。研究表明:CO2池沸腾换热效果优于管内沸腾换热效果;同时,结合实验数据,拟合在饱和压力3.2和3.4MPa下的CO2池沸腾换热系数与热流密度和对比压力之间的关联式。     

水平窄空间沸腾换热的数理模型研究

窄空间只有在间距小于汽泡脱离直径时，对沸腾传热强化才有比较显的效果。窄空间沸腾强化传热的机理在于较大的泡底微层加速了蒸发传热和窄空间中被加热的液体周期性地与池液进行容积交换。水平圆盘窄空间中的汽泡生长分为性质完全不同的自由生长期和抑制长大期；在一个周期内，加热面的总传热量等于壁面传导给窄空间液体的热量与通过合体泡底微层蒸发潜热之和。在对圆形水平窄空间的沸腾传热的现象和机理进行分析的基础上，提出了窄空间的沸腾换热过程的数理模型；进而对窄空间沸腾的本质规律在理论上进行了初步探索，并得到分析解。理论计算结果与实验数据比较表明，该分析解适合于中低壁面过热度的情形。由于问题的复杂性，该模型仍需不断完善。     

超声波对池沸腾换热的影响

本实验针对池沸腾传热,以去离子水为工质,在不同过冷度、超声功率和辐射距离条件下,研究了超声波对池沸腾换热的影响,揭示了池沸腾系统中声空化和声流效应的作用机理;在旺盛沸腾区,由于沸腾系统本身汽泡核化所产生的气液界面削弱了到达加热面的超声能量,使得声空化效应减弱;同时,核化汽泡的生长和脱离所引起的对流效应远大于超声波声流引起的宏观对流效应,使得超声波在旺盛沸腾区的强化传热作用并不显著。     

满液型海水淡化蒸发器的换热特性研究

海水淡化装置，太阳能或余热吸收式制冷机中的蒸发换热器目前使用管排外降膜式蒸发方式。如将传热管束紧凑排列置于饱和状态液体中则变为满液式蒸发换热器，利用传热管束间受限空间内早期沸腾强化换热机理，将中小热负荷条件下的自然对流换热转化为核沸腾换热，在间隙尺寸适宜时，其换热性能可能优于降膜式蒸发换热器。该研究以盐水为实验工质，对紧凑传热管束受限空间的沸腾换热进行了实验研究，确认了满液式蒸发换热器也具有很好的换热性能，在中小热负荷条件下甚至超过降膜式蒸发换热器。     

水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍；临界热流密度有所降低。     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

Effects of pool subcooling on boiling heat transfer in a vertical annulus with closed bottom

Effects of inlet subcooling on pool boiling heat transfer in a vertical annulus with closed bottom have been studied experimentally. For the test, a tube of 19.1 mm diameter and the water at atmospheric pressure have been used. Up to 50 K of pool subcooling has been tested and results of the annulus are compared with the data of a single unrestricted tube. The increase in pool subcooling results in much change in heat transfer coefficients. As the heat flux increases and the subcooling decreases, a deterioration of heat transfer coefficients is observed. The governing mechanisms are suggested as single-phase heat transfer and liquid agitation for the single tube while liquid agitation and bubble coalescence are the major factors at the bottom closed annulus.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017     

Enhanced Boiling Heat Transfer by Using Micro-Pin-Finned Surface in Three Different Test Systems

For efficiently cooling electronic components, experiments were conducted to study heat transfer performance of FC-72 over a silicon chip with micro-pin-fins in three different test systems including pool boiling, natural circulation flow boiling, and forced flow boiling. A smooth chip (chip S) and a micro-pin-finned chip having fin thickness of 30 μm and fin height of 60 μm (chip PF30-60) were tested. The micro-pin-fins were fabricated on the surface of square silicon chip (10 × 10 × 0.5 mm³) by use of dry etching technique for enhancing boiling heat transfer. The experiments were conducted at fluid velocities of 0.5 and 1 m/s for the forced flow boiling and the liquid subcoolings of 25 and 35 K for all three test systems. With regard to the three test systems, the micro-pin-finned surface shows a considerable heat transfer enhancement compared to the smooth surface and shows a sharp increase in the critical heat flux (CHF). For the same chip, the boiling curves are almost the same for the pool boiling and the natural circulation flow boiling, while the boiling curves shift toward a smaller wall superheat for the forced flow boiling. The critical heat flux was the highest for the forced flow boiling at a fluid velocity of 1 m/s.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Pool boiling heat transfer on copper foam covers with water as working fluid

Pool boiling heat transfer with porous media as the enhanced structure is attractive due to its simple geometry and easy operation. However, the available studies focus on low porous porosities. Metallic foams provide large porous porosities that have been less studied in the literature. In this paper a set of copper foam pieces were welded on the plain copper surface to form the copper foam covers for the pool boiling heat transfer enhancement. Water was used as the working fluid. Enhancement of pool boiling heat transfer compared with plain surface depends on the increased bubble nucleation sites, extended heat transfer area, and resistance for vapor release to the pool liquid. Effects of pores per inch (ppi) of foam covers, foam cover thickness, and pool liquid temperatures are examined. It is found that temperatures at the Onset of Nucleate Boiling (ONB) are significantly decreased on copper foam covers compared with on plain surfaces. Heat transfer coefficients of foam covers are two to three times of the plain surface. A large ppi value provides large bubble nucleation sites and heat transfer area to enhance heat transfer, but generates large vapor release resistance to deteriorate heat transfer. Therefore an optimal ppi value exists, which is 60 ppi in this paper. Generally small ppi value needs large foam cover thickness, and large ppi value needs small foam cover thickness, to maximally enhance heat transfer. Effect of pool liquid temperature on the heat transfer enhancement depends on the ppi value. For small ppi value such as 30 ppi, lower pool liquid temperature can dissipate higher heat flux at the same wall superheat. However, the heat transfer performance is insensitive to the pool liquid temperatures when large ppi values such as 90 ppi are used.     

Study on boiling heat transfer in liquid saturated particle bed and fluidized bed

An investigation on the effects of solid particles on boiling heat transfer enhancement is performed. The range of particle diameter is from millimeter to nanometer. The experimental results show that boiling heat transfer can be enhanced greatly by adding the solid particle into the liquid whether in fixed particle bed or in fluidized particle bed. The boiling enhancement is closely related to the particle size, the initial bed depth and the heat flux applied. The experiments show that boiling characteristics are greatly changed when a particle layer is put on the heated surface. The major effects of fixed particle bed on nucleate pool boiling heat transfer are the nucleation, bubble moving and thermal conductivity effect. A boiling heat transfer correlation is obtained to predict the boiling heat transfer coefficients in a liquid saturated porous bed. A volumetric convection mechanism of boiling heat transfer enhancement by fluidized particles is proposed. The calculated results from the model suggested in this paper agree reasonably with the experimental values.     

Similarities and Differences Between Flow Boiling in Microchannels and Pool Boiling

Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected, as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This article explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of these phenomena will help in overcoming these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels.