表面微结构对阵列微射流沸腾换热的影响

与单相射流相比,阵列式微射流沸腾换热耦合了分布式射流与气液相变两种高效传热模式,在高热通量电子器件冷却领域具有重要的应用前景。本文创新性提出一种具有顶部浸入式阵列射流柱与底部微针肋阵列结构耦合的微射流沸腾换热系统,采用无水乙醇为工质,研究了入口过冷度、入口Re、热通量对射流沸腾换热影响特性;采用电刷镀制备了镍/石墨烯微纳复合结构,研究了该复合结构对微针肋阵列表面射流沸腾换热的影响规律,揭示了镍过渡层引入的附加热阻以及蘑菇状微纳复合结构对气泡脱离的抑制是换热削弱的主要原因。为克服上述弊端,采用激光对镍/石墨烯微纳复合结构表面进行了刻蚀,发现激光刻蚀消除了镍/石墨烯微纳复合结构导致的附加热阻及其气泡脱离抑制效应,其最大传热系数达到30787.0W/(m2·K),较镍/石墨烯微纳复合结构表面和针肋阵列光滑表面传热系数分别提高了140.7%和119.8%。本文的研究结果表明,微纳复合结构对沸腾换热的影响取决于制备工艺及其结构形貌,激光刻蚀较电刷镀形成的微纳复合结构在微射流沸腾换热强化方面更具优势,为表面微纳结构强化沸腾换热系统设计、制备和运行提供科学参考。     

润湿性对微纳复合结构表面池沸腾换热的影响

为揭示润湿性对微纳复合结构表面池沸腾传热的影响,采用电刷镀工艺和表面改性技术在紫铜表面制备了接触角分别为6.5°和148.6°的超亲水性和超疏水性微纳复合结构,通过实验对比研究了不同表面的饱和池沸腾传热特性,结果表明:(1)超亲水性和超疏水性微纳复合结构的最大换热系数较光表面分别提高了3倍和1.5倍;(2)在q580k W×m~(-2)的低热流密度区,超疏水性微纳复合结构的换热系数最大;当q580 k W×m~(-2)时,超亲水性微纳复合结构的传热性能开始优于超疏水性微纳复合结构;(3)超亲水性微纳复合结构表面的临界热流密度较光表面和超疏水性微纳复合结构分别提高了110%与60%;微纳复合结构显著增加了受热表面的气泡核化密度,而亲水性微纳复合结构的毛细吸液能力要显著强于疏水性微纳复合结构,是临界热流密度增大的主要机理。     

两种烧结通道的沸腾传热和阻力特性对比

多孔材料对沸腾换热的强化是能源化工领域的重要主题。本文针对两种不同的烧结结构——并联微通道和扁平通道（仅有烧结底层）,以去离子水为工质,进行了过冷流动沸腾换热实验对比研究。研究发现：并联微通道的传热系数和临界热流密度远高于扁平通道,这和并联微通道优异的毛细供液性能相关。底厚粒径比对并联微通道的沸腾换热性能影响较大,过大的底厚粒径比会造成换热性能的下降。质量通量对小粒径样品的沸腾曲线和换热性能均影响较大,对大粒径（d=120μm）样品的沸腾曲线影响较小。烧结并联微通道的平均压降大于扁平通道。相同底厚下,平均压降随着微通道粒径的增大而增大。可视化观察表明：两种通道在中高热流密度流型不同,其主要相变机制均为薄液膜蒸发模式。     

连通微通道内过冷流动沸腾传热强化机理分析

连通微通道（平行主通道由支流通道连通）流动沸腾传热具有优越的换热性能,但其传热传质强化机理尚不够明确,限制了其实际应用。鉴于此,本文基于流体体积函数（VOF）方法,对连通微通道内过冷流动沸腾进行二维非稳态数值模拟,研究了流场扰动、脱落汽泡与壁面间的薄液膜分布对微通道当地传热系数的影响规律。结果表明,连通微通道存在两种强化换热机理：支流通道脱落汽泡可增强主通道流场扰动,进而促进了通道热边界层再发展;脱落汽泡与热壁面间可形成薄液膜,该薄液膜减小了换热热阻。同时研究了支流通道倾角（θ）对连通微通道强化换热的影响,结果发现,不同θ时,连通微通道整体平均传热系数提高10.51%~17.66%,单个主通道平均传热系数最高可提升27.94%,且θ=45°时连通微通道具有最佳换热特性。该研究有望为芯片高效冷却结构的设计提供指导。     

结构参数对烧结微通道流动沸腾性能的影响

微通道沸腾冷却在电子器件方面的应用近年备受关注。将多孔烧结微通道作为微电子器件的有效冷却方案进行了流动沸腾传热性能的实验研究,重点围绕热通量和通道宽度对流动沸腾特性的影响。烧结微通道采用铜粉加压烧结的方法,使用150μm树枝状铜粉进行烧结,制备了三种通道宽度分别为1.8、0.6和0.2 mm的并联微通道,对应的槽数分别为11、22和33槽。研究发现:存在最优通道宽度,其综合沸腾换热效果达到最优。在4 L/h流量下,中等宽度样品最高传热系数可达200 kW/(m²·K),临界热通量可达到170 W/cm²左右。可视化研究发现:通道宽度对压力脉动曲线会造成很大影响,适中的通道宽度压力脉动曲线更为有序,大大缓解压力脉动从而提升微通道的沸腾换热性能。     

微纳复合表面HFE-7100/水的BRT现象及沸腾传热特性

HFE-7100/水作为非共沸不互溶工质可以拓宽核状沸腾传热的有效温区,目前关于其在微纳复合表面的沸腾传热特性和气泡运动机理尚不明晰。利用气泡模板电沉积法在铜基表面上制备了具有微纳孔洞的复合结构,测试了HFE-7100/水的沸腾传热特性,并通过可视化探究了沸腾工质转换（BRT）过程中两相工质在表面的润湿状态和气泡运动现象。结果表明,微纳复合表面上HFE-7100/水的BRT过程中,气泡先后经历小气泡聚并、气膜膨胀、轻工质接触壁面核化三个过程。在BRT过程中,HFE-7100与水对热壁面的润湿性存在竞争关系,随着过热度增加,薄的HFE-7100液层难以维持稳定的重工质沸腾,上层水工质可以穿过HFE-7100层对热壁面实现完全润湿,完成BRT过程。与单一工质相比,常压下HFE-7100/水混合工质体系可以在343~423 K下实现高效的核状沸腾传热。该研究揭示了HFE-7100/水在微纳复合表面的沸腾传热特性,为沸腾强化表面设计提供了思路。     

多孔壁面微通道换热性能的实验研究

微通道散热器作为一种高效散热器件,广泛应用于微电子、光电、汽车、航天国防、能源等领域。针对传统光滑微通道传热面积小、换热性能偏低、沸腾迟滞等问题,本文提出一种多孔壁面微通道结构,并采用激光直写方法实现微通道多孔壁面的高效、稳定生成。该多孔壁面微通道显著增大了换热面积、促进流体的扰动、提供大量稳定沸腾核心,从而强化单相与两相沸腾传热。通过搭建微通道换热性能测试系统,测试对比了多孔壁面微通道与光滑微通道的单相对流、两相沸腾传热性能。发现多孔壁面微通道的Nu数相对于光滑微通道提升了21%~31%。在两相沸腾换热过程中,其粗糙多孔结构促进了沸腾气泡成核,其核态沸腾起始温度相比于光滑微通道降低了35%。同时粗糙多孔结构可以保证沸腾过程中的液体持续供给,从而大幅提升了沸腾换热能力,避免了干涸现象的提前发生,其两相沸腾换热系数相对于未处理的光滑微通道最大提升了83%。此外,还开展了不同流量下多孔壁面微通道的沸腾传热性能测试,发现在质量流率为G=500kg/(m²·s)下的沸腾换热系数相对于G=200kg/(m²·s)情况下最大提升了30%。     

微细通道内CO2沸腾换热与干涸特性

CO₂作为一种天然制冷剂在微通道内应用具有很大的换热优势,然而由于微尺度效应及其物性,在低干度区容易发生干涸,严重影响换热效果。为研究微细通道内CO₂流动沸腾换热与干涸特性,搭建了相应实验装置,对内径分别为1mm、2mm、3mm以及内表面粗糙度为16μm的不锈钢管,在CO₂制冷剂热流密度2~34kW/m²、质量流率50~1350kg/(m²·s)、饱和温度-10~15℃下进行换热性能与干涸实验对比研究。结果表明:常规管径换热特性在微细通道内不再适用;热流密度的增加对于强化核态沸腾换热具有显著影响,高于临界热流密度(critical heat flux,CHF)则发生干涸;质量流率对于核态沸腾区换热系数的影响则较小;不同饱和温度时换热特性有所不同,高饱和温度下换热系数随其升高而提高,低饱和温度下则相反;干涸过程对总换热系数的影响占34%。研究结论为CO₂微通道换热器的研究开发提供理论依据。     

浅谈微通道换热器

微通道换热器体积换热系数大、换热效率高,并且有着优良的耐压性能、不俗的抗腐蚀能力、紧密的结构以及低廉的价格,已经成为换热器领域研究热点。此篇综述介绍了微通道换热器内流体的流动特性、微尺度效应与入口段效应以及与常规换热器相比微通道换热器所具有的结构简单、成本低廉、换热效率高、抗压抗腐蚀等优势。总结了微通道换热器在微电子领域以及车用空调领域的应用,并展望其前景。     

非均匀润湿性微通道表面池沸腾换热特性

采用高温热氧化与表面改性技术并结合电火花线切割工艺在紫铜表面制备了3类非均匀润湿性微通道表面,微通道顶部接触角分别为8.6°、88.1°、156.1°,通道内部接触角为113.2°。经饱和池沸腾试验表明,具有超亲水性顶部（θ=8.6°）和超疏水顶部（θ=156.1°）的微通道表面临界热通量分别较紫铜表面（θ=88.1°）提高了61%和35%,最大传热系数分别提高了2.3倍和6倍。气泡动力学可视化研究表明：非均匀润湿结构能够显著抑制气泡的合并与团聚,使得气泡之间存在的间隙成为液体补充路径,这是临界热通量提高的主要机理。     

Bubble breakup and boiling heat transfer in Y-shaped bifurcating microchannels

Three-dimensional simulations are performed to study the bubble breakup and boiling heat transfer in Y-shaped bifurcating microchannels with different heat fluxes and bifurcating angles. Results show that the breakup regime for continuously growing bubble changes from tunnel breakup to obstructed breakup as the heat flux increases. Interestingly, the pinch-off stage of bubble breakup becomes inconspicuous in small acute-angle bifurcating microchannel. The flow structure changes from smooth mode to twining mode as the bifurcating angle increases. The bubble transit leads to the shrink or disappearance of vortex. The heat transfer is tightly associated with bubble dynamics. The negative heat transfer enhancement is observed at low heat flux and it is eliminated at high heat flux. The heat transfer is enhanced with the increase in heat flux, while the effect of bifurcating angle is relatively complex. The present study provides new insights into the two-phase flow in bifurcating structures.     

仿蜂巢分形微管道网络中的流动与换热

受自然界中蜂巢结构分形特征的启发,设计和加工了仿蜂巢分形微管道网络,并进行了参数优化.在微管道截面参数、对流传热系数、传热温差均相同的条件下,对流动与换热特性的理论分析表明：加热底面积相同时,仿蜂巢分形微管道网络所能带走的热量可达平行阵列微管道网络的5倍以上;不计分流、合流效应,总换热量一定时,仿蜂巢分形微管道网络所需的泵送功率约为传统平行阵列微管道网络的1/10.恒定热流条件下的去离子水层流对流换热实验也证明：仿蜂巢分形微管道网络比传统的平行阵列微管道网络具有更高的Nusselt数和更低的流动压降.这种分形微管道网络除用于电子器件冷却,还可用于微燃料电池极板、微混合器、微生化反应器等微化工系统结构设计.     

电火花改性表面池沸腾换热特性

通过电火花成型加工技术在铜基换热表面制备微纳结构改性表面,以自制换热表面性能测试装置进行改性表面的池沸腾换热性能实验。改性表面随加工电流改变而具有不同粗糙度、孔隙率和粗糙度因子,表面接触角范围在117.4°~133.5°。实验结果表明,改性表面的微纳结构提高换热面的池沸腾换热效果,临界热流密度较光滑铜表面提高了26%~87.8%,最大传热系数提高了48.1%~213%。改性表面的传热系数随着粗糙度增大而减小,而临界热流密度则是先增大后减小;孔隙率的增大使得改性表面的传热系数也随之增大,临界热流密度则是随着孔隙率的增大而先增大后减小;临界热流密度随着粗糙度因子的增大而降低,传热系数则是先增大后降低。粗糙度对沸腾换热的强化效果较小,孔隙率和粗糙度因子是强化池沸腾换热的关键,孔隙率和粗糙度因子分别影响了气泡核化密度和实际接触面积,提高了气泡脱离频率,带走更多的热量,但两者间存在互相制约的平衡关系。     

An experimental study of air-water Taylor flow and mass transfer inside square microchannels

Flow and mass transfer properties under air-water Taylor flow have been investigated in two square microchannels with hydraulic diameters of 400 and 200 μm. Experimental data on Taylor bubble velocity, pressure drop and liquid side volumetric mass transfer coefficient (k_La) have been presented. It was shown that the measured Taylor bubble velocity in square microchannels could be well interpreted based upon an approximate measurement of the liquid film profile therein. Then, the obtained two-phase frictional pressure drop values in both microchannels were found to be significantly higher than the predictions of the correlation proposed by Kreutzer et al. [2005b. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. A.I.Ch.E. Journal 51, 2428-2440] when the liquid slug was very short, which can be explained by the inadequacy of their correlation to describe the excess pressure drop caused by the strong inner circulation in such short liquid slugs. An appropriate modification has been made to this correlation in order to improve its applicability in microchannels. Finally, the experimental (k_La) values in the microchannel with hydraulic diameter of 400 μm were found to be in poor agreement with those predicted by the existing correlations proposed for capillaries with diameters of several millimeters. The observed deviation was mainly due to the fact that mass transfer experiments in this microchannel actually corresponded to the case of short film contact time and rather poor mixing between the liquid film and the liquid slug, which was not in accordance with mass transfer assumptions associated with these correlations. A new empirical correlation has been proposed to describe mass transfer data in this microchannel.     

常/微重力下微结构表面强化沸腾换热研究进展

表面改性是提高沸腾换热性能的重要手段。本文以自主开发的微结构表面为基础，简述了近三年来常重力条件下的微/纳结构表面强化池沸腾换热、临界热流密度预测模型及经验关联、微重力条件下（重力水平为10^-2～10^-3 g ₀，g ₀=9.8m/s²）加热面尺寸对沸腾换热的影响和气泡动力学等方面的研究进展。对柱状微结构参数和排布方式进行优化后的多尺度复合微结构表面相比柱状微结构表面和光滑表面，其壁面温度可分别降低8K和30K以上，而临界热流密度（CHF）则分别提高了28%和119%以上。体积分数为0.02%的乙醇/银纳米流体相对于单纯的乙醇工质，相同条件下换热壁面温度可降低8～15K，而机械作用对CHF约有25%的提高。通过对柱状微结构的几何参数以及临界发生时的供液机理研究，建立了考虑柱状微结构参数的CHF关联式、微/纳结构表面考虑液体毛细芯吸作用的CHF预测模型以及考虑液体铺展速度的CHF预测关联式。根据微重力下加热面尺寸对沸腾的影响的研究，提出了基于恒定热流密度的换热预测关联式。考虑微重力条件下主气泡和小气泡的表面张力，对传统的气泡脱离直径预测的力平衡模型进行了改进，进一步提高了微重力下气泡的脱离半径的预测精度。此外，对近年来以FC-72为工质的其他强化池沸腾换热微结构表面的研究成果进行了总结，并与自主研发的微结构表面换热性能进行了对比与分析，为今后的研究方向和应用指出了方向。     

Pool boiling fouling and corrosion properties on liquid‐phase‐deposition TiO2 coatings with copper substrate

Fouling on the heat transfer surfaces of industrial heat exchangers is an intractable problem, and several techniques have been suggested to inhibit fouling. Surface coatings are of such techniques by which the adhesion force between fouling and heat transfer surface can be reduced with low surface free energy thin films. In this article, liquid phase deposition was applied to coat titanium dioxide thin films on the red copper substrates with film thickness in micro‐ or nano‐meter scale. Coating thickness, contact angle, roughness, surface topography, and components were measured with X‐ray diffraction, contact angle analyzer, stylus roughmeter, scanning electron microscopy, and energy dispersive X‐ray spectroscopy, respectively. Surface free energy of coating layers was calculated based on the contact angle. Heat transfer and fouling characteristics in pool boiling of distilled water and calcium carbonate solution on coated surfaces were investigated. Heat transfer enhancement was observed on coated surfaces compared with untreated or polished surfaces due to the micro‐ and nano‐structured surfaces which may increase the number of nucleation sites. The nonfouling time on the coated surfaces is extended than that on the untreated or polished surfaces due to the reducing of the surface free energy of coated surfaces. Corrosion behavior of coated surfaces soaked in the corrosive media of hydrochloric acid, sodium hydroxide alkali, and sodium chloride salt solutions with high concentration at room temperature a few hours was also explored qualitatively. Anticorrosion results of the coated surfaces were obtained. The coatings resisted alkali corrosion within 7.2 × 10⁵ s, acidic corrosion within 3.6 × 10⁵ s and salt corrosion within 2.16 × 10⁶ s. The present work may open a new coating route to avoid fouling deposition and corrosion on the heat transfer surfaces of industry evaporators, which is very important for energy saving in the related industries. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Nucleate boiling performance on nano/microstructures with different wetting surfaces

ABSTRACT: A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.