磁场调控磁性纳米流体流动和传热研究进展

磁性纳米流体在实现能量高效和可控传递领域极具发展潜力。本文综述了磁场作用下磁性纳米流体对流换热及沸腾换热的最新进展,主要包括强制对流换热、混合对流换热、自然对流换热、池沸腾换热及管内沸腾换热等方面的实验研究,分析了磁场类型、强度、梯度、频率、方向及磁铁位置等对磁性纳米流体流动和热传输特性的影响,指出可通过改变外加磁场来实现对磁性纳米流体流动和传热的控制,并探讨了磁性纳米流体流-磁耦合作用下的传热机理以及目前所面临的挑战。在此基础上,提出了未来磁场调控磁性纳米流体对流换热和沸腾换热的主要发展方向：制备稳定的磁性纳米流体,建立系统有效的流动和传热理论模型,并从微介观尺度诠释热-流-磁耦合协同换热机理。     

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。     

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%。     

微通道中纳米流体流动沸腾换热性能研究

为研究微通道中纳米流体流动沸腾的换热性能,设计了一种水力直径为143 μm的矩形硅基微通道,搭建了研究微通道中纳米流体流动沸腾换热的高速测量和光学可视化实验平台.研究了质量分数为0.2%的Al2O3纳米流体及纯水在微通道中的流动沸腾换热性能.通过比较在两种换热工质中系统压降和壁温,并结合流型的同步变化分析了纳米流体的流...     

纳米孔链双层复合表面过冷池沸腾传热特性

微纳复合多孔结构对相变换热的强化是能源化工领域的重要主题。基于气液协同输运的概念，通过飞秒激光正交扫描加工，在硅片上生成二维嵌套的纳米孔链双层复合多孔结构，实验研究了其对HFE-7100过冷池沸腾传热特性的影响。实验结果表明，相比于光滑表面，多孔表面在35K过冷池沸腾条件下的起始过热度从16.7K下降到12.3K，降低26.3%，最大临界热通量提高128.7%。同时利用高速摄影观察气泡行为来研究强化沸腾传热机理。研究发现，双层多孔结构表面和内部形成的大量连通孔穴大幅度增加了有效成核位点，纳米孔和双层连通结构提供垂直和水平方向的液体补充通道，在高热通量下气泡尺寸更小，脱离更快。有效汽化核心密度增加以及气液自适应协同输运增强了多孔网络中的微液膜蒸发和微对流作用，从而有效提升沸腾换热能力和临界热通量。     

微肋阵通道流动沸腾换热与压降特性

为探究不同截面微肋阵通道内的流动沸腾换热机理,以去离子水为工质,在质量流速为96～224 kg·m^-2·s^-1,有效热通量为10～240 W·cm^-2的范围内,对圆形、菱形、椭圆形微肋阵通道内流动沸腾换热及压降特性进行了实验研究,同时对微通道内流动沸腾的不稳定性进行了分析。通过实验发现：在低热通量下,核态沸腾占主导地位,而在中高热通量下,薄膜蒸发对流换热为主要沸腾机制;沸腾传热系数随着热通量和出口干度的增加而减小,两相压降随着热通量和出口干度的增加而增大;微肋阵肋间形成的次级通道宽度对换热和两相压降有很大的影响,次级通道越宽,气泡越容易脱离,换热效果越好,压降越大;微肋的存在抑制了气泡的反向流动,减小了沸腾不稳定性,推迟了临界热通量的发生,椭圆形微肋阵通道的流动沸腾稳定性最好,而圆形微肋阵通道的流动沸腾稳定性最差。     

微肋阵通道流动沸腾换热与压降特性

为探究不同截面微肋阵通道内的流动沸腾换热机理,以去离子水为工质,在质量流速为96～224 kg·m~(-2)·s~(-1),有效热通量为10～240 W·cm~(-2)的范围内,对圆形、菱形、椭圆形微肋阵通道内流动沸腾换热及压降特性进行了实验研究,同时对微通道内流动沸腾的不稳定性进行了分析。通过实验发现:在低热通量下,核态沸腾占主导地位,而在中高热通量下,薄膜蒸发对流换热为主要沸腾机制;沸腾传热系数随着热通量和出口干度的增加而减小,两相压降随着热通量和出口干度的增加而增大;微肋阵肋间形成的次级通道宽度对换热和两相压降有很大的影响,次级通道越宽,气泡越容易脱离,换热效果越好,压降越大;微肋的存在抑制了气泡的反向流动,减小了沸腾不稳定性,推迟了临界热通量的发生,椭圆形微肋阵通道的流动沸腾稳定性最好,而圆形微肋阵通道的流动沸腾稳定性最差。     

时空调控微柱表面浸润性强化单气泡沸腾换热

微结构耦合浸润性调控是目前强化核态沸腾换热的主要手段,针对水工质在单晶硅微柱表面的核态沸腾过程,采用CFD-VOF三维数值模拟方法,对比研究时间及空间分别调控表面浸润性对沸腾气泡动力学、相界面形变及传热性能的影响。结果表明：亲水性增强使得气泡界面曲率增大、合力增强,促使气泡的脱离;空间调控主要表现为增大气泡体积,时间调控则主要表现为优化气泡动力学过程,提高热流较大的生长阶段在整个气泡周期内的占比,从而强化换热;本实验工况下,空间梯度浸润表面以及在生长阶段提高壁面亲水性,均可大幅度提高单气泡沸腾换热性能,平均热流最大可提高42.7%;考虑微尺度下梯度浸润性加工难度,时间调控浸润性强化沸腾换热具有更好的发展前景。     

Ni/Ag微纳结构强化顶部连通型微通道沸腾换热

微通道换热器较大的比表面积使其具有较高的热质传输效率,在化工、能源等领域具有广泛的应用前景。针对微通道流动沸腾换热强化,本文设计了一种具有Ni/Ag微纳复合结构表面的顶部连通型微通道换热器,该顶部连通型微通道由11条并联微通道组成,微通道的截面为400μm×400μm的正方形,并联通道上方连通空间的高度也为400μm;采用电刷镀技术在顶部连通型微通道表面制备了Ni/Ag微纳米复合结构,以无水乙醇为工质,开展了普通并联微通道（regular microchannel, RMC）、顶部连通型微通道（top-connected microchannel,TCMC）以及具有微纳复合结构表面的顶部连通型微通道（TCMC-Ni/Ag）内流动沸腾换热对比实验研究。结果表明：TCMC-Ni/Ag表面的最大局部换热系数达179.84kW/(m²·K),较RMC的最大局部换热系数提高了4.1倍。可视化研究发现,对于TCMC-Ni/Ag,强亲水性的微纳复合结构表面同时提高了核化密度和核化频率,中低热流条件下形成气相汇聚于顶部连通区域,微通道表面仍然产生大量气泡的流型结构,在高热流密度条件下,强亲水性微纳复合结构的毛细吸液作用使得通道内产生了薄液膜对流蒸发换热模式,是其换热性能大幅提高的主要机理。     

超疏水表面微通道内水的传热特性

微通道传热效率高但流动阻力大,超疏水表面因其与水具有滑移边界而表现出低流阻的特征,在微过程中具有应用前景。利用化学刻蚀法制备出具有微纳米阶层结构的铝基超疏水表面微通道(内径为0.68mm)。在超疏/亲水微通道内进行了水的流动传热实验研究,并将结果进行对比。研究发现存在于超疏水表面微纳米结构里的气泡层减小了水的流动阻力,也降低了表面传热系数,但降低程度明显小于流动阻力的降低,传热系数高于考虑纳米气泡层计算的传热系数。因此认为在水的滑移速度作用下,凹穴中微纳米级气泡内产生了气体的涡旋流动,一定程度上增强了传热效果。     

基于双流体模型的液氢流动沸腾数值模拟

基于双流体模型,建立了液氢管内流动沸腾的数值模型,在液体Reynolds数67000～660000、壁面热通量16300～317800 W/m²、饱和温度22～29 K、入口过冷度0～8 K的范围内,对管径5.95和6.35 mm的圆管内液氢流动沸腾开展了数值模拟研究,并与试验结果进行了对比。对比显示,液氢流动沸腾传热系数的模拟结果与试验数据的平均误差（MAE）为7.79%,94%的模拟数据都在±20%误差带范围内。     

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。     

Identification of Physical Phenomena Governing Nucleate Boiling Heat Transfer of Binary Mixtures

A model based on first principles is presented to compute nucleate boiling heat transfer coefficients. It includes microscale heat and mass transfer phenomena in the so‐called micro region, i.e., the thin film area where the liquid vapor phase interface approaches the wall. The model is verified by comparing calculated to measured heat transfer coefficients. Parameter studies allowed to identify physical phenomena governing the typical reduction of binary mixture heat transfer coefficients compared to the ideal heat transfer coefficient, i.e., the molar average of the heat transfer coefficients of the two pure components of the mixture. These are bubble site density and departure diameter that deviate from the corresponding values of the two pure components. Furthermore, overall heat transfer is decreased by strong concentration gradients in the micro region.     

Population balance modelling for bubbly flows with heat and mass transfer

Population balance equations combined with a three-dimensional two-fluid model are employed to predict bubbly flows with the presence of heat and mass transfer processes. Subcooled boiling flow belongs to this specific category of bubbly flows is considered. The MUSIG (MUltiple-SIze-Group) model implemented in CFX4.4 is further developed to account for the wall nucleation and condensation in the subcooled boiling regime. Comparison of model predictions against local measurements near the test channel exit is made for the radial distribution of the bubble Sauter diameter, void fraction, interfacial area concentration and gas and liquid velocities covering a range of different mass and heat fluxes and inlet subcooling temperatures. Additional comparison was also performed against existing boiling model in CFX4.4 and the modified model developed in our previous work (Int. J. Heat Mass Transfer 45 (2002) 1197). Good agreement is better achieved with the local radial bubble Sauter diameter, void fraction, interfacial area concentration and liquid velocity profiles against measurements using the newly formulated MUSIG boiling model over the simpler boiling models. However, significant weakness of the model is still evidenced in the prediction of the vapour velocity. Work is in progress to circumvent the deficiency of the model by the consideration of additional momentum equations or an algebraic slip model to account for bubble separation.     

铜基正弦波微通道内流动沸腾传热特性试验研究

基于超快激光技术加工铜基正弦波弯曲型微通道,以去离子水为流动工质,在不同质量流量和热通量条件下,对弯曲型微通道内流动沸腾特性进行试验研究。基于温度/压力数据和流动可视化结果,发现通道传热系数随出口干度增大,呈迅速增大后减小并趋于稳定趋势,正弦波微通道相较直微通道具有更好的换热性能,传热系数最大提高127.7%,压降仅增加14.4%。波状通道结构能明显抑制流动沸腾中不稳定现象发生。通过可视化试验发现,随热通量增大,流型经历泡状流-弹状流-环状流的转变,换热主导机制由核态沸腾逐渐过渡到薄液膜蒸发。     

基于MUSIG模型的竖直圆管内液氮过冷沸腾数值模拟研究(英文)

Multiple size group （MUSIG） model combined with a threedimensional twofluid model were em ployed to predict subcooled boiling flow of liquid nitrogen in a vertical upward tube. Based on the mechanism of boiling heat transfer, some important bubble model parameters were amended to be applicable to the modeling of liquid nitrogen. The distribution of different discrete bubble classes was demonstrated numerically and the distribu tion patterns of void fraction in the wallheated tube were analyzed. It was found that the average void fraction in creases nonlinearly along the axial direction with wall heat flux and it decreases with inlet mass flow rate and sub cooled temperature. The local void fraction exhibited a Ushape distribution in the radial direction. The partition of the wall heat flux along the tube was obtained. The results showed that heat flux consumed on evaporation is the leading part of surface heat transfer at the rear region of subcooled boiling. The turning point in the pressure drop curve reflects the instability of bubbly flow. Good agreement was achieved on the local heat transfer coefficient aalnst experimental measurements, which demonstrated the accuracy of the numerical model.     

Influence of nanoparticle concentrations on flow boiling heat transfer coefficients of Al2O3/R141b in micro heat exchanger by direct metal laser sintering

Al2O3/R141b+Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investi-gate the influence of nanoparticle concentrations on flow boiling heat transfer of Al2O3/R141b+Span-80 in micro heat exchanger by direct metal laser sintering.Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathemat-ical statistics.The heat transfer performance of Al2O3/R141b+Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141b.The heat transfer coefficients of 0.05 wt.%,0.1 wt.%,0.2 wt.%,0.3 wt.% and 0.4 wt.% Al2O3/R141b+Span-80 nanorefrigerant respectively increase by 55.0%,72.0%,53.0%,42.3% and 39.9% compared with the pure refrigerant R141b.The particle fluxes from viscosity gradient,non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling.This mi-gration motion enhances heat transfer between nanoparticles and fluid.Therefore,the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing.The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly.There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation.The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%.The channel surface with wettability increasing needs more energy to produce a bubble.Therefore,the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%.In addition,a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat trans-fer performance.The new correlation can effectively predict the heat transfer coefficient.     

A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.