方形微通道热沉的耗散优化分析

基于CFD软件建立了两种不同结构的方形微通道热沉,并对其进行数值计算,模拟得到热沉的温度场和压力场。在此基础上,研究了不同微通道分布方式、不同质量流率和不同热通量对热沉的温度、压降的影响,同时基于耗散理论对比分析来获得方形微通道热沉换热效果较好的优化方案,在固定边界热流条件下,耗散越小,换热效果越好。计算结果表明：随着质量流率的增大,热沉温度逐渐降低,进出口压差逐渐增大,PEC逐渐增大,耗散逐渐减小;随着热通量的增大,热沉温度逐渐升高,进出口压差逐渐降低,PEC逐渐增大,耗散逐渐减小。微通道分布方式为上层内切圆半径-下层外接圆半径分布时热沉的温度更低,PEC更大,耗散更小,传热效率更高。     

带有周期性扰流结构的微通道内流动与传热特性

设计了一种周期性扰流微结构,由布置在微通道侧壁的凹穴和微通道中心的针肋组成。研究了该热沉内流动和传热特性,分析了扰流结构几何参数对热沉不可逆损失和散热效率的影响,利用热阻和强化传热因子评价综合性能。研究表明,等腰梯形凹穴的底边相对长度（RL）对热沉性能具有显著影响。雷诺数（Re）较大时,减小RL能够明显减小凹穴内部的旋涡,从而减小流动摩擦损失,降低通道压降和流动不可逆性。同时,减小RL有利于增强流体对凹穴收缩段的冲刷,减小凹穴内的层流滞止区,将凹穴处的热量及时带走,从而提高热沉的散热效率。与传统光滑微通道（SM）相比,周期性扰流结构能够显著减小热沉的总熵产和热阻,增大强化传热因子,提高热沉的综合性能。综合考虑传热和流动阻力,较低泵功条件下,RL=0.3的热沉综合性能最优;较高泵功条件下,RL=0的热沉综合性能最佳。周期性扰流结构能够提高微冷却系统的效率和经济性,在微型器件冷却领域具有广阔的应用前景。     

微通道形状对自相似热沉综合性能的影响分析

自相似微通道热沉(SSHS)作为一种新的换热结构设计,比一般的微通道热沉(MHS)具有更好的综合性能。前期工作通过将内部分流通道改为渐缩式斜坡结构,使内部流量分配不均受到了很大抑制。本文在此基础上,利用数值模拟方法进一步分析了微通道(溢流通道)形状对SSHS流动及传热过程的影响。计算结果表明,原设计中溢流通道内存在较大的低流速区,导致了不均匀的换热过程;将矩形截面微通道改为梯形截面,可以减小溢流通道内低速区的流通面积,从而使微通道内的流动和换热过程更加均匀;进一步对比分析发现,随着溢流通道顶部宽度mw减小,低流速区域占比进一步减小,换热均匀性有所提高;该结构优化方案对流动阻力的影响较小,流量在0.18~1.8kg/h范围内,流动阻力增加平均不超过5%,但换热均匀性平均提高15%左右。     

新型金属泡沫-微柱群复合热沉内流动传热性能分析

高效热管理技术是大功率微电子设备安全运行的可靠保障。为进一步强化高功率电子器件的冷却效果,本文提出了一种新型泡沫铝-微柱群复合热沉结构。采用实验和数值模拟相结合的方法对新型水冷泡沫铝-微柱群复合热沉内的流场分布、壁面温度分布、阻力系数、换热性能及柱鳍与泡沫铝间的耦合传热规律等开展了深入分析。研究结果表明,与传统微柱群热沉相比,20PPI泡沫铝-微柱群复合热沉的壁面最高温度大幅降低,平均换热性能提升了33.9%~41.5%。然而,微柱群内填充泡沫铝却导致流动阻力增大,增加了7.9~10.5倍。泡沫铝-微柱群复合热沉的强化换热机理为：微柱群热沉内填充高热导率泡沫铝提升了热沉整体的有效导热性能,热量可通过金属泡沫固体骨架迅速传递,同时多孔界面较强的传热能力能够保证热量及时被冷却流体散除。本文相关研究成果可为高热流密度电子器件散热装置的研发提供理论指导。     

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。     

恒壁温下矩形微通道热沉换热特性分析

建立了恒壁温条件下矩形硅微通道热沉的三维模型,对微通道内单相层流的换热和流动特性进行了数值模拟研究,分析结果表明:沿流动方向,热沉流体域的温度梯度大于固体域的温度梯度,且最大的温度梯度出现在入口段;除恒温热沉顶面外,通道顶面的温度最大,通道底面和热沉底面的温度近似趋于定值。通道内的换热研究发现,通道侧壁面的Nu数最大,顶面与底面相差很小,角落处的Nu数趋近于0。     

微通道临界热通量的基础理论与提升技术研究进展

临界热通量（CHF）是微通道流动沸腾换热的限制参数之一,当热通量大于CHF时,换热性能急剧恶化,换热设备易发生烧毁与故障,因此CHF对于微通道换热的安全运行具有重要影响。微通道换热是目前电子冷却的主流技术,然而近年来电子设备热通量不断提高,CHF已成为限制微通道应用的关键参数之一。针对微通道CHF的研究进展,详细阐述微通道CHF的形成机理,分析工况参数和通道尺寸对微通道CHF的影响机制,总结微通道CHF的预测模型,详述微通道CHF提升的各类技术方法与原理,探讨学术界观点差异和今后研究方向。该综述为微通道在高热通量条件下安全可靠运行提供了研究借鉴。     

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。     

微通道狭缝冲击射流热沉换热及热应力模拟

为研究纳米流体种类及浓度、热沉材料、狭缝长宽比对热沉换热性能及热应力的影响,建立了微通道狭缝冲击射流热沉三维模型.利用数值模拟得到纳米流体在通道形状为圆形和矩形热沉中的压降、热阻、泵功及壁面与流体之间的温差大小,同时也得到不同材料的热沉由于温度分布不均匀而引起的形变量的大小,研究结果表明:通道形状为圆形的热沉在导致泵功...     

梯形微槽道表面池沸腾换热性能研究

池沸腾换热表面的结构对其沸腾换热性能具有重要影响。为了进一步强化在较低表面过热度时池沸腾换热的性能,提出了新型梯形微槽道池沸腾换热表面,采用可视化实验方法研究了饱和温度下去离子水在该表面的池沸腾换热性能。结果表明：与光滑平面相比,梯形微槽道表面可以降低起始沸腾表面过热度;在相同表面过热度时,随着下底长度的增大、下底角角度的减小,梯形微槽道表面的热通量增加,换热能力增强。下底长度为1.2 mm、下底角度为45°的梯形微槽道表面具有最低的起始沸腾表面过热度（1.4 K）;在表面过热度为8.3 K时,其热通量能达到1.2×10⁶ W·m^-2,为相同表面过热度时光滑表面的24.0倍。较大的下底长度和较小的下底角角度有利于增强梯形微槽道表面的池沸腾换热性能。     

PREDICTION OF VOID FRACTION AND VELOCITY PROFILES OF BUBBLY FLOWS IN VERTICAL PIPES

Adopting a similar approach to Beyerlein et al. (1985), void fraction distributions in turbulent two-phase bubbly air/water upflows and downflows in vertical pipes were analyzed using a simple transport model which was based on the assumptions that the lateral shear-induced lift force acting on bubbles (Thomas et al., 1983) is balanced by bubble dispersion, and that bubbles in the flow are conserved i.e. no bubble breakup or coalescence occurs. The model shows the importance of considering the lateral lift force experienced by bubbles as they move relative to the liquid phase in a non-uniform velocity field. This force causes the bubbles to accumulate near the wall forming a high concentration for upward flow, while the concentration increases toward the centre of the pipe for downward flow. The eddy diffusivity, as widely used in calculation of single-phase flow, can be extended to include the effect of pseudo-turbulence (Lance and Balaille, 1991) due to bubbles, and thus can be linked with the bubble dispersion coefficient. It is also demonstrated that the transverse or radial pressure gradient induced by the Reynolds stress exerts a lateral force on the bubbles, and thus affects their distribution in the flow. A comparison of the model predictions with experimental data from Serizawa et al. (1975) for upflows and Wang et al. (1987) for both upflows and downflows shows that our model predicts void fraction peaking near the wall for upflows and coring at the centre-line for downflows. Compared with similar investigations (e.g., Drew and Lahey, 1982; Lopez de Bertodano et al., 1990) of the same problem, our model approach appears to be simpler and more suitable for engineering calculations.     

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%。     

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study, forced flow and heat transfer of water/NDG (Nitrogen-doped graphene) nanofluid in nanoparticles mass fractions (φ) of 0, 2% and 4% at Reynolds numbers (Re) of 10, 50, 100 and 150 are simulated in steady states. Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection. Temperature of inlet fluid equals with T_c=293 K and hot source of microchannel is under the influence of oscillating heat flux. Also, in this research, the effect of the variations of attack angle of triangular rib (15°, 30°, 45° and 60°) on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L/H=28 and nanofluid jet injection is investigated. Obtained results indicate that the increase of Reynolds number, nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop. By increasing fluid viscosity, momentum depreciation of fluid in collusion with microchannel surfaces enhances. Also, the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient. At low Reynolds numbers, due to slow motion of fluid, variations of attack angle of rib, especially in angles of 30°, 45° and 60° are almost similar. By increasing fluid velocity, the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently. In general, by using heat transfer enhancement methods in studied geometry, heat transfer increases almost 25%.     

PREDICTION OF GAS-PHASE HOLDUP IN A BUBBLE COLUMN

Hydrodynamic properties of bubble columns play a significant role in many chemical and biochemical processes. Recent theoretical and experimental work conducted by Krishna et al. (1991, 1994), and Wilkinson et al. (1992) have been examined in conjunction with a bubble column and data for the air-water system operating at ambient conditions. The bubble column is 0.108 m in internal diameter, has a 1.70 m tall test section, and is equipped with a perforated plate distributor having 91 holes of 0.8 mm diameter. The data are taken for five values of the slumped water column height in range from 0.79 to 1.15 m, and for superficial air velocities up to about 0.4 m/s.

The data accord to the qualitative aspects of Krishna et al. model but lead to different values of the bubble swarm rise velocity, and superficial transition air velocity characterizing the transition from homogeneous bubbly flow regime to heterogeneous churn-turbulent flow regime. The quantitative reproduction by the model expressions of these recent works of the experimental data is poor. This may be partly attributed to the geometry of the column, diameter and distributor design.

The qualitative features of Krishna et al. model for the two regimes are confirmed by the present data. For quantitative predictions of gas-phase holdup, a new model is proposed in which the large bubble flow in the churn-turbulent regime is formulated following the drift-flux theory. The proposed theory and experimental data are in good agreement.     

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.     

不同压力下HFE-7100在光滑铜基表面的饱和池沸腾传热实验

池沸腾是重要的传热模式之一,广泛应用于诸多工业领域。饱和压力的变化会影响传热工质的热物性,进而引起表面核化及气泡动力学参数的改变,因此饱和压力对池沸腾传热性能具有显著影响。本文在不同饱和压力（0.07MPa、0.10MPa、0.15MPa及0.20MPa）工况下对HFE-7100工质在纳米级粗糙度光滑铜基表面的池沸腾传热及可视化实验进行了研究,针对饱和压力对池沸腾传热的影响机制进行了深入探讨,同时采用相关池沸腾传热及临界热通量预测模型对传热性能曲线进行了对比分析。光滑铜基表面的平均粗糙度为19nm,HFE-7100工质在其上的静态接触角为9.83°。可视化图像展现了沸腾孤立气泡生成、充分发展合并及核化沸腾向膜状沸腾转换的过渡状态。实验数据表明,饱和压力的提升可强化池沸腾传热能力及提升临界热通量。相较于0.07MPa低压池沸腾,0.10MPa、0.15MPa及0.20MPa条件下池沸腾的最大传热系数分别提升29%、59%及75%,传热系数的平均提升率分别为24%、50%和63%,而临界热通量分别增加27%、48%及64%。相对而言,Forster和Zuber（1955）建立的池沸腾传热预测模型及Guan等（2011）建立的临界热通量预测模型较为准确地预测了本研究操控条件下的池沸腾实验数据。     

Identification of the morphology of a thin film by fluorescence laser-scanning confocal microscopy

This study is an extension of our previous paper (Hayashi, M. et al. Polymer, 1998, 39, 299) dealing with the phase separation and structure formation of a polysulfone (PSU)/polyamide (PA) blend as observed by laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). Here we use fluorene-labelled PSU in order to apply fluorescence LSCM for more detailed morphological investigations of the internal structure of a thin film as-cast from solution.