电场作用下矩形微槽群润湿特性数值分析

以预测电场作用下竖直矩形微槽群热沉内液体润湿特性为目的,基于自适应理论,建立一维轴向模型,研究了电场强度、热通量以及微槽尺寸对润湿特性的影响。结果表明：电场作用下润湿长度随热通量增加逐渐降低。当热通量较低时,电场强度对润湿长度的强化较大,但随着热通量的增大强化程度减弱。电场强度对矩形微槽群热沉适应长度的强化较小,而对于边角流动区域长度的强化较为显著。电场作用下润湿长度随槽深和槽宽的增加分别呈增加和下降的趋势。与较小槽深槽宽相比,当槽尺寸较大时,电场强度对微槽内液体润湿强化更为显著。     

电场作用下矩形微槽群润湿特性数值分析

以预测电场作用下竖直矩形微槽群热沉内液体润湿特性为目的,基于自适应理论,建立一维轴向模型,研究了电场强度、热通量以及微槽尺寸对润湿特性的影响。结果表明:电场作用下润湿长度随热通量增加逐渐降低。当热通量较低时,电场强度对润湿长度的强化较大,但随着热通量的增大强化程度减弱。电场强度对矩形微槽群热沉适应长度的强化较小,而对于边角流动区域长度的强化较为显著。电场作用下润湿长度随槽深和槽宽的增加分别呈增加和下降的趋势。与较小槽深槽宽相比,当槽尺寸较大时,电场强度对微槽内液体润湿强化更为显著。     

电场对竖直微槽润湿及毛细流动特性影响

竖直微槽群毛细结构广泛应用在重力热管、蒸发器等散热设备内,但受重力等因素影响易达到毛细极限。引入电场的主动强化方式来提高竖直微槽的毛细极限,并通过实验和建立数学模型研究电场对竖直微槽内液体润湿及毛细流动特性的影响。结果表明,电场可以提高竖直微槽内液体润湿高度,当电场为5.0 kV时与无电场时相比,润湿高度强化比可达到30.0%。同时,电场作用下流体在微槽道内的毛细润湿流动呈分段效应：润湿流动初期,润湿高度与时间的1/2次方呈线性关系,即h-t^1/2,润湿速率与润湿高度的倒数呈线性关系,即v-1/h;润湿流动中后期,润湿高度与时间的1/3次方呈线性关系,即h-t^1/3,润湿速率与润湿高度平方的倒数呈线性关系,即v-1/h²,且润湿速率随时间呈下降趋势。     

电场对竖直微槽润湿及毛细流动特性影响

竖直微槽群毛细结构广泛应用在重力热管、蒸发器等散热设备内,但受重力等因素影响易达到毛细极限。引入电场的主动强化方式来提高竖直微槽的毛细极限,并通过实验和建立数学模型研究电场对竖直微槽内液体润湿及毛细流动特性的影响。结果表明,电场可以提高竖直微槽内液体润湿高度,当电场为5.0 kV时与无电场时相比,润湿高度强化比可达到30.0%。同时,电场作用下流体在微槽道内的毛细润湿流动呈分段效应：润湿流动初期,润湿高度与时间的1/2次方呈线性关系,即h-t^1/2,润湿速率与润湿高度的倒数呈线性关系,即v-1/h;润湿流动中后期,润湿高度与时间的1/3次方呈线性关系,即h-t^1/3,润湿速率与润湿高度平方的倒数呈线性关系,即v-1/h²,且润湿速率随时间呈下降趋势。     

电场分布对R123沸腾换热的影响

采用6种不同电极布置方式，进行了不同电势和热流密度下的R123池沸腾换热的试验研究。通过数值分析，计算了不同电极布置下换热面上的电场强度及分布。不同的电极布置，会导致换热面上电场强度和电场均匀性两方面的变化。结合试验和电场分布的计算结果，分析了电场均匀性、电场强度、热流密度与沸腾换热效果之间的关系。结果表明，在低热流密度下，电场分布对沸腾换热影响较大;而在高热流密度下，影响较小。电水动力学（EHD）强化换热效果是电场强度和电场均匀性综合作用的结果。     

竖直毛细微槽群热沉中蒸发液体的干涸特性

利用宽视场体视显微镜和CCD摄像系统对纯蒸发换热情形下竖直放置的矩形毛细微槽群热沉中的液体沿微槽槽道方向的流动情况和干涸点高度（润湿高度）进行了观察测量，并对微槽几何尺寸、工质等因素对润湿高度的影响进行了实验研究。实验结果表明：纯蒸发情形下的液体润湿高度随着输入加热功率的增加而陡降；一定热负荷下，微槽较深、较窄以及微槽群密度较大时液体的润湿高度较高；甲醇和乙醇在较低输入加热功率条件下的润湿能力要强于蒸馏水；竖直毛细微槽中液体的润湿特性受重力的影响严重。     

超亲水微纳复合结构表面热沉强化润湿与传热实验

利用碱辅助的表面氧化法在紫铜微槽群热沉表面生成了氢氧化铜纳米棒阵列结构,制备出一种全新的超亲水微纳复合结构表面热沉。并以蒸馏水为液体工质,进行了纯蒸发条件下微槽群热沉、微纳复合结构表面热沉和超亲水微纳复合结构表面热沉的润湿及传热特性的对比实验研究。实验结果表明：氢氧化铜纳米棒阵列结构使得原始亲水表面的亲水性更好,随着表面纳米棒数量的不断增多,接触角不断减小,最低为9.5°,可以进一步形成超亲水微纳复合结构表面。与无纳米结构的微槽群热沉相比,在相同输入加热功率下,微纳复合槽群热沉具有更高的液体润湿高度和更好的传热性能,而超亲水微纳复合结构表面热沉的形成会进一步提高强化润湿和传热效果,相比于紫铜微槽群热沉,超亲水微纳复合结构表面热沉内液体的润湿高度提高了300%,表面温度降低了15℃左右。     

电场对三角形微槽热沉轴向干涸特性的影响

将电流体动力学(EHD)主动强化换热技术应用到微槽换热器中是解决化工、能源领域散热问题的一种有效手段。参考自适应理论建立了一维轴向无量纲分析模型,并将库仑力、介电电泳力和电致伸缩力都引入进了模型中,研究EHD效应对三角形毛细微槽热沉内液体干涸特性的影响。研究发现,电场能有效提升微槽内液体干涸长度。三角微槽内液体的干涸高度以及自适应长度都随着电场强度的增加而增加。本文也得到了不同电场强度下三角微槽内液体曲率半径沿轴向的变化,发现随着电场强度的增加,相同截面上的液膜曲率半径增加,且离微槽入口越远的位置,曲率半径增加幅度更大。     

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。     

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。     

Direct numerical simulation of surfactant solution flow in the wide‐rib rectangular grooved channel

The turbulent flow of surfactant solution in the wide‐rib rectangular grooved channels was studied by direct numerical simulation. Moreover, the variations of near‐wall streamwise vortices with time were discussed and the distributions of streamwise vortex radius, swirling strength and density were quantitatively investigated. It was found that the influence of microgrooves on the fluid mainly occurred within the buffer layer and microgrooves could induce numerous streamwise vortices with small size and swirling strength within the grooved valleys. The drag‐reducing enhancement mechanism of microgroove in the surfactant solution could be mainly considered as the competing results between the “restriction effect” and “tip effect” of microgroove, and the essential factor should be the numerous secondary streamwise vortices with small size and swirling strength within the grooved valleys. Furthermore, a predicted method for the optimal drag‐reducing size of microgroove was proposed, and the prediction values agreed well with the numerical results. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2898–2912, 2018     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

Gravity–capillary evaporation regimes in microgrooves

The gravity–capillary evaporation phase change in microgrooves inside a heat pipe is experimentally investigated by a visualization system. The evaporation regime, startup temperature response, and evaporation thermal resistance for gravity–capillary evaporation are evaluated and compared with those for capillary evaporation. The results identify oscillatory motion of vapor–liquid interface for gravity–capillary evaporation in rectangular microgrooves arising from the competition between gravity, the capillary force, and the shear force. In addition, the presence of gravity introduces the transition from fin–film evaporation regime to corner–film evaporation regime in rectangular microgrooves, and hence results in an overshoot startup and temperature oscillation during the process of gravity–capillary evaporation. Interestingly, the corner–film evaporation improves the evaporation phase change performance in the microgrooves. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1119–1125, 2019     

受限微结构对低表面张力液滴合并弹跳的影响

设计并制备了CuO纳米结构和矩形微槽相结合的层级微槽超疏水表面,采用去离子水、质量分数为8%和16%的乙醇溶液为工质,研究了三种表面张力的单个液滴在微槽内的受限生长特征以及槽内变形液滴与槽外正常液滴的合并弹跳行为,探讨了受限微结构对较低表面张力液滴合并弹跳的影响。结果表明,在Laplace压力差的驱动下,微槽内受限变形的水滴发生自弹跳行为,随着溶液中乙醇浓度的提升,液体表面张力减小,表面对液滴的吸附增强,乙醇质量分数为8%和16%的槽内变形液滴不发生自弹跳,而是爬升并悬浮于微槽上方。受限微结构对液滴合并弹跳的强化作用随液体表面张力的减小而减弱,与去离子水相比,乙醇质量分数为8%和16%的受限变形液滴与槽外正常液滴的合并弹跳速度分别降低了26.7%和75.9%,能量转化效率分别降低了17.8%和90%。     

Surface microstructure evolution of Cf/SiC ceramic matrix composite microgrooves processed by ultrafast laser

Modern aviation components have higher requirements for high temperature resistance, high strength and lightweight materials, and ceramic matrix composites have superior overall performance. However, its high brittleness and anisotropy lead to a challenge for manufacturing. In order to understand the formation conditions and the evolution of surface microstructures of the C_f/SiC microgrooves processed by ultrafast laser comprehensively, we designed a single-factor experiment and performed sensitivity analysis. The experiment results showed that the pulse energy had great effects on the depth of the microgroove, and the intense ablation caused more active oxidation of SiC to occur, generating more SiO(g). However, too much pulse energy may cause the material removal mechanism to be more due to the photothermal effect rather than the plasma effect. Low repetition frequency caused a large number of laminated connections in the microgroove and the oxide gradually changed from lumpy to flocculent as the repetition frequency increased. The more scanning times, the more ablation products sputtered onto the sample surface, including unablated carbon fibers. Shallow depth and ablation residues remained in the microgroove occurred under few scanning times. Although too fast scanning speed leaded to a rapid decrease in the microgroove depth, too slow scanning speed also generated more unablated carbon fibers sputtering out of the microgrooves. The microgroove depth had the highest sensitivity to the repetition frequency, followed by the pulse energy and scanning speed. The pulse energy and scanning speed had a greater effect on the oxide layer height, the repetition frequency affected the oxide layer width, and the scanning speed affected the microgroove width significantly. According to the processing requirements and the hot spot map, the processing parameters that can be adjusted effectively will be able to be obtained.