不同温度下纳米流体对微槽群热沉毛细性能的强化差异研究

基于一维的自适应理论,采用经典的毛细上升法,在室温、40℃、60℃情况下比较分析温度对Fe3O4水基纳米流体强化微槽群热沉毛细性能的影响.结果表明:3个温度下,纳米流体所产生的强化效果随颗粒体积分数的变化规律是一致的,并存在统一的最佳体积分数.纳米流体对热沉毛细性能的平均强化比例随温度的提高呈现先减小后增大的趋势.低体...     

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。     

电场对竖直矩形微槽群润湿及表面温度的影响

微槽群在热流密度较大时会达到其毛细极限,可通过主动换热方式之一——电水动力学效应对其进行强化。本文为了研究电场对微槽群表面润湿特性和温度分布的影响,采用平板电极提供电场,蒸馏水作为工质,使用高速相机拍摄微槽内液体润湿长度,测量误差为2.97%～7.46%;使用红外热像仪拍摄电场作用下微槽群表面温度分布,测量误差为2.1%～2.39%。热流密度测量误差范围是9.66%～11.11%。结果表明：电场通过驱动微槽内流体向加热区域流动而提升其润湿性能,且较低热流密度下提升更好。因润湿性能的提升,微槽表面温度得以下降。随着电场增强,微槽横向温度分布的“波峰”、“波谷”差别加大,微槽纵向温度明显降低。当热流密度加大时,温降更为显著,1.4W/cm²热流密度、6kV电压下温降可达到30℃以上。温降的增加反映了电场对微槽的强化润湿进一步提升了微槽换热性能,且电场对较高热流情形下的微槽换热有着更为显著的强化效果。     

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。     

微/小圆柱针肋热沉传热特性

圆柱形微/小针肋热沉热阻由底板及微/小肋热传导热阻、冷却液与肋间对流换热热阻以及冷却液吸热焓变热阻等组成。以去离子水为冷却液,恒定热流下,进行了叉排排布的不同尺寸的微/小圆柱形针肋热沉的单相强制对流传热实验,实验Re为100～1000,加热功率为50～300 W。实验表明：微/小圆柱形针肋热沉的Nu随Re和加热功率的增大而增大,当Re=600时对     

不同管径毛细管的毛细蒸发特性实验

以无水乙醇为工质,采用3种不同管径的圆形毛细管进行毛细蒸发实验。研究结果表明：毛细管毛细蒸发过程中,蒸发界面移动后会延展形成一层液膜,这层液膜可以大幅提高液体蒸发速率。这是因为液膜蒸发引起的温度变化会通过Marangoni效应转化为表面张力差,使液膜在蒸发的同时得到不断补充,形成高效的蒸发-补充机制。随管径增大,液膜厚度增加,蒸发稳定性变好,速率变化幅度减小。工质整体蒸发速率随管径变大呈递减趋势。     

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。     

电场强化微槽道结构毛细芯蒸发器的传热特性

该实验以R141b作为工质,采用圆柱形电极,在不同运行压力条件下,实验研究了电场强度对微槽道结构换热表面蒸发/沸腾传热特性的影响。实验结果表明:随着电场强度的增加,传热强化效果越明显;运行压力对强化换热的效果具有重要的影响,在施加相同电场强度的条件下,不同运行压力的强化传热效果明显不同,传热系数最大可强化为未施加电场条件下传热系数的1.28倍。     

电场强化微槽道结构毛细芯蒸发器的传热特性

该实验以R141b作为工质,采用圆柱形电极,在不同运行压力条件下,实验研究了电场强度对微槽道结构换热表面蒸发/沸腾传热特性的影响。实验结果表明：随着电场强度的增加,传热强化效果越明显;运行压力对强化换热的效果具有重要的影响,在施加相同电场强度的条件下,不同运行压力的强化传热效果明显不同,传热系数最大可强化为未施加电场条件下传热系数的1.28倍。     

燕尾形轴向微槽道热管的传热特性(英文)

A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance. The effect of heat load on the axial distribution of capillary radius, and the effect of working temperature and wick structure on the maximum heat transfer capability, as well as the effect of the heat load and working temperature on the total thermal resistance are all investigated and discussed. It is indicated that the meniscus radius increases non-linearly and slowly at the evaporator and adiabatic section along the axial direction, while increasing drastically at the beginning of the condenser section. The pressure difference in the vapor phase along the axial direction is much smaller than that in the liquid phase. In addition, the heat transfer capacity is deeply affected by the working temperature and the size of the wick. A groove wick structure with a wider groove base width and higher groove depth can enhance the heat transfer capability. The effect of the working temperature on the total thermal resistance is insignificant; however, the total thermal resistance shows dependence upon the heat load. In addition, the accuracy of the model is also verified by the experiment in this paper.     

变孔隙毛细芯平板热管性能

毛细芯是热管的核心部件,单一均匀孔隙毛细芯往往难以同时兼顾高性能热管对于毛细抽吸力和渗透率两方面的需求,变孔隙毛细芯则可以根据需要分别设置变化的内部孔隙分布,同时满足所需要的毛细抽吸力和渗透率。本文以纤维毡为主要材料制备不同孔隙分布的变孔隙毛细芯,搭建变孔隙毛细芯平板热管实验台,通过实验研究毛细芯的孔隙分布等参数对平板热管的启动性能和传热性能的影响,研究结果表明：含有变孔隙复合毛细芯的平板热管比含均匀孔隙毛细芯的平板热管性能更好,启动所需的时间更短,热管的传热性能也更好。其他条件相同,倾斜角度为45°时,回流方向孔径递减毛细芯平板热管能承受的加热功率比均匀毛细芯平板热管高约2W,比重力热管高3～4W。     

The effect of liquid film vaporization on natural convection heat and mass transfer in a vertical tube

A numerical analysis was carried out to investigate the effects of film vaporization on natural convection heat and mass transfer in a vertical tube. Results for interfacial Nusselt and Sherwood numbers are presented for air-ethanol and air-water systems for various conditions. Predicted results show that heat transfer along the gas-liquid interface is dominated by the transport of latent heat in association with the vaporization of the liquid film. Additionally, the predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Chang et al. (1986) and Yan and Lin (1990) is only valid for systems with small liquid mass flow rates. For systems with a high liquid film Reynolds number, Re₁₀, the assumption of an extremely thin film is seriously in error.     

垂直管内三相流化床沸腾传热特性

研究了三相流化床沸腾传热的特性和影响传热系数的诸因素。在传热过程中，由于固体粒子的存在，强化了传热。以玻璃球粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热的二倍。以铜粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热系数的３倍。     

Thermal and hydrodynamic characteristics of constructal tree‐shaped minichannel heat sink

A three‐dimensional thermal and hydrodynamic model for constructal tree‐shaped minichannel heat sink is developed. The heat and fluid flow in the constructal heat sink with an inlet hydraulic diameter of 4 mm are numerically analyzed, taking into consideration conjugate heat transfer in the channel walls. The pressure drop, temperature uniformity, and coefficient of performance (COP) of the constructal tree‐shaped heat sink are evaluated and compared with those of the corresponding traditional serpentine flow pattern. The results indicate that the constructal tree‐shaped minichannel heat sinks have considerable advantages over the traditional serpentine flow patterns in both heat transfer and pressure drop. The strong and weak heat flow can be effectively allocated in tree‐shaped flow structures; hence, the inherent advantage of uniform temperature on the heating surface in the constructal tree‐shaped heat sink is demonstrated. And in tree‐shaped flow structures, the local pressure loss due to confluence flow is found to be larger than that due to diffluence flow. In addition, an aluminum constructal tree‐shaped minichannel heat sink is fabricated to conduct the verification experiment. The experimentally measured temperature distribution and pressure drop are in agreement with the numerical simulation, which verifies that the present model is reasonable. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

垂直上升管内超临界CO2 流动传热特性研究

在压力为7.5～21 MPa，热通量为50～413 kW·m^-2，质量流速为519～1500 kg·m^-2·s^-1的实验参数范围内，对超临界CO₂在内径为10.0 mm的垂直上升管内的流动传热特性进行了均匀加热条件实验研究。分析了热通量、压力和浮升力对圆管内传热特性的影响规律。实验结果表明：随着热通量的增加，传热出现恶化现象，并且随着热通量的增加壁温峰值点向入口段移动。传热恶化发生在流体温度小于拟临界温度而壁面温度大于拟临界温度附近。增大压力时由于物性的变化趋于平缓，传热恶化被抑制。当传热恶化发生时，浮升力对传热恶化有明显的影响。基于实验数据，综合考虑物性变化和浮升力对传热的影响，建立了新的超临界二氧化碳传热关联式，在实验工况范围内，预测值与实验值的平均偏差和标准差分别为1.2%和16.29%。