高孔隙率泡沫金属相变材料储能、传热特性

以高孔隙率泡沫金属材料作为骨架制备而成的新型复合相变储能材料的导热系数将大大高于相变材料本身的导热系数,在储能过程中具有更好的传热效果。给出了较通用的高孔隙率泡沫金属材料等效导热系数的估算公式,并利用准稳态方法建立了复合相变材料在凝固过程的数值模型,对其凝固过程的传热特性进行了理论分析。以铝—石蜡和铜—石蜡复合材料作为研究对象。分析表明,采用复合储能材料可以使得其传热性能得到很大提高,但是也会使复合材料的储能能力有所降低。提出了一种平衡储能能力和传热性能的方法,当泡沫金属处于平衡孔隙率时,在传热性能得到极大提高的同时也使得其储能能力降低不多。同时,分析得到了外部换热环境对储能能力、传热性能以及平衡孔隙率的影响,即较大的对流换热时,若要取得适当的储能能力和传热性能,则需要较小的孔隙率。     

泡沫铜内石蜡凝固的孔隙尺度实验研究

对泡沫铜内石蜡凝固相变进行孔隙尺度实验研究。采用高分辨率相机与红外热像仪对凝固过程相场与温度场进行可视化,并通过热电偶测量石蜡与泡沫铜骨架局部温度以获得相变过程热响应及热非平衡特性。揭示了泡沫铜孔隙内凝固相变中包括固液相界面移动、液态石蜡流动及石蜡体积收缩等多个物理过程。研究表明:在多物理过程交互影响下,泡沫铜可高效扩展凝固相界面、提升样品热响应速率,采用孔隙率为0.974的泡沫铜可将石蜡凝固相变速率提升至2.8倍;泡沫铜能有效避免石蜡凝固过程由体积收缩引起的裂缝问题,消除由其引起的热阻;石蜡与泡沫铜骨架间存在局部热非平衡性,且在相变阶段尤为明显。     

基于梯度孔隙率金属泡沫的复合相变单元储热性能数值模拟北大核心CSCD

向相变材料中添加金属泡沫可以解决相变材料低导热率引起的换热效果较差等问题,提高系统的整体蓄热效率。然而,复合相变材料的传热性能受金属泡沫孔隙率分布的影响较显著,为进一步提高相变储能单元的传热性能,本工作基于低孔隙率金属泡沫-相变材料(PCM)复合储能系统,建立了一种新的梯度孔隙率金属泡沫结构,通过数值模拟方法,对蓄热单元熔化过程中的熔化率、储能速率、储能总量进行分析,系统研究了孔隙率沿加热方向负梯度分布、正梯度分布对复合相变材料熔化速度和储热性能的影响。研究结果表明,负梯度孔隙率结构可以进一步提高储能系统的储热效率,其中,孔隙率梯度为0.12(案例S-6)时增强效果最显著。在熔化周期的不同阶段,负梯度孔隙率对复合材料的传热均有不同程度增强,对于S-6,在1000 s、2000 s、2600 s时,熔化率相较于均匀孔隙率结构分别增加了0.67%、2.31%、9.90%;随着孔隙率梯度的增加,相变材料的热性能提高越显著,与均匀孔隙结构相比,改进的负梯度孔隙率结构其完全熔化时间最高可缩短7.32%,储热速率可提高8.02%。对于正梯度孔隙率结构,其对熔化速度没有显著影响,但是储热总量可提高0.49%。     

梯度孔密度金属泡沫的池沸腾传热性能研究

实验研究了梯度孔密度通孔金属泡沫的池沸腾传热性能。工质为去离子水,梯度孔密度金属泡沫材质为铜和镍, 孔隙率为0.98,泡沫厚度为4-14 mm。实验结果表明：相比于单层泡沫,梯度孔密度金属泡沫显著的增强了沸腾传热能力,但增强程度受孔密度变化梯度、泡沫厚度和材料的影响;梯度孔密度泡沫的池沸腾传热性能随着表面活性剂SDS浓度的增大而减小,而且SDS降低了梯度孔密度金属泡沫的临界热流密度; 添加Al2O3纳米颗粒严重的削弱了梯度孔密度铜泡沫的池沸腾传热能力。     

地下水源热泵运行时渗透系数对含水层的影响

为减少地下水源热泵运行对地下空间的影响,运用颗粒迁移理论研究了地下水源热泵长期运行时渗透系数对含水层参数变化特性的影响。结果表明,随着地下水源热泵的长期运行,含水层孔隙率和渗透系数的变化特性与渗透系数的初始值无关,而承压水头的变化特性则受渗透系数的影响,渗透速度为影响含水层参数变化的重要因素之一,较小的渗透速度可有效减少地下水源热泵运行对地下空间的影响。     

形貌对通孔金属泡沫辐射性能的影响

本文借助PerkinElmer 光学测试系统实验研究了不同形貌通孔金属泡沫的漫反射率和漫透射率。实验结果表明,材质对金属泡沫的漫反射率有重要的影响。铜泡沫的漫反射率随着孔密度的增大而减小,而镍泡沫的漫反射率变化趋势则相反。无论是铜泡沫还是镍泡沫,吸收率皆随着孔密度的增大而增大,消光系数随着孔密度的增大而增大。对于烧结有铜板的铜泡沫,吸收率随着孔隙率的增大而增大。     

高孔隙率泡沫金属复合相变材料有效热导率的预测

基于十四面体泡沫金属的单元晶胞结构,提出了一种高孔隙率开孔泡沫金属复合相变材料有效热导率的预测模型。热量沿热流方向在金属骨架和填充介质中并行传递,推导得出包含结构参数f(节点边长与韧带半径之比)和d(韧带半径与韧带长度之比)的有效热导率计算式。在孔隙率0.889 0≤ε≤0.977 0,针对单元晶胞的十六分之一结构,采用数值模拟的方法并结合实验数据的对比分析,校正了与有效热导率紧密相关的两个无量纲参数f和d,并以孔隙率的二阶多项式拟合出f变量的函数关系,以孔隙率的三阶多项式拟合出d变量的函数关系,从而确定出有效热导率的预测模型。通过与其他几种预测模型和实验数据的比较分析,结果表明：验证了当前预测模型具有较高的正确性和有效性,与实验值的平均相对误差ARD和相对均方根误差RMS分别为2.93%和3.50%,相对其他模型具有更高的预测精度。     

TPS法测定泡沫铜/石蜡复合相变材料热物性

运用瞬态平面热源法(Transient Plane Source-TPS)对4种孔隙率的泡沫铜/石蜡复合材料热物性进行了测量。以10μm厚的镍金属按双螺旋线布置作为测量探头。泡沫铜材料孔隙率分别为ε=97.79%、ε=96.17%、ε=94.94%和ε=93.26%,经线切割加工后向内灌入液态石蜡,凝固后作为测试样品。在室温(25±1℃)和常压下对复合材料的等效导热系数、热容及热扩散率进行了测量。测试结果表明:复合材料导热系数和热扩散率因泡沫铜的加入而大幅提高,在孔隙率ε=93.26%时,等效导热系数已达到单纯石蜡的25倍,而复合材料等效热容则由于铜金属加入的绝对量较少相对原石蜡热容变化较小。以比例加成的方法对泡沫铜/石蜡复合材料的等效热容进行了计算,并利用实验数据拟合了其等效导热系数的计算公式,运用这些公式对复合材料物性的计算结果与实验结果非常吻合。     

均匀及梯度泡沫金属复合PCM熔化特性模拟研究

基于六面通圆孔的均匀泡沫金属结构,构建了泡沫金属复合相变材料(PCM)三维模型,采用高性能计算显卡(GPU)加速的多松弛时间格子玻尔兹曼方法模拟了均匀及梯度泡沫金属复合PCM的瞬态熔化过程。结果表明：随着均匀泡沫金属孔隙率的降低,复合PCM的传热速率提高,潜热储能的能力减弱;对于固定平均孔隙率的不均匀泡沫金属,孔隙率沿导热方向上递增的模型具有最佳的强化传热效果,其完全熔化时间比填充均匀骨架模型和孔隙率在导热方向上递减的骨架模型分别缩短了4.2%和25%,当孔隙率梯度变化方向与导热方向一致时,在高温壁面附近填充低孔隙率泡沫金属能显著强化传热;当两者方向垂直时,熔化速率取决于平均孔隙率,与梯度分布几乎无关。     

复合相变材料调控机理与强化换热的实验研究

为获取相变温度、潜热和导热性都比较合适的相变材料,本文使用高熔点的固态石蜡(熔点为70℃)和低熔点的液态石蜡(熔点为5℃)按照不同比例进行配比实验,来获得不同相变范围的相变材料,以适应实际应用的要求。制备了五种复合相变材料样品,使用差式扫描量热仪(DSC)测试其参数。并选用孔隙率均为95%,孔隙密度(pores per inch, ppi)分别为15,30和50 ppi的三种泡沫铜,采用熔融浸渍法将石蜡填充其中制备复合相变材料,进而探究泡沫铜对石蜡强化换热的效果。实验结果显示比例为A1(20%5℃+80%70℃)、A2(35%5℃+60%70℃)和A5(80%5℃+20%35℃)时只有一个熔化峰,其起始点分别为56.6℃,53.2℃和3.7℃,表明通过物理方法将两种石蜡混合可以调控复合相变材料的熔点与潜热。热导率测试结果表明当孔隙率为95%孔隙密度分别为15,30和50 ppi时泡沫铜可以提高石蜡导热率3-7倍。     

Capillary Performance of Micropillar Arrays in Different Arrangements

In this study, permeability and capillary pressure of copper micropillar structures (height: 50 µm, diameter: 50 µm) in different arrangements (hexagonal, rectangular, and square) and different porosities (0.45/0.5, 0.6, 0.7, 0.8) are compared experimentally and numerically. The micropillar structures are fabricated on copper clad printed circuit board with electroplating, and the samples are coated with silica nanoparticles to enhance wettability. A forced liquid flow test is used to measure permeability of the samples, and capillary rate-of-rise measurement technique is used to determine the capillary pressure of the wicks. In the permeability model, the effect of meniscus curvature is considered, and the results are compared with other permeability models. Capillary pressure is predicted by using surface energy minimization tool, Surface Evolver. The test results show that the micropost array in rectangular arrangement have the highest permeability, and similar capillary pressure compared to other pillar arrangements with the same porosity, and thus show the highest capillary performance parameter. The effect of gravity on the sample characterization with capillary rate-of-rise test is also studied to investigate the feasibility of applying Washburn’s equation to test data.     

Microtomography-Based Simulation of Transport through Open-Cell Metal Foams

Important heat transfer parameters of aluminum foams of varying pore sizes are investigated through CT-scanning at 20 micron resolution. Small sub-samples from the resulting images are processed to generate feature-preserving, finite-volume meshes of high quality. All three foam samples exhibit similar volumetric porosity (in the range ～91–93%), and thereby a similar thermal conductivity. Effective tortuosity for conduction along the coordinate directions is also calculated. Permeability simulations in the Darcy flow regime with air and water show that the foam permeability is isotropic and is of the order of 10^?7 m². The convective heat transfer results computed for this range of Reynolds numbers exhibit a dependence on the linear porosity, even though the corresponding volumetric porosity is the same for all the samples considered.     

泡沫铝通道内瞬态流动的平均换热系数

针对泡沫铝金属填充矩形通道内的对流换热开展了瞬态实验研究,分析了泡沫铝孔径(孔隙率)、流体流量(流速)等关键参数的影响。为了有效地处理实验数据,重新定义并推导了平均换热系数的计算公式,得到了泡沫铝通道内流动的平均换热系数,并引入了基于渗透率的雷诺数和达西数,确定了相关换热、流动准则数关系。实验研究表明,流速的增大有利于对流换热的强化:而平均换热系数对泡沫金属孔径较敏感;对于低孔隙率泡沫金属,渗透率成为影响换热强度的主要因素,相同或接近的孔隙率下,孔径越大,渗透率和达西数越大,越有利于换热,且压损减小。     

Passive thermal management using metal foam saturated with phase change material in a heat sink

An electronic passive thermal management system was designed. The system featured a hybrid heat sink with parallel fins sintered onto its top and copper metal foam–paraffin composite saturated in its hollow basement. The other two types of basement patterns for thermal dissipation were also employed: (1) a hollow basement saturated with pure paraffin; (2) a solid copper basement. The experimental results showed that the use of the copper metal foam reduced the surface temperature and the time required to reach the melting point of the paraffin. Lower surface temperature can be achieved by either reducing foam porosity or foam pore density. During the melting process, temperature increased more linearly for the foam–PCM composite than for the case of pure paraffin since the enhancement in thermal conduction caused by the metal foam exceeded the level of its suppression to natural convection of melted paraffin.     

Investigation of functionally graded metal foam thermal management system for solar cell

Concentrated photovoltaic cell (CPV) is a solar energy harvesting device that converts solar energy into electrical energy. However, the performance and efficiency of the CPV are heavily dependent on the temperature. Besides, nonuniformity of temperature distribution on the CPV will lead to thermal aging and affects the cycle life. Hence, an effective cooling system is required to remove excess heat generated to ensure that the CPV operates at optimum operating temperature with minimum variation of temperature. Metal foam is a new class of material that possesses huge potential for thermal management. In this study, a functionally graded metal foam is proposed for the CPV thermal management system. Computational thermal fluid dynamic analysis is conducted to investigate the effect of porosity and pore density on the flow field and thermal performance of the aluminum foam heat sink. The investigation results revealed that 10 PPI functionally graded aluminum foam heat sink with two stages of porosity gradient 0.794 and 0.682 produced the lowest pressure drop and highest thermal performance. Temperature difference of 3.9°C was achieved for a solar cell with total heat generation of 900 W under water mass flow rate of 20 gs⁻¹.     

Computational analysis of gas permeability of slip flow through microannulus replete by based-circular Sierpinski porous medium

There are wide applications for flow in a microporous medium. In this study, a computational analysis of airflow through a porous microannulus constructed by circular-based Sierpinski has been performed in a slip flow regime, where several parameters played an important role in the flow characteristics. These parameters are the Knudsen number, the average friction factor, radius ratio, and porosity. The impacts of these parameters on permeability and the gas flow characteristics are examined and analyzed thoroughly. The ranges of the investigated parameters are as follows (0.001 ≤ Kn ≤ 0.1 and the porosity range is 0.75 ≤ ε ≤ 0.95). The results showed that porosity has a significant impact on the velocity distribution and Darcy number. The Knudsen number has also a direct effect on the velocity distribution, while it has a positive logarithmic proportionality with a dimensionless permeability but the radius ratio does have a neglected effect on the Darcy number. Moreover, the effect of the average friction factor has an inverse proportional relationship to the Darcy number.     

Study on heat transfer characteristics of porous metallic heat sink with conductive pipe under bypass effect

The work investigated the forced convection heat transfer of the heat sink situated in a rectangular channel by considering the bypass effect. The fluid medium was air. The relevant parameters were the Reynolds number (Re), the relative top by‐pass gap (C/H), and the relative side by‐pass gap (S/L). The size of the heat sink was 60 mm (L)×60 mm(W)×24 mm(H). Two heat sinks were employed as test specimens: (A) the 0.9‐porosity aluminum foam heat sink and (B) the 0.9‐porosity aluminum foam heat sink with a 20 mm diameter copper cylinder. The copper cylinder was used as a conductive pipe of heat sink. The average Nusselt number was examined under various forced convection conditions. Experimental results demonstrate that increasing by‐pass space decreased the Nusselt number. Besides, the average Nusselt number of mode B heat sink was higher than that of mode A heat sink by 30% for the case without by‐pass flow. The heat transfer enhancement by the copper cylinder would decline as the by‐pass space grew. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20247     

Hydrodynamic Characterization of Nickel Metal Foam—Effects of Pore Structure and Permeability

The structural characterization of chemical vapor deposition (CVD) nickel metal foam is presented in this study. Scanning electron microscope and post image processing were utilized to analyze the surface of the nickel metal foams. Measured data on foam unit cell, ligament thickness, projected pore diameter, and averaged porosity were obtained. The unit cell and projected pore diameters of CVD nickel metal foam possess Gaussian-like distribution. Characteristics of pore structure and its effect on permeability in the Darcian flow regime were analyzed. Results indicate that the permeability and the viscous conductivity of the CVD processed metal foam are highly affected by the porosity and ligament thickness.     

Experimental investigation of a new thermal energy storage system using thermo‐sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.     

Analysis of the parameters of the sintered loop heat pipe

The purpose of this paper is to establish an experimental formula for sintered dendritic nickel powder. For this reason, wick structures with different porosity ranging from 65 to 80% were fabricated by cold pressing sintering process at fixed porosity and their parameters that included porosity, pore radius, and permeability were also measured. According to both the capillary limitation and the present experimental formula of the sintered dendritic nickel powder, the wick structure parameters that would affect the heat transfer capacity of the loop heat pipe (LHP) were analyzed theoretically and then investigated experimentally. The results showed that there exists an optimal combination of wick structure parameters by which the performance of the LHP would achieve optimization. The maximum heat transfer capacity was up to 500 W and the thermal resistance was 0.12^°C/W at the allowable working temperature 80^°C. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 515–526, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20034