金属泡沫内石蜡固液相变蓄热/放热实验

针对解决太阳能热利用过程中所面临的辐射强度不稳定、不连续和不均匀等关键问题,相变蓄热技术常与太阳能热利用系统耦合协同匹配,以实现稳定连续的热量输出.为了强化固液相变蓄热/放热过程、提高系统热储能效率,对金属泡沫内石蜡类相变材料(PCMs)在不同蓄热流体温度下的固液相变蓄热/放热特性开展了实验研究.设计并搭建了相界面可视...     

泡沫铜对相变蓄热系统蓄热性能影响的实验研究

石蜡作为一种有机固液相变材料,因其具有高潜热值、无毒、无腐蚀、性能稳定等优点被广泛应用于热蓄存、电子冷却及建筑温控等领域。但在蓄热过程中,因石蜡导热系数较低,导致蓄热时间过长、温差过大。实验按照1∶3的比例将泡沫金属铜均匀分布在石蜡箱体中,探究泡沫铜对石蜡相变速率的影响。结果显示:加入泡沫铜后,有效地提升了石蜡的相变速率,缩短了石蜡相变的时间;同时加入泡沫铜后,石蜡内部温差明显减小,温度分布更加均匀,并且有效缓解了自然对流造成的顶部过热和底部不熔化现象。     

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。     

同心套管石蜡相变熔化数值模拟分析

以相变材料石蜡作为研究对象,采用数值模拟方法分析同心套管蓄热装置(以下简称蓄热装置)内换热管与石蜡相变的温差(以下简称换热温差)对石蜡相变传热的影响。研究石蜡在不同换热温差下随时间变化的液相率和熔化速率。6种工况下的相变传热过程均为自然对流传热与导热的共同作用,自然对流传热对熔化过程起关键作用,加快了石蜡的熔化速率,上半部分的熔化速率远大于下半部分,造成石蜡液相率分布的不均匀性。换热温差越大,相同时刻的石蜡液相率越大。下半部分未熔化的石蜡未受到浮升力的作用,且未熔化的石蜡离加热管壁面越来越远,导致热阻越来越大,熔化需要的时间越长。采用3/4熔化时间能够提高蓄热效率。     

泡沫金属内石蜡相变凝固的数值模拟

研究了泡沫金属中相变材料的相变熔化过程,由于金属骨架和相变材料传热性能的巨大差异,建立了骨架和相变材料的双温度模型,采用显热容法进行了数值模拟。模拟结果显示,相变材料中填充泡沫金属,能有效改善相变材料的温度分布;在相变时,骨架与相变材料的温差较大,局部热不平衡明显;泡沫金属孔隙率越小,石蜡熔化越快。     

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。     

圆柱形等距螺旋盘管式相变蓄热装置蓄热性能研究

文章设计了一种以石蜡为相变材料的圆柱形等距螺旋盘管式相变蓄热装置,并通过实验分析了该装置的传热特性,以及传热流体入口温度、入口流量对石蜡的融化特性、相变蓄热装置的蓄热量及相变蓄热系统总传热系数的影响。分析结果表明:融化后期,石蜡的融化速率会明显加快;当传热流体入口温度一定时,随着入口流量逐渐增大,蓄热装置的最终显热蓄热量略微升高;与传热流体入口流量相比,传热流体入口温度对石蜡融化速率影响较大;相变阶段,石蜡的传热性能较强,传热流体入口温度越高,石蜡的传热性能越不稳定。     

太阳平板集热/储能相变传热的数值模拟

提出了一种高效储能平板集热器模型.该平板集热器中采用相变材料作为储能介质,不需储能水箱及防冻保护装置,因此节约了空间和费用.建立了太阳平板集热器内相变材料(石蜡)二维相变传热及自然对流模型,运用显热容法模拟了平板集热器内泡沫铝中石蜡相变融解传热过程.与平板集热器内填充纯相变石蜡的融解传热过程进行比较发现,泡沫铝可极大地提高相变石蜡的相变传热性能.计算结果为高效储能型平板集热器的开发提供了理论指导.     

金属蜂窝/石蜡复合相变材料融化储热性能研究

针对金属蜂窝/石蜡复合相变材料融化储热过程中,金属蜂窝热传导与液相自然对流传热的竞争关系,基于流-固-热三场耦合理论,建立相变材料融化储热计算模型。开展相变石蜡融化试验,验证计算模型的正确性。进一步分析液相自然对流和金属蜂窝热传导传热的增强效应,以及两者间的竞争关系。结果表明：底部加热下的密闭方腔内相变石蜡融化储热过程可分为热传导、稳定增长、过渡和紊流等四个阶段;各阶段占总融化储热时间的比例分别为0.8%、2.3%、13.6%和83.3%。热量随着液相石蜡的自然流动实现无障碍传输,达到提升相变石蜡融化储热效率的目的。自然对流传热的增强效应随尺寸减小而显著降低,在尺寸小于2 mm后可忽略不计。金属蜂窝通过增大热传导性和传热面积,达到提升相变石蜡融化储热效率的目的。嵌入金属蜂窝后,相变材料储热过程中存在多层共融现象,且在融化区形成温度梯度。与纯石蜡储热效率相比,金属蜂窝作用呈现先增强后抑制的规律,当融化分数超出临界值0.77后,金属蜂窝将进入抑制阶段。     

椭圆管强化固液相变蓄热器的数值模拟

由于蓄热材料融化后受自然对流的影响,为了改善固液相变蓄热器上下融化不均的情况,采用椭圆管强化蓄热器中的传热,利用Fluent软件模拟了相同面积的圆管和椭圆管这2种结构中石蜡相变的融化过程,得到了石蜡融化过程温度场分布及融化过程中液相分数线的规律,根据这些规律分析了椭圆管对增强蓄能效果的影响,并且分析了不同椭圆管长短半轴比对加快石蜡蓄热时间的影响。     

Numerical model of the passive thermal management system for high-power lithium ion battery by using porous metal foam saturated with phase change material

A two-dimensional transient model for a passive thermal management system was developed for commercial square lithium ion battery by using the phase change material (PCM) of paraffin saturated in metallic copper foam. This model combined the thermo-electrochemical model for the battery and a model that characterized the solid–liquid phase change of paraffin in copper foam. The thermo-electrochemical model was composed of species conservation, charge conservation, and energy balance equations. In the model of phase change in metal foam, the non-Darcy, natural convection of melted paraffin, and local thermal non-equilibrium effects were considered. The thermo-electrochemical performance of the battery and convective heat transfer behavior of the foam-PCM composite were investigated. The predicted results were in agreement with experimental data. Compared to the air convection and adiabatic modes, the thermal management by foam-PCM composite has dramatically reduced battery surface temperature to the allowable range at 1C and 3C discharge rates.     

Laminar film condensation driven latent thermal energy storage in rectangular containers

This paper focuses on numerically analyzing the thermal transport phenomena in the transient conjugate problem of melting and laminar film condensation. The key focus is to identify an optimum container aspect ratio/shape and conditions for which the heat storage time and the storage capacity are minimum and maximum respectively. Since most solid–liquid phase change materials (PCMs) suffer from poor thermal conductivities, the major resistance to heat transfer comes from PCM. Hence, high thermal conductivity, low-cost metal foam is suggested for use along with PCM to minimize this resistance. The conjugate transient problem of film condensation driven solid–liquid phase change of PCM impregnated inside porous metal foam is numerically analyzed. An effective heat capacity formulation is employed for modeling the transient PCM phase change in porous foam and is solved using finite element method. It is coupled with laminar film condensation on the outside of the storage container. The model is then used for selecting the best aspect ratio for thermal energy storage (TES) containers that enables to store comparatively the maximum heat. The results of the developed model showed that the major resistance to heat transfer and hence efficient thermal energy storage depends strongly on the aspect ratio of the PCM storage containers.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

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.     

Role of metal foam in solidification performance for a latent heat storage unit

The current latent heat storage (LHS) units are usually poor in energy charging and discharging efficiency. Given this, a two dimensional (2D) numerical model of the energy discharging process is presented and comprehensively analyzed to predict the role of metal foam in the solidification performance of LHS units. In the model, the fractal geometry reconstructed by the fractal Brownian motion is utilized for the pore characterization of the metal foam. The proposed model is validated through a melting experiment in copper foams from the reference. The temperature dynamic response and the solidification front evolution in metal foam are analyzed and compared to those in a corresponding cavity. The roles of the fractal dimension and porosity in the solidification behaviors are quantitatively analyzed. The results report that the presence of metal foam enhances the solidification performance. For the main goal of maximizing the latent storage, the appropriate porosity of an LHS unit is dependent on the duration time for the heat discharging process in the real application of thermal energy storage. Even if the porosity is the same, the fractal dimension also affects the solidification performance. A decrease in the fractal dimension (lower degree of disorder for pore distribution) provides greater access to heat flow through the phase change material-foam composite and thus leads to improvement in the interstitial heat transfer, which in turn accelerates the rate of heat release. The fractal dimension is expected to be less than 1.5 for superior solidification performance.     

铝/石蜡复合相变材料蓄热性能的数值模拟

相变储能材料由于其具有周期性储存和释放能量的特点,在电池热管理、太阳能发电等领域存在着广泛的应用。然而由于导热系数低的原因限制了其进一步的应用。高导热率泡沫材料的添加为解决这一不足提供了一种有效的方法。文章采用三周期性极小曲面（TPMS）生成泡沫铝骨架,基于孔隙尺度数值模拟了铝/石蜡复合相变材料相变蓄热的变化规律。结果表明：铝骨架的添加强化了蓄热,缩短了融化时间,在复合相变材料孔隙率为0.90、0.85、0.80时,相比于纯石蜡,完全融化时复合相变材料的融化时长分别缩短了68％、75％和80％,而且蓄热过程中温度场更加均匀;验证了铝骨架与石蜡之间由于热导率存在较大的差异,存在的热非平衡效应,且铝/石蜡复合相变材料孔隙率越低,此效应越明显     

Thermal analysis of void cavity for heat pipe receiver under microgravity

Based on theoretical analysis of PCM (Phase Change Material) solidification process,the model of improved void cavity distribution tending to high temperature region is established.Numerical results are compared with NASA (National Aeronautics and Space Administration) results.Analysis results show that the outer wall temperature,the melting ratio of PCM and the temperature gradient of PCM canister,have great difference in different void cavity distribution.The form of void distribution has a great effect on the process of phase change.Based on simulation results under the model of improved void cavity distribution,phase change heat transfer process in thermal storage container is analyzed.The main goal of the improved designing for PCM canister is to take measures in reducing the concentration distribution of void cavity by adding some foam metal into phase change material.     

Investigation of the effect of metal foam characteristics on the PCM melting performance in a latent heat thermal energy storage unit by pore-scale lattice Boltzmann modeling

Latent heat thermal energy storage (LHTES) has many advantages such as high energy density and phase change at a nearly constant temperature compared with sensible thermal energy storage or chemical energy storage techniques. However, one of its major drawbacks is the low thermal conductivity of phase change materials (PCMs) which impedes the heat transfer efficiency. High thermal conductivity metal foams could be added into the LHTES to enhance the heat transfer speed. Under this case, the investigation of the effects of metal foam porosity and pore size on the melting process is essential for improving the heat storage capability of LHTES. In this article, a pore-scale modeling of melting process in a LHTES unit filled with metal foams is carried out by enthalpy-based multiple-relaxation-time lattice Boltzmann method. The quartet structure generation set is used to generate the morphology of metal foams. In addition, a Compute Unified Device Architecture (CUDA) Fortran code is developed in this work for executing highly parallel computation through graphics processing units. The melting process in the PCMs is investigated in terms of porosity, pore size, nonuniform metal foam, hot wall temperature, and initial subcooled temperature to optimize the design of LHTES filled with metal foams.