相变材料相变点温度热物性的测试及误差分析

固-液相变过程中移动的相界面曲线与两相热物性如比热、密度、导热系数以及相变潜热是密切相关的。本文提出通过测定相界面的移动速率来确定相变材料固-液相变温度点导热系数等多个热物性参数的方法，并设计了相应的测试装置。对测试系统的测量误差进行了定量分析，发现采用数值计算与实验相结合的方法测试的系统误差不超过3％。利用研制的测试系统对几种材料的导热系数和热扩散系数进行了测定，得到了满意的结果，表明本文所提出的测试方法是可信的。     

固液相变蓄能中有效导热系数的数值分析与实验研究

引入有效导热系数经验公式[3],使固液蓄能数学模型得以简化为仅用能量方程加以描述。运用热焓法求解能量方程,得到计算相变位置随时间变化曲线。并通过实验测得相变过程的实际温度场,得到实验相变位置随时间变化曲线。二者比较,证明了自然对流对固液相变换热的影响不可忽略,验证了该公式在Ra<1010,Pr<105下的正确性。     

基于热焓方程的相变材料凝固时间分析

对相变导热的研究,以往大多集中于固液边界层的移动速率,间接推导相变时间。对相变材料焓值与温度建立数学模型,并基于数学模型和热力学第一定律,建立一种新的相变导热热焓方程。运用该方程,可直观分析相变材料发生相变凝固所需时间,并分析影响固液相变时间的具体因素。计算表明,在零维非稳态导热中,相变时间与相变材料质量、相变潜热、换热面积、对流换热系数、相变材料与周围冷却介质温差有关。     

相变材料特性对建筑围护结构传热的影响

采用有限容积法对选用相变材料时建筑围护结构的传热问题进行数值研究。利用"焓法模型"进行求解,得出如下结论:随着相变层的增厚,相变层内侧温度变化越小,稳定时间在增加;随着相变温度的升高,相变层内侧温度变化比较大,稳定时间在缩短,相变温度主要影响相变层的稳定时间;随着相变潜热量的增加,相变层内侧温度变化较小,稳定时间增长。由此可见,不同相变材料的潜热量对传热的稳定时间有直接影响;当导热系数较小时,固体层内侧温度变化较小,稳定时间比较长;当导热系数较大时,全天都可以较快地进行热传递,从而导致温度变化比较大;相变半径的影响可以不予考虑。     

新型定形板状相变材料的蓄/放热特性

对填充了新型定形板状相变材料的蓄热槽的蓄／放热特性进行了数值计算分析和实验比较。根据数值计算，对影响蓄热槽蓄／放热特性的主要因素——相变材料的几何尺寸、相变材料的导热系数、流体流速、对流放热系数、相变材料填充率等的影响规律进行了分析研究，计算出了蓄热槽内温度分布随时间的变化；并在实验台上测试了蓄热槽初始温度、流人和流出蓄热槽流体温度、作为蓄热体的相变材料测点温度随时间的变化，计算结果与实验结果具有较好的一致性。     

基于内燃机冷却循环的相变蓄能器传热特性分析

本文主要借助Fluent软件对相变蓄能器的蓄、释能特性和相变过程进行模拟分析。基于内燃机冷却循环的余热利用,通过对某一简单的相变蓄能结构进行传热流动分析,得到了相变蓄能材料在蓄能、释能过程中液相分数及温度随时间的变化规律,掌握了其固液界面运动的规律,同时本文也研究了不同工况对相变过程的影响。     

石蜡/膨胀石墨复合相变储热材料的研究

以膨胀石墨为基体,石蜡为相变储热介质,利用膨胀石墨对石蜡良好的吸附性能,制备出了石蜡／膨胀 石墨复合相变储热材料。由于毛细作用力和表面张力的作用,石蜡在固-液相变时,很难从膨胀石墨的微孔中渗 透出来。实验结果表明,石蜡/膨胀石墨复合相变储热材料没有改变膨胀石墨的结构和石蜡的固-液相变温度, 且其结合了石墨高的导热系数和石蜡大的相变潜热,因而储热密度较高,导热性能好。其相变潜热与对应质量 分率下的石蜡相当,储/放热时间比石蜡明显减少。     

石蜡基碳纳米管复合相变材料的热物性研究

以多壁碳纳米管为填料,制备了不同质量分数(1%～5%)的石蜡基纳米复合相变材料。采用差示扫描量热技术对所制备复合相变材料的相变特性进行了表征,其导热性能则通过瞬态热线法导热仪进行了测量。实验结果发现,虽然复合相变材料的相变温度几乎不变,但其相变焓则随碳纳米管的加载量的增加而近似线性下降。在质量分数为5%时,相变焓较纯石蜡下降了约15%。复合相变材料的导热系数大致随温度的升高而降低,而在30和50℃时分别由于固固和固液相变的作用,导热系数测量值出现了较大程度的突增。此外,导热系数随质量分数呈线性增长的趋势,在质量分数为5%时,最大的相对提升率接近40%,展现了良好的导热强化效果。     

固液相变蓄热技术的研究进展

综述了相变蓄热材料、相变传热问题求解方法、典型相变传热过程以及相变潜热蓄热系统(LHTES)优化设计及强化传热等诸多固液相变蓄热技术相关问题的研究进展情况     

相变储能石膏板传热过程测试及有限元分析

利用Hot Disk导热系数测试仪对相变材料的热性能随温度变化的相关参数进行测试,制备"三明治"结构的相变储能石膏板,通过实验测试和ANSYS有限元分析相变储能石膏板的传热性能。研究结果表明:相对于普通石膏板,相变材料的潜热储能效应可增加相变石膏板的冷热面温差,且随着相变层厚度的增大,冷热面温差逐渐增大。利用ANSYS有限元方法分析相变储能石膏板的传热过程,模拟结果表明相变材料的加入可延缓石膏板冷面温升速率,提高室内环境舒适度。     

Structure and thermal properties of expanded graphite/paraffin composite phase change material

Different contents of expanded graphite (EG) composite phase change material (PCM) were prepared by the melt mixing method, taking paraffin as the PCM and EG as the supporting material. Phase compositions of EG, paraffin, and EG/paraffin composite were investigated using X-ray diffraction (XRD). Microstructures of EG and EG/paraffin composite PCMs with different EG contents were observed by a scanning electron microscope (SEM). Thermal properties, such as phase-transition temperature and latent heat of the materials, were determined by differential scanning calorimetry (DSC). Mass loss and thermal properties after 100 heating cycles were measured. The results show that physical absorption exists between paraffin and EG. EG is beneficial for the PCM composite to reduce leakage of paraffin, decrease the phase change temperature and latent heat, and strengthen the thermal stability. The solid–liquid phase change latent heat of materials is larger than that of the solid–solid one. The heating cycle has little effect on the phase-transition temperature and latent heat.     

Three—Dimensional Heat Transfer Analysis for A Thermal Energy Storage Canister

IlltroductionSolar dynamic power modules (SDPM) with phasechange material (PCM) is a vital solution to ensureuninterrupted power supply for low-earth orbitapplication. The advantage of SDPM is its longerlifehme and higher efficiency. Longer lifetime results insubstanhal savings in hardware replacement, launch, andon-orbit installation costs. Because of SDPM's higherefficiency, its solar collection area is only about 25percent of that for a PV system. This would allowspacecraft operatin…     

Microstructure and thermal properties of a paraffin/expanded graphite phase-change composite for thermal storage

A paraffin/expanded graphite phase-change composite for thermal storage was prepared and its thermal properties were studied using differential scanning thermal calorimetry. The paraffin was uniformly absorbed in the porous network of the expanded graphite. Results showed that the phase-change temperature did not change with a change in the amount of paraffin present, whereas the latent heat of the phase change was increased with increasing paraffin content. There was no exudation of paraffin liquid during the solid–liquid phase change.     

相变蓄热同心套管传热模型和性能分析

建立了一种简便的相变蓄热同心套管传热模型,用来求解相变材料在相变过程中流体温度和相变界面随时间和向位置的变化规律,模型中考虑了相变材料导热热阻和有效传热面积随时间和位置的变化,适用于流体入口温度和流量随时间变化的情况,计算值与有关文献值吻合,验证了模型的正确性。藉此模型对文献「７」中相变贮能换热器的储、放热性能进行了模拟分析。     

Thermal energy storage properties of the capric acid–stearic acid binary system and 48# paraffin–liquid paraffin binary system

In this paper, the phase change temperature, latent heat and thermal stability of the capric acid–stearic acid binary system and 48# paraffin–liquid paraffin binary system were experimentally studied. The experimental results showed that the phase change temperature and phase change latent heat change with the content of the component. The phase change temperature of binary mixtures changes in a wide range, so they can be used in different fields by adjusting mixing ratio. The phase change latent heat of fatty acid mixtures is higher than that of paraffin mixtures. The thermal stability of fatty acid mixtures is better than that of paraffin mixtures. The mixtures used in the phase change material (PCM) wall or the PCM floor as energy storage materials are given in the paper.     

Thermal energy storage properties of a capric acid/stearic acid binary system and a 48# paraffin/liquid paraffin binary system

In this paper, the phase change temperature, latent heat and thermal stability of a capric acid/stearic acid binary system and a 48# paraffin/liquid paraffin binary system were experimentally studied. The experimental results showed that the phase change temperature and phase change latent heat change with the content of the component. The phase change temperatures of binary mixtures change in a wide range, so they can be used in different fields by adjusting the mixing ratio. The phase change latent heat of fatty acid mixtures is higher than that of paraffin mixtures. The thermal stability of fatty acid mixtures is better than that of paraffin mixtures. The mixtures used in the phase change material wall or the phase change material floor as energy storage materials were given in the paper.     

Phase transition temperature ranges and storage density of paraffin wax phase change materials

Paraffin waxes have been used in many latent thermal energy storage applications because of their advantageous thermal performances. In this paper, the liquid–solid phase diagram of the binary system of tetradecane and hexadecane has been used to obtain information of the phase transition processes for cool storage applications. The analysis of the phase diagram indicates that, except for the minimum-melting point mixture, all mixtures melt and freeze in a temperature range and not at a constant temperature. The latent heat of fusion evolves throughout this temperature range. Differential scanning calorimetry (DSC) was used to determine the thermophysical properties of the binary system. Depending on the DSC settings throughout the measurements, varying results were obtained. For example, when the DSC runs at a high heating/cooling rate, it will lead to erroneous information. Also, the correct phase transition temperature range cannot be obtained simply from DSC measurement. By combining phase equilibrium considerations with DSC measurements, a reliable design method to incorporate both the heat of phase change and the temperature range is presented.     

Granular phase change materials for thermal energy storage: Experiments and numerical simulations

The present paper reports on the utilization of granular phase change composites (GPCC) of small particle diameter (1–3 mm) in latent heat thermal energy storage (LHTES) systems. The phase changing parameters (phase change temperature, latent heat, and energy storage capacity) of GPCC have been determined using differential scanning calorimeter (DSC) and temperature-history methods. Further analysis of measurement results has been conducted to describe the evolution of latent heat with temperature during phase change in terms of liquid fraction–temperature relationships. Charging and discharging packed bed column experiments have been also carried out for different operating conditions to analyze the potential of GPCC for packed bed thermal energy storage. The present column results clearly demonstrate the dependence of temperature variation along the packed bed and the overall performance of the storage unit on the phase change characteristics of GPCC. Small and non-uniform particles diameters of GPCC and heterogeneity of the bed material complicate the phenomena of heat transfer and evolution of latent heat in the packed bed. Mathematical modeling of the packed bed that considers the GPCC and air as two separate phases with inter-phase heat transfer is presented. Comparisons between experimental and numerical results are used to evaluate the sensitivity of numerical simulations to different model parameters.     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

Phase change characteristics of ethylene glycol solution‐based nanofluids for subzero thermal energy storage

Nanofluids, particularly water‐based nanofluids, have been extensively studied as liquid–solid phase change materials (PCMs) for thermal energy storage (TES). In this study, nanofluids with aqueous ethylene glycol (EG) solution as the base fluid are proposed as a novel PCM for cold thermal energy storage. Nanofluids were prepared by dispersing 0.1–0.4 wt% TiO₂ nanoparticles into 12, 22, and 34 vol.% EG solutions. The dispersion stability of the nanofluids was evaluated by Turbiscan Lab. The liquid–solid phase change characteristics of the nanofluids were also investigated. Phase change temperature (PCT), nucleation temperature, and half freezing time (HFT) were investigated in freezing experiments. Subcooling degree and HFT reduction were then calculated. Latent heat of solidification was measured using differential scanning calorimetry. Thermal conductivity was determined using the hot disk thermal constant analyzer. Experimental results show that the nanoparticles decreased the PCT of 34 vol.% EG solution but minimally influenced the PCT of 12 and 22 vol.% EG solutions. For all nanofluids, the nanoparticles decreased the subcooling degree, HFT, and latent heat but increased the thermal conductivity of the EG solutions. The mechanism of the improvement of the phase change characteristics and decrease in latent heat by the nanoparticles was discussed. The nanoparticles simultaneously served as nucleating agent that induced crystal nucleation and as impurities that disturbed the growth of water crystals in EG solution‐based nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.