相变储热微胶囊储热调温效果的研究

采用原位聚合法用蜜胺树脂包覆了一种相变点为24℃，相变热为225．5J／g的有机复合相变材料，并对所制备的相变储热微胶囊的储热调温效果进行了评价。采用扫描电镜对微胶囊表面形态进行了观察；采用示差扫描量热仪对微胶囊的相变点和相变热进行了测定；绘制了微胶囊的步冷曲线。实验结果表明：微胶囊呈球形，粒径分布均匀，平均粒径为5—6μm；表面光洁且致密；相变储热微胶囊基本上不影响相变材料的相变点和相变热；相变微胶囊节能效果实验表明，相变材料的贮能过程具有可逆性和连续性，节能效果达到15．6％。初步证明相变储热微胶囊具有很好的储热调温效果。     

相变微胶囊悬浮液强制对流换热实验研究

提出了一种可以同时作为储能介质与传热流体的新型相变微胶囊悬浮液(MPCS),设计和搭建试验台,分别在层流和湍流条件下在等热流密度的光滑圆管中对MPCS进行了强制对流换热实验,研究了悬浮液浓度、流量、泵送功率和加热速率对其流动及传热特性的影响。结果表明:对于质量分数为5%的MPCS,当微胶囊中相变材料分别处于固体、固体-液体和液体状态时,对应的努塞尔数平均增大了23.9%、20.5%和9.1%;与纯水相比,MPCS作为在热力系统应用的传热流体可以实现强化传热,但是需要在传热实验中控制好相变过程才能使MPCS的传热性能优于水。     

Preparation and characterization of microcapsules energy storage materials with carbon nanotube modified

将碳纳米管加入以硬脂酸丁酯作为囊芯的材料,脲醛树脂作为囊壁材料的原位聚合微胶囊体系,合成具有储热性能的碳纳米管微胶囊材料,研究了微胶囊技术与碳纳米管技术对硬脂酸丁酯相变材料储热性能的影响.采用扫描电镜观察样品表面形貌与分布,傅里叶红外光谱仪对样品进行结构表征,采用差式量热扫描仪对样品进行热性能分析,采用Hotdisk热物性分析测试仪对样品的导热系数进行测量.结果表明,碳纳米管均匀分散在囊芯硬脂酸丁酯中,提高了该材料的相变潜热和导热特性.制成的微胶囊呈球形,完整包覆了硬脂酸丁酯碳纳米管复合材料,提高了相变材料的热稳定性.碳纳米管复合材料的热物性能和储热性能与碳纳米管的分散方法,尺寸和品质,甚至测量方法和仪器有关系,本实验结果对所采用的相变材料和碳纳米管的相关作用进行了研究,提供了这方面的实验数据.     

纳米铜/微胶囊复合体系相变过程及传热特性

针对相变材料的微胶囊化过低的导热性能限制了其大规模应用,通过将具有优良导热性能的纳米铜粉末(CNPs)填充在微米级相变微胶囊颗粒间,构建CNPs质量分数1.0%～8.0%的复合相变体系,研究复合体系相变过程和相变传热特性。研究结果表明,相比于微胶囊相变材料,复合相变体系有效导热系数随CNPs含量增加而提高,有效热扩散系数、相变速率与相变焓值随CNPs含量增加而先减小后增大,且有效热扩散系数与相变速率变化趋势基本保持一致,均在CNPs质量含量1.0%时出现增长的转变,且在CNPs质量含量低于3.0%时两者的数值均低于微胶囊相变材料,而复合体系相变温度则不受CNPs影响。     

（相变）复合材料瞬态导热性能的简化计算方法

一般情况下复合材料不存在有效导温系,其瞬态热性能需藉三维非稳态传热模型分析。如掺温材料中涉及相变材料,由于其等效比热容与温度强烈相关,使得描述复合材料瞬态传热性能的热扩散方程呈现很强的非线性,更增加了求解的难度。该文探讨了利用等效一维均质上变材料模型分析（相变）复合材料瞬态传热特性的可行性,结合算例,分析了该方法的适用范围。结果表明,当掺混材料的相对尺寸不太大时,（相变）复合材料可当作均质（相变）材料,其瞬态传热性能可藉等效热物性计算。对于许多实际问题,上述方法可简化分析,节省计算量。     

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。     

高温相变蓄热的研究进展

从如下两个方面总结了高温相变蓄热的研究现状：①在高温相变材料(PCM)方面，重点介绍了高温相变材料的一些重要性能及其测量，高温相变材料的封装，高温复合相变材料及高温相变材料的应用；②在传热分析方面，主要介绍了相变过程的数值模拟和相变蓄热系统(LTES)的热力学优化。     

高温熔融盐纳米固液相变复合材料研究进展

熔融盐是一种非常有前景的高温液体传热蓄热工质,在太阳能热发电、余热回收及工业热利用方面有显著的优势,但是熔融盐本身存在导热性能不高等问题。本文对纳米复合相变材料固液相变储能过程的若干最新研究进行了回顾,综述了熔融盐纳米固液相变复合材料国内外研究现状及发展趋势,最后对纳米复合相变材料固液相变储能过程的未来发展和重点研究方向进行了展望,认为主要解决纳米复合材料内熔化相变传热双温度模型的建立及求解、NC-PCM的制备工艺、金属纳米粒子的团聚性及NC-PCM蓄热器的热循环实验等方面的问题是未来研究的重点。     

正交各向异性相变材料的无网格法传热模型及应用

利用无网格迦辽金(EFG)法建立正交各向异性相变材料的传热计算模型,基于该模型编程完成各向异性材料太阳能相变蓄热水箱和管壳式相变蓄热单元的相变传热分析,并探讨热导率因子和材料方向角对复合材料相变传热特性的影响.研究表明:在相同节点布置下EFG法的温度场和相界面计算精度均高于有限元法,EFG法在动态相界面追踪方面具有明显...     

Research and application of microencapsulated phase change materials

相变材料微胶囊很好地解决了相变材料的使用问题.本文介绍了制备微胶囊可采用的芯材和壁材以及它们的优缺点,介绍了相变材料微胶囊化的几种常用方法:原位聚合法,界面聚合法,复凝聚法,悬浮聚合法,喷雾干燥法;总结了相变材料微胶囊在建筑,热流传导,纤维纺织,军事伪装,电子器件冷却等领域的应用;指出了相变材料微胶囊以后的研究目标和方向.     

Investigation of energy transportation capability of a phase change slurry through a cold storage‐cooling coil system

Microencapsulated phase change materials slurries (MEPCM slurries) have the advantage that they can be used as heat transport and storage medium, and therefore provide improved continuity of energy transport. Energy transportation capability of a MEPCM slurry based on Rubitherm RT6 through a cold storage‐air handling system was investigated for various concentrations and pump powers. The transport properties of the MEPCM have been identified. The optimum concentration for maximum energy transportation capability and energy transportation effectiveness of the system has been found. This investigation is intended for determination of the optimum concentration of MEPCM slurry for an air conditioning system that uses MEPCM slurry as a secondary cooling medium directly into air handling units and stored for load shifting. Copyright © 2009 John Wiley & Sons, Ltd.     

Research progress on heat transfer enhancement technology of phase change energy storage

相变储能是通过相变材料吸/放热过程来实现能量储存的技术,它能够解决热量供需时间、空间和强度上的不匹配,并以其高储能密度成为储能领域的研究热点,但由于相变材料的热导率较低,使其应用受到限制。针对相变储能材料熔化/凝固过程中热导率低引起的传热速率慢的问题,从优化储能设备结构、添加剂提高相变材料热导率以及联合强化传热技术三方面综述国内外相变材料储能强化传热技术的最新进展。通过比较各种强化传热方式的优劣,实验和模拟均显示复合强化传热即可解决相变材料热导率低,又增大传热面积,从而提高相变材料的传热性能;多孔金属作为导热添加剂增强导热效果更好;并提出了相变储能强化传热技术未来需要解决的相关技术难题。     

Numerical study on the melting thermal characteristics of a microencapsulated phase change plate

ABSTRACT

The melting thermal characteristics of a microencapsulated phase change material (MEPCM) plate under constant heat flux are numerically investigated. The effective property relations of the MEPCM plate related to the parameters of base materials are first established and verified by the measured thermal properties of 10 samples. A one-dimensional phase change thermal model based on the enthalpy method is built, and the predicted results are also verified with test data. The temperature profile and phase interface movement in the MEPCM plate are discussed, and the effects of Stefan number, phase change temperature range, particle and core fraction, and additive fraction are also analyzed. Three states of solid, mushy, and fluid phases of phase change material (PCM) core divide the melting process of the MEPCM plate into five stages as follows: Fo ≤ 0.1, 0.1 < Fo ≤ 0.2, 0.2 < Fo ≤ 0.9, 0.9 < Fo ≤ 1.2, and Fo ≥ 1.2. The time point of regular regime for heat transfer in the MEPCM plate is Fo = 0.2. The addition of conductivity additive homogenizes the temperature profile in the MEPCM plate. Under the combined effects of thermal diffusivity and Ste number, an optimum additive fraction exists for the MEPCM plate to achieve the minimum melting completion time or the maximum latent storage efficiency.     

Micro-channel heat sink with slurry of water with micro-encapsulated phase change material: 3D-numerical study

This study investigates the influence of using micro-encapsulated phase change material (MEPCM) on the thermal and hydraulic performance of micro-channel heat sinks used for heat dissipation of high power electronic devices. A three-dimensional, one-phase, laminar flow model of a rectangular channel using water slurry of MEPCM with temperature dependent physical properties was developed. The results showed a significant increase in the heat transfer coefficient under certain conditions for heat flux rates of 100 W/cm² and 500 W/cm² that is mainly dependant on the channel inlet and outlet temperatures and the selected MEPCM melting temperature. Lower and more uniform temperatures across the electronic device can be achieved at less pumping power compared to using water only as the cooling fluid.     

Experimental study on cooling performance of minichannel heat sink using water-based MEPCM particles

Experimental study was performed to investigate the cooling performance of a minichannel heat sink (Mini-CHS) with microencapsulated phase change material (MEPCM) particles/water as the coolants. Emphasis was focused on the evaluations of the hydraulic and thermal performance of water-based MEPCM particles in a Mini-CHS from the results of friction factor, average Nusselt number, heat transfer efficacy, temperature control effectiveness, and figure of merit with the Reynolds number ranging from 133 to 1515. The measured results reveal that the thermal performance can be enhanced by addition of MEPCM particles in water with low values of latent-sensible heat ratio (ω_pcm/Ste_bf^⁎) and using a low flow rate of coolant as compared to those of the pure water. For instance, an increment of 52% heat transfer effectiveness can be achieved for Re_bf = 133 and ω_pcm/Ste_bf^⁎ = 0.0472. It is also found that the case with lower flow rate and latent-sensible heat ratio has a better improvement of thermal resistance compared with that for the pure water. However, results also indicate that the enhancement effect may be reduced by increasing the values of latent-sensible heat ratio and the thermal performance of the MEPCM particles with higher latent-sensible heat ratio is not necessarily better than that of pure water.     

Heat Transfer in Coiled Square Tubes for Laminar Flow of Slurry of Microencapsulated Phase Change Material

Passive heat transfer enhancement using a slurry of microencapsulated phase-change material (MEPCM) flowing in a laminar regime through a coiled duct of square cross section was evaluated. The phase-change material is n-octadecane. The flow behavior and heat transfer performance of water and MEPCM suspensions in various configurations (conical spiral, in-plane spiral, and helical spiral) of coiled tubes of square cross section was investigated. The results are compared with those for water as the base fluid flowing through a straight tube. A computational fluid dynamics (CFD) approach is used to simulate the laminar flow of water with MEPCM suspension in these geometries. The liquid suspension properties are expressed as functions of the volumetric concentration of MEPCM particles and the temperature. Improved heat transfer performance was obtained as the concentration of MEPCM suspension increased from 1 to 10%. However, the overall performance in terms of the pumping power consumed for unit heat transferred worsened.     

Preparation and thermal properties of microencapsulated phase change material for enhancing fluid flow heat transfer

Microencapsulated phase change material (MEPCM) is formed by packing PCM into a microcapsule with a solid but flexible shell. MEPCM can be used to enhance liquid cooling performance considerably. In this paper, experiments on the preparation of MEPCM with a double‐layered shell have been conducted. An in‐situ polymerization microencapsulation process was used to prepare the MEPCM with melamine resin as the shell material and n‐Docosane (C₂₂H₄₆) as the core material. Interesting parameters like the size of the prepared MEPCM, the core mass fraction in the MEPCM, and the thermal storage capability of the prepared MEPCM have been measured and analyzed. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(1): 28–37, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20138     

Review on the low melting point alloys for thermal energy storage and heat transfer applications

低熔点合金具有导热系数高,储能密度大,使用温度范围广,性能稳定等特点,是一种潜在的宽温域传热工质和中低温相变储热材料.结合低熔点合金的相变温度,相变潜热,热导率及相变稳定性等热物理性能,综述了低熔点合金相变储热材料的研究进展;介绍了液态低熔点合金传热材料的蒸汽压,表面张力,黏度及比热容等性能,以及低熔点合金在高温下与容器材料的相容性;对低熔点合金传热储热材料的下一步研究进行了展望.