纳米金属/石蜡复合相变蓄热材料的实验研究

以石蜡为相变材料基体、纳米金属铜、镍、铝、铁和锌为导热增强剂、油酸为分散剂,采用超声波震荡法制备纳米金属/石蜡复合相变蓄热材料体系。通过复合蓄热体系的步冷曲线分析,结果显示纳米铁为有效导热增强剂。对不同质量分数纳米铁/石蜡复合相变蓄热体系进行DSC和导热系数测试分析,结果表明:随着纳米铁质量分数的增加,复合材料的导热系数逐渐增大,相变潜热值逐渐降低,相变温度变化不大;纳米铁质量分数为0.1%时,复合材料的固态导热系数可增大2.8倍,相变潜热值下降1.1%。     

RT6/纳米铝相变蓄冷材料的热物性研究

石蜡类相变材料RT6相变潜热高,相变温度低,化学性质稳定,但其具有导热系数低,蓄能时间长的缺点,固采用纳米粒子强化换热。通过粒度观测法选取分散稳定性较好的纳米Al粒子制备纳米相变材料,实验分析强化后的相变过程、纳米流体石蜡/纳米铝的导热性和相变特性。实验结果表明,复合材料稳定性好,相变温度基本不变为4～8℃,相变焓值略有下降,但导热系数明显提高,可以作为蓄冷系统的新型蓄冷材料广泛应用。     

纳米材料增强石蜡相变乳状液在太阳能中的应用

潜热型石蜡相变乳状液是一种新颖的储传热工质。该研究采用"两步法"制备一种石蜡质量分数为30%,并掺入0.1%Cu纳米粒子的复合相变石蜡乳状液,并将这种复合相变乳状液作为太阳集热器的集热-储热-传热工质。通过与纯Cu纳米流体热工质比较,研究两者的集热-放热规律,分析并计算以复合相变乳状液作为工质的集热器的日平均热效率。结果表明:复合相变乳状液热工质具有优异的热物性,适合用作太阳集热器的热工质,其日平均集热效率可达60%以上。     

建立余热回收中纳米工质的导热系数预测模型

采用两步法制备质量分数为1%的Cu/Al2O3-H2O/EG混合纳米流体。首先,研究其导热系数随温度和基液混合比的变化情况。然后,根据多项式回归理论建立Cu/Al2O3-H2O/EG混合纳米流体的导热系数预测模型。实验结果表明,纳米流体的稳定性随乙二醇含量的增大而增强,由于不同种类粒子间的分子吸附力不同,导致相同种类粒子容易结合形成团聚体,而Cu粒子与Al2O3粒子的团聚体则较少。导热系数随着温度的升高非线性升高,随基液中水含量的增大而下降。根据实验数据,拟合了导热系数与温度及基液混合比的多项式预测模型,回归系数R2达0. 998,精度较高可以很好地预测Cu/Al2O3-H2O/EG混合纳米流体的导热系数。该模型可以指导工程应用。     

热适应复合相变材料的制备与热性能

根据电子器件散热技术领域对热适应复合材料的性能要求,选取导热系数高且密度低的膨胀石墨作为无机支撑材料,石蜡作为有机相变材料,制备出高导热系数和储热密度的热适应复合相变材料.采用扫描电镜(SEM)、差示扫描量热仪(DSC)、偏光显微镜(POM)和Hot Disk热常数分析仪等多种测试技术,对复合相变材料进行分析研究;通过储/放热实验和1000次热循环实验研究了复合相变材料的传热性能和热稳定性.实验结果说明该复合相变材料具有形状稳定、导热率高、储热密度大等特点,并具有良好的热稳定性和使用寿命.     

Paraffin based composite phase change materials for thermal energy storage: Thermal conductivity enhancement

为提高石蜡作为固-液相变储热材料的导热性能,在石蜡(PW)中掺加高导热系数的碳纳米管(CNTs),制备了碳纳米管-石蜡复合相变材料(PW-CNTs).为进一步增强PW-CNTs的传热性能,通过内置金属网结构,利用金属网的高导热性,加快PW-CNTs作为相变材料的充放热速率.测试了PW-CNTs的熔点和相变潜热,导热系数以及置入金属网前后的充放热时间.结果显示,PW-CNTs的导热系数较石蜡得到显著提高,其中掺加10%(质量分数)CNTs的复合材料的固态,液态导热系数平均分别提高31.4%,40.2%.置入金属网结构后,PW-CNTs的充放热时间至少分别缩短了40.3%和30.2%.此外,碳纳米管在石蜡中易发生团聚沉积,针对这一特点,对PW-CNTs进行了多次热循环,研究了热循环对PW-CNTs导热系数的影响.     

纳米铝粉/石蜡/膨胀石墨复合相变材料的制备及性能研究

将具有导热系数高,与石蜡相容性较好特点的纳米铝粉加入到液体石蜡中,形成纳米铝粉/石蜡流体,利用膨胀石墨特有的网络状孔隙结构以及对石蜡的高吸附性能,制备出了纳米铝粉/石蜡/膨胀石墨复合相变材料,解决了纳米铝粉在液体石蜡中容易发生团聚和沉降的问题,并通过实验研究了其热物性能。研究结果表明:当石蜡与膨胀石墨质量百分比例为93/7,加入纳米铝粉的质量分数低于3%时,膨胀石墨可以稳定的吸附纳米流体,经反复循环蓄、放热,纳米流体不会出现泄漏问题,且对复合相变材料的体积和蓄热能力没有影响;膨胀石墨的网络状孔隙结构可以抑制纳米铝粉的团聚现象,但随着纳米铝粉含量的增加,纳米颗粒仍会发生团聚现象,复合相变材料的导热系数,蓄、放热速度均呈非线性增加。应控制纳米铝粉的加入量,当纳米铝粉质量分数为2%时,纳米铝粉颗粒未发生明显团聚现象,复合相变材料的热性能较好。     

基于模板法制备氧化铝纤维及其石蜡复合相变材料热性能

石蜡是一种高储热密度的有机相变材料,但是热导率低和易泄漏的缺点限制其进一步发展.为提高石蜡的导热和防泄漏性能,本研究以天然纤维为模板制备了具有高导热性的纤维状氧化铝导热填料,通过真空浸渍混合法制备了氧化铝纤维/石蜡复合相变材料,并对其形貌、热导率、相变循环稳定性、防泄漏性能以及热响应性能进行测试.结果表明,随着填料含量...     

Influence of 2-D nanofillers on the thermal conductivity of composite PCMs

为了探究两种不同二维纳米填料对复合相变材料导热系数的影响,分别制备了以石墨烯纳米片和六方氮化硼纳米片为填料的石蜡基复合相变材料.采用瞬态平面热源法在20 ℃时测量了不同添加量下复合相变材料的导热系数.结果显示,石蜡基复合相变材料的导热系数随纳米填料添加量近似线性增长;六方氮化硼纳米片对复合相变材料导热系数的提升远低于石墨烯纳米片.此外,利用基于有效介质模型的预测公式与试验值进行了比较,计算发现形状,大小和导热系数相近的两种纳米材料,六方氮化硼纳米片的界面热阻却高出石墨烯纳米片两个数量级,是后者具有更显著强化效果的原因之一.     

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。     

十四烷纳米流体导热系数影响因素探究

十四烷是工业中最常用的液态烷烃之一,常被用于有机溶剂，有重要的应用价值。相比于纯烷烃，烷烃基纳米流体具有许多 优异的性质，特别是导热系数的增强。本文采用实验与理论模型对比的方法，对一些影响十四烷基纳米流体导热系数的因素进行研究，包括纳米颗粒种类、浓度、温度以及稳定性。结果表明，本文中纳米流体的有效导热系数随纳米颗粒体积分数的增加而增加，随温度的升高而下降；在各种纳米颗粒中，碳纳米管对导热的增强最为显著，且碳纳米管流体具有最好稳定性。     

泡沫铜强化石蜡相变蓄热特性的数值分析

通过构建泡沫金属内固液相变传热模型,对方腔蓄热单元中泡沫铜强化石蜡相变蓄热特性进行数值分析。数值模型采用考虑泡沫金属真实结构的等效导热系数通用模型,并兼顾石蜡在融化前后与金属骨架之间的热非平衡效应。通过求解模型得到方腔内石蜡固液界面演化规律与温度分布,进而对蓄热过程进行火用分析。结果表明:当前数值模型能较好地预测泡沫铜内固液相变传热;泡沫铜显著改善了石蜡相变的空间均匀性,减小了蓄热区温度梯度,使蓄热速率和火用效率得到有效提高。     

Micro-encapsulated paraffin/high-density polyethylene/wood flour composite as form-stable phase change material for thermal energy storage

Six novel polymer-based form-stable composite phase change materials (PCMs), which comprise micro-encapsulated paraffin (MEP) as latent heat storage medium and high-density polyethylene (HDPE)/wood flour compound as supporting material, were prepared by blending and compression molding method for potential latent heat thermal energy storage (LHTES) applications. Micro-mist graphite (MMG) was added to improve thermal conductivities. The scanning electron microscope (SEM) images revealed that the form-stable PCMs have homogeneous constitution and most of MEP particles in them were undamaged. Both the shell of MEP and the matrix prevent molten paraffin from leakage. Therefore, the composite PCMs are described as form-stable PCMs. The differential scanning calorimeter (DSC) results showed that the melting and freezing temperatures as well as latent heats of the prepared form-stable PCMs are suitable for potential LHTES applications. Thermal cycling test indicated the form-stable PCMs have good thermal stability although it was subjected to 100 melt–freeze cycles. The thermal conductivity of the form-stable PCM was increased by 17.7% by adding 8.8 wt% MMG. The results of mechanical property test indicated that the addition of MMG has no negative influence on the mechanical properties of form-stable composite PCMs. Taking one with another, these novel form-stable PCMs have the potential for LHTES applications in terms of their proper phase change temperatures, improved thermal conductivities, outstanding leak tightness of molten paraffin and good mechanical properties.     

Fabrication and characterization of electrospun fatty acid form-stable phase change materials in the presence of copper nanoparticles

Latent heat storage system using phase change materials (PCMs) has been recognized as one of the most useful technologies for energy conservation. In this study, a novel type of fatty acid eutectic of methyl palmitate (MP) and lauric acid (LA)/polyacrylonitrile (PAN) composite phase change fiber is prepared by single electrospinning method. Additionally, copper nanoparticles (CNPs) with different mass ratio are combined for improving the thermal conductivity of the PCM. The structure and morphology of the fabricated composite PCMs are observed by scanning electron microscopy (SEM), and the thermal properties and performance are also characterized. SEM results show that the liquid fatty acid has been fully stabled by the three-dimensional structure of the fibers. Good compatibility among the components of the composites is also demonstrated. Besides, the addition of nanoparticles leads to an improved thermal conductivity by over 115.2% and a phase transition temperature 21.24 °C as well as a high latent heat of 85.07 J/g. Moreover, excellent thermal reliability of the phase change fiber is confirmed by multiple thermal cycles. Hence, the composite PCM prepared in this study shows a promising potential for thermal energy system such as building insulating and thermal mass regulating textiles.     

Nanofluids with encapsulated tin nanoparticles for advanced heat transfer and thermal energy storage

Novel high‐temperature heat transfer fluids (HTFs) with incorporated phase change nanomaterials were synthesized and tested for heat transfer and thermal energy storage. The advanced thermal properties were achieved by preparing a nanofluid consisting of core/shell silica encapsulated tin (Sn/SiO₂) nanoparticles dispersed in a synthetic HTF Therminol 66 (TH66) at loadings up to 5 vol%. Tin nanoparticles were synthesized by modified polyole reduction method followed by sol–gel silica encapsulation process. The measured increase in thermal conductivity of the nanofluid (~13% at 5 vol%) was in agreement with Maxwell's effective medium theory. Latent heat of phase change during melting of Sn core added ~11% increase to the volumetric thermal energy storage of the nanofluid when cycled in between 100°C and 270°C. The value could be further improved if thermal cycling is conducted in a narrower temperature range. The experimental results demonstrated dual functionality of the engineered nanofluids as desired for Concentrated Solar Power systems. Viscosity and stability of the nanofluids as well as thermal stability of core/shell nanomaterials) were investigated in a wide temperature range to obtain a perspective on any additional pumping power requirements for the nanofluid over the base fluid. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation and thermophysical properties of nanoparticle-in-paraffin emulsion as phase change material

In this study, phase change material (PCM) embedded by nanoparticles was prepared by emulsifying alumina (Al₂O₃) nanoparticles in paraffin (n-octadecane) by means of a non-ionic surfactant. The formulated nanoparticle-in-paraffin emulsions contain the nanoparticles of 5 wt.% and 10 wt.%, respectively; their effective thermophysical properties, such as latent heat of fusion, density, dynamic viscosity, and thermal conductivity, were investigated experimentally. The experimentally measured density of the emulsions agrees excellently with that predicted based on the mixture theory. The measured thermal conductivity and dynamic viscosity for the nanoparticle-in-paraffin emulsions formulated show a nonlinear increase with the mass fraction of the nanoparticles compared with that for the pure paraffin, depending on the temperature.     

增强石蜡相变材料传热性能的实验研究

添加高导热颗粒和增大换热面积是当前增强石蜡相变材料传热性能的主要研究方向。以此为基础搭建试验台结合数据采集系统对石蜡在圆管外的熔化凝固过程进行了实验测试,并对各测点的温度变化趋势进行分析,研究了添加不同纳米颗粒和加入金属肋片对换热过程的影响。结果表明:在石蜡溶液中添加纳米颗粒能够起到减小过冷度的效果同时有效增强相变材料的传热性能,添加纳米氧化铜颗粒的传热性能增强效果要优于添加氧化锌颗粒和二氧化硅颗粒;在储热系统中加入肋片能够显著提高相变储能系统的热性能,强化换热过程。     

ZrC nanoparticle-modified microencapsulated phase change materials with enhanced thermal conductivity and photo-thermal conversion performance

A photo-thermal energy storage microcapsule modified by KH550-treated ZrC nanoparticles was prepared via in situ polymerization. ZrC nanoparticles were treated by KH550 to stabilize the paraffin-ZrC emulsion, which could ensure ZrC nanoparticles were incorporated into the microcapsule shell. In this article, microcapsules modified by different mass fractions of modified ZrC nanoparticles (paraffin@MUF-ZrC) were prepared to investigate the influence of ZrC nanoparticles on microstructure, thermal storage, thermal stability, thermal conductivity, and photo-thermal conversion behavior. According to the SEM images, the obtained paraffin@MUF-ZrC microcapsules exhibited good morphology with regular spherical shape and uniform particle size distribution. DSC, TGA, and thermal conductivity analysis were used to characterize the thermal storage properties, thermal stability, and thermal conductivity of paraffin@MUF-ZrC microcapsules, respectively. It showed that these three thermal performance indices improved as the mass fraction of modified ZrC nanoparticles increased. Specifically, the thermal properties of paraffin@MUF-4%ZrC microcapsules were better than paraffin@MUF microcapsules. As a result, specific enthalpy was slightly increased by 14.92% (121.74 J/g) and thermal conductivity increased by 225.16% (0.4962 W/m·K). Moreover, the optical absorption capability also increased. The paraffin@MUF-4%ZrC microcapsules showed remarkable optical absorption capability of 70.23%, which was a 247.67% improvement compared to the 20.20% of the paraffin@MUF microcapsules. Moreover, it was found that paraffin@MUF-4%ZrC microcapsules could heat up from 30°C to 78°C under simulated solar radiation, showing excellent photo-thermal conversion behavior. Based on good thermal storage properties, thermal conductivity, and light absorption capability over the full spectrum, the new photo-thermal energy storage microcapsules have good application prospects in solar thermal conversion and energy storage systems.