石墨填料的形态和面向尺寸对复合相变材料传热特性影响的实验研究

发布时间：2018-09-12 20:48

【摘要】：热能存储是应对能源危机、提高热能利用效率的有效手段。采用有机相变材料的相变潜热进行能量存储具有安全稳定、相变过程近似恒温等优点,得到迅速发展。然而,有机相变材料导热系数低(-0.2W/(m·K))的缺陷严重限制了其使用范围。目前,将高导热的颗粒添加到有机相变材料中制备复合相变材料是增强其传热能力最受关注的方法。石墨因其导热系数高(100W/(m·K))、密度小等特点,日益成为添加物的首选材料。首先利用真空吸附法制备不同饱和吸附量膨胀石墨(EG)／十六醇复合相变材料,综合运用SEM、XRD、FT-IR等手段研究了材料的形态、结构和化学稳定性,对比研究了吸附量对导热系数的影响。EG的多孔结构对液态相变材料具有较强的吸附定型作用,吸附量越大,导热系数越小。以十二酸为基体材料时变化趋势相似。超声震荡(300W,10s)得到的石墨纳米片(GNS)对导热系数的强化效果优于EG。在此基础上,通过控制超声震荡时间得到不同面向尺寸的GNS,研究了GNS的添加量、尺寸和温度对十六醇基复合相变材料导热系数的影响。导热系数随着GNS添加量的增加均近似线性增长,面向尺寸越大,强化效果越好。Nielsen模型在形状因子A取100至180时可以较好的预测实验值。导热系数对温度几乎无依赖性。此外,对比研究了石墨的形状和种类对复合物传热特性的影响,片层状的鳞片石墨效果最好,多孔碳最差,二维平面结构的石墨烯(GNP)强化效果优于碳纳米管。综合对比以上各种石墨材料强化导热效果,虽然基体材料不同,超声震荡30min得到的GNS-30导热强化效果与GNP相当。其次,针对上述导热强化效果最好的两种石墨材料(EG、GNS),实验中利用DSC对相关样品的相变温度(T)、相变潜热(L)进行测试,并解释了相变潜热变小的原因。石墨的加入对相变温度的的影响无明显规律,相变焓减小,小于计算值。GNS的尺寸对相变温度和相变焓的作用规律不明显。最后本文搭建了相变传热实验台,采用时间-温度法分别测试纯十六醇及复合相变材料储热单元的相变传热过程,探究石墨的吸附量、尺寸、加载量和石墨种类对传热过程的影响。石墨材料的加入使得复合相变材料的吸、放热速率得到很大提高,在有效提高复合相变材料导热系数的同时,增大粘度,减少相变传热过程中的自然对流,使得低加载量下的强化效果不明显。
[Abstract]:Thermal energy storage is an effective means to deal with the energy crisis and improve the efficiency of thermal energy utilization. The use of latent heat of organic phase change materials for energy storage has the advantages of safety and stability, and the process of phase transition is similar to the constant temperature, so it has been developed rapidly. However, the low thermal conductivity (-0.2 W / (m K) of organic phase change materials severely limits its application range. At present, adding high thermal conductivity particles to organic phase change materials to prepare composite phase change materials is the most concerned method to enhance their heat transfer ability. Because of its high thermal conductivity (100W/ (m K), density, etc.), graphite is becoming the preferred additive material. Firstly, the expanded graphite (EG) / hexadecanol composite phase change materials with different saturated adsorption amounts were prepared by vacuum adsorption method. The morphology, structure and chemical stability of the materials were studied by means of SEM,XRD,FT-IR and so on. The effect of adsorption amount on thermal conductivity. The porous structure of EG has a strong adsorption setting effect on liquid phase change materials. The larger the adsorption amount, the smaller the thermal conductivity. The trend of change is similar when dodecanoic acid is used as base material. The enhancement of thermal conductivity of graphite nanocrystalline (GNS) obtained by ultrasonic oscillation (300W / 10s) is better than that of EG.. On this basis, the effects of GNS addition, size and temperature on the thermal conductivity of hexadecanol based composite phase change materials were studied by controlling the ultrasonic oscillation time and GNS, with different dimensions. The thermal conductivity increases linearly with the increase of GNS content. The larger the size is, the better the strengthening effect is. Nielsen model can predict the experimental value better when the shape factor A is between 100 and 180. The thermal conductivity is almost independent of temperature. In addition, the effects of graphite shape and type on the heat transfer characteristics of the composites were compared. The lamellar flake graphite had the best effect, and the porous carbon was the worst, and the two-dimensional graphene (GNP) was better than the carbon nanotube. Comprehensive comparison of the thermal conductivity of various graphite materials above, although the matrix material is different, the GNS-30 obtained by ultrasonic shock 30min is comparable to that of GNP. Secondly, for the two kinds of graphite materials (EG,GNS) with the best thermal conductivity strengthening effect, DSC was used to measure the phase transition temperature (T),) latent heat (L) of the related samples, and the reason why the latent heat of phase transition became smaller was explained. The effect of graphite addition on the phase transition temperature is not obvious, but the enthalpy of phase transition decreases, but the size of GNS is less than the calculated value, and the effect of the size of GNS on the phase transition temperature and enthalpy is not obvious. Finally, a phase change heat transfer test bench was set up. The phase change heat transfer process of pure hexadecanol and heat storage unit of composite phase change material was measured by time-temperature method, and the adsorption capacity and size of graphite were investigated. The effect of loading quantity and graphite type on heat transfer process. With the addition of graphite, the absorption and exothermic rate of the composite phase change material is greatly increased. The viscosity of the composite phase change material is increased while the heat conductivity of the composite phase change material is increased, and the natural convection in the process of phase change heat transfer is reduced. The enhancement effect under low load is not obvious.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB34

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