纳米复合相变材料熔化过程数值模拟

相变储能技术在航空航天等领域具有广泛的应用前景，但是相变材料导热性能差制约了其工程化应用。高导热的纳米材料能够有效提高相变材料的导热性能。为了对其相变现象进行更精细的模拟分析，基于Maxwell-Garnett 等效介质理论（EMT）建立3种具有代表性结构的纳米复合相变材料详细物性参数，将流体体积（VOF）模型与焓-多孔介质模型相耦合，在考虑相变材料体积膨胀的情况下，数值模拟了纯石蜡、添加不同体积组分金刚石纳米粒子（ND）、单壁碳纳米管（SWCNT）和石墨烯纳米片（GnP）的纳米复合相变材料在定温边界条件下的固液相变过程。结果表明：相变材料熔化过程中对流效应主要分布在临近固液相界面、临近加热壁面及临近气液两相交界面这3个区域；3种纳米粒子中GnP的导热强化效果最佳，相比纯石蜡，添加体积分数为3% 的GnP纳米复合相变材料固相导热系数提高了486%，相变材料的熔化时间缩短了69%；升高壁面温度能够有效缩短复合相变材料的熔化时间。

Numerical simulation of melting process of nanoparticle-enhanced phase change materials

The latent heat thermal energy storage can be applied widely in aerospace domain and many other industrial fields. However, phase change materials suffer from low thermal conductivity that constrains their engineering application. Nano materials with high thermal conductivity can effectively improve the thermal conductivity of phase change materials. For simulating the melting process in more detail, the physical properties of paraffin composited with three representative kinds of nano materials were founded based on the Maxwell-Garnett type effective medium theory (EMT). The volume of fluid (VOF) model and the enthalpy-porosity model were coupled to simulate the melting process of the pure paraffin and the paraffin composited with nano diamond (ND), single-walled carbon nanotube (SWCNT) and grapheme nano platelets (GnP) under a constant wall temperature. Meanwhile the volume expansion was taken into account. Numerical calculations show that the natural convection is mainly distributed at the region closed to the solid-liquid interface, the region closed to the heating surface and the region adjacent to air-liquid interface. Among these three kinds of nano materials, GnP is the most promising additive that can enhance thermal conductivity of phase change material. For a fixed GnP loading of volume fraction of 3%, the solid phase heat conductivity coefficient of nanoparticle-enhanced phase change materials increases by 486% compared to that of pure paraffin, and the melting time of phase change materials decreases by 69%. Meanwhile, the melting process of the nano-composite phase change materials can be significantly shortened by raising the temperature of the heating surface.