基于分形的多孔介质复合相变材料的储热特性

本文利用膨胀石墨和纳米颗粒来强化相变储热系统的传热性能。在膨胀石墨基体中填充含纳米颗粒的相变材料，用焓-孔隙度法模拟材料的相变过程。针对不规则的膨胀石墨孔隙结构，用三维W-M分形函数修正膨胀石墨孔隙率波动，以研究不同的孔隙率和有效导热系数比对固态显热蓄热阶段相变材料熔融速率的影响。在液态显热蓄热阶段时探讨膨胀石墨孔隙率以及纳米颗粒体积分数对相变储热系统中对流传热的影响。研究结果表明，分形分布的孔隙结构能有效地抑制纳米颗粒的自由运动从而降低了纳米颗粒的局部团聚的可能性，所以利用三维W-M分形函数修正的膨胀石墨比采用平均孔隙率能更好地模拟相变材料的熔融速率。在固态显热蓄热阶段，膨胀石墨孔隙率为0.8的相变材料熔融速率比孔隙率为0.85和0.9显著增加，另外，膨胀石墨与纳米颗粒-相变材料的有效导热系数比为100的熔融速率也明显比有效导热系数比为80和60的快。当相变材料处于液态显热蓄热阶段时，其在膨胀石墨孔隙中产生对流，对流传热速率随着膨胀石墨的孔隙率增大而增大，纳米颗粒体积分数的增加也会提高对流传热速率。

Study on heat storage characteristics of porous media composite phase change materials based on fractal

In this paper, expanded graphite and nanoparticles are used to enhance the heat transfer performance of latent heat thermal energy storage system. Phase change materials containing nanoparticles are filled in expanded graphite matrix and the enthalpy-porosity method is used to simulate the phase transition. In order to study the effect of different modified porosity and effective thermal conductivity ratio on melting rate of phase change materials in the stage of sensible heat storage, the porosity fluctuation of foam metal was corrected by using three-dimensional W-M fractal function. The effects of the porosity of expanded graphite and the volume fraction of nanoparticles on convective heat transfer in latent heat thermal energy storage system are investigated. The results show that, the pore structure with fractal distribution can effectively inhibit the free movement of nanoparticles, which reducing the possibility of local agglomeration of nanoparticles. As the result of that, the expanded graphite modified by the three-dimensional w-m fractal function can better simulate the melting rate of phase change materials than the average porosity. In the phase of solid sensible heat storage, the melting rate of expanded graphite with a porosity of 0.8 was significantly higher than that of 0.85 and 0.9. In addition, the melting rate of expanded graphite with effective thermal conductivity ratio of 100 is also significantly faster than that of 80 and 60. Convection occurs in the pores of the expanded graphite when the phase change material is in the phase of sensible heat storage in the liquid state, in which that the convective heat transfer rate increases as the porosity of the expanded graphite increases, and an increase in the volume fraction of the nanoparticles also increases the convective heat transfer rate.