球形相变胶囊内凝固传热过程数值模拟

Numerical Simulation on Solidification Heat Transfer of Spherical Phase Change Capsule

以海藻酸钙为壁面,石蜡/高密度聚乙烯定形材料为囊芯的球形相变胶囊为对象,建立了凝固传热过程的相变模型。模型中同时考虑了石蜡的体积变化和空腔分布,并根据囊芯材料的微观结构特征,基于分形理论对其等效导热系数进行了有效表征。采用了显热容法模拟了相变胶囊凝固过程中的传热和相界面位置的移动。模拟显示:考虑相变材料的体积变化对初始阶段的凝固过程几乎没有影响,但此后随着固相率逐渐增加,热传递速率明显减缓;初始空腔率不仅影响了相变传热速率,也降低了单位体积胶囊的相变潜热;越大的胶囊尺寸斯蒂芬数对其完全凝固时间所造成的差异愈加显著。该结果对如何提高相变胶囊的储存量和储存速率具有一定的指导意义。

The present study explored numerically the process of solidification of spherical phase change capsule, which spherical high-density polyethylene （HDPE）/paraffin shape stabilized phase change material （PCM） encapsulated by Calcium alginate. The mathematical model was solved numerically by using apparent heat capacity method. In simulation, it described the pore microstruc- ture by the fractal geometry after the paraffin in capsule was extracted, and the volume change of paraffin in phase transformation and the cavity caused by manufacturing technology were also considered. Based on the fractal characterization, as one of the important physical properties of the HDPE/paraffin, the effective thermal conductivity was determined. The results show that The initial solidification process is almost unaffected when the volume change of the PCM is considered. However the heat transfer rate begins to shift to a significantly slower with the increment of solid rate. The initial cavity ratio influences the rate of the phase change heat transfer and reduces the phase change latent heat of each unit volume of capsule. The larger the size of spherical capsule, the greater the impact of Ste number on the complete solidification time. The conclusion would have important guidance significance to how to improve heat storage capacity and heat storage rate of the PCM.