Abstract:

Phase change materials (PCMs) are mainly used to provide high storage densities. The spherical geometry is one of the most interesting cases for heat storage applications. The present study used the lattice Boltzmann method (LBM) to investigate constrained melting process of PCMs in a spherical capsule, which can be useful for the study on phase change phenomenon of microencapsulated PCM slurry in the future. The phase interface is traced by updating the total enthalpy, while the moving interface is treated by the immersed moving boundary scheme. The computational results of the melting process of PCMs are analyzed at different scales. The numerical simulations at macro-scale are compared with the published experimental data, and the results clearly show that the thermal stratification is in the upper of the sphere capsule while the waviness phase front is at the bottom of the solid PCM. Quantitative analysis of the temperatures at nine points, eight points along the vertical axis and the other one near the inner shell, further indicates the existence of chaotic convective motion at the bottom of the sphere capsule. In addition, the effect of the natural convection on the melting process is reduced as the decrease of capsule sizes. When the diameter of capsule is less than 3 mm, the natural convection can be ignored.

Key words: phase change, multiscale, numerical simulation, thermodynamics, lattice Boltzmann method

摘要：

为了准确预测球形胶囊内部材料的相变性能，为后续相变微胶囊浆体的多尺度研究提供微观相变信息，采用格子Boltzmann方法（LBM），引入浸入式移动边界处理方案，借鉴糊状区和热焓理论，构建了适于相变模拟的数值模型，模拟了球形胶囊内部固液相变过程，讨论了不同粒径尺度下熔化机制的区别。结果表明，利用LBM方法得到的预测结果与可视化实验数据吻合较好，清晰地呈现出球体上部温度热分层和下部液相强对流共存的特性。随着粒径尺度的不断减小，胶囊内部对流作用逐渐减弱，甚至当粒径小于3 mm时，其内部对流作用可忽略。

关键词: 相变, 多尺度, 数值模拟, 热力学, 格子Boltzmann方法