Natural convection plays a dominant role in the melting dynamics of phase change materials (PCMs). However, natural convection is not present at the beginning of the melting process of a fully solid PCM. Instead, it starts playing a role after some PCM has melted by conduction. This study was undertaken to explore the conditions for the onset of natural convection in subcooled PCMs in a 2-dimensional experimental system. Melting of n-octadecane and dodecanoic acid in horizontal cylindrical enclosures with center-tube diameters of 18, 27, and 36 mm were studied for PCMs initially subcooled by 2.5, 7.5, 15, and 22.5 °C. At these conditions, the difference between the melting temperature of the PCMs and the center-tube wall temperature, $\text{Δ}{T}_{\text{m}}$, was maintained at 8.4, 16.9, 25.3, 33.8, and 42.2 °C. The melt volume at the onset of natural convection was found to increase with the center-tube diameter, with shorter times and less liquid PCM required for the onset of convection as the $\text{Δ}{T}_{\text{m}}$ -based Stefan number increased. Subcooling of the PCMs delayed the occurrence of convection onset, with a geometric, $\text{Δ}{T}_{\text{m}}$ -based Stefan number, and PCM dependence observed for the impact of subcooling on melt volume needed for the onset of convection.

自然对流在相变材料 (PCM) 的熔化动力学中起主导作用。然而，在完全固体 PCM 的熔化过程开始时，不存在自然对流。相反，它在某些 PCM 通过传导熔化后开始发挥作用。本研究旨在探索二维实验系统中过冷 PCM 中自然对流开始的条件。对于最初过冷 2.5、7.5、15 和 22.5 °C 的 PCM，研究了正十八烷和十二烷酸在中心管直径为 18、27 和 36 毫米的水平圆柱形外壳中的熔化。在这些条件下，PCM 的熔化温度与中心管壁温之间的差异，$\text{Δ}{吨}_{\text{米}}$，保持在 8.4、16.9、25.3、33.8 和 42.2 °C。发现自然对流开始时的熔体体积随着中心管直径的增加而增加，对流开始所需的时间更短，液体 PCM 更少，因为$\text{Δ}{吨}_{\text{米}}$基斯特凡数增加。PCM 的过冷延迟了对流开始的发生，具有几何形状，$\text{Δ}{吨}_{\text{米}}$ 基于斯特凡数和 PCM 依赖性观察到过冷对开始对流所需的熔体体积的影响。