Experimental study on melting/solidification and thermal conductivity enhancement of phase change material inside a sphere

In this paper, the melting and solidification processes and the thermal conductivity enhancement of the phase change material (PCM) inside a sphere have been experimentally investigated to facilitate a greater understanding and improvement of the phase change heat transfer of PCM inside a sphere. The experiments are conducted by inserting a sphere filled with organic PCM into a constant temperature bath of hot/cold water. Through measuring and analyzing the temperature field inside the sphere under conditions of various initial and final temperatures, the thermal characteristics of melting and solidification processes of PCM are obtained. To determine the thermal conductivity enhancement effect of aluminum powder, the PCM-aluminum composite material in which the mass fractions of aluminum are 1% and 2%, respectively, is tested. And the experimental data of PCM-aluminum composite material are compared with those of pure PCM. Results indicate that the PCM in the upper part of the sphere melts faster than that in the lower part, and the PCM near the inner wall solidifies faster than that in the center of the sphere. The initial PCM temperature has little effect on the solidification process of the PCM inside the sphere when Stefan number (Ste) is small and Rayleigh number (Ra) is large. The thermal conductivity of the PCM inside the sphere is enhanced by adding aluminum powder. And the sedimentation of aluminum powder is more beneficial to accelerate the heat transfer of the whole sphere in the melting process, compared with the uniform diffusion of aluminum powder in the PCM.