Experimental study of heat transfer characteristics of phase change material emulsions in a horizontal circular tube

•Heat transfer characteristics of phase change material emulsions were investigated.•Secondary convection due to phase change material particles melting was observed.•The secondary convection significantly improved the heat transfer performance. Phase change material emulsions have been attracting attention for their potential to be utilized as efficient thermal energy storage and transport media. To understand their heat transfer performance, the heat transfer experiments in a horizontal circular tube were performed. The horizontal circular tube possessed thermocouples embedded in a circumferential direction and a constant heat flux incident on the flowing phase change material emulsion. A significant temperature distribution was observed when phase change material emulsion flowed with phase change material particle melting. Moreover, it was found that the local Nusslet number at the upper position of the horizontal tube was significantly enhanced and the Nusselt number of phase change material emulsion was up to about 2.5 times higher than that of the single-phase fluid. On the other hand, no temperature distribution was observed when phase change material emulsion flowed upward, even with phase change material particle melting. These results imply the presence of a secondary convection induced by phase change material particle melting owing to the density difference between the solid and liquid phase change material particles. The present findings suggest that the flow direction should be considered when using phase change material emulsions as thermal energy storage and transport media.