Study on the competition effects between thermal conductivity and viscosity of graphene reinforced octadecane phase change material

Abstract

In this study, a combination of microscale molecular dynamics (MD)-based and macroscale computational fluid dynamics (CFD)-based simulations are employed to investigate the effects of graphene with varying mass ratios on the thermal conductivity and viscosity of octadecane, revealing the nature of the competition effects of thermal conductivity and viscosity in the heat transfer process. The results show that graphene can significantly increase the average thermal conductivity of octadecane. But when its synergy angle in the direction of heat flux is close to 90°, the thermal conductivity of nanocomposite phase change materials (NCPCMs) is only 0.125 W·(m·K)−1, which is lower than that of pure octadecane by 18.83 %. The viscosity of NCPCMs decreased significantly with the decrease of graphene weight ratio and the increase of temperature, and the viscosity of NCPCMs with 9.15 wt% and 28.71 wt% at different temperatures was 812.65 % (max) and 313.75 % (min), respectively. Finally, the equilibrium point between the thermal conductivity and viscosity competition effects favors the viscosity side as the graphene mass ratio increases. Accordingly, natural convection became more crucial in the heat transfer process as the graphene mass ratio increased. Consequently, NCPCMs with the best heat transfer performance should possess a low graphene mass ratio, which results in a high Rayleigh number, providing them with high synergistic efficiency.