Pore-scale study on Rayleigh-Bénard convection formed in the melting process of metal foam composite phase change material

Abstract

Rayleigh-Bénard convection significantly affects the melting process of phase change material (PCM). It has been verified to exist in the model with bottom heating. Nevertheless, researchers rarely focus on the Rayleigh-Bénard convection in the metal foam composite PCMs (MFCPCMs). This paper took numerical investigations to study the melting behavior of the MFCPCMs when Rayleigh-Bénard convection was involved, and a visualized experiment was used to verify their accuracy. By analyzing the number of Bénard cells, dimensionless parameters of Rayleigh number and Nusselt number, we found three regimes, including the conductive regime, linear regime, and coarsening regime, forming in the models. The liquid fraction contour captured the evolution of the melting front, and the state of convective heat transfer was characterized by Bénard cells. Results show that the intensity of Rayleigh-Bénard convection in the MFCPCM is obviously weaker than that in the pure PCM. Moreover, the porosity of metal foam has a significant influence on the development of Rayleigh-Bénard convection. With the increase of the porosity, the suppression of liquid PCM flow is weakened, so the convection is more developed. The inhomogeneous evolution of the melting front is more pronounced. Besides, to study the effect of model size on the melting behavior, we built three groups of models with width-height ratios (γ) of 2, 1, and 0.5, respectively. Results show that the model with large γ has more significant inhibition on the development of Rayleigh-Bénard convection. The solid/liquid interface is flat or with a slight fluctuation. On the contrary, the model with small γ has more developed Rayleigh-Bénard convection. The interface of it has a violent fluctuation, which shows as a humped arch. This work interprets the internal heat-transfer characteristics of the MFCPCM model with Rayleigh-Bénard convection. It paves the way for the structural optimization of the metal skeleton to gain efficient heat storage.