Non-Newtonian phase change study of nano-enhanced n-octadecane comprising mesoporous silica in a porous medium

Poor thermal conductivity of phase change materials (PCMs) is the principal barrier to their widespread application. As such, numerous single or hybrid techniques have been proposed to lower the thermal resistance of PCMs. In the present paper, a hybrid approach, including the dispersing of nano-sized particles and embedding in a porous medium, was employed to augment the rate of heat transfer and melting process of the resulting nano-enhanced PCM (NePCM). Aluminum foam was used as the solid matrix, and phase-change heat transfer of a NePCM comprising n-octadecane as the phase change substance and nano-sized particles of mesoporous silica (MPSiO₂) was studied. Previous experimental studies have shown that, although n-octadecane behaves as a Newtonian fluid, the mentioned NePCM behavior deviates from that of Newtonian fluids. Hence, the thermal and hydrodynamic behaviors of the non-Newtonian suspension of the NePCM in porous media were investigated numerically. Governing equations, including mass and momentum conservation for the liquid NePCM and energy equations for both solid and liquid phases, were solved using the Galerkin finite element method. The deformed mesh technique was employed to address the movement of the melting front. The finite element code was validated against several experimental and numerical works and was sufficiently accurate. Results showed that owing to the alteration of the rheological and thermophysical characteristics of the NePCM, increasing concentration of nanoparticles reduced the average Nusselt number and the normalized melt volume fraction (NMVF). It was also found that porosity played an important role in the phase-change rate. Although the steady-state condition was achieved faster in the case of lowest porosity, the maximum NMVF was achieved in the case of the highest porosity after Fourier number = 0.02.