Pore-scale numerical study on the heat transfer process in porous media under different thermal conditions

The convective heat transfer coefficient is a key parameter when investigating the heat transfer process in the porous volumetric solar receiver. Previous researches pay little attention to the influence of thermal conditions on the determination of this value. In this paper, a fully coupled pore-scale heat transfer model is proposed and three different thermal conditions such as constant surface temperature, constant heat flux, and incident solar radiation are investigated. The computational domain is reconstructed by the X-ray computed tomography technique and the governing equations are solved directly with finite volume method in FLUENT. The energy source due to solar radiation is modeled by Beer's law while the radiation transfer inside the porous media is solved with discrete ordinates method. The results show that the convective heat transfer coefficient increases and the Nusselt number decreases as the intensity of thermal conditions increases in all cases. When the average temperature or average heat flux of the porous volumetric solar receiver is imposed, the Nusselt number under constant temperature thermal boundary condition is a good approximation to that of the porous volumetric solar receiver but the constant heat flux thermal boundary condition leads to a smaller value. The detailed temperature fields also demonstrate that the heating processes are similar in cases under constant temperature and incident solar radiation. As a result, the heat transfer characteristics under different thermal conditions in porous media are revealed and the correlation of Nusselt number versus Reynolds number for the porous volumetric solar receiver is proposed.