Analysis of variable porosity, thermal dispersion, and local thermal non-equilibrium on two-phase flow inside porous media

Abstract

This work involves the numerical simulations of two-phase flow inside porous media and the associated phase change processes in order to investigate the effects of thermal dispersion and variable porosity near the wall boundary. The modified enthalpy formulation has been employed. The mathematical model for the conservation of energy is based on the assumption of Local Thermal Non-Equilibrium (LTNE) condition. The relevant parameters such as free stream porosity, pore diameter, thermal conductivity of the solid phase, heat flux and Reynolds number have been used to analyse the significance of the above mentioned effects. The governing equations have been discretised using the Finite Volume Method (FVM). The developed two-dimensional code has been validated against experimental data, which definitely display that there is a good agreement between them. The numerical results indicated that considering the effects of thermal dispersion and variable porosity is significant, particularly near the heated wall. It is also observed that those two effects become more pronounced for the large values of pore diameter, free stream porosity and Reynolds number. The effect of variations in the free stream porosity, Reynolds number and the pore diameter becomes substantial near the wall when thermal dispersion and variable porosity effects are included, whereas the effect of variations in the heat flux and thermal conductivity of the solid phase have minor influence on the temperature distribution near the heated wall. It is evident that adding the effect of variable porosity and thermal dispersion have significant impact on the initiation and termination of phase change process, especially in the superheated vapour region as compared to the results that obtained by excluding those two effects. In general, it has been found that the present model provides a lower maximum fluid temperature near the wall as compared to that obtained by excluding those effects. Therefore, excluding these effects may lead to inaccurate predictions of two-phase flow with phase change process inside porous media. Most importantly, the thermal dispersion in the transverse direction plays an important role in the dispersion phenomenon inside porous media owing to the thermal boundary layer growth is more dependent on the transverse diffusion as compared to the axial diffusion.