Porous media heat exchangers can facilitate efficient transfer and reliable storage of heat, which is essential to keep up with the growing demand for renewable energy. Turbulence transport in porous media is determined by the geometry of the solid obstacles at the microscale. The focus of the present study is to understand the influence of solid obstacle surface roughness on the heat transfer characteristics of the porous medium through a systematic study. Surface roughness is often unavoidable during fabrication, or it can develop over time due to corrosion, and it is an important consideration in design. In this study, the Reynolds-averaged microscale flow field is simulated by explicitly representing the surface roughness geometry by adding square roughness particles on the solid obstacle. The roughness particle height and spacing, the porosity, and the Reynolds number are varied systematically. The trends of the macroscale variables such as the pressure drag, viscous drag, and surface averaged Nusselt number with respect to the roughness particle height and spacing are analyzed. The primary focus of the analysis is the microscale flow physics caused by the roughness particles that is underlying in the macroscale variables.
We have identified two distinct flow regimes for the variation of surface roughness particle height (k_s) and spacing (w). The roughness particles cause the formation of recirculation vortices that are similar in size to the roughness particle height. Roughness particle heights of k_s = 0.01d and 0.005d (where d is the solid obstacle hydraulic diameter) are considered fine roughness whose influence on heat transfer is limited to the change in the width of the boundary layer, causing a heat transfer decrease compared to the smooth case. Roughness particle heights of k_s = 0.1d and 0.05d are considered coarse roughness because they modify the geometry of the solid obstacle and the flow patterns around them, causing a heat transfer enhancement. Increase in the roughness particle spacing increases the tortuosity of the flow, but the number of roughness particles on the surface decreases. The two competing factors gives rise to two regimes, which transition from the prevalence of roughness to smoothness. We also note that the effect of surface roughness is enhanced when the porosity is lowered and the Reynolds number is increased. The study concludes that the surface roughness of solid obstacles in porous media should be small and sparsely distributed to decrease its detrimental effects on heat transfer.