Pore-Scale Simulation of a Tubular Solar Absorber Partially Filled With Raschig Ring Porous Medium for Efficiency Enhancement Purposes

This study considers a partial filling of a novel porous insert to ease the burden of the excessive pressure drop and customize the porous structure with the absorbed flux distribution in a tubular solar absorber. This arrangement would suggest an enhanced porous integration with solar absorbers, in which the fraction of porous material with higher thermal contribution is kept, while the side with lower heat contribution is removed to maintain the pressure drop at low levels. To achieve this goal, a numerical investigation was performed on a novel porous medium made of metallic Raschig Rings (RR), with three different filling percentages of 10, 20, and 30% of the tube cross-section. Simulations were conducted using CFD and validated against the experimental data acquired for a 100% RR porous filling in a tubular absorber tested in a solar furnace. Up to an 80% increase in the pressure drop was observed using partial filling instead of a clear tube while the heat transfer performance in terms of Nu was improved up to > 20%. Moreover, studying the fluid behavior in the porous and non-porous regions revealed that the fluid in the porous zone is pushed away from the matrix into the non-porous region, making a complex behavior at this interface. The influences of different filling percentages on the flow behavior at the interface and the consequence cooling effects on the absorber are also addressed.