Internal thermal redistribution and thermo-fluid performance across cavity macro-geometries in high-temperature volumetric solar receivers

High temperature volumetric solar receivers are central to concentrated solar power applications because their performance depends on radiative absorption, internal energy redistribution, fluid heating, and thermal losses to surrounding structures. Although cavity shape affects receiver behaviour, the thermal and flow effects associated with different cavity geometries, and their relation to internal energy redistribution and heat transfer towards the surrounding insulation, remain insufficiently quantified. This study presents a receiver-level numerical investigation of spherical, octagonal, and cylindrical cavity geometries for a high-temperature volumetric solar receiver incorporating a silicon-infiltrated silicon carbide (SiSiC) reticulated porous ceramic absorber under fixed total solar input. A three-dimensional computational fluid dynamics (CFD) model with Monte Carlo (MC) radiation treatment was developed in ANSYS Fluent and validated against experimental outlet air temperature data for the octagonal case at 4.1 and 4.9 kW. Beyond outlet temperature and thermal efficiency, the analysis uses the Insulation Heat Loss Fraction (IHLF) to quantify heat transfer towards the ceria insulation layer, while the static pressure difference between the receiver inlet and outlet reflects the overall hydraulic response. The spherical receiver achieved the highest efficiencies of 80.4% at 4.1 kW and 79.7% at 4.9 kW and maintained the highest thermal performance across the investigated operating range. IHLF trends were also associated with variations in porous flow area and internal flow intensity. At their respective highest efficiency operating points, the octagonal configuration exhibited a higher inlet to outlet static pressure difference than the spherical configuration under both input conditions. These findings show clear differences among the three receivers in thermal efficiency, heat-transfer behaviour, internal thermal redistribution, and hydraulic response.