Convective heat transfer characteristics of supercritical CO2 in mini-channels of compact solar receivers under unilateral heating conditions

The compact solar receiver based on mini-channels is one of the most attractive options for the directly heated S-CO2 solar receiver due to its excellent thermal performance and outstanding pressure bearing ability. It is of great significance to obtain the convective heat transfer characteristics of S-CO2 in mini-channels under unilateral heating conditions for the design and optimization of the compact solar receiver, unfortunately, which has not been investigated thoroughly. This paper numerically investigates the buoyancy effect and thermal acceleration effect influencing convective heat transfer characteristics of S-CO2 in mini-channels under unilateral heating conditions. The results indicate: (1) the bulk average convective heat transfer coefficient under the unilateral heating condition is slightly lower than that under the bilateral heating condition, while for the local heat transfer, the unilateral heating condition has an adverse effect on the convective heat transfer on the heated side by reducing the turbulent kinetic energy and increasing the dynamic viscosity, which is opposite on the unheated side; (2) the buoyancy effect deteriorates the heat transfer of S-CO2 with a maximum value of 21.93% influence in the upward flow, which is mainly caused by the thickening thermal boundary layer associated with enlarged viscosity and the reduced Pr number near the wall, and the lower viscosity and the higher Pr number caused by buoyancy effect enhance the convective heat transfer in the downward flow with a maximum value of 17.37% influence; (3) the thermal acceleration effect deteriorates the heat transfer of S-CO2 in both upward and downward flows with a maximum value of 25.06% influence, which is also attributed to the thickening thermal boundary; (4) the buoyancy effect and thermal acceleration effect are weakened as the inlet velocity increases in both downward flow and upward flow, and strengthened due to the increased heat flux.