Studies on thermal-hydraulic characteristics of supercritical CO2 flows with non-uniform heat flux in a tubular solar receiver

Supercritical CO2 (SCO2) Brayton cycle is one of the best approaches for concentrated solar power tower, with tubular solar receiver being one of the most attractive options to directly heat SCO2. The superposition of drastically variable properties of SCO2 and uneven distribution of solar irradiance flux poses a great challenge to the design and optimisation of solar receiver, thus the studies on thermal-hydraulic characteristics of SCO2 in solar receiver become crucial to deal with the challenge. In this study, the thermal-hydraulic characteristics of SCO2 in a vertical receiver tube under axially and circumferentially non-uniform heat-flux conditions are numerically studied with average heat flux ranging from 150 kW m−2 to 210 kW m−2 and mass flux ranging from 400 kg m−2 s−1 to 700 kg m−2 s−1, and the influences of buoyancy on the flow and heat transfer characteristics are evaluated. The non-uniform distribution of the axial heat flux leads to 1.2%–20.5% increase in the overall heat transfer coefficient, while more severe local heat transfer deterioration is observed. The buoyancy effect changes the flow structure, resulting in suppression of turbulence intensity and formation of thick momentum and thermal boundary layers, which is closely related to the severe heat transfer deterioration of SCO2 under non-uniform heat-flux conditions. Bo* could well predict the buoyancy effect in a vertical tube under non-uniform heat-flux conditions. Compared with upward flow, downward flow could effectively alleviate heat transfer deterioration and reduce wall temperature gradient especially for axially non-uniform heat-flux condition. The present work could provide guideline for the design and optimisation of tubular solar receiver and solar power system with SCO2 as heat transfer fluid.