Buoyancy effects on convective heat transfer of supercritical CO2 and thermal stress in parabolic trough receivers under non-uniform solar flux distribution

The buoyancy influenced heat transfer characteristics of S-CO2 have been investigated widely in previous studies, but limited to the conditions with constant wall temperature or constant heat flux. The present study numerically investigates the buoyancy effects on the convective heat transfer of S-CO2 and the thermal stress in parabolic trough receivers (PTRs) under the non-uniform solar flux distribution based on a coupled optical-thermal-fluid-mechanical numerical model. The results indicate that the drastic density variation associated with the large temperature gradient of the fluid occurs on the cross-section under the non-uniform solar flux distribution, which further induces the secondary flow. As the secondary flow improves the synergy between the velocity vector and the temperature gradient, the bulk average heat transfer is prominently enhanced due to the buoyancy effect. However, the buoyancy effects on the local heat transfer characteristics are more complex. The buoyancy effect enhances the heat transfer at the bottom but impairs the heat transfer at the top, and the heat transfer enhancement area is expanded along the flow direction. The temperature distribution in the absorber tube wall is greatly flattened due to the buoyancy effect, and the thermal stress is consequently reduced. The buoyancy effects on the thermal and mechanical performances become less significant with the increase in the operating pressure, the inlet velocity and the inlet temperature but with the decrease in DNI. The buoyancy effect in the PTR under non-uniform solar flux distribution can be ignored when Se/Re<0.3, Ri<0.04, Bu-p<0.002, Bu-j<80, or Gr(q)/Gr(th)<70. (C) 2021 Elsevier Ltd. All rights reserved.