Experimental investigation and theoretical analysis on the heat transfer deterioration of supercritical CO2 in mixed and forced convection in vertical-straight tube with downward flow

The heat transfer deterioration of supercritical fluids in vertical-upward flow has been extensive studied since 1930s. However, few studies focused on the heat transfer deterioration of supercritical CO2 in vertical-downward flow. In this paper, the sudden rise of wall temperature for supercritical CO2 in mixed convection with vertical-downward flow is observed firstly in the experiments, except for in forced convection. The effect of operation parameters on the heat transfer deterioration are studied, covering pressures (7.5 similar to 9 MPa), mass flow rates (240 similar to 1120 kg/(m(2)center dot s) and heat fluxes (45-200 kW/m(2)). A new shear stress redistribution model is proposed by considering the thermal acceleration in the boundary layer with the convection term of motion equation. The new model is verified with multiple sets of independent experimental data from the public literatures. The effect of thermal acceleration not only includes the acceleration in the average mass bulk, but also has the conspicuous acceleration of fluids in the boundary layer. The experimental results demonstrate that the occurrence of heat transfer deterioration in vertical-downward flow is mainly induced by the effect of thermal acceleration in the boundary layer. (c) 2022 Elsevier Ltd. All rights reserved.