Experimental and numerical investigation on heat transfer of supercritical CO2 in a horizontal U-tube under thermal boundary of immersed tube

The heat transfer behaviors of supercritical CO2 (sCO(2)) under uniform heat flux have been widely investigated due to the excellent thermophysical properties. However, the heat transfer behaviors of sCO(2) in an immersed tube as the heat exchanger of the pressurized fluidized bed have not been reported so far. In this work, we studied them by means of a pressurized fluidized bed-sCO(2) closed-cycle heat transfer platform and a heat transfer model under circumferential and axial non-uniform heat flux with thermal boundary characteristics of immersed tube. The effects of operating parameters, circumferential and axial heat flux and wall thickness were analyzed. The experimental results revealed that the heat transfer coefficient (HTC) of sCO(2) decreased sharply with the increase of the bed temperature. Mass flow rate can enhance heat transfer more significantly than inlet pressure. The simulation results showed that the unique circumferential heat flux of the immersed tube was beneficial to restrain buoyancy. And the increase of wall thickness can improve the circumferential uniformity of wall temperature. Axial non-uniform heat flux increased the risk of tube bursting. By comparing with experimental and simulated data, it is found that the supercritical-boiling-number and Liao-Zhao correlation can predict the heat transfer deterioration and the HTC in the immersed tube, respectively. This work can provide an understanding of the heat transfer behavior of sCO(2) in an immersed tube.