Dual-effect evaluation of heat transfer deterioration of supercritical carbon dioxide in variable cross-section horizontal tubes under heating conditions

• The diverging tube has better the heat transfer performance. • Proposed the concepts of quasi-air film and eddy blockage. • Proposed the dimensionless criterion number for eddy blockage prediction. • Explained the dual-effect mechanism of heat transfer deterioration. Heat transfer characteristics analysis of supercritical carbon dioxide (sCO 2 ) in variable cross-section tubes is critical for its promotion and application. This paper proposed three horizontal heat exchange tubes, i.e., straight tube, converging tube, and diverging tube. On this basis, we used numerical simulation to analyze the convective heat transfer performance of sCO 2 in the horizontal tube with variable cross-section under heating conditions. The diverging tube effectively enhances the overall heat transfer performance compared with the uniform cross-section tube, and the heat transfer rate is increased by 19.26% compared with that of the straight tube. In contrast, the converging tube weakens the heat transfer ability. Moreover, we proposed quasi-air film and eddy blockage concepts to reveal the fluid's heat transfer deterioration mechanism near the top bus. Wherein the quasi-air film is full of sCO 2 , which has a low thermal conductivity close to that of air at room temperature, and generates near the top bus. Besides, the criterion factor, Bu t was proposed by double corrections of density and temperature for judging the occurrence of eddy blockage, and the Nu-based evaluation shows that the eddy blockage can deteriorate the heat transfer performance at the top of the horizontal tube by 17%. The physical mechanism of heat transfer deterioration under the heating condition is clarified by the dual-effect of quasi-air film and eddy blockage on this account.