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Abstract Supercritical heat transfer mechanisms and heat transfer correlations are firstly reviewed in present paper. The heat transfer characteristics of S-CO 2 in circumferentially non-uniform heated vertical upward flow are then numerically studied and the fundamental mechanisms are discussed. The abnormal heat transfer for S-CO 2 is believed to be caused by the comprehensive effect of thermophysical property variations. For S-CO 2 with non-uniform heating and large mass flow rate, the enhanced heat transfer is mainly related to the large specific heat in the near wall region while the deteriorated heat transfer is dominated by the thickening viscous sublayer induced by the viscosity increase. Besides, the buoyancy effect induced by the density variation is still prominent to axial velocity distribution even in the forced convection. Finally a new correlation with higher accuracy is developed for S-CO 2 circumferentially average heat transfer by introducing the correction parameters of viscosity, specific heat, axial flow-acceleration and non-uniform heat flux. Highlights • Supercritical heat transfer mechanisms and correlations are comprehensively reviewed. • Heat transfer of S-CO 2 under non-uniform heating is numerically studied. • The relation between thermophysical property variation and abnormal heat transfer is examined. • An average heat transfer correlation for S-CO 2 under non-uniform heating is proposed and evaluated. [ABSTRACT FROM AUTHOR]

The supercritical-CO 2 (S–CO 2) cooler is one of the main components in S–CO 2 power system. This work aims at revealing the cooling mechanism of S–CO 2 under wide working conditions to assist the S–CO 2 cooler design. The pseudo-condensation is proposed with the analogy between supercritical cooling and subcritical condensation. From the pseudo-condensation and the traditional turbulent field analysis, the specific heat is found to be the dominant parameter for the enhanced heat transfer in S–CO 2 cooling. The disappearing of enhanced cooling heat transfer, in the view of pseudo-condensation, is mainly due to the thickening of pseudo liquid film, which is dominated by the gravity force, inertial force and interfacial force in low mass flux flow while the gravity force effect is negligible in high mass flux flow. However, from the turbulent field analysis, the disappearing of enhanced cooling heat transfer is mainly caused by the thickening boundary layer due to the local increase in density and viscosity in addition to the decrease in specific heat. Finally, based on the S–CO 2 cooling mechanism, two correlations for predicting heat transfer are proposed and evaluated. The present work can significantly enhance in-depth understanding on S–CO 2 cooling and promote engineering application. • The S–CO 2 cooling mechanism is analyzed with pseudo-condensation and turbulent field. • Pseudo-condensation is based on analogy between supercritical cooling and subcritical condensation. • Turbulent field analysis focuses on selection of key influencing thermophysical properties. • Two more accurate correlations are developed based on the obtained cooling mechanism. • The pseudo-condensation for S–CO 2 cooling process is proved to be with high reliability. [ABSTRACT FROM AUTHOR]

• The seriousness of the HTD problems in large-diameter tubes is highlighted experimentally. • SST k-ω model for S-CO 2 heat transfer prediction is experimentally validated within ±15% error range. • HTD mechanism is revealed by comprehensive comparisons between simulations and experiments. • Pseudo-nucleate boiling concept is proposed to further develop the pseudo-boiling theory for S-CO 2. • Conical strips insert can prevent HTD problems and achieve high PEC of 1.21-1.35. Heat transfer deterioration (HTD) problems can undermine the thermal safety of heating tubes for supercritical carbon dioxide (S-CO 2) flows. Relevant experimental evidence is highly desired, but most cases were investigated for small tube diameters (below 10 mm) and under limited operation parameters, far from the industry-scale applications. In the present work, large tubes (24 mm) are investigated experimentally with pressures of 7.5–15 MPa, mass flow rates of 100–1200 kg·m−2·s−1 and heat fluxes of 30–350 kW·m−2. The seriousness of HTD in large tubes was experimentally confirmed, and the mechanism is explored via a carefully-designed comprehensive comparison between present numerical simulations and experimental tests. Detailed analysis of vapor-like film development inspires us to mitigate the problem using structured-inner-surface, from the viewpoint of "supercritical pseudo-boiling". This inspiration is further evaluated numerically: five types of enhancement structures are proposed to interfere with the development of vapor-like film. It was found that the proposed methods can fully prevent HTD problems, and especially the conical strips can achieve a high heat transfer performance (with performance evaluation criteria PEC of 1.21–1.35) and are thus recommended for HTD prevention in vertical large-diameter tubes under near-critical conditions. Overall, the experimental tests and numerical explorations can deliver more evidence on the theory and applications of supercritical fluid flow and heat transfer. [ABSTRACT FROM AUTHOR]