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• 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]

The S–CO 2 Brayton cycle has been considered as a promising alternative in coal-fired power plants due to its high efficiency and high compactness. However, the high inlet temperature and low heat transfer coefficient of S–CO 2 in the cooling wall tubes arranged in the furnace could lead to thermal fragile in practical operation. In present study, thermal-hydraulic-structural evaluation on S–CO 2 cooling wall tubes is investigated. Firstly, it is found that the stress distribution is mainly dominated by the circumferential temperature gradient. Secondly, a new criterion, the thermal deviation factor (TDF), is proposed to evaluate the tube thermal stress. It indicates the plastic deformation if exceeding the critical value of TDF = 1, which is more efficient than conventional structural analysis. Thirdly, based on the TDF , two typical methods of inserting twisted tapes and unilaterally arranging dimples inside the tube are proposed to reduce the thermal stress. A novel configuration of elliptic dimple with rounded corners is finally proposed to reduce the local stress concentration efficiently. The results show that the elliptic dimples have the highest comprehensive performance, which is recommended as the heat transfer enhancement structures in S–CO 2 cooling wall tubes. • Thermal-hydraulic-structural performance of S–CO 2 cooling wall tube is investigated. • A new thermal deviation factor (TDF) is proposed to evaluate thermal stress. • Twisted tapes and unilaterally arranged dimples are used to reduce thermal stress. • A novel elliptic dimple can release local stress concentration and improve comprehensive performance. [ABSTRACT FROM AUTHOR]