Thermal-hydraulic-structural evaluation of S–CO₂ cooling wall tubes: A thermal stress evaluating criterion and optimization

The S–CO₂ 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–CO₂ 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–CO₂ 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–CO₂ cooling wall tubes.