Investigation on reverse flow characteristics in UTSGs with coupled heat transfer between primary and secondary sides

Under natural circulation conditions, the reverse flow phenomenon may occur within some U-tubes. This will result in the flow resistance coefficient of the steam generator (SG) obviously larger than that under forced circulation conditions, and the decrease of nature circulation capability in the primary circuit (PC). The heat transfer characteristics between the secondary circuit (SC) and PC in the SG are very complicated. Expressly, there is an obvious preheating section due to a certain fluid under-heat degree at the inlet of the ascending passage in the SC. In this paper, a type of marine SG is studied and a theoretical model of the SC single channel is established for the preheating sections (PS) corresponding to the ascending section (AS) and the descending section (DS) of U-tubes. Based on this model, the reverse flow characteristics of U-tubes are researched, and the influence of SC operating pressure, U-tubes pitch and circulation ratio on the U-tube reverse flow characteristics are calculated and analyzed. The results show that compared with the existing reverse flow calculation methods, the temperature distribution in the PS of SC increases exponentially and remains unchanged in the boiling section, and the critical flow rate is relatively higher. The increase of SC operating pressure and U-tubes pitch will decrease the mass flow rate of reverse flow, and the influence of circulation ratio has the opposite conclusion. (C) 2019 Elsevier Ltd. All rights reserved.