Investigation on pressure drop and characteristics of flow-pattern transition of steam-water two-phase flows in helically coiled tubes

Helically coiled tube (HCT) steam generators are widely used in chemical engineering, aerospace industry and nuclear engineering, etc., and especially, in small modular nuclear reactors because of their high heat transfer efficiency, compact structure and free thermal expansion behavior. The special geometry of the HCT leads to complex flow characteristics such as secondary flows and the resulting special phase distribution and flow patterns in the tube and relatively high pressure drop compared to that in straight tubes. The difference of geometrical parameters usually leads to the significant difference in the characteristics of gas-liquid flow in HCTs. In the present study, the pressure drop and flow-pattern transition characteristics of subcritical pressure water-steam two-phase flow in an HCT with a special small tube diameter were investigated by experiments and numerical simulations. It can be concluded that the frictional pressure drop firstly increased with the increase in thermal equilibrium quality, and reached a peak at the quality of 0.75, and then decreased gradually. This was due to the flow regime transforms from annular flow to dispersed flow (or mist flow) when the quality equaled to 0.75, which led to the reduction of frictional pressure drop. The flow regimes of high-pressure steam-water two-phase flow in HCTs can be divided into bubble flow, intermittent flow, annular flow and dispersed flow (or mist flow). The transition criteria between bubble flow and intermittent flow was at the quality of 0.038, the transition criteria between intermittent flow and annular flow was at the quality of 0.500, the transition criteria between annular flow and dispersed flow (or mist flow) was at the quality of 0.751, and the dry-out point was at the quality of 0.93. This study can provide guidance for the design and safe operation of HCT steam generators. © 2025 Chemical Industry Press Co., Ltd.. All rights reserved.