Visual experimental research on flow pattern transition mechanisms in vertical helically-coiled tube under liquid-solid-liquid coupled heat transfer conditions

Abstract

Helically-coiled tube steam generators are important equipment in nuclear power plants. The fluid on the primary side flows over the helically-coiled tube bundles, and exchanges heat with the fluid on the secondary side inside the helically-coiled tube steam generator. Therefore, the secondary side fluid in the helically-coiled tube is under a liquid-solid-liquid coupled heat transfer heating condition. Furthermore, the flow pattern characteristics and transition mechanisms of the secondary side fluid in a helically-coiled tube steam generator play a crucial role in improving the heat transfer efficiency of the generator and the secure and stable operation of nuclear power plants. Therefore, a visual experiment system of a helically-coiled tube steam generator was set up in this paper to observe and study the flow pattern characteristics and transition mechanisms of the secondary side fluid under liquid-solid-liquid coupled heat transfer condition. It was found that the flow patterns of the secondary side fluid include bubble flow, slug flow, stratified-wavy flow and stratified flow in helically-coiled tube. The convergence, contact, collision, coalescence of boiling bubbles and the growth of bubbles along the top of tube were the main flow pattern transition mechanisms under liquid-solid-liquid heat transfer condition. Additionally, the influence of parameters on flow pattern characteristics and transition mechanisms were analyzed, and it was concluded that an increase of heating water temperature in the primary side or an increase of mass flux of secondary side fluid promoted flow pattern transition. However, an increase of mass flux of heating water in the primary side had no obvious effect on the flow pattern transition. Finally, the flow pattern map and the prediction correlations of flow pattern transition boundary curves of the helically-coiled tube were established under liquid-solid-liquid coupled heat transfer conditions based on the experimental results. This flow pattern map was similar to the map obtained by Li (2018), Zhang and Chen (1983), Liu et al. (1976) and Li et al. (2024) based on the water–air experiment in the helically-coiled tube in terms of the shape of some flow pattern transition boundary curve. However, it was significantly different from the flow pattern maps obtained by Mandhane et al., 1974 in a horizontal tube, Mishima and Ishii, 1984 in a vertical tube and Barnea et al. (1980) in a inclined tube.