Experimental investigation on two-phase flow instability induced by direct contact condensation in open natural circulation

Direct contact condensation (DCC) is a phenomenon that occurs frequently in nuclear and steam power systems, causing two-phase flow instability and water hammer events. This, in turn, results in system oscillations and, in some cases, leads to structural failure. This paper aims to characterize and highlight the mechanisms and features of the DCC-induced two-phase flow instability in an open natural circulation system employing visual experiments. To aid this, a laboratory-scale experimental rig is established based on the working principle of open natural circulation. In the experiments, the transient two-phase flow images are captured by a high-speed camera. Five kinds of flow instability modes and evolution characteristics are analyzed quantitatively and qualitatively using image processing techniques. The results indicated that the frequency of instability is influenced by the amount of heating power, and the instability mode is mainly influenced by the subcooled-water temperature. Moreover, the steam condensation rate was found to have a direct impact on the instability extent. These results obtained in this study offer insights into the full characterization and understanding of two-phase flow instability in open natural circulation systems, as well as towards the prevention of DCC-induced water hammer in offshore nuclear power ships.