Numerical investigation on steam bubble condensation and water hammer effects in cold seawater pipe for passive heat removal

Direct contact condensation (DCC) between saturated steam and cold-water in the passive heat removal pipe is liable to condensation induced water hammer (CIWH) phenomena, which can endanger the integrity of the related equipment. The development mechanisms and transient characteristics of the CIWH were numerically investigated in this paper. Firstly, the numerical model for the CIWH inside pipe was established by combining user-defined functions (UDF), and then validated through natural circulation experiments and Paks Modular Simulator-2 (PMK-2) experiments. The numerical results were in good coincidence with the corresponding experimental results. Then, the mechanisms of formation and collapse of isolated steam bubble were analyzed. The interfacial steam condensation triggered a counterflow of cold water and formed isolated steam bubbles, resulting in rapid bubble condensation, collapse, and consequent pressure peaks. Finally, the transient characteristics of the CIWH under different steam velocity, volume fraction and temperature difference were analyzed. The results indicate that the steam condensation rate determines the reverse flow rate of cold water, and the pressure peak decreases continuously with the increase of steam velocity and water temperature. In addition, the increase in steam volume fraction significantly increases the frequency of pressure peaks.