Inclination and rolling effects on water-steam flow instability in residual heat removal pipe

Small modular reactors are increasingly being utilized on offshore platforms as part of the transition to low-carbon energy. A crucial safety system component is the passive residual heat removal system, which effectively eliminates residual heat from the reactor core during power failures. The process of residual heat removal involves direct contact condensation of steam, leading to various flow instabilities, such as subcooled-water reverse flow, Kelvin–Helmholtz instability, and condensation-induced water hammer (CIWH). Marine motions introduce additional complexity to these instabilities. This study developed a rolling experimental setup to simulate an offshore residual heat removal system featuring adjustable inlet steam mass flux, a visualizable rolling test section, and a temperature-controlled water tank. Experiments were conducted with subcooled-water at 30 °C under two steam mass fluxes of 15 and 11 kg/h across 22 scenarios, including horizontal, inclined, and rolling configurations. Visual results show that a liquid column formed by subcooled-water reverse flow is necessary for CIWH. The liquid column's motion is modulated by condensation driving force, flow inertial force, and gravity. In downward-inclined pipes, gravity limits the reverse flow distance, with CIWH typically occurring when subcooled-water floods the pipe. Conversely, the reverse flow distance increases in upward-inclined and rolling pipes, raising flow instability frequency. At 11 kg/h, the flow pattern in the upward-inclined and low-frequency rolling pipes shifts to intermittent slug flow. The bubbles condense during the flow process, and the low subcooling at the interface slows the shrinkage rate, which prevents CIWH.