Study on the Density Wave Instability in Natural Circulation System under Rolling Conditions

In the ocean environment, the thermal fluid system installed on the ship moves with the waves. This unsteady transient motion imposes additional body forces on the liquid. Thereby, the thermohydraulic properties of the thermal fluid system are changed, especially for natural circulation systems with a low driving force. Flow instability related to system security is a common two-phase flow system phenomenon. It is an essential subject in the study of thermal-hydraulic characteristics. In this paper, the density wave instability in a low-pressure natural circulation system with several parallel heating channels in the circumstances of rolling motion is studied by PNCMC (Program for Natural Circulation under Motion Condition). PNCMC is a newly developed program by adding extra body force induced by ship motions in two-phase's momentum equations. The mechanism of rolling motion and its impact on flow oscillation and unstable boundary power are investigated. The supercooled boiling caused by the higher surface heat flux firstly promotes the occurrence of density wave oscillations between parallel heating channels. When enough steam enters the riser, the system density wave oscillation occurs under the gravity pressure drop-flow rate-vapor fraction feedback. A more complicated compound oscillation is created by superimposing the interchannel oscillation and the system density wave oscillation. If the rolling motion has little change to the natural circulation flow, when density wave instability arises, the flux oscillation rule and the instability boundary power are basically consistent with the vertical conditions. When the rolling angle is large, the fairly large amplitude flow oscillation generated by rolling will prevent density wave instability from occurring and force the flow oscillation to obey the law of rolling effects.