Study of density wave instability in natural circulation with parallel channels under inclined and heaving conditions

Nuclear reactors used in warships, commercial ships and floating nuclear power plants will move synchronously with the carrier due to the action of waves or surges. Under ocean conditions, flow oscillations due to additional external motions can be superimposed with flow instabilities in the system resulting in larger amplitude oscil-lations or even resonances. Sustained compound oscillation will lead to fatigue failure of components, deteriorate local heat transfer, and affect the safe operation of the reactor system. The PNCMC (Program for Natural Cir-culation under Motion Condition) program is utilized in this research to explore the density wave instability (Type-I) in parallel channels during ocean circulation, and stability boundaries under different thermal-hydraulic parameters as well as ocean conditions to support the safe operation of the system. Through our study, it is found that under the condition of small heating channel inlet resistance coefficient, the unstable boundary powers in vertical case of natural circulation according to the order from small to large are the initial boundary of the parallel three-channels out-of-phase oscillation, the initial boundary of system's density wave oscillation, the boundary of the terminated system oscillation of density wave, as well as boundaries of three parallel channels where out-of-phase oscillations terminate, respectively; The inclined condition's stable boundary is generally lower than that in the vertical case. Unstable boundary power for the natural circulation in the heaving situation according to the order from small to large are the initial boundary of density wave oscillation in whole system are the initial boundary of oscillation of the density wave in system, the initial boundary of the parallel three -channels out-of-phase oscillation, boundary for three parallel-channels where out-of-phase oscillations termi-nate, and the boundary of the terminated system density wave oscillation, respectively; The oscillations caused by heaving motions, the system oscillations of density wave, as well as oscillations out of phase between the channels were overlapped to cause flow oscillations of very complex compound inside the heating parallel channels. At the same time, the increase of the resistant coefficient at inlet of heating channels can significantly suppress out-of-phase oscillation between the channels.