Experimental Investigation On Single-phase Oscillation Characteristics Of Parallel Channels Of Inverted U-tube Steam Generator

The U-tube steam generator（UTSG）is a widely used heat exchanger in the nuclear reactor system.Under accident conditions,with the decrease of primary side flow and increase of inlet temperature,single-phase oscillation between pipes could occur in UTSG.The oscillation,on the one hand,can cause the fluctuations of flow rate,pressure drop and temperature in the U-tube,which will affect the system stability and cause the mechanical fatigue of the system,and on the other hand,it will lead to the early occurrence of flow reversal in U-tube,which affects the accuracy of flow reversal prediction.Therefore,it is of great significance to study the characteristics of oscillation parameters in the U-tube steam generator and the relationship between the oscillation and the flow reversal to improve the safety analysis of the reactor system under accident conditions.In this paper,the oscillation characteristics and key parameters in the U-tube were studied based on the single-phase oscillation experiment among three U-tubes at 1MPa,the influences of oscillation on flow reversal were analyzed,and the applicability of N_sub-N_pch dimensionless unstable boundary map to the single-phase oscillation between U-tubes was also discussed.According to the experimental results,it was found that the flow rate,temperature and pressure drop of U-tubes presented a diverging trend with time when oscillation occurred,and the flow rate and temperature oscillated in-phase for the outer long pipe and the middle pipe,but out-of-phase for the inner pipe and the other two.With the increase of inlet temperature,the flow rate at the onset of flow rate oscillation increased linearly and the oscillation period decreased linearly.With the increase of secondary side flow rate,the onset of flow rate oscillation showed a small change,which increased first and then decreased,and the oscillation period gradually increased.With the increase of inlet resistance coefficient,the onset of flow rate oscillation decreased,and the oscillation period increased slowly and then increased rapidly.By comparing the self-programming calculation results of the flow reversal critical flow rate with experimental data,we found that under accident conditions,the oscillation between U-tubes always took place ahead of flow reversal,and ratio of flow rate was between 1.05 and 1.2.Through the Fast Fourier transform（FFT）of the oscillation flow rate,it was found that the oscillation amplitude of the primary side flow rate increased with the increase of the inlet temperature,and decreased with the increase of the secondary side flow rate,and decreased first and then increased with the increase of the inlet resistance coefficient.The ratio of flow time to period is up to 1.3 times.The single-phase oscillation among tubes had the characteristics of density wave oscillation.The phase difference between the outlet temperature and the middle part temperature of the U-tube was 40.5°.The N_sub-N_pch dimensionless unstable boundary map was used to describe the single-phase cooling oscillation of U-tubes.It was found that the system stability boundary diagram presented a shape of"I"rather than a traditional shape of"L".The N_sub-N_pch dimensionless unstable boundary map may be applicable to the change of primary side parameters,but not proper for those of secondary side.