| The narrow space channel is widely used in advanced nuclear reactor engineering and electronic cooling due to the advantages of compact structure and high heat transfer efficiency,but the decrease of narrow space channel size leads to the increase of flow resistance.Especially in boiling two-phase flow,the flow pattern transition in narrow space is accompanied with the sharp change of pressure drop,which is more likely to cause oscillation of boiling two-phase flow due to the limitation of channel size.As the flow oscillation appears in the channel,it will affect the stability of the system due to the oscillations of system flow rate and pressure,on the other hand,it will result in mechanical fatigue with the continuous behavior of system oscillation,and even lead to system failure.Therefore,it is of great significance to study the characteristics of the flow oscillation and its influencing factors in the narrow space channel to improve the safety of the system design and operation.In this paper,the visualized experimental research on the characteristics of boiling two-phase flow oscillation is carried out with two channel structures of single channel and bypass single channel.The influence of working parameters and bubble dynamic on the characteristics of flow oscillation is analyzed with different working conditions combined with visualization.The research find that in the single channel,the period of flow oscillation is dozens of seconds,but the amplitudes of pressure drop and the mass flus are small,which belongs to the low frequency and low amplitude oscillation.The wall temperature,mass flux and pressure drop have the same period of flow oscillation,and the oscillation parameters obviously show three different stages in a cycle.Bubble flow and slug flow are observed at the outlet of experimental section with the visualization method.The continuous flow pattern transition is the main factor to determine the oscillation period.While the period of flow oscillation in narrow channel with bypass is only a few seconds,the amplitude of mass flux oscillations is much larger than that in single channel,which belongs to high frequency and high amplitude oscillation.With the increase of heat flux,the system condition transforms immediately from steady state to boiling critical without flow oscillations as the inlet temperature is 60 ?.The inlet water temperature has little influence on the characteristics of oscillation,and the amplitude of mass flux oscillation increases nonlinearly with the increase of heat flux.The mass flux has a great influence on the amplitude of mass flux oscillation.At the same time,bubble flow,slug flow,mixing flow,annular flow and are observed at the outlet of the experimental section.As the flow oscillation occurs,the boiling critical appears and flow pattern enters to the mist flow.Not only the transition of flow pattern will affect the oscillation period,but also the influence of vapor and liquid interface fluctuation near the heating wall can not be ignored. |
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