Study On Vapor Phase Behavior Characteristics Of Low Pressure Natural Circulation Flow Flashing Process

For the low-pressure natural circulation system,the flow instability induced by flash often occurs,which has a serious impact on the heat exchange efficiency and safety of the system.In order to suppress flash induced flow instability,based on the mechanism of flash heterogeneous nucleation,flash occurred in advance,a method was proposed to install inserts in flash segment or inject non-condensate gas into the inlet of ascending segment,so as to eliminate flash induced flow instability and improve the heat transfer efficiency of the system.In this paper,visual images and experimental data are combined to study the influence of the position,quantity and insertion depth of inserts in flash section on the instability of flash flow under different heating power and different water tank level conditions.The effects of different gas injection rates on flash flow instability were studied.Combined with the visual images,the system flow state,flow pattern evolution,flash start point change,and the radial and axial distribution of cavitation share during flash flow were analyzed.The results show that the insertion in the flash section can eliminate the instability of flash flow and improve the heat transfer efficiency of the system.Insert the insert into the tube wall l=D.When the system flow is in an unstable state,the insert can inhibit the flow instability at different positions in the flash section.The closer the insertion is to the flash start point,the stronger the inhibition effect is,and the maximum increase of the mean value of the circulating flow is 10%.When the system flow is in a stable state,there will be no harmful effects on the system due to the presence of inserts.When multiple inserts are simultaneously inserted into the tube wall l=D,compared with the single insert,the cavitation share increases due to the increase of the vaporization core,and the mean value of the circulating flow rate increases by12% at the maximum.Although there will be bubbles in the insert,it has little effect on the size,radial and axial distribution of cavitation fraction without changing the length of flash section.When the length of flash section is changed,the size of cavitation fraction at the coaxial position of the phase changes only because the location of flash start changes,but the distribution form remains unchanged.Injecting a small amount of non-condensable gas into the inlet of the ascending stage can also eliminate flash flow instability and improve the heat transfer efficiency of the system.After gas injection,the position of flash start point moves down,which induces the thermodynamic non-equilibrium fluid in flash section to produce more bubbles,increasing the driving force of the system.When the system flow is in an unstable state,gas injection can eliminate the instability of flash flow,and the mean value of circulating flow can be increased by 28% at most.When the system flow is in a stable state,the gas injection will not change the system flow state,and the average increase of the circulating flow is within 10%.