Study On The Characteristics Of Flashing Flow In Low Pressure Natural Circulation System Under Heterogeneous Nucleation Conditions

The operating pressure of the low pressure natural circulation system is low,and when the hot fluid flows upward in the riser,the flashing flow phenomenon will occur due to the decrease of the static pressure and the overheating of the fluid.In addition to fluid superheat,impurities,non-condensable gases,or other perturbations contained in the fluid will provide nucleation conditions for flashing.In order to clarify the characteristics of low pressure natural circulation flashing flow under heterogeneous nucleation conditions,the experimental study on the flashing flow bubble behavior,flow pattern and void fraction distribution under the effect of inserts was carried out.The operating characteristics of the low pressure natural circulation system under gas injection conditions were analyzed.In this paper,based on the visual observation and experimental data analysis,the flow characteristics induced by flashing phenomenon in the low pressure natural circulation loop is investigated in detail.A low pressure natural circulation experiment system was set up to analyze the effect of inserts on the flashing flow process under different heating powers.According to the flashing mechanism,the bubble behavior at the flashing inception,the flow patterns and evolution processes in the flashing section,the axial and radial distribution characteristics of the void fraction in the two-phase section and the system flow behaviors were studied under different experimental conditions.Results indicate that,in the rising section,the effect of the inserts on the natural circulation flashing flow process is obvious.When superheated liquid flows through the inserts,flashing is induced to occur in advance.Under low power conditions,intermittent flashing is formed at the inserts positions,and bubbly flow and churn flow occur in the flashing section from the flashing inception to the rising section outlet.The flashing continues on the insert surface under high power conditions,and the length of the flashing section increased significantly compared with low power conditions.The bubbly flow,churn flow and pre-annular flow appeared in the flashing section.Increasing the inserting depth at different positions significantly changes the bubble behaviors on the surface of the inserts,affects the occurrence of the flashing inception and the development of vapor phase behavior in the subsequent flashing process.When the inserting depth is less than the half pipe diameter(r ? 10 mm)with 32 kW heating power,a typical slug flow pattern appears.Combined with the bubble behaviors of flashing inception and the flow patterns in the flashing section,the axial instantaneous distribution of the void fraction presents various distribution forms such as a periodic distribution and a continuous distribution.The radial distribution of the void fraction mainly presents a "double peak" distribution and a "core peak" distribution.On this basis,the flow characteristics of flashing natural circulation system are further studied.Results indicate that four stages,such as single-phase flow stage,flow oscillation stage,flow rate rise slowly stage and stable operation stage,are observed from start-up to stable operation in the low pressure natural circulation system.By analyzing the axial changing tendencies of fluid temperature,it is found that the fluid temperature decreases once the flashing occurs in the rising section.Therefore,the position of the flashing inception can be identified by the axial mainstream temperature change.Finally,an attempt was made to analyze the effects of gas injection on flashing-driven two-phase flow instability.It is found that gas injection can significantly improve the system driving head and the system stability.When the gas injection rate is only 0.1kg/h,the direct gas injection method can enhance the system flow rate by approximately 14%.By investigating the influence of different gas injection methods(direct gas injection and porous media gas injection)on the circulation flow rate,it is found that the direct gas injection method is more effective in improving the flow rate than the porous medium gas injection method.Under the condition of gas injection rate 0?1.5kg/h,the difference between the two is 5%?10%.