| The open natural circulation system with a long straight riser is prone to flow flashing phenomenon due to the low system pressure.The occurrence of flow flashing has an impact on the operational characteristics of the open natural circulation system.In order to clarify the flow flashing characteristics in the open natural circulation system,This paper employs a research methodology combining visual observation and analysis of experimental data to investigate the occurrence conditions of stability flashing inception and bubble behavior under undisturbed conditions.Furthermore,the paper investigates the influence of inserts and gas injection on the characteristics of flashing in flowing liquids.Research has found that the occurrence conditions of flashing inception are determined by the rising section inlet temperature and the circulation velocity.The commonly used flashing inception models cannot predict the flashing inception of the open natural circulation systems.A modified model for the flashing inception of the open natural circulation systems was established using the modified classical nucleation theory.A comparison with experimental data revealed good applicability of the model,with a relative error within 20%.The larger the degree of superheat,the faster the bubble growth rate.The axial velocity of the same bubble increases with its diameter,but there are slight differences in axial velocity between different bubbles at the same diameter.Under stable flashing conditions,when different shapes,materials,and roughness of inserts are inserted into the single-phase region,it triggers premature flashing inception,alters the position of flashing inception,accelerates the evolution rate of flow patterns,slightly increases the average void fraction and system circulation flow rate in the axial direction.When inserts are placed in the two-phase region of flashing,it does not change the position of flashing inception,bubble behavior,and flow patterns,but it accelerates the evolution rate of flow patterns and slightly increases the average void fraction and system circulation flow rate in the axial direction.Inserting inserts in an unstable flow regime has little impact on the system circulation flow rate.Under long-term unstable flow conditions,inserting differentshaped inserts in the single-phase region increases flow instability for cylindrical and prismatic inserts,while the cuboid insert stabilizes the flow.When inserts are placed in the flashing region,all three types of inserts suppress flow instability,with the cuboid insert being the most effective.Before introducing gas into the rising section,the flow characteristics of the open natural circulation system were studied.The study found that the system undergoes four stages during the transition from single-phase to two-phase stability: the single-phase stable flow stage,intermittent flashing unstable flow stage,sustained flashing unstable flow stage,and stable flashing flow stage.The flow characteristics of each stage were described by analyzing the circulation flow rate,rising section inlet temperature,and heater inlet temperature.Subsequently,the impact of gas injection on the unstable flow stage was investigated.It was found that gas injection can suppress unstable flow and increase the circulation flow rate.The larger the amount of gas injection,the more pronounced the suppression effect and the greater the increase in circulation flow rate.Moreover,the closer the gas injection point to the rising section,the more significant the suppression effect and the increase in circulation flow rate.Lastly,the impact of gas injection on the stable flow stage was studied.It was found that gas injection had no effect on the system’s flow state,and the increase in system circulation flow rate was within 14.3%. |
PDF Full Text Request