| The open natural circulation system has been widely used in the field of passive safety because of its simple structure,strong heat discharge capacity and good safety.However,open natural circulation systems are prone to flash flow instability under long-term operating conditions,which will adversely affect the heat removal performance and safe operation of the system.Therefore,in this paper,the gas injection to the ascending section is adopted to improve circulating driving force of the system,increase the heat discharging capacity and make the system run more stably.In this paper,the characteristics of open natural circulation flow driven by gas injection was experimentally studied under cold and hot state conditions respectively.Based on the cold experiment,the evolution of flow pattern under different gas injection modes was analyzed in detail.It was also found that the fragmentation effect of porous media on the injected air is the main reason for the difference of flow pattern.Combined with Wire-Mesh sensor measurement system,the variation law of void fraction with gas injection rate under different gas injection modes was summarized.Compared with the commonly used calculation models of void fraction,the formulas for gas injection in porous media pipe were deduced.In addition,the corresponding relationship between the typical void fraction distribution and the flow pattern was established according to the time-varying characteristics and the radial distribution of the void fraction,and the internal mechanism that influenced the distribution was analyzed from the perspective of the bubble force.Finally,the main factors that affected the system circulation flow were analyzed.According to the resistance characteristics of two-phase section,the ratio of natural circulation driving force and two-phase drag ratio was put forward as the evaluation basis for enhancing the effect of natural circulation capacity.The results show that for direct gas injection,the optimal gas injection rate of natural circulation system should be that the two-phase flow pattern is at the lower limit of the slug flow.For the gas injection method of porous media pipe,the best gas injection rate should be that two-phase flow pattern is at the lower limit of the cap-disperse bubble flow.Under the condition of thermal experiment,for the open natural circulation system of constant power,the system has experienced single-phase steady flow,two-phase oscillatory flow and two-phase steady flow successively.The two-phase oscillatory flow is mainly caused by the flow flashing of high-temperature liquid during the ascending process.The mechanism of flash unstable flow was analyzed in detail.Combined with the visual observation,the flash flow pattern of two-phase steady flow stage was described,and the flow pattern was drawn.In addition,the effect of gas injection on different operating phases of the system was also studied.Gas injection can not only significantly inhibit the occurrence of two-phase oscillatory flow,but also make the circulatory system advance into the two-phase stable flow phase.And the maximum lifting flow rate in the experiment can reach 44%.However,due to the limitation of the length of the ascending section,the rate of increase of the circulating flow of the system gradually slowed down as the amount of gas injected increased.In this case,different gas injection modes were changed,and it was found that gas injection using porous media pipe did not significantly increase the system flow rate compared to the direct gas injection mode. |
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