Study On The Natural Circulation System Characteristics Under Rolling Motion Condition

As a typical passive system,the natural circulation system is used in advanced nuclear power reactor systems because of its capability to enhance the inherent safety of nuclear power units.However,due to the small space available for arranging nuclear power equipment,the driving pressure heads of natural circulation systems are generally small.And thus influenced by rolling motion,the thermal and hydraulic characteristics of such a system will change,affecting the operating characteristics of the system.In this paper,a study was conducted on the characteristics of the natural circulation system in a rolling motion condition by combining experiments with numerical simulation and theoretical analysis.An experimental study was conducted on influences of rolling motion on single-phase flow and heat-transfer characteristics of narrow rectangular channels,with phase relationship between flow rate and frictional pressure drop,instantaneous and cycle-average frictional resistance,and heat-transfer characteristics being obtained.With these as the basis,as well as the calculation results and theoretical analysis results provided by an independently developed system analysis program,it was pointed out that the nonlinear relationship between parameters was the key influencing mechanism leading to the changes of cycle-average frictional resistance and heat-transfer characteristics in the rolling motion condition.The instantaneous radial velocity distribution and temperature distribution in the rectangular channel in the rolling motion condition were studied with FLUENT software,which,together with the contrastive analysis of the experimental results against the calculation results of the system analysis program,got the mechanism of how the rolling motion influenced the instantaneous frictional resistance and heat-transfer characteristics.An experimental study was carried out on the influences of rolling motion on the system characteristics of single-phase natural circulation,with the pattern of how rolling motion parameters influenced the flow rate and temperature fluctuation being obtained.On this basis,combined with the calculation results of the system analysis program,the main mechanism about the fluctuation characteristics of the flow rate and temperature were obtained.Meanwhile,according to the above conclusions plus the results of theoretical analysis,the mechanism of how the rolling motion influenced the cycle-average flow rate,the peak flow rate,and the valley flow rate was obtained.It was pointed out that the inconsistencies between the decrease of cycle-average flow rate and the rangeability of peak and valley flow rates were mainly caused by the superlinear relationship between flow rate and resistance pressure drop.It was also argued that the change of the heat-transfer characteristics at the turning point was the main cause of multiple peaks showing up in the temperature fluctuation curve.Based on the analysis of the characteristics of parameters' changes in the process of starting the rolling motion,an empirical model was proposed,which could be used to predict the changes of parameters when the rolling motion was being started.The reliability of the calculation results of the system analysis program was verified with experimental data,based on which,the axial temperature distribution in narrow rectangular channels and adiabatic sections was studied using this program in the rolling motion condition and in the single-phase natural circulation working condition.The results showed that in constant heat flow rate condition,if the instantaneous flow rate in the cycle was always greater than zero,the temperature distribution in the heating zone showed an approximately linear change trend.A length limit existed,if which was exceeded by the distance from the outlet of the heating zone,the temperature in the adiabatic section tended to be distributed evenly.With the method of theoretical derivation being adopted and the momentum conservation equation being taken as a starting point,a posterior model of peak flow rate and valley flow rate in predictable rolling motion condition was proposed.Accordingly,with the posterior model being simplified reasonably according to the study results of the rolling motion's influence on the characteristics of the natural circulation system,a prior model predicting peak flow rate and valley flow rate was worked out.Then,the reliability of the model predicting results was verified with the experimental results.An experimental study was done on how the rolling motion influenced the flow rate and heat-transfer characteristics when highly sub-cooling boiling occurred in the narrow rectangular channel,with instantaneous frictional resistance and heat-transfer characteristics being obtained through comparison between the results with the calculation results delivered by the system analysis program.It was pointed out that the calculation method in vertical working condition could be used to predict the instantaneous frictional pressure drop in the rectangular channel when highly sub-cooling boiling occurred in the rolling motion condition.With visualized experimental study method being adopted,a study was made on the mechanism of how the flow rate changed when instability was caused in vertical and rolling motion conditions,with the mechanism of the occurrence of flow instability in different types in the natural circulation system being acquired based on the results of the system analysis program.Meanwhile,it was pointed out that the size of the gaseous space of the voltage stabilizer had important impacts on the flow instability period of the coupled pressure drop and thus it could facilitate the design of a closed natural circulation system.