Experimental And Theoretical Investigations On Reverse Flow Characteristics In U-tube Steam Generator

U-tube steam generator is widely used in many commercial nuclear steam supply systems and pressurized water reactor nuclear power plants.The primary side of UTSG includes several parallel U-tubes.Many researches at home and abroad have found that some U-tubes will have the phenomenon of reverse flow under natural circulation or forced circulation with low flow.When the reverse flow occurred in U-tube,the flow resistance in U-tube increases and the heat transfer capacity weakens,and the influence on the normal operation of steam generator and reactor system can’t be ignored.Therefore,it has great significance to study the reverse flow characteristics of U-tube steam generator.This paper is based on visual experiments and system program analysis,the reverse flow characteristics of U-tube steam generator are studied.And the theoretical analysis model and judgment criteria of the reverse flow characteristics in U-tube steam generator are established.Based on visual experiment of the reverse flow characteristics of U-tube steam generator,the visual image of reverse flow in U-tube is obtained.The visual image reflects clearly the whole period physical process of the fluid which flows from stable forward to partial inversion in the U-tube.Through analyzing the results of the cold-state experiment,the resistance characteristics of the experimental loop and the resistance characteristics of the electric control valve are obtained.Through the processing of experimental data such as the total pressure drop of the loop,the pressure drop of the loop friction and the pressure drop of the U-tube at both ends,it is found that with the increase of the primary inlet temperature,the pressure drop of the U-tube decreases at both ends.At the same time,the height difference of the cold and heat sources increases in the loop.The heat loss characteristic of the system is obtained by the extrapolation of heat loss,and the heat loss characteristic curve of the system is fitted.The R~2 is 0.983,and the obtained relation is in good agreement with the experimental results.The working conditions of the typical experimental are analyzed.It is found that when a U-tube occurs reverse flow,the flow rate in this U-tube decreases quickly below zero,but the flow rate in the other two U-tubes increases quickly at the same time.At the same time,the cold water exiting the primary outlet flows back to the inlet,which causes the temperature of the fluid at the inlet of the U-tube occurring the situation of sharp drop.In addition,for the working condition with small resistance at the cold end of the loop,a significant flow pulsation phenomenon appears before the critical time of reverse flow.Then,the parameter influence of the reverse flow characteristics is analysed.It is found that the temperature of primary inlet and the total flow rate of the primary loop have great influence on the critical point of the reverse flow and the starting point of pulsation.With the increase of the resistance coefficient of the primary loop,the starting point of the pulsation is close to the critical point of the reverse flow.And within the working condition range of this experiment,the cooling water flow rate of secondary side has very small influence on the critical point of the reverse flow and the starting point of pulsation.Based on experiments results,three conservation equations and the equal pressure of the U-tube at the inlet and outlet of UTSG,the theoretical analysis model of the reverse flow characteristics of U-tube steam generator is established.The dimensionless analysis method is used to establish the control equation of the fluid in U-tube.Combined with the small disturbance theory,Re_c which is the critical Reynolds number is obtained,and the judgment criteria of the reverse flow of the U-tube steam generator is proposed.When Re is greater than Re_c,the flow is stable in the U-tube,and the reverse flow won’t occur;when Re is less than Re_c,the flow is unstable in the U-tube,and the reverse flow will occur.Based on theoretical analysis model and judgment criteria,the reverse flow characteristics of U-tube steam generator are analyzed by system program.In order to further optimize the model,according to the experimental data,the calculation relation of the flow friction resistance coefficient is fitted in the transition region of 2161<Re<3000,and R~2 is 0.830.And the calculation relation of the heat transfer coefficient of primary side is fitted in the transition region of 7960<Re<10538 and307<Re<7960,R~2 is 0.979 and 0.977 respectively.The obtained relations are all in good agreement with the experimental data.Then the critical point of the reverse flow which is calculated by system programs is verified with the experimental results.The calculated results are in good agreement with the experimental results,the maximum error is less than 8%.The optimized one-dimensional system program is used for extended calculation,It is found that the inner diameter and the length of the U-tube,the temperature of the secondary side and the heat transfer coefficient have greater influence on the critical point of reverse flow,but the wall thickness,the bend radius and the pressure of primary side of the U-tube have small influence on the critical point of reverse flow.