| In the event of a loss-of-coolant-accident in a pressurized water reactor nuclear power plant,the pressure of the primary circuit will be reduced,and a large amount of steam will be generated in the reactor pressure vessel.The steam enters the steam generator through the heat pipe section and is condensed in the steam generator.The condensate flows back into the reactor pressure vessel through the heat pipe section so that the core heat is discharged.The vapor and condensate form a gas-liquid two-phase countercurrent flow in the heat pipe section.However,the gas-liquid two-phase countercurrent flow is only stable in a certain steam flow range.If the steam flow is too large and exceeds the countercurrent flow limitation(CCFL),the reflux of the condensate will be blocked,which greatly reduces the flow of the reflux condensate and adversely affects the safe operation of the reactor.Based on the above research background,this paper conducts a visual experimental study on the reverse flow flooding phenomenon of air-water in a horizontally connected inclined tube and the characteristics of CCFL.The CCFL curve when the flooding phenomenon occurs and the CCFL characteristic curve in the partial transport state are obtained by dimensionless processing of the apparent velocity of gas phase and liquid phase.The gas-liquid two-phase flow in the experimental tube was photographed by a high-speed camera,and different flow phenomena in the tube before and after the occurrence of CCFL were found.The flow phenomena during the occurrence of flooding and partial transport were analyzed.The probability density analysis and power spectral density analysis of the pressure difference signals at both ends of the experimental tube were carried out by using statistical methods.The mean,standard deviation,skewness,kurtosis and fluctuation frequency of the pressure difference signals under different working conditions and different experimental tube geometries were obtained.Finally,the CFD method was used to simulate the flooding phenomenon in the horizontally connected inclined pipe.The simulation results can accurately show the gas-liquid interface characteristics and two-phase flow conditions in the process of flooding.The CCFL curve when flooding occurs is divided into three parts.The apparent velocity of gas phase decreases first,then increases and then decreases again with the increase of dimensionless liquid phase apparent velocity.The flooding phenomenon in the tube is caused by a hydraulic jump.The water forms a hydraulic jump under the shear action of the gas,then develops into a rolling wave,adjusts the direction of motion,and finally rushes out of the experimental tube from the inclined section.The variation trend of the mean,amplitude and frequency of the differential pressure signal is opposite to that of the CCFL curve when the flooding occurs.In the partial transport state,when the air flow rate is large,the flow in the inclined section is mainly annular flow,and the flow in the horizontal section is mainly wave flow and stepped surge,which may lead to slugging at the elbow.When the air flow is small,the flow is dominated by slug flow and water seal,and sometimes wave flow occurs.The CCFL characteristic curve in partial transport state decreases with the increase of dimensionless liquid superficial velocity.The mean,amplitude and frequency of the pressure difference signal decrease with the increase of the dimensionless gas superficial velocity.When the dimensionless gas superficial velocity is the same,the larger the diameter of the pipe,the shorter the length of the horizontal section,the larger the dimensionless liquid superficial velocity,the lower the mean and amplitude of the pressure difference signal;in addition,the liquid level height of the high water tank has little effect on the CCFL characteristic curve,but the mean and standard deviation of the differential pressure signal increase with the increase of the liquid level height of the high water tank.The numerical simulation obtained the occurrence process of the air-water reverse flow flooding phenomenon in the horizontally connected inclined tube,and the numerical simulation results were in good agreement with the experimental results,A hydraulic jump appeared at the end of the horizontal section,and the hydraulic jump developed into a rolling wave and was swept out of the experimental tube by the gas in the tube.After the development of the hydraulic jump,the flow state in the horizontal section changes from supercritical flow to subcritical flow.The momentum of water flowing from the inclined section is continuously consumed,and the water on the surface will be wrapped by the gas to change the flow direction and form an eddy current. |
