Simulation Of Gas-liquid Two-phase Flow Of CAP1400 Reactor Coolant Pump Under Small Breakwater Accident

Nuclear reactor coolant pump(referred to as nuclear main pump)is an important part of the nuclear power plant circuit.In the accident of a small break water loss in the loop,the coolant leaks and the pipeline pressure drops sharply.All of them cause the pipeline coolant in the rapid vaporization and the nuclear main pump running under the vapor-liquid two-phase flow conditions.As the change of water vapor is very complex,with reference to the relevant research,this paper uses air instead of water vapor for vapor-liquid two-phase flow numerical simulation.Comparing the experimental data and simulation data,the result shows that the two fit well under the condition of two liquid phase,and the degree of coincidence under the condition of low gas content is better than that under the condition of high gas content.The steady and unsteady numerical simulations of the existing mature core pump models under different gas-bearing conditions are carried out to get a better understanding of the flow characteristics of the main pump.The constant numerical simulation revealed the variation law of the external characteristic curve,the internal gas distribution and the velocity vector cloud of the core pump under different gas content.It shows that the gas accumulation phenomenon is easy to occur due to the centrifugal force and the pressure gradient.The structure of the twisted radial space guide vanes is easy to cause the gas retention phenomenon.Through the unsteady numerical simulation,the influence of the gas content on the pressure pulsation was revealed.Under the condition of higher gas content,the uneven distribution of pressure near the impeller exit is more obvious,and the fluctuation amplitude of the pressure fluctuates near the impeller face.The increase of gas content has a certain inhibitory effect on the fluctuation range of pressure fluctuation in guide vanes.In order to further understand the kinematic characteristics of the nuclear pump under the condition of the two-phase flow,the pump hydraulic performance and the distribution of the gas volume in the runner are analyzed and analyzed.With the increase of the flow rate,the descending speed of the core pump head curve and the torque curve is slowed down.As the flow rate increases,in the reverse countercurrent braking condition and the reverse pump condition,and the reversal of the positive flow braking condition is just the opposite.In the reverse countercurrent braking condition and the reverse pump condition,the high gas content is mainlyconcentrated in the front area of the impeller inlet.The volume fraction of gas in the runner is extremely uneven.When a loss of water occurs in the first loop,the nuclear pump will be in the vapor-liquid two-phase flow conditions,then the performance of the main pump will changes complexly.In order to explore the performance of the law of the nuclear main pump,the transient simulation of a small fault water loss was carried out.Through the numerical simulation,we obtained the influence of gas content on the unsteady state of the main pump.With the gas content increasing,the flow rate decreased,the shaft torque and efficiency of the main pump decreased,and the lift value variation increased first and then decreased.With the gas content gradually increases,a higher degree of gas accumulation occurs at the inlet of the impeller,the middle rear cover and the outlet suction surface.The fluctuation amplitude of the gas flow rate in the flow direction is gradually reduced and the followability is obvious.The transient radial force decreases gradually with the increase of the gas content and the difference between the two types of radial force becomes smaller.The pressure in the vane is increasing and the fluctuation degree is weakened.With the increase of the gas content,the low velocity region in the volute is expanding,which shows that the spherical volute is not conducive to the smooth flow of gas.In order to design the core main pump hydraulic model with excellent hydraulic performance under different gas-bearing conditions,seven pairs of nuclear main pump external factors was selected to conduct the larger orthogonal design,including the Impeller inlet diameter Dj,the blade angle ?,the blade outlet width b2,the impeller outlet D2,,the impeller exit angle ?2,the blade exit angle ? and the blade number Z.Through the design,we obtained a set of optimal combination of geometric parameters of the impeller and the influence of seven factors on the external characteristics of the nuclear main pump in the liquid phase and the lower gas content.According to the parameters sequence table of the influence on characteristics of the pump,we selected the exit angle and angle as the further optimization of the parameters.And through further optimization,we obtained the optimal combination of geometric parameters of the impeller.