Analysis On The Inception Of The Large-scale Distorted Magnetohydrodynamics Flow In The Annular Linear Induction Pump Based On A Three-dimensional Full-scale Model

Annular Linear Induction Pump(ALIP)is the ideal pump type for the main coolant pump of the fourth-generation nuclear power reactor using liquid metal as coolant.The commercial liquid metal cooled fast breeder reactor requires a large flow rate for the main coolant pump.However,the designed large-size ALIP can suffer from serious magnetohydrodynamic(MHD)flow instability when the pump runs at off-design working condition.Therefore,it is of great importance in studying the mechanisms of the MHD flow instability in the design of large-size ALIP.In this paper,a three dimensional full-scale numerical model is built with the reference to the ALIP-2 pump of the Russian D.V.Efremov Institute by using the modified software COMSOL.Based on this model,the conducting flow in the ALIP-2 pump was simulated under various flow rates,and its hump pressure curve and the large-scale distorted MHD flows and the pressure head over a wide range of flow rate were accurately predicted.Finally,the mechanism of the occurrence of the large-scale distorted MHD flows was deeply revaled,and the main research conclusions are as follows:(1)Strong reverse flows occur in the pump channel at smaller flow rates when an azimuthal non-uniformity of the external magnetic field with a variation of 10% is defined in the simulations.The size and range of the reverse flow are increased with the reducing flow rate.The effects of the forward and reverse flows are opposite on the magnetic field intensity in the channel.The reverse flow can suppress the magnetic field.In addition,the reverse flow can also attenuate the end effect of the axial Lorentz force.The pressure pulsation of double-supply frequency is therefore reduced with the reducing flow rate since it is caused by the end effect.Moreover,the pressure pulsation of low-frequency is predicted by the simulations and its formation is due to the occurrences of large-scale periodic vortices in ALIP pump.The amplitude of the pressure pulsation is increased as the flow rate reduces.(2)The large-scale vortices only occur on the azimuthal plane of the pump channel while reverse flow which is correlated with the vortices is observed on the meridian plane.The reverse flow is initially found from the outer wall,indicating that the large-scale vortex initially incepts near the outer wall.Based on the analysis of the fluid kinetic energy equation,it is found that the reason for occurring the highly heterogeneous velocity field along the azimuthal direction is attributed to the mismatch between non-uniform induced Lorentz force and pressure gradient in the fluid.An analytical formula of the axial Lorentz force is derived,and it is found that the azimuthal non-uniformity of the magnetic field and the amplification mechanism of the MHD flow corresponding to the non-uniform disturbance are the original reasons for the formation of large-scale vortices.(3)Based on the analyses of the magnetic energy and kinetic energy equations,the mechanisms of the energy conversion in the ALIP are summarized.It is found that the main factor leading to the low efficiency of ALIP is the Ohmic dissipation caused by the large-scale unstable flows in the conducting flow.The growth of the vortex scale and the nonlinear interaction between the magnetic and fluid fields are the main factors in the formation of the hump pressure curve.(4)The stability of the MHD flow in ALIP are analyzed based on the magnitude analysis of the derived magnetic induction equation and the one-dimensional small-perturbation theory.The index H is proposed to judge the flow instability and its value is determined by the magnetic Reynolds number,the wave number of disturbance,the axial position and the radius of the central cylinder of the channel.In the region of H>0,the disturbance in magnetic and fluid fields will be suppressed by the MHD system of the ALIP.In the region of H<0,the disturbance will be amplified,resulting in severe unstable flows in the pump.Therefore,the unstable region of ALIP can be accurately identified by drawing the H-z curve.The finding provide a theoretical method for the design and optimization of ALIP.