Transient Thermal Simulation And Safety Assessment Of A Canned Motor Under Water Loss And Speed Increase Conditions

The canned motor of nuclear main pump is an active rotating equipment in the main circuit of nuclear power plant,which drives the continuous circulation of coolant.Transient accidents such as loss of normal water supply,decrease of temperature of water supply and excessive increase of load may occur in the production process.The occurrence of transient accidents will seriously affect the normal operation and safety of power plant.Based on the background of secondary cooling water loss of a nuclear canned motor and the 120% increase of the speed of the canned motor,the thermal simulation of the transient working condition of the motor and the safety assessment of the related faults are carried out to ensure the normal circulation of coolant in the reactor coolant system of the motor and to prevent the accident of the nuclear power plant.Firstly,the three-dimensional physical model of the calculation domain of the canned motor 1/8 is established,the mathematical model is established,and the conditions for solving transient conditions are set up.Several important aspects,such as the time-space region discretization method and the influence of ‘Fo' number on the calculation time step,are briefly discussed.The steady-state results are analyzed as the initial field and the influence of the three pressurevelocity coupling algorithms on the transient calculation,and the grid independence and time step independence are verified to ensure the accuracy of the calculation results.The results show the results of steady state simulation should be taken as the initial field of unsteady state calculation,the results of the three pressure-velocity coupling algorithms for two transient conditions are almost the same,‘Fo' number characterizes the depth of the unsteady state process and affects the choice of time step.Secondly,the safety assessment of motor secondary water loss is made.Through the analysis of the whole machine temperature field,it is determined that the temperature changes of two cooling water lines and outer cooling jacket,casing and stator components are obvious,and then the temperature field changes with time are analyzed separately.The results show that after secondary water loss,the thermal conductivity decreases and the heat can not be taken away in unit time,which leads to the temperature increase of secondary water itself,outer cooling jacket and shell solid parts,the heat generated by stator parts transfers along the axis and radial direction,which leads to the continuous rise of the temperature of the straight end and primary water at the back of the winding,the peak temperature of the winding increases very little in 3 minutes,the primary water exceeds the alarm temperature,and the electric power is used.The machine can't run safely.Finally,numerical thermal simulation is carried out to analyze the change of clearance flow field,cooling water circuit and winding temperature field of shield sleeve with time under the condition of 120% increase of motor spee d.At the same time,the variation law of temperature field of thrust bearing and upper guide bearing at different time is given,which can provide reference for analyzing the change of thermal stress,wear and lubrication of load components under the transient condition of motor.The results show that the circumferential velocity of clearance water in shield sleeve increases gradually after the motor speed increases by 120%,and the forced convective heat transfer in clearance increases continuously,but the decrease of winding,bearing parts and primary water temperature is not obvious.In this paper,by simulating the variation of thermal flow field of canned motor under different transient conditions,it has guiding significance for thermal control and thermal management in engineering practice.In addition,when solving the transient conditions of canned motor numerically,the important aspects such as time-space region discretization,‘Fo' number influence on calculation time step are discussed,and the pressure-speed coupling algorithm is validated.The numerical solution of unsteady flow and heat transfer problems in engineering practice has important reference and guiding significance.