Analysis Of Rotor System Dynamic Characteristics Of Hightemperature Long-Axis Molten Salt Pump Based On Fluid-Solid-Thermal Coupling

High temperature molten salt pump is one of the key equipment in solar photothermal power generation system,and its main function is to drive the circulating heat transfer of high temperature molten salt in the system.Due to the long structure of the vertical multi-stage long-shaft centrifugal pump,up to 17m,and the long-term operation of the pump in the molten salt environment of300℃-565℃,the molten salt temperature is high,the density and viscosity are high,and the rotor system is easy to be"locked"due to thermal expansion.this poses a technical challenge to the safe and stable operation of the high temperature molten salt pump unit.In order to improve the operation reliability of high temperature multistage molten salt pump in solar photothermal power generation system,supported by Jiangsu Province Scientific and technological Achievement Transformation Project（BA2016167）,this paper takes the prototype of molten salt pump used in a domestic 50MW photothermal demonstration project as the research object,and studies the internal flow field flow characteristics,rotor structure strength,bearing rotor dynamics and modal characteristics of molten salt pump under extremely high temperature conditions based on fluid-solid-thermal multi-field coupling theory.The main contents and conclusions are as follows:（1）Based on the numerical simulation of the internal flow field of the high temperature molten salt pump,the external characteristic parameters of the high temperature molten salt pump prototype under different temperature media and the internal velocity and pressure distribution are analyzed.The numerical simulation results show that the external characteristics of the clear water medium are basically consistent with the experimental results,which verifies the accuracy of the simulation,and the density and viscosity of the medium have little influence on the external characteristics.By changing the medium temperature and pump flow conditions,the variation trends of fluid velocity and pressure distribution,blade surface load distribution,impeller outlet velocity and pressure in all levels of molten salt pump are higher,and the internal flow field of each stage impeller is similar.With the increase of temperature,the density of molten salt medium decreases,so that the pressure difference between impeller passage and inlet and outlet decreases.With the increase of the flow rate,the difference of the physical parameters of the medium on the outlet velocity distribution will be weakened,and the position of the maximum pressure difference will be transferred from the middle of the front and rear edge of the blade.（2）Aiming at the rotor system of high temperature molten salt pump,based on the unidirectional fluid-solid-thermal multi-field coupling analysis method,the stress and deformation law of impeller in rotor system under different temperature medium and different flow conditions are analyzed,and the stress intensity is checked.The results show that the dominant stress in the impeller is the flow field and mass force load.with the increase of the flow condition to 4.52MPa,the overall stress of the impeller has been in a small stress range less than 1.2Q_opt,but the maximum equivalent stress at the trailing edge of the blade suddenly increases and exceeds 36MPa,and there is a phenomenon of local stress concentration.As the molten salt temperature increases from 300℃to 565℃,the molten salt density decreases,and the maximum equivalent stress corresponding to the temperature of the impeller decreases by 13%.The temperature load is the primary cause of the impeller deformation.with the increase of the temperature of the molten salt medium,the material expands when heated,and the deformation value increases gradually,and the change trend is gradually larger from the upper end of the pump shaft to the lower end of the pump shaft.this is caused by the superposition of axial displacement.According to the calculation,the material strength of the molten salt pump meets the safety standard.（3）By simplifying and numerically simulating the sliding bearing of high temperature molten salt pump rotor system,the pressure,speed and bearing capacity distribution of bearing liquid film under different temperature medium conditions are analyzed.The results show that the effect of the density of the medium on the liquid film pressure distribution of the bearing is more obvious than that on the velocity distribution.When the bearing is not offset,the bearing liquid film is subjected to minimal pressure;with the increase of eccentricity,the pressure on the bearing liquid film increases,and the absolute value of the maximum positive pressure is about 0.9 times that of the maximum negative pressure,and the area of the positive pressure area is larger than that of the negative pressure zone.on the whole,the pressure balance is achieved.The maximum velocity distribution on the bearing liquid film increases with the increase of eccentricity and does not fluctuate obviously with the change of medium temperature.The load of the bearing liquid film increases with the decrease of the temperature of the molten salt medium,with the increase of the eccentricity,the load gap of each medium increases,the Sommerfeld number（So）decreases,and the bearing capacity of the bearing decreases.The relevant simulation results can provide theoretical support for rotor design.（4）Based on the simulation analysis of whether the high temperature molten salt pump rotor system is prestressed or not and the modes under different shafting lengths,the influence of various factors on the modal characteristics of the rotor structure is discussed.the safe operation range of the rotor system is determined by critical speed analysis.The simulation results show that when the shafting length is equal,the natural frequency of the rotor system tends to increase with the increase of order;under the same order,the natural frequency with prestress is generally higher than that without prestress,no matter whether prestress is applied or not.The amplitudes of each order are consistent to a certain extent.The overall trend of the inherent frequency of each order of the shafting decreases with the increase of the length of the rotor shafting.Under different conditions,there is a big gap between the critical speed of each order of the rotor and the running speed of the rotor system,and the natural frequencies of each order are not in the danger range of130-159 Hz and 261-319 Hz where the blade frequency is 1 and 2 times the blade frequency.It is considered that the design of the rotor system of each length meets the requirement of critical speed and avoids the resonance phenomenon of the rotor system during operation.