Research On The Internal Flow And Running Stability In A High Temperature Molten Salt Pump For Concentrating Solar Power Plant

The high temperature molten salt pump?MSP?for concentrating solar power?CSP?plant is a kind of long shaft liquid pump,used to transport molten salt with temperature up to 400?to 700?.As a core equipment of CSP plant,it affects the operational reliability of the thermal storage island,but mainly depends on import currently.The internal unsteady flow is complicated in MSP with multi-stage vertical structure,it is noted that the pressure pulsations and hydraulic excitation force induced by unsteady flow would lead to the pump body vibration in large.The pump shaft belongs to the slender rotor with L/D>100,and it cannot be ignored that influence on the running stability in terms of critical speed and transient excitation.In addition,the heat of high temperature medium delivered by the pump is transferred to the upper bearing along the shaft and solid structure,which can lead to serious mechanical failure,such as lubrication failure and shaft seizure accident.Therefore,it is necessary to study the internal flow mechanism and running stability of the high temperature molten salt pump,and the research results can provide reference for the hydraulic and structural optimization of the pump.In this paper,a hot salt pump used in a 100MW CSP plant in China is chosen as the research object.The internal unsteady flow and its excitation characteristics,pump rotor critical speed and transient excitation,bearing cooling and its reliability,have been researched and analyzed.The main work and conclusions are as follows:1.According to the actual operation requirements of the CSP plant,the hydraulic design of the two-stage impellers and diffusers of the MSP was carried out to obtain the main geometric parameters,and the structural of the pump rotor and its support system,the upper bearing cooling system were optimally designed.Finally,the model prototype was obtained to meet the requirements of the performance.2.Based on the RNG k-?turbulent model,the internal flow field of the MSP was numerically calculated,the pressure pulsations and radial force pulsations resulting from rotor/stator interaction?RSI?and other complex flow structures were discussed,and the unsteady vortex structures motion and development process captured by the vortex recognition technique were analyzed and correlated with pressure pulsations.The results show that rotor/stator interaction?RSI?stimulate blade passing frequency(f_BPF)and diffuser passing frequency(f_DPF)signals,which dominate in pressure pulsation spectra of diffusers and the impellers respectively.The pressure pulsation amplitude varies with the operating conditions and the positions in the pump,and the low-frequency peak signals in the secondary diffuser are more prominent under low flow rate condition.There are multiple evolutions of unsteady vortex structures in diffusers captured by Q-criterion,including trailing edge shedding vortex?2³f _R?,suction surface shedding vortex?3f_R?and impacting vortex?f_R?.Correlation analysis shows that the pressure pulsations amplitude of the vortex evolution core region increase significantly,where frequency is consistent with the vortex shedding frequency.It is indicatied that the periodic evolution of the vortex structures is a important excitation source of the pressure pulsations except RSI in the pump.3.In order to explore the operational stability of the slender shaft rotor of the high temperature molten salt pump,the rotating parts were modeled and assembled.Based on the Workbench and Comsol Multiphysics platforms,the critical speed of the rotor under different support stiffness conditions was calculated,and the transient response characteristics were analyzed when rotor is start-up and affected by fluid force.The results show that the critical speed of the rotor is more than 1.5 times of the working speed,so there is no resonance phenomenon in the range of start-up to working speed.The guide bearing can be equivalent to rigid support when the support stiffness value is greater than 1×10⁹N/m.In the start-up process,the displacement of the nut and the first-stage impeller is much higher than in other positions,and its maximum displacement is also increased linearly with increasing the eccentric mass of the rotor.As the stiffness of guide bearings increasing in the range of?1 to 10?1×10⁷N/m,the maximum response displacement is exponentially reduced.When considering fluid excitation force,the displacement of the first-stage impeller is an order of magnitude larger than that of the secondary impeller.However,the transient response caused by impeller radial force is minimal and the effect on rotor stability is negligible.4.Based on Workbench and Fluent platforms,a heat-fluid-solid coupling analysis was conducted on the upper bearing cooling system.It was found that there is a strong convective heat exchange along channel between air and solid wall under the forced drive of the wind turbine,the temperature gradually increased.The effect of the air inlet-velocity on heat exchange in a certain range,not the greater the velocity,the heat exchange effect is better.The upper bearing temperature is more affected by the medium transfer heat than its own frictional heat.The critical inlet-velocity of bearing temperature less than 80?is proposed by interpolation method.According to the fourth strength theory,the strength requirements are satisfied by the verification of the equivalent stress and deformation of the solid domain.