Analysis Of Solid-liquid Two-phase Flow And Structural Dynamic Characteristics For The Molten-salt Pump

The molten-salt pump is used to transport high-temperature membrane caustic soda,carbonate,nitrate,phthalic anhydride and other mediums in industrial loops.The temperature of the medium in the molten-salt pump is generally above 400 °C.Chloride ions in the molten salt impose a strong corrosion effect on various steels such as the stainless steel;some carbonates in molten salt will decompose into metal oxides and peroxides due to high temperature.These metal oxides will travel with molten salt in the form of solid particles and might transform the flow in the molten-salt pump into solid-liquid two-phase flows.The presence of solid particles not only affects the heat exchange efficiency of the medium,but also causes wear or even damage to the components of the molten-salt pump,aggravating the structural vibration induced by unsteady flows in the pump.Therefore,investigation of the solid-liquid flow mechanism and structural dynamic characteristics related to the molten-salt pump is greatly necessitated.In this dissertation,solid-liquid two-phase flows in a molten-salt pump were studied using theoretical analysis and numerical simulation.The influence of solid particle parameters on flow characteristics of the pump was explained,and the structural dynamic characteristics of the pump were analyzed as well.Main work and the conclusions obtained are as follows:(1)Steady flows in the molten-salt pump for different solid-phase parameters were simulated using commercial computational fluid dynamics(CFD)code ANSYS CFX,and the distributions of the concentration and the relative velocity of the solid phase near the impeller shroud,hub and blades were described.The results show that the range covered by the solid phase over the impeller shroud and hub is narrowed with the increase in the particle diameter.The solid phase concentration on the impeller shroud and hub increases apparently with the increase in the particle volume fraction.The variation of the relative velocity of the solid phase near the impeller shroud and the hub is not significant for different particle diameters and volume fractions.The solid phase concentration increases gradually with the increase in the particle diameter and the volume fraction,and the concentration of solid phase in the rear part of the suction surface is higher than that associated with the pressure surface.From blade inlet to blade outlet,the solid-phase concentration decreases firstly and then increases.The maximum relative velocity of the solid phase arises at the outlet of the blade pressure under all operation conditions.In general,the most serious wear in the impeller appears at the inlet of blade pressure surface and the downstream area of suction surface.(2)Unsteady flows in the molten-salt pump for different solid phase parameters were simulated.Distributions of static pressure under different operation conditions were obtained.Meanwhile,the effects of particle diameters and the volume fraction of the solid phase on temporal and spectral characteristics of pressure fluctuations were analyzed.The results indicate that static pressure increases gradually with the increase in the particle volume fraction,and the change of the particle diameter has a slight influence on the static pressure distribution in the molten-salt pump.The pressure fluctuations at each monitored point deployed in the pump volute exhibit periodic features,which are in accordance with the blade number.The pressure fluctuation in the molten-salt pump is mainly affected by the blade passing frequency and its second harmonic frequency;at the blade passing frequency,the pressure fluctuation amplitude near the volute tongue is considerably higher than its counterparts at other positions.The influence of the shaft frequency surfaces as the particle diameter and the volume fraction of the solid phase increase consistently.(3)Using the one-way thermal-fluid-solid coupling method,the structural dynamic characteristics of the pump rotor were studied.The stress and deformation of the impeller and the pump shaft for different particle parameters were examined;and the modal characteristics of the rotor were analyzed.The results demonstrate that the influence of temperature deserves a full consideration during the pump design in view of high temperature gradients arising at both ends of the rotor.The maximum equivalent stress on the impeller and the shaft increases gradually with the increase in the solid-phase volume fraction,and stress concentration is witnessed at blade outlet.The maximum deformation of the pump rotor takes place in the impeller,as is shared by each operation condition.The change in particle parameters slightly influences the deformation of the rotor.The structure strength of the pump rotor is verified under each operating condition.The deformation of the pump rotor is concentrated on the impeller,which is determined by the supporting manner and operation characteristics of the pump rotor.Natural frequencies corresponding to each mode of the pump rotor deviate considerably from the blade passing frequency and its second harmonic frequency as well.Therefore,the resonance caused by hydraulic excitations will not occur for the molten-salt pump considered.