Study Of Fluid Structure Interaction With Flexible Stirring Structures In Stirred Vessels

The flow field in stirred vessels timely exhibits macro-instabilities with low frequency,periodicity with multiples of speed frequency,turbulence with high frequency.And it also exhibits main circulations in the bulk flow,trailing vortices behind the impeller blades,and turbulent eddies in space.The unstable and inhomogeneous flow field due to the spatiotemporal instability exerts unstable reactive loads on the structures such as the stirring shaft,impeller blades,baffles,and vessel wall,leading to the structures instantaneous deformation,which in turn disturbs the surrounding fluid motion,enhancing the inhomogeneity of flow field.Such interaction between the fluid and the structures is called fluid structure interaction(FSI).The research in this thesis includes the FSI of the lateral vibration of shaft and the normal vibration of impeller blades.For the FSI of the lateral vibration of shaft,we focused on the moment(torque and bending moment)acting on the overhung shaft,because the bending moment is a key factor for the design and strength analysis of the shaft.The shaft moment generated by radial disk turbines with different blade curvature(Rushton turbines,RT;half circular blades disk turbine,CD;half elliptical blades disk turbine,HEDT;and parabolic blades disk turbine,PDT)and the pitched blade turbine under different operation methods(up-and down-pumping configurations)in a gas-liquid stirred vessel was experimentally measured by a moment sensor.The results show that the decline degree of relative power demand(RPD)decreases with the increase of blade curvature for the disk turbines.The amplitude distribution of torque is more dispersive with the increase of blade curvature and gas flow rate.Under the situation without resonance,the relative shaft bending moment(RMB)manifests a three-staged variation(rising-falling-rising)over gas flow number for the four disk turbines.The shaft bending moment amplitude for the four disk turbines can be fitted by an asymmetric Weibull distribution,which is attributed to the asymmetric shaft lateral deflection because of the shaft elastic resilience.The gas loading ways for pitched blade turbine under up-and down-pumping configurations result to the different moment characteristics.The RPD and RMB both decrease with the increase of gas flow number under up-pumping configuration,but manifest "falling-falling-rising" and"rising-falling-rising" over gas flow number,respectively under down-pumping configuration.The structural and fluid components of the loads acting on the shaft and impeller cannot be measured directly.The FSI in the stirred vessel agitated by a pitched blade turbine was simulated by CFD and CSD.The bending moment acting on the shaft and the lateral force,axial force,and bending moment acting on the impeller were obtained,and the fluid and structural components of the loads were separated and extracted.The results show that the impeller mass imbalance increases the shaft bending moment and the lateral force on the impeller and their structural components,but has little influence on the axial force and bending moment on the impeller.The dimensionless fluid components of the four loads are rarely influenced by the impeller mass imbalance and the rotation speed.The shaft bending moment mainly results from the impeller lateral force.For the FSI of the normal vibration of impeller blades,we focused on the characteristics of blades vibration and the effect of it on the shaft moment and flow field.The vibration of flexible blades made of hyperelastic nitinol alloy was recorded by a high speed camera.The corresponding flat-rigid and curved-rigid blades were made and the profiles were same as the flexible blades at rest and the equilibrium position,respectively.The results show that under the same rotation speed,the torque and power is highest for the flat-rigid impeller,followed by the flexible impeller and lowest for the curved-rigid impeller.The shaft bending moment is almost the same for the flexible and curved-rigid impeller and is lower than that of the flat-rigid impeller.The flow fields agitated by the above three impellers under a same rotation speed were experimentally measured by 2D particle image velocimetry(PIV).Compared with the flat-rigid blades,the curved-rigid blades with curvature weaken the radial flow in the region far away from the blades,while the flexible blades with normal vibration enhance the radial flow near the blade tip.The calculated turbulent kinetic energy(TKE)by the pseudo-isotropic assumption based on the fluctuating radial and axial velocities is slightly higher than that by the three fluctuating velocities for the flexible and curved-rigid impellers,and the difference between the two calculations is smaller for the former impeller.Compared with the curved-rigid blades,the flexible blades with normal vibration not only consume more power and produce higher mean TKE in the vessel,but also disturbs the periodic motion of the vortices behinds the blades,and the trailing vortices and high TKE region is further from the blades in radial direction,enhancing TKE transport from the blades to the bulk region of the vessel.However,the enhance effect on the mean TKE by blades vibration is weakened under the same power consumption.The synchronous measurements of the flexible blades vibration and the corresponding flow field were also performed.The results show that with the rise of blade deformation,the flow discharge angle is increased and the flow velocity in the discharge direction is enhanced.With the rise of vibration amplitude,the vortices and mean velocity hardly changes,but the flow turbulence is enhanced.The flow fields agitated by the three impellers under a same rotation speed were experimentally measured by volumetric PIV,and the obtained velocity agrees well with that by the stereo-PIV;however,the TKE is lower than that by the stereo-PIV.The 3D velocity isosurface expands in the tangential direction from the low phase to the high phase and in the radial direction from inside to outside.The fluctuating tangential velocity is higher than the fluctuating radial and axial velocities near 20°;but the result is opposite near 0° and 50°.The TKE is high in the interaction area of the two vortices and the isosurface expands in the tangential direction from the low phase to the high phase and in the radial direction from inside to outside.