Flow Field Characteristics In A Stirred Vessel With Flexible Blades

The flow field in the stirred vessel is uneven and randomly changes.If the blade stiffness is not enough(flexible blades),the deformation is easy to occur under the action of unstable flow field.This paper mainly studies the influence of blade deformation on the flow field.In this paper,super-elastic nickel-titanium alloy is used to make easily deformable stirring impeller,and its high-speed camera is used to capture the vibration characteristics of its blade.According to the profile of the flexible blade at rest and equilibrium position,the flat-rigid impeller and the curved-rigid impeller are respectively made of tungsten steel and spring steel.High-frequency two-dimensional particle image velocimetry(PIV)phase analysis is used to measure the flow field characteristics of three kinds of impellers in the same rotation speed(210 rpm),such as flow velocity,wake vortex,pulsation and turbulent kinetic energy.The results show that the mean flow velocity of the flexible impeller is larger and the turbulent diffusion intensity is higher.The vibration of the flexible impeller leads to the radial position of the trailing vortex more outward than the rigid propeller,and the three-direction velocity pulsation of the flexible propeller is higher than that of the rigid curved-rigid impeller.In order to verify the applicability of the isotropic assumption of the flow field in the stirred tank to the flexible impeller,the picture areas of PIV are placed vertically and horizontally separately and under the same experimental conditions(210 rpm,with the same blade).The real value of the flow field characteristics obtained by the three velocity components at the coincident straight line of the two picture areas in the flow field is compared with the assumed value of the flow field characteristics obtained by the isotropic assumption.It was found that the true value and the assumed value of the turbulent kinetic energy of the flexible impeller are closer than those of the rigid-curved impeller,but the difference between the two impellers is very small,so the isotropic assumption is also applicable to the flexible impeller.High-speed cameras and PIV are used to simultaneously photograph the vibration and flow fields of the flexible impeller.The PIV data are grouped according to different blade shapes or blade amplitudes,and the resulting flow field characteristics are compared.It was found that the area of the high speed zone(Vel/Vtip?0.65)is positively related to the amount of blade deformation and negatively related to the amplitude.When the blade is located near the mean deformation and medium amplitude,the velocity distribution,turbulent kinetic energy and wake vortex distribution are closest to the overall phase-analysis distribution.With the increase of the deformation,the radial distribution and radial distribution of the core area of the upper and lower wake vortexes increase in turn.The area of the upper high flow energy area gradually increases,and the area of the lower high flow energy area gradually decreases.With the increase of the amplitude,the radial distribution range of the wake vortex and the distance between the upper and lower vortexes are larger,and the area of the high flow energy area is also larger.