Volumetric three-component velocimetry measurements of the turbulent flow around a Rushton turbine

Abstract

Volumetric 3-component velocimetry measurements have been taken of the flow field near a Rushton turbine in a stirred tank reactor. This highly unsteady, three-dimensional flowfield is characterized by a strong radial jet, large tank-scale ring vortices, and small-scale blade tip vortices. Approximately 15,000 3d vectors were obtained in a cubic volume; these data offer the most comprehensive view to date of this flow field, especially since they are acquired at three Reynolds numbers (15,000, 107,000, and 137,000). The fluid dynamics video shows various animations and combinations of the velocity and vorticity data. The volumetric nature of the data enable tip vortex identification, vortex trajectory analysis, and calculation of vortex strength. In the video, three identification methods for the vortices are compared based on: the calculation of circumferential vorticity; the calculation of local pressure minima via an eigenvalue approach ('λ 2 ') ; and the calculation of swirling strength again via an eigenvalue approach ('λ ci '). A 'swirl strength' criterion overall provides clearest identification of the tip vortices. The visualization of tip vortices up to 140 degrees past blade passage in the highest Reynolds number case is notable and has not previously been shown.The associated video can be viewed in mpeg-2 format and mpeg-1 format. The related article currently appears in the online first section

Study of fluid flow in baffled vessels stirred by a Rushton standard impeller

“…8 shows that the trailing vortices can be detected in a range up to 100 o . This is in an agreement with results of Sharp et al [26] who observed the trailing vortices beyond blade passage up to approximately 140 o . A comparison of our data with data of Sharp et al [26] is made in Fig.…”

“…A lot of areas which are evidently not related to the vortical structures of the trailing vortices can be seen. This is in relation with conclusions given by Sharp et al [26]. To overcome such inconsistencies we propose to use a threshold value of λ 2 which could remove these undesirable areas.…”

“…9. The comparison shows that the trailing vortices measured by Sharp et al [26] are closely to the impeller than our data. This can be explained by the fact, that Sharp et al [26] used an unbaffled cylindrical tank.…”

“…The comparison shows that the trailing vortices measured by Sharp et al [26] are closely to the impeller than our data. This can be explained by the fact, that Sharp et al [26] used an unbaffled cylindrical tank. The location of the trailing vortex for r * = 0.85 (behind the blade) was obtained from the PIV measurements performed in the vertical plane perpendicularly to the blades.…”

“…Using this method with only twodimensional data they obtained very similar results when compared with tip vortex identification based on three-dimensional data. Sharp et al [26] performed instantaneous volumetric measurements of the flow inside an unbaffled cylindrical tank driven by a Rushton impeller. They compared various methods of tip vortex identification including arbitrary cut-offs of normalized vorticity, identification of pressure minima and associated vortex cores based on an eigenvalue criterion for the pressure tensor (λ 2 < 0), calculation of swirl strength based on an eigenvalue criterion but associated with the velocity gradient matrix and calculation of the principal strain rates as measures of stretching and compression.…”