Flow and turbulence in the discharge stream of different agitators for stirred tanks

University dissertation from Sveriges Lantbruksuniversitet

Abstract: In this thesis, the fluid dynamic properties of the conventional Rushton turbine and another impeller type, the Scaba Radial flow Gas dispersing Turbine (with six concave blades) are compared. For comparison, point-wise velocity measurements were made in the discharge of the two turbine types using Hot-Wire Anemometry in constant temperature mode (CTA) with a split film probe. For the time-averaged flow and turbulent properties, good agreement was found for the turbines using both a representation of data based on an analogy to a jet, and the power input. These comparisons were not available when a larger 6SRGT was studied, due to the presence of the axial pumping impellers. These turbines lift the radial jet in the 6SRGT discharge, making the tangential velocity in the impeller centreline significantly lower. Because of the effect of the rotating blades, the measured time series is considered to be the superposition of the periodic (pseudo-turbulent) and the random turbulent signal. These two parts were separated by eliminating or retaining the associated frequencies in the Fourier spectrum. Using plane projections of the phase portraits of the deterministic part, the dynamics of pseudo-turbulence were found to be very similar for the 6SRGT turbine regardless of the diameter or agitation rate. Somewhat slightly different projections were obtained for the 6RT turbine. From the profile measurement, the lack of trailing vortices was concluded in the case of the 6SRGT turbines, and the turbulence was found to be characterised by the same integral length scales and Eulerian time scales in both cases at pilot scale.

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