A Digital Particle Image Velocimetry Investigation of Flow Field Instabilities of Axial-Flow Impellers

Abstract

Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched-blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well below the blade passage frequency (periods ranging from 40 to over 300 impeller revolutions have been observed). This result has important implications for computational modeling because current descriptions of agitated vessels are based upon time-averaged flow fields with superimposed turbulence. This modeling approach may not accurately capture the mixing associated with the low-frequency phenomena observed in this study.

Flow Instabilities in Mechanically Agitated Stirred Vessels

“…In addition they noticed that the fraction of the total kinetic energy carried by the flow instabilities (relative magnitude) varied with the location inside the stirred vessel, they being stronger in the central and wall regions below the impeller but weaker in the discharge flow from the impeller. Myers et al (1997) used digital PIV to investigate flow instabilities in a stirred tank equipped with two different impellers: a 4-bladed 45° PBT of D = 0.35T and a Chemineer HE-3 of D = 0.39T. The PBT was set at C = 0.46T and 0.33T, whereas the Chemineer HE-3 was set at C = 0.33T.…”

PIV measurements of flow in an aerated tank stirred by a down- and an up-pumping axial flow impeller

“…In the turbulent flow regime, Sheng et al [17] used PIV to determine the mean flow field and turbulence characteristics produced by a down-pumping 4-bladed pitched blade turbine and compared these results with imposed boundary condition CFD simulations. Meyers et al [18] investigated flow instabilities induced by a down pumping 4-bladed pitched blade turbine and a down-pumping HE-3 (Chemineer), and compared these instantaneous flow fields with timeaveraged data. Gas-liquid flows in agitated vessels provide additional complications with respect to data acquisition and processing, and therefore published work in this area is very scarce.…”

Investigation of Horizontal Velocity Fields in Stirred Vessels with Helical Coils by PIV

“…As a nonintrusive measuring technology to investigate complex flow fields, the application of PIV in science and industry gained significant importance during the past two decades [10]. PIV-studies on the flow field for a pitched blade turbine have been conducted by Sheng et al [11], Myers et al [12], and Bakker et al [13], for the Rushton turbine by Hill et al [14], Sharp and Adrian [15], Deen and Hjertager [16], Hill et al [17], and Wang et al [18], with increasing focus on the small scale flow around the impeller. Therefore, Atibeni et al [19] studied the adverse effect of inserts in stirred vessels by use of 2D-PIV and showed the applicability of the technique for investigating flow fields in stirred vessels.…”