The mean flow field produced by a 45° pitched blade turbine: Changes in the circulation pattern due to off bottom clearance

Abstract

The impeller discharge stream and bulk circulation flow produced by a pitched blade turbine (PBT) in a fully baffled stirred tank have been defined, using a combination of flow visualization and laser Doppler anemonietry measurement techniques. Two distinct bulk circulation patterns have been identified, one for higher off bottom clearances, and one for lower clearances. The circulation patterns are shown to have a substantial impact on the discharge stream, even very close to the edge of the impeller blades. These findings indicate that the action of the impeller is not independent of the tank geometry, but contains a substantial feedback component from the bulk flow. This conclusion has substantial implications for the modelling and prediction of stirred tank flow fields. ~ On a analysC le courant de dCcharge et I'kcoulement principal produits par une turbine a aubes inclinee dans un rCacteur agitC chicanes, en combinant des techniques de visualisation de I'Ccoulement et de mesure anCmomCtrique laser Doppler. Deux profils de circulation globale distincts ont CtC observCs, I'un pour des dCgagements dans le fond du reservoir importants, l'autre pour des petits dkgagements aux parois. On montre que les profils de circulation ont un effet marquC sur le courant de dCcharge, mime i proximiti des aubes de la turbine. Ces dkcouvertes indiquent que I'action de la turbine n'est pas indkpendante de la gComCtrie du rkservoir et qu'elle subit une influence non nkgligeable de l'kcoulement global. Cette conclusion a des implications importantes pour la modklisation et la prkdiction des champs d'kcoulement dans les rkservoirs agitks.Keywords: stirred tank, pitched blade impeller, LDA, flow visualization, refractive index correction. lthough stirred tanks in their various forms comprise 42

Flow characteristics of Newtonian and non-Newtonian fluids in a vessel stirred by a 60° pitched blade impeller

“…Investigations of [6] and [7] used 2-D and 3-D PIV techniques in mixing vessels stirred with 45°pitched blade and Rushton impellers, respectively, to estimate the turbulent dissipation rate and made comparison with previous estimated values made from single-point velocity measurement techniques. In particular, the former investigation showed that for a coarse spatial resolution of 5 mm the estimated rate of dissipation agrees well with that of the dimensional analysis employed by [5]. The latter study showed the feasibility of direct measurement of the dissipation distribution using both 2-D and 3-D PIV methods by measuring the mean velocities and Reynolds stresses simultaneously from which the dissipation rate was determined.…”

“…The results of [3] were more detailed and revealed that the velocities scaled with rotational speed and that the hydraulic efficiency increases with the blade pitch. The studies of [4][5] also used 45°pitched-blade impellers and showed the bulk flow patterns are different for low and high off-bottom clearances. Detailed time-resolved LDV measurements by [1] for a 60°pitched blade impeller revealed an axial jet which originated beneath the impeller and formed a large ring vortex covering up to two thirds of the vessel height, a small vortex was shed from the tip of each blade which decayed rapidly, a swirl velocity beneath the impeller proportional to radius and that the turbulence in the discharge region was in general anisotropy.…”

Flow Instabilities in Mechanically Agitated Stirred Vessels

“…For the higher clearance the impeller stream turned upwards before reaching the base of the vessel, generating also a reverse flow directed radially from the walls towards the vessel axis at the base of the vessel. Kresta & Wood (1993) measured the mean flow field of a vessel stirred with a 4-bladed 45° PBT for two impeller sizes, i.e. D = T/3 and D = T/2, and varying the impeller clearance systematically from T/20 up to T/2.…”

Macro-instability: a chaotic flow component in stirred tanks

“…The existence of MI has been confirmed by various experimental methods (e.g. Kresta & Wood 1993;Brůha et al 1996;Montes et al 1997;Bittorf & Kresta 2000;Nikiforaki et al 2003). The presence of the MI in the flow pattern is displayed by distinct peaks in the low-frequency part of the power spectrum of a signal measured in the tank.…”

“…This difference arises from the different structure and properties of fluid streams in these two regions (e.g. Kresta & Wood 1993). Figures 3 and 4 give more detailed views of the spatial distributions of the l max and d c values over the grid of the experimental points.…”