Enhanced mixing of Newtonian fluids in a stirred vessel using impeller speed modulation

Abstract

This paper reports on an experimental study of mixing intensification using speed modulation of a six-blade Rushton turbine in a stirred vessel. Mixing times were measured using a non-intrusive technique based on direct visualisation of an acid-base reaction in a Newtonian fluid. The impeller speed modulation was achieved by using two waveforms: a square wave and a sine wave. The amplitude was fixed between a maximum Reynolds number of Re max = 60 and minimum Reynolds numbers of Re min = 40 or 30. The wave periods were varied (10, 20, or 40 s) in order to compare the effects of unsteady stirring on mixing performance. It was observed that a square wave protocol with the shortest wave period and the larger amplitude resulted in the shortest time to destroy the observed isolated mixing regions (IMRs), which are known to exist in stirred vessels operating at low Reynolds number. However, the sine wave protocol led to a slow diffusive mechanism in which IMR structures reached an asymptotic volume and remained visible even after several hours. The results are presented and discussed using digital photographs taken at different time intervals during experimentation.Ce papier présente uneétude experimentale concernant l'intensification du mélange en modulant la vitesse d'une turbine de type Rushton a six palettes dans une cuve agitée. Les temps de mélanges sont mesurés avec une technique non-intrusive basée sur la visualisation directe d'une réaction acide-base au sein d'un fluide Newtonien. La modulation de la vitesse de la turbine aété realisée en utilisant deux formes d'ondes: une onde carrée et une onde sinusoidale. L'amplitude de chaque onde aété fixée entre un nombre de Reynolds maximal de Re max = 60 et un nombre de Reynolds minimal de Re min = 40 ou 30. Les périodes des ondesétaient variées (10, 20 ou 40 s) dans le but de comparer les effets du mélange non-stationnaire sur les performances du systeme. Il aété observé que le protocole suivant l'onde carrée avec la periode la plus courte et l'amplitude la plus large produisait le temps le plus court necessaire pour la destruction des zones de mélanges isolées (IMRs), dont l'éxistense est connue dans les cuves a mélange opérant aux nombres de Reynolds bas. Toutefois, le protocol suivant une onde sinusoidale a conduit a un mechanisme de diffusion lent dans lequel les IMRs atteignies un volume symptotique et sont restées visibles après plusieurs heures. Les résultats sont présentés and discutés en utilisant des photographes digitales prisesà des intervales de temps differents durant leséxperiences.

Flow regimes in the mixing of municipal sludge simulant using submerged, recirculating jets

“…The tank is situated in a larger cubical tank (B), with the gap between the two filled with water. This is to reduce the effects of optical distortion due to the curvature of the cylindrical tank in keeping with established methodology for flow visualization experiments [23]. The liquid was drawn from the tank through a glass pipe of diameter d s = 10 mm placed at the edge of the tank with the help of a peristaltic pump (C) (Masterflex Ò L/S Ò , Cole Parmer, USA) with a variable speed drive.…”

“…The flow visualization procedure follows that used previously [23] and involves increasing the pH of the liquid body using 15 ml of 1 M NaOH, and adding a fluorescence dye until a light green colour is observed. The vessel is then stirred mechanically to ensure uniform pH and distribution of the dye.…”

Reduced IMRs in a mixing tank via agitation improvement

“…Yao et al (1998) also introduced a similar method using unsteady RPM as an effective method in destroying the IMRs in low Reynolds numbers. Other methods associated with the elimination of IMRs include large object insertion (Takahashi and Motoda, 2009), novel perturbation waveforms (Yek et al, 2009) and installation of eccentricstirred impellers (Cabaret et al, 2008;Takahashi et al, 2012).…”

Enhanced Micromixing of Non-Newtonian Fluids by a Novel Zigzag Punched Impeller

“…In order to enhance mixing under laminar condition, many works have been reported about the advantage of utilizing unsteady impeller motion. − Yao et al adopted two unsteady stirring approaches, coreverse periodic rotation and time-periodic variation of rotational speed, to enhance global mixing in a stirred tank with highly viscous fluid. Nomura et al also revealed that coreverse rotation of the impeller prevented the formation of a segregated mixing region and reduced mixing time.…”