The mean flow field generated by a pitched blade turbine: Changes in the circulation pattern due to impeller geometry

Abstract

The mean flow field in a tank stirred with a pitched blade turbine was measured using a two-component Laser Doppler Anemometer system (LDA). The effects of impeller clearances and impeller geometries (number of blades, blade angle and blade size) on the mean flow field have been studied. The primary pumping number, induced pumping number of the primary circulation loop and the induced pumping number of the secondary circulation loop, which often has been ignored, are reported. The flow patterns and circulation loops are more complex than those traditional ones, which vary with the geometries of the PTD and the clearances.On a mesure le champ d'ecoulement moyen dans un reservoir agite muni d'une turbine a pales inclinees a I'aide d'un anenometre laser Doppler bicomposantes. Les effets du degagement de la turbine par rapport aux parois et de la geometrie de la turbine (nombre, angle et dimension des pales) sur le champ d'ecoulement moyen ont ete etudies. Le nombre de pompage primaire, le nombre de pompage induit de la boucle de circulation primaire ainsi que le nombre de pompage induit de la boucle de circulation secondaire, qui sont souvent ignores, sont presentes ici. Les profils d'ecoulement et les boucles de circulation sont plus complexes que ceux des cas classiques, qui varient selon la geometrie de la turbine a pales inclinees (TPI) et les degagements par rapport aux parois.Keywords: stirred tank reactors, PTD, flow pattern, pumping number, Laser Doppler Anemometry.echanically agitated reactors are very versatile and M are commonly used in practically all the small and large scale chemical process industries. The stirred tank is almost our 'tool-of-all-work' (Villermaux, 1993), and there are several functions of mechanical agitation such as solid suspension, gas-liquid dispersion, liqui&liquid dispersion, gas-liquid mass transfer, solid-liquid mass transfer and liquidliquid mass transfer. They are achieved by introducing mechanical energy into the tank with the aid of a rotating impeller and converting this energy into hydrodynamic motion. Therefore, detailed understanding of the hydrodynamics of the tanks (velocities and stress fields, turbulence characteristics, etc.) is essential for the confident design of stirred tanks. The characteristics of the flow strongly depend on the types and geometries of the impellers. Recently, pitched blade downflow type turbines (PTD) have been found to be more efficient with respect to liquid phase mixing and solid suspension than other impellers (Pandit and Joshi, 1983;Raghav Rao et al., 1988). The aim of this work is to study the three dimensional, circulating turbulent flow field formed by an impeller in order to be able to predict the flow field formed by a given impeller in the new geometric configuration, and to be able to characterize the important features of the flow field in all parts of the tank. Previous workMost of the available data about flow in stirred tanks is for the standard disc turbine; information for pitched blade turbines has only appeared recently ...

Analysis of the Flow Agitated by Disc Impellers with Pitched Blades

“…For a pitched-blade turbine with diameter D ¼ T=3 the experimental study of Jaworski et al [4] showed that when the offbottom clearance changes from T=4 to T=2, the strength of the dominant circulation loop decreases and a secondary circulation loop rotating in the opposite direction of the main one forms in the bottom region. A similar observation was reported by Mao et al [5] that with a D ¼ T=2 pitched-blade turbine located at clearance C ¼ T=2 a secondary loop is clearly visible in the bottom region. However, as the clearance or the impeller is reduced to T=3 only one circulation loop prevails.…”

Flow Instabilities in Mechanically Agitated Stirred Vessels

“…Rutherford et al (1996a) investigated the flow pattern generated by a dual Rushton impeller and observed different circulation patterns depending on the impeller clearance of the lower impeller and the separations between the two impellers, observing three stable flow patterns: "parallel flow", "merging flow" and "diverging flow" patterns. Mao et al (1997) measured with LDA the flow pattern generated from various PBT of different sizes in the range of 0.32 < D/T < 0.6 and number of blades varying from 2 to 6 in a stirred vessel in turbulent regime (Re > 20,000). They used two impeller off-bottom clearances, C = T/3 and C = T/2, observing a secondary circulation loop with the higher clearance.…”

Experimental Study of Influence of the Impeller Spacing on Flow Behaviour of Double‐Impeller Stirred Tank

“…These studies involved not only the flows with simple agitators and at low Reynolds numbers (Smith et al, 1990;Lu et al, 1995), but also the flows with complex agitators and at high Reynolds numbers (Derksen et al, 1999;Myers et al, 1999;Bittorf et al, 2000). On the other hand, experimental methods have also been widely adopted to study the flow in the stirred tank (Li et al, 1996;Mao et al, 1998, Schäfer et al, 1998Bittorf et al, 2000Bittorf et al, , 2001 and the results have been used to improve the agitator design and scale-up design in engineering applications.…”