An experimental study of double‐to‐single‐loop transition in stirred vessels

Abstract

The velocity characteristics of the flows in a fully baffled vessel of diameter T = 290 mm stirred by a Rushton impeller of' diameter D = TI3 were investigated by means of laser-Doppler anemometry measurements. The effects of clearance and rotational speed on the flow patterns in the vessel were studied. It was found that at impeller clearances from the bottom of the vessel (C) around 0.2 Tthe characteristic double-loop flow pattern undergoes a transition to a single-loop one with the impeller stream direction becoming partly axial and being inclined at around 25 to 30" to the horizontal. The impeller stream inclination varied with radial distance from the impeller, as well as with angular position between blades (blade angle). Impeller speed was found to have no effect on the flow pattern or the mean velocities and turbulence levels normalized by V,iR for C/T> 0.20 or CIT 20.15. The flow structure measured with C = 0.1ST is described in detail and the implications of the data for fluid mixing in stirred vessels are discussed. luid motion is of paramount importance for the mixing F processes in stirred tanks but, notwithstanding their frequent use in process plants, the design of stirred reactor units is most often based on empirical knowledge and "rules of thumb" and may be subject to substantial approximations and uncertainties which cause financial losses estimated in the region of a billion dollars per annum in the USA alone (Tatterson, 1994). This is particularly true when mixing of multiphase systems is involved; for instance, in solid-liquid processes, suspension of the solids from the bottom of the vessel is normally required and impeller selection must reflect this. Axial flow impellers are normally used to lift solids off the vessel bottom, but the effects of impeller clearance and rotational speed on the mean flow pattern must be known a priori if efficient suspension is to be achieved. This is complicated, however, by quantitative and qualitative differences in the mean flow and turbulence fields which are sometimes observed with changes in the impeller speed andlor clearance, even with commonly-employed impellers such as the Rushton and the pitched blade turbine.Impeller Reynolds number or speed may affect the circulation pattern in stirred vessels and such flow changes have been previously reported for pitched blade turbines (PBTs) by Nouri and Whitelaw (1990) and Hockey (1990), who observed that the impeller stream with 45" and 60" PBTs changed direction with increasing Re from primarily radial to primarily axial flow at Reynolds numbers of 490 and 650, *Author to whom correspondence may be addressed. E-mail address: observed that the downward flowing jet from a PBT depends strongly on clearance and that impeller location is one of the critical factors for agitator design.

Flow Instabilities in Mechanically Agitated Stirred Vessels

“…They used two impeller off-bottom clearances, C = T/3 and C = T/2, observing a secondary circulation loop with the higher clearance. Montante et al (1999) provided a detailed investigation of the flow field generated by D = T/3 RT placed at different off-bottom clearances varying from C = 0.12T to C = 0.33T. They found that the conventional radial flow pattern (termed "double-loop" pattern) occurred for C = 0.20T, but it was replaced by an axial flow pattern (termed "single loop" pattern) as the clearance was decreased to C = 0.15T.…”

Dense solid–liquid off-bottom suspension dynamics: Simulation and experiment

“…A standard six bladed Rushton turbine was used in the suspension experiments. A close off-bottom impeller clearance was chosen in order to ensure a "single-loop" flow configuration [12]. The agitator speed here investigated (380 rpm) was sufficient to get fully suspended conditions at steady state.…”

CFD modeling of pilot-scale pump-mixer: Single-phase head and power characteristics

“…Impeller off-bottom clearance is an important geometric parameter and its effect on flow patterns and power consumption in batch stirred tanks has been a subject of several experimental and computational investigations (Nienow, 1968;Conti et al, 1981;Ibrahim and Nienow, 1995;Armenante and Nagamire, 1998;Montante et al, 1999Montante et al, , 2001a. The most important finding of these studies is that below a certain value of clearance even a radial flow impeller behaves like an axial flow impeller.…”