The present paper treats the agitation of shear thinning fluids in a cylindrical unbaffled vessel with a flat-bottom. Two kinds of impellers have been used: simple and double helical ribbons. This work is achieved with the help of CFD package (CFX-12.0), which is based on the finite volume method to solve the continuity and momentum equations. The effect of impeller rotational speed, fluid rheology, impeller size, impeller clearance from the tank bottom on the flow fields and power consumption have been investigated. Finally, a comparison between the two impellers is made. It was found that the best performance is obtained with double helical ribbon impeller placed at the middle of the tank, with moderate blade size and operating at Reg >  . Our results have been compared with70590

those of other literatures and a satisfactory agreement is observed.

Helical ribbon impeller • Shear thinning fluid • Numerical computation • Hydrodynamic • Power consumption • Stirred vessel

Introduction

Mixing is a dynamic process whose driving force is the pressure difference in various tank regions that is produced by a rotating impeller [1]. The aims of mixing can be different. They can include, for instance, the intensification of chemical reactions, production of a homogeneous mixture (emulsion or suspension) and enhancement of heat or mass transfer.

Mixing of highly viscous fluids is mainly carried out in the laminar regime. It is often associated with poor bulk mixing, an inhomogeneous distribution of the various phases to be

processed, and the presence of rheological complexities. In the laminar regime, mixing is obtained by a stretching- folding-breaking mechanism of the secondary phase and not by highly energetic eddies like in the turbulent regime, making the design of the mixer a challenging task.

To respond to the needs of industrial processes, various impellers have been developed and a number of works studied various impeller characteristics have been reported. Among the different impellers available, the helical ribbon is considered to be more efficient for the agitation of highly viscous liquids. In the literature, some works have devoted to this impeller.

Dieulot et al. [2] investigated the way of improving the mix- ing of highly viscous Newtonian fluids in a tank using suit-

 able operating conditions (unsteady stirring  approaches).

with an anchor at the bottom. They found that the use of

time-dependent rotational speed during the mixing pro- cess allows energy savings. For the unsteady stirring approaches tested, energy savings can reach up to 60% compared to the energy required to obtain the same mix- ing time with constant impeller rotational speed. They provided a model which allows the prediction of the mixing process of the agitated system with Newtonian fluids for both steady and unsteady stirring approaches.

Delaplace et al. [3] investigated, by experiments and CFD method, the heat transfer for several Newtonian and non- Newtonian liquids agitated in a rounded bottom vessel equipped with an atypical helical ribbon impeller.

Yao et al. [4] analyzed numerically the local and total dispersive mixing performance of large type impellers, a standard type of Maxblend and double helical ribbons impellers. The results indicated that a standard type of Maxblend has a satisfactory local dispersive mixing per- formance, especially in the grid region where the local dispersive mixing efficiency is high to near 1. However, when the Re is somewhat low, the total dispersive mixing performance is not as satisfactory as that operated under a moderate Re number. The double helical ribbon impeller can’t provide a promising local mixing performance, al- though it can induce a good total circulation throughout the stirring tank.

Rieger [5] showed the effect of the Reynolds number on the pumping efficiency of screw agitators for a wide range of Reynolds number values from creeping to the turbulent flow region.