Engineering Applications of Computational Fluid Mechanics Vol. 5, No. 3, pp. 416–429 (2011) Received:1 Dec. 2010; Revised:19 Apr. 2011; Accepted:25 Apr. 2011 416 FLOW CHARACTERISTICS IN MIXERS AGITATED BY HELICAL RIBBON BLADE IMPELLER Yeng-Yung Tsui* and Yu-Chang Hu Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Chinese Taipei * E-Mail: yytsui@mail.nctu.edu.tw (Corresponding Author) ABSTRACT: The main concern of this study is to investigate the flow mixing generated by helical ribbon blade impellers and to show that with the help of CFD the performance of the mixing system can be significantly improved by optimizing the geometric configuration of the impeller. To fulfill this objective, a numerical model is developed to solve the Navier-Stokes equations for the flow field. However, difficulties arise due to the rotation of the impeller in the vessel. In order to ease the problem, the velocity field is assumed to be in a quasi-steady state and the multiframe of reference is adopted to tackle the rotation of the impeller. For discretization the fully conservative finite volume method, together with unstructured grid technology, is incorporated. It is shown that the flow in the mixer can be regarded as a flow in an open channel with a wall moving at an angle with respect to the channel. The influences of the blade pitch, the blade width, and the clearance gap between the blade and the surrounding wall are examined. The mechanism to cause these effects is delineated in detail. It is demonstrated that after optimization of the blade geometry, the circulating flow rate induced by the impeller is largely increased, leading to significant reduction in mixing time. In addition, the power demand is reduced. It is also evidenced that by enlarging the clearance, it is difficult for the fluid in this region to be mixed. Keywords: mixing flows, stirred mixers, helical ribbon blade impellers, multiframe of reference, unstructured- grid methods 1. INTRODUCTION The mixing of fluids is a common operation encountered in productions of polymer, food, paint, and greases, to name a few. Poor mixing may result in formation of dead zones, hot spots, and temperature and concentration gradients, which will affect the quality of the final products. The selection of mixing systems depends on operating conditions such as agitation speeds and fluid properties. When the viscosity of the fluid is low, the rotational speed of the agitator can be high enough to produce turbulent flows. Most of these systems involve the use of turbine impellers such as Rushton turbines or pitched blades. For highly viscous liquids, the flow is more likely in the laminar regime because, otherwise, an extremely high demand of power is required. The use of small turbine impellers becomes inefficient as stagnant zones may be formed in the region at far distance from the impeller. To obtain adequate mixing under laminar flow conditions, close- clearance impellers are usually adopted. Impellers such as anchors, gates, or paddle impellers, which produce mainly circumferential flow, perform poorly in mixing because of lack of axial flow to sweep through the entire vessel. In an agitating system with helical ribbon impellers, mixing proceeds first in the region near the blades and the vessel wall where the fluid is subject to high shear strains. Fluid homogenization is then fulfilled by the axial vortex flow induced by the rotation of the ribbon impeller. It has been shown that this kind of impeller is very effective in mixing high viscous fluids (Gray, 1963). To characterize performance of mixing systems two parameters are usually adopted: the power number and the mixing time. One kind of power number is defined in terms of viscosity  as * 2 3 / p N P ND   , where D is the diameter of the impeller, N the rotational speed and P the power consumption. A more common definition of the power number is 3 5 / P N P ND   . These two dimensionless numbers are related by * Re p p N N  (1) where 2 Re / ND    is the Reynolds number. For low-Reynolds number flows the power consumption of the agitator is proportional to the square of the rotational speed. As a consequence, * p N is independent of rotational