Experimental Investigation of Flow Regimes in an SMX Sulzer Static Mixer K. Hirech, A. Arhaliass, and J. Legrand* GEPEA-UMR CNRS 6144, University of Nantes, CRTT-IUT Saint Nazaire, BP 406, 44602 Saint Nazaire Cedex, France An experimental study of the flow pattern in the wall region of an SMX static mixer was performed by using electrochemical shear rate sensors. Electrochemical signals were related to the local shear stress and the fluctuating rate of the velocity gradient. Signal fluctuation analysis allowed one to define the flow regimes inside the static mixers. The laminar flow was characterized by a time-constant evolution of the velocity gradient, while the turbulent flow corresponded to the stabilization of the fluctuating rate of the velocity gradient for high values of pore Reynolds number, defined by considering the static mixer as a porous medium and by using a capillary model. The transition from a laminar flow to an intermediate flow occurred at a pore Reynolds number of about 200. The turbulent flow was observed at a pore Reynolds number of between 1500 and 3000. Introduction Mixing or dispersing operations are necessary in many processes and at different scales, from laboratory to industrial scale. However, the choice of agitated systems to realize these operations depends essentially on their capacity and efficiency to ensure mixing or dispersing action with respect to their energy consump- tion. In this context, the static mixer technology is increasingly used in most areas of chemical engineering such as homogenization, liquid-liquid dispersion, heat and mass transfer, and chemical reactions. These mix- ers present at the same time an alternative and a complementarity to the more traditional stirred vessel. 1 The SMX mixer consists of a series of inclined baffles. Elements of the mixer are rotated by 90° and arranged successively in a tube. The more important properties of static mixers over conventional dynamic mixers are no moving parts, simple design, lower capital and operating costs, lower energy requirements, etc. How- ever, despite their performances and growing uses in different operations, the working mode of these mixers as well as the physical phenomena governing the mechanisms of these operations remain to be elucidated. Many works have been devoted to the determination of flow regimes in porous media. Different criteria have been used to characterize flow regimes. Jolls and Hanratty, 2 Wegner et al., 3 and Dybbs and Edwards 4 followed the streamlined evolution from a visualization technique with a dye tracer. Jolls and Hanratty, 2 Latifi et al., 5 Rode et al., 6 and Seguin et al. 7,8 have analyzed the time evolution of the local mass transfer measured by an electrochemical method. Hall and Hyatt 9 have determined the turbulence intensity measured by laser anemometry, while Mickly et al. 10 and Van der Merve and Gauvin 11 have used the film anemometer technique. On the contrary, there is practically no experimental work concerning the local flow investigation in an SMX static mixer. However, numerical simulations of the flow through SMX static mixers are available in the litera- ture. Fradette et al. 12 have calculated the energy dis- sipation and the elongation inside the SMX mixer, and Rauline et al. 13 have determined the axial shear rate evolution. This paper deals with an experimental study of the flow pattern in the wall region of an SMX Sulzer static mixer by an electrochemical technique. The goal is to better understand the hydrodynamic phenomena governing the flow in static mixers through flow-regime characterization, spectral analysis, and mean velocity gradient evolution. The limits of laminar and turbulent regimes determined from both local (electrochemical technique) and global (pressure drop) measurements will be compared. The results were analyzed by consid- ering the static mixer as a porous medium. 14 Experimental Section Experimental Setup. A schematic view of the experimental setup used in the present work is given in Figure 1. The working fluid stored in a tank (1) was pumped by a centrifugal pump (2). At the exit of the flowmeter (3), the fluid entered the test section, which consisted of a honeycomb (4), playing the role of a calming section, and a test cell containing the static mixer element (5). At the exit of the test cell (5), the fluid was returned to the storage tank (1). The working fluid temperature was controlled thanks to heating (8) and cooling (9) systems. The test cell (5) contained a four Sulzer SMX static mixer elements of 51.6 mm nominal diameter. The geometrical characteristics of the SMX static mixer are 14 ǫ ) 0.90, d p ) 15.15 mm, and τ ) 1.46. Nineteen electrochemical sensors were mounted flush to the surface of the static mixer housing tube (Figure 1b). Their axial locations with respect to the static mixer inlet are given in Table 1. Experimental Technique. The electrochemical tech- nique allows the determination of the local flow, i.e., the local wall shear rate and the turbulence characteristics. Its principle is based on the determination of mass transfer by diffusion-controlled conditions of an active ion to a nonintrusive electrochemical probe. It consists of measuring the mass flux at the electrode where a * To whom correspondence should be addressed. Tel: 33 240 17 26 33. Fax: 33 240 17 26 18. E-mail: jack.legrand@gepea. univ-nantes.fr. 1478 Ind. Eng. Chem. Res. 2003, 42, 1478-1484 10.1021/ie0206195 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003