MRI OF FLOW REGIMES INSIDE A TAYLOR-COUETTE CELL WITH APPLICATIONS IN ELECTRHOCHEMISTRY.

CARPINELLA, MARIELA; VELASCO, MANUEL I.; OVEJERO, JUAN M.; DASSIE, SERGIO A.; ACOSTA, RODOLFO H.

Rosario

Workshop; Workshop Frontiers in Magnetic Resonance, from Materials to Biological Systems; 2013

Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET)

Electrochemical cells (EC) and Rotating Disks Electrodes (RDE) are extensively used to describe chemical reactions and investigate transport mechanisms. In order to obtain quantitative data from the cell is necessary to know precisely the flow distribution inside it. More than 50 years ago, Cochran [1] and Levich [2] developed analytical expressions for the approximate speeds on a RDE using several simplifications. In these expressions, the cell dimensions are endless and the electrode thickness negligible. Despite this, the expressions are still widely used to calculate the limiting currents in this type of devices. A Couette cell is a device composed of two concentric rotating cylinders and represents a simplified model of an EC. In particular, we focused on a cell in which the inner cylinder rotates while the outer one remains stationary. In a simplified description, when the rate of rotation of the inner cylinder exceeds a certain critical value, given by the Critical Taylor number, the laminar azimuthal flow is modified by the appearance of counter-rotating vortices flow (TVF) superimposed on the tangential rotation of the liquid. In the presence of Taylor vortices the device is denominated Taylor-Couette cell (T-C). Despite the big number of works devoted to the study of the flow patterns generated in this kind of devices by means of simulations or Computational Fluid Dynamics [3,4], there are a numerous theoretical inconsistencies that often cannot be clarified because of the difficulties in the experimental measurement of these flows. A precise experimental determination of the velocity field near the electrode through modern techniques such as Particle Imaging Velocimetry becomes difficult due to cell geometry and electrode dimensions, and in many cases requires optically transparent flows [5]. Performing NMR imaging (MRI) has many advantages for this purpose, as it is noninvasive and does not require transparent fluids. MRI has been used for the characterizationof the flow inside T-C cells, whose rotating inner cylinder is in contact with the bottom of the cell [6,7]. Althoughthese works are very useful for understanding the flow patterns that are generated, this mechanism is a simplification of the real problem, where the rotating rod length is smaller than the length of the EC. With the purposed of characterizing the hydrodynamics behavior of the liquid inside a real cell we performed 2D velocity maps using MRI Pulsed Gradients Spin Echo sequence in a T-C cell filled with water, for different heights of the inner cylinder and different rotational speeds. The NMR results are compared with finite element simulations using COMSOL software. Maps were performed in a centered longitudinal plane of the cell of 2 mm thickness with pixel sizes of 0.156 x 0.156 mm.      REFERENCES 1.     Cochran W.G. Proc. of the Cambridge Philosophical Society 30, (1934), 365?375. 2.     Levich B. Acta Physicochimica Urss 17, (1942) 257?307. 3.     Gonzalez J. et al. Journal of Electroanalytical Chemistry 651 (2011) 150?159 4.     Alexiadis A. et al. Journal of Electroanalytical Chemistry 669 (2012) 55?66 5.     Desmet G. et al. Chem Eng. Sci., VoL 51, No. 8, pp., (1996) 1287?1298, 6.     Vallatos A. et al. EPL, 99 (2012) 68001 7.     Broadbent A. et al. Appl Magn Reson 42, (2012) 137?152