Hydrodynamic electrodes flow regimes

The fundamentals of the electrochemical response at electrodes operating in a regime of forced convection, hydrodynamic electrodes, and the information that can be obtained have been reviewed [23, 24]. Some of these electrodes are good candidates for direct introduction into flow systems, in particular tube/channel electrodes and impinging jet (wall-jet and wall-tube) electrodes. Particular practical advantages of these flow-past hydrodynamic electrodes are that there is no reagent depletion while the sample plug passes the electrodes, and there is no build-up of unwanted intermediates or products. Recent advances in instrumentation also mean... 

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

Consider a fluid moving through a pipe in the laminar flow regime. The wall of the pipe contains an electrode, located at a certain distance from the entry (Figure 4.26). The flow rate at the wall is zero. In the vicinity of the walls, viscous forces slow down the fluid as soon as it enters the pipe. Thus a gradient in flow rate is established across a layer referred to as the hydrodynamic boundary layer. Its thickness increases with the distance from the inlet. The boundary layers of opposing walls eventually meet after a distance L, called the hydrodynamic entrance length. From this point onward, the flow profile is observed to be parabolic. For a tube, Lh has a value of about 70 times its diameter. 

In this work, we determine constraints on the dimensionless parameters of the system (dimensionless electrode widths, gap size and Peclet number), first qualitatively and then quantitatively, which ensure that the proposed flow reconstmction approach is sufficiently sensitive to the shape of the flow profile. The results can be readily applied for identification of hydrodynamic regimes or electrode geometries that provide best performance of our flow reconstmction method. 

The characteristics of the fluid velocity depend on the design of the hydrodynamic cell and the flow pattern. The latter is said to be laminar when the solution flows smoothly and constantly in parallel layers such that the predominant velocity is that in the direction of the flow. Laminar flow conditions are desirable since accurate descriptions of the solution hydrodynamics are available. On the other hand, under turbulent flow conditions the solution motion is chaotic and the velocities in the directions perpendicular to that of the flow are significant. The transition between the laminar and turbulent regimes is defined in terms of the dimensionless Re5molds number, Re, that is proportional to the relative movement rate between the electrode and solution, and the electrode size, but inversely proportional to the kinematic viscosity of the solution. Thus, for low Re values the flow pattern is laminar and it transits to turbulent as Re increases. For example, in a tubular channel the laminar regime holds for Re < 2300. 

THE PROBLEM Electrosynthesis is to be carried out in a parallel plate reactor with electrodes 10 cm wide, the electrolyte gap being 0.5 cm. The synthesis is to take place in a fully developed turbulent hydrodynamic regime so that mass transfer characteristics will be well defined. Calculate the minimum flow rate required and the value of the mass transfer coefficient if the electrolyte density is llOOkg/m and its viscosity is 3 X 10 Ns/m. The diffusivity of the diffusing species is 0.92 x 10 m /s. 

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