Rotating-disc electrode radial flow

In many respects, similar to the diffusion layer concept, there is that of the hydrodynamic boundary layer, <5H. The concept was due originally to Prandtl [16] and is defined as the region within which all velocity gradients occur. In practice, there has to be a compromise since all flow functions tend to asymptotic limits at infinite distance this is, to some extent, subjective. Thus for the rotating disc electrode, Levich [3] defines 5H as the distance where the radial and tangential velocity components are within 5% of their bulk values, whereas Riddiford [7] takes a figure of 10% (see below). It has been shown that... 

To understand this, we must consider the flow velocities of the liquid near the electrode surface. Convective flow near a rotating disc electrode is described by three velocity components (a) the radial velocity v, which moves the liquid from the center of rotation outward, in a plane parallel to the surface, (b) the perpendicular velocity v, ... 

Fig. 73. Flow patterns at a rotating disc. (A) Perpendicular view of the electrode surface of a circular velocity Uc = cOrRD he radial distance Rq (B) side view of the stream lines concentric around the z-axis u velocity along the rotation axis, u centrifugal velocity, perpendicular to the rotation axis.

It was also explained in section 4.1.1 that the rotation of the electrode structure induces a particular flow pattern to the solution. The solution is pulled up at the centre of the rotating structure (i.e. to the disc electrode) and then thrown out radially across the surface of the structure (see also Fig. 

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