Effect of microscale protrusions on local fluid flow and mass transport in the presence of forced convection
Abstract
1. Three-dimensional creeping flow around single, axisymmetric protrusions is studied numerically using the boundary-integral technique. Emphasis is placed upon cylindrical protrusions on plane walls for various height-to-radius (h-to-a) aspect ratios, but cones and sections of spheres protruding from plane walls are also briefly examined. The presented items include shear- stress distributions, shear-stress contours, extents of the fluid-flow disturbance, total forces and torques on the cylinders, streamlines, and skin-friction lines. Also included is a discussion of flow topology around axisymmetric geometries. 2. Mass transfer around insulating, cylindrical protrusions on an electrode surface is studied both numerically and experimentally, and good agreement is obtained between the two. Both the experimental and numerical findings suggest that the wake region is the least favorable for mass transport, except for the experiments at Re p>~50 for a single, insulating protrusion. For this exception, the mass flux 2.41 cylinder radii (r cyl) upstream from the center of the protrusion is comparable to the mass flux between ~5 and ~9 r cyl downstream from the center of the protrusion. At similar Reynolds numbers for a protrusion array, the mass flux is more favorable in front of the protrusion than downstream of the protrusion by a factor of about 1.5. The numerical results at 0.56<Rep<5 suggest that the single protrusion redistributes the reaction instead of increasing or decreasing the total mass flux to the electrode. That is, regions of enhanced mass transport appear to balance regions of diminished mass transport so that the total mass flux is the same with or without the single protrusion. Over a large area around the protrusion, mass transport is impeded because the shear stress at the electrode surface is attentuated compared to its undisturbed value far from the protrusion. Immediately in front of the protrusion, mass transport is enhanced owing to convection toward the electrode, and, immediately behind the protrusion, mass transport is diminished owing to convection away from the electrode. In addition to the enhancement in front of the protrusion, the diminished mass transport around the protrusion is also balanced by a slight mass-transport enhancement farther downstream of the protrusion. (Abstract shortened by UMI.)
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1997
- Bibcode:
- 1997PhDT.......151M
- Keywords:
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- Engineering: Chemical, Physics: Fluid and Plasma