Local Mass Transport in Two‐Dimensional Cavities in Laminar Shear Flow

Abstract

The local mass‐transfer rate was investigated along the bottom wall of two‐dimensional cavities of nominally rectangular shape, 100 μm wide, and aspect ratio (depth/width) 0.5. Experimental measurements were made by an electrochemical limiting current technique based on reduction of at gold ultramicroelectrodes, each 10 μm wide and spaced 1 μm apart. Experiments were carried out in the presence of laminar fluid flow in the range between Stokes flow and turbulent flow (0.068 < Re < 2400). Numerical computations of two‐dimensional, laminar, convective diffusion were carried out with use of (a) a research grade finite‐difference code (ERMES) and (b) a commercial grade finite‐element program (FIDAP). In addition, ERMES was implemented for numerical computations of two‐dimensional, multispecies transport by convection, diffusion, and migration under laminar flow. The simulations carried out for both rectangular (ERMES) and undercut (FIDAP) cavity shapes indicated that the hydrodynamic profiles consisted of a major recirculating vortex in the cavity and a pair of relatively small eddies inside the two corners at all flow rates studied. The simulations indicated that above an aspect ratio of 0.33 the hydrodynamic flow pattern for a rectangular cavity changed from a major single eddy to isolated corner eddies. In the convective regime, experimental and theoretical results agreed in average mass‐transport rates to the surface and in the local flux distributions but differed in predicting the location of the maximum. © 2000 The Electrochemical Society. All rights reserved.