To investigate the mass transfer enhancement of vortexes by the external laminar channel flow in cavity holding a uniform concentration at bottom cavity wall, numerical simulations were carried out at various aspect ratios (depth/width), Reynolds numbers and Schmidt numbers in laminar flow. The numerical technique was based on an upwind differenc method of the governing differential equations that were first integrated over control volumes surrounding the node points in a rectilineal, nonuniform grid system.
Results were presented for a comprehensive numerical analysis of the two-dimensional mass transfer in cavity of rectangular profile. As the aspect ratio increased, the number of vortex in cavity increased. The distribution of the local mass transfer rate was affected by the direction of the vortex. For each aspect ratio, the relation of the average mass transfer rate, Reynolds number and Schmidt number, was correlated by the following equations:
$Sh_m = KRe^x Sc^y$
As the aspect ratio increased, the constant K value in the above equation rapidly increased and the values of x and y monotonously decreased. But when the aspect ratio was above five, the constant K value asymptotically approached to one. Because the thickness of the concentration boundary layer in cavity was approximately equal to the depth of cavity, the Colburn $j_D$-factor representing the relation of the momentum and concentration analogy could not be applied to this system.
In case that the aspect ratio was one, the results were compared with experimental data measured by the limiting current method and the relative error of mean Sherwood number was within twenty percent.
