(The) effects of turbulence promoters on mass transfer

Abstract

The effects of turbulence promoters on mass transfer were investigated numerically. The zigzag and cavity types of geometries were selected, for while mass transfer and flow visualization studies were made at Reynolds numbers of laminar flow ranges. Upwind differences scheme was used for the better convergence of solutions at moderately high Reynolds numbers and also non-uniform grid was adopted to account for the sharp changes in concentration inside the boundary layers. The variation of Reynolds numbers or Peclet numbers did not affect the rate of convergence greatly. However the accuracy of the solutions was highly dependent on the grid size near the wall. Careful examinations of the solutions obtained showed that turbulence promoter enhances the mass transfer by forming the circulating flow which breaks the concentration boundary layer and increases the wall shear stress in the main stream. The following empirical correlations were obtained for the mass transfer of zigzag type promoter of which the ratio of distance between the centers of the neighboring promoters to channel height is 5. $$Sh = 0.518 Sc^{0.4}Re^{0.5}$$ $$Sh = 0.816 Sc^{0.34}Re^{0.43}$$ The flow separations shown by visualization were in a good agreement with the predictions by the numerical analysis at Reynolds numbers from 20 to 150. Also the visualization studies showed that the unsteady flow begins at Reynolds numbers of 250 to 300.