Mass-transfer distribution in rotating, two-pass, ribbed channels with vortex generators

Local and global effects of cylindrical vortex generators on the mass-trans fer distributions over the four active walls of a square, rib-roughened rot ating duct with a sharp 180-deg bend are investigated. Cylindrical vortex g enerators (rods) are placed above, and parallel to, every other rib on the leading and trailing walls of the duct so that their wake can interact with the shear layer and recirculation region formed behind the ribs, as well a s the rotation-generated secondary flows. Local increases in near-wall turb ulence intensity resulting from these interactions give rise to local enhan cement of mass (heat) transfer. Measurements are presented for duct Reynold s numbers Re in the range 5 x 10(3)-3 x 10(4) and for rotation numbers in t he range 0-0.3. The rib height-to hydraulic diameter ratio e/Dh is fixed at 0.1, and the rib pitch-to-rib height ratio P/e is 10.5. The vortex generat or rods have a diameter-to-rib height ratio die of 0.78, and the distance s eparating them from the I ibs relative to the rib height s/e is 0.55, Mass- transfer measurements of naphthalene sublimation have been carried out usin g an automated acquisition system and are correlated with heat transfer usi ng the heat/mass transfer analogy. The results indicate that the vortex gen erators tend to enhance overall mass transfer in the duct, compared to the case where only ribs are present, both before and after the bend at high Re ynolds and rotation numbers. Local enhancements of up to 30% are observed o n all four malls of the duct, At low Reynolds numbers, for example, 5 x 10( 3), the insertion of the rods often leads to mass-transfer degradation. lit high Reynolds numbers, for example, 3 x 10(4), the enhancement due to the rods occurs on the surfaces stabilized by rotation (trailing edge on the in let pass and leading edge on the outlet pass) and the side walls. The enhan cement is more pronounced as the rotation number is increased. The detailed measurements in a ribbed duct with vortex-generator rods clearly show loca lized regions of enhanced mass (heat) transfer at Reynolds and rotation num bers within the envelope of practical interest for gas turbine blade coolin g applications.