Mass-Transfer Distribution in Rotating, Two-Pass, Ribbed Channels with Vortex Generators

Local and global effects of cylindrical vortex generators on the mass-transfer distributions over the four active walls of a square, rib-roughened rotating duct with a sharp 180-deg bend are investigated. Cylindrical vortex generators (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 as the rotation-generated secondary flows. Local increases in near-wall turbulence intensity resulting from these interactions give rise to local enhancement of mass (heat) transfer. Measurements are presented for duct Reynolds numbers Re in the range 5 X 10 exp 3 -3 X 10 exp 4 and for rotation numbers in the 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 generator rods have a diameter-to-rib height ratio d/e of 0.78, and the distance separating them from the ribs relative to the rib height s/e is 0.55. Mass-transfer measurements of naphthalene sublimation have been carried out using an automated acquisition system and are correlated with heat transfer using the heat/mass transfer analogy. The results indicate that the vortex generators 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 Reynolds and rotation numbers. Local enhancements of up to 30% are observed on all four walls of the duct. At low Reynolds numbers, for example, 5 X 10 exp 3 , the insertion of the rods often leads to mass-transfer degradation. At high Reynolds numbers, for example, 3 X10 exp 4 , the enhancement due to the rods occurs on the surfaces stabilized by rotation (trailing edge on the inlet pass and leading edge on the outlet pass) and the side walls. The enhancement is more pronounced as the rotation number is increased. The detailed measurements in a ribbed duct with vortex-generator rods clearly show localized regions of enhanced mass (heat) transfer at Reynolds and rotation numbers within the envelope of practical interest for gas-turbine blade cooling applications.