Turbulent heat/mass transfer enhancement with V-shaped ribs in a square channel

Abstract

This dissertation presents a comprehensive experimental investigation of the effects of V-shaped ribs on the heat transfer enhancement and the increased pressure drop for turbulent air flow in a square channel. The test section, with V-shaped rib arrays on two opposite walls, models roughened coolant flow passages in modern gas turbine blades. First, heat transfer experiments are conducted with rib angles-of-attack of 45° 60° 120° and 135°, and air flow rates corresponding to Reynolds numbers between 10 000 and 60 000. The 60° V-shaped ribs with a pitch-to-height ratio of 10 cause the highest average heat transfer on both the ribbed walls and the smooth walls, and give the highest heat transfer per unit pumping power. Naphthalene sublimation technique is employed to determine the detailed distributions of the local mass transfer coefficient on the ribbed walls and the smooth walls. By applying the analogy between heat transfer and mass transfer, the local distributions identify the regions of high and low heat transfer on the channel walls. Oblique ribs cause significant spanwise variations of the local mass transfer coefficient on the exposed surfaces of the ribbed walls, as a result of secondary flows. In the 60° full rib case, secondary flows over a ribbed wall move from one smooth wall toward the other smooth wall in an oblique direction. The vigorous mixing between the main flow and secondary flows enhances the overall mass transfer on the ribbed wall more than 90° transverse ribs. For 45° and 60° V-shaped ribs, secondary flows move from the centerline of a ribbed wall toward the two smooth walls. The local mass transfer distributions in the two cases differ significantly from each other. In contrast, in the 135° V-shaped rib case, secondary flows over a ribbed wall move from the two smooth walls toward the centerline of the ribbed wall. The overall mass transfer coefficients on the ribs are higher than those on the exposed surfaces of the ribbed walls.