Vane Suction Surface Heat Transfer in Regions of Secondary Flows: The Influence of Turbulence Level, Reynolds Number and the Endwall Boundary Condition

Abstract

Secondary flows in vane passages sweep off the endwall and onto the suction surface at a location typically close to the throat. These endwall/vane junction flows often have an immediate impact on heat transfer in this region and also move any film cooling off the affected region of the vane. The present paper documents the impact of secondary flows on suction surface heat transfer acquired over a range of turbulence levels (0.7% through 17.4%) and a range of exit chord Reynolds numbers (500,000 through 2,000,000). Heat transfer data are acquired with both an unheated endwall boundary condition and a heated endwall boundary condition. The vane design includes an aft loaded suction surface and a large leading edge diameter. The unheated endwall boundary condition produces initially very high heat transfer levels due to the thin thermal boundary layer starting at the edge of heating. This unheated starting length effect quickly falls off with the thermal boundary layer growth as the secondary flow sweeps up onto the vane suction surface. The heat transfer visualization for the heated endwall condition shows no initial high heat transfer level near the edge of heating on the vane. The heat transfer level in the region affected by the secondary flows is largely uniform, except for a notable depression in the affected region. This heat transfer depression is believed due to an upwash region generated above the separation line of the passage vortex, likely in conjunction with the counter rotating suction leg of the horseshoe vortex. The extent and definition of the secondary flow affected region on the suction surface is clearly evident at lower Reynolds numbers and lower turbulence levels when the suction surface flow is largely laminar. The heat transfer in the plateau region has a magnitude similar to a turbulent boundary layer. However, the location and extent of this secondary flow affected region is less perceptible at higher turbulence levels where transitional or turbulent flow is present. Also, aggressive mixing at higher turbulence levels serves to smooth out discernable differences in the heat transfer due to the secondary flows.