Heat Transfer and Fluid Mechanics Measurements in Transitional Boundary Layers on Convex-Curved Surfaces

Abstract

An experimental investigation of the effects of convex curvature and free-stream turbulence on momentum and heat transfer in a boundary layer undergoing natural transition was performed. The test section was designed for providing a two-dimensional boundary layer flow on a uniformly heated curved surface. The unique feature of the test wall was its flexibility to be bent to various radii of curvature R. Three cases of R = ∞, 180 cm, and 90 cm, are presented, each with two free-stream turbulence intensity levels, 0.68 and 2 percent. Mean velocity and temperature profiles and local Stanton number data were taken in a heated flow and streamwise Reynolds normal stress profiles were taken in an isothermal flow. Mild convex curvature (R = 180 cm) is shown to delay transition. Bending the test wall to a smaller radius of curvature, 90 cm, results in no significant further delay of transition. Cases with both curvature and higher free-stream disturbance effects show a pronounced dominance of the latter. Heat transfer rates and mean temperature profiles are shown to change more slowly after transition than wall shear stress values and velocity profiles. This can be observed as higher values of turbulent Prandtl number and lower values of the Reynolds-analogy factor, 2St/Cf, in the transitional and early-turbulent flows than values known to apply for mature turbulent flows. The results of this study increase the data base for development of transition prediction models which can be used in gas turbine blade design.