Effect of transverse convex curvature on heat transfer from uniformly heated surface with backward-facing steps.

Abstract

Local heat transfer coefficients have been obtained through measuring the temperature distribution along the separating, reattaching, and redeveloping regions of axisymmetric turbulent boundary layers on the wall of cylinders with constant heat flux. Measurements have been made with different configuration of three cylinders and five ellipsoidal steps on the leading face. The velocity of the turbulent flow was changed stepwise from 5 to 30 $m/s.$ The range of Reynolds number, based on either the step height or diameter of convex curvature, is between 4,000 and 100,000. In addition, a surface-streamline flow visualization technique was tried to identify the reattachment loci, as well as the axisymmetry of the temperature distribution. It was found that the points of maximum heat transfer and reattachment are coincident within an uncertainty of $\pm$0.5 inch. As a common feature, the rate of heat transfer undergoes a drop within a short length distance right after separation, followed by a rise leading to a sharp maximum at the reattachment neighborhood. The study demonstrates that the maximum Nusselt number and heat transfer rate increase with either decreasing cylinder radius, or increasing step diameter. The effect of step height is similar to that of the step diameter. An empirical equation, correlated as a function of Reynolds number and aspect ratio of cylinder-step diameters, is proposed for the maximum Nusselt number.