Heat Transfer Augmentation Through Wall-Shape-Induced Flow Destabilization

Experiments on heat transfer augmentation in a rectangular cross-section water channel are reported. The channel geometry is designed to excite normally damped Tollmien-Schlichting modes in order to enhance mixing. In this experiment, a hydrodynamically fully developed flow encounters a test section where one channel boundary is a series of periodic, saw-tooth, transverse grooves. Free shear layers span the groove openings, separating the main channel flow from the recirculating vortices contained within each cavity. The periodicity length of the grooves is equal to one-half of the expected wavelength of the most unstable mode. The remaining channel walls are flat, and the channel has an aspect ratio of 10:1. Experiments are performed over the Reynolds number range of 300 to 15,000. Streakline flow visualization shows that the flow is steady at the entrance, but becomes oscillatory downstream of an onset location. This location moves upstream with increasing Reynolds numbers. Initially formed traveling waves are two dimensional with a wavelength equal to the predicted most unstable Tollmien-Schlichting mode. Waves become three dimensional with increasing Reynolds number and distance from onset. Some evidence of wave motion persists into the turbulent flow regime. Heat transfer measurements along the smooth channel boundary opposite the grooved wall show augmentation (65 percent) over the equivalent flat channel in the Reynolds number range 1200 to 4800. The degree of enhancement obtained is shown to depend on the channel Reynolds number, and increases with the distance from the onset location.