| An experimental investigation of passive flow destabilization and heat transfer enhancement in grooved channels is performed. The channel geometry is designed to excite normally-damped Tollmien-Schlichting modes resulting in early transition to three-dimensional unsteady flow. In this experiment, a hydrodynamically fully developed flow encounters an isolated section where one channel boundary is a saw-tooth series of periodic, transverse grooves. Attention is given to both the grooved section, where unsteadiness appears, and a flat recovery region downstream of the grooves, where unsteadiness eventually decays. The experimental measurements are made for a range of Reynolds numbers, 300 {dollar}le{dollar} Re {dollar}le{dollar} 15,000. These results are compared to measurements in the same test section with the grooved wall replaced by a flat surface. Both air and water are used as working fluids.; Flow visualizations at Re = 1000 in long, grooved channels show that unsteadiness appears within a few hydraulic diameters of the first groove. The onset point for unsteadiness is closely correlated with locations where measurable increases in heat transfer and pressure drop are observed. Fully developed results indicate that heat transfer is enhanced by as much as a factor of 4.6 at equal Reynolds numbers and 3.5 at equal pumping powers relative to flat passages.; The local heat transfer coefficient and pressure gradient drop off from the grooved channel values at different rates in the flat, downstream recovery passage. In the Reynolds number range, 1500 {dollar}le{dollar} Re {dollar}le{dollar} 3000, the pressure gradient relaxes to within 10% of the flat channel value in roughly ten hydraulic diameters. The heat transfer decay length is approximately twice that distance. Intermittently grooved channels may, therefore, offer even greater practical improvement in the heat transfer/pumping power performance of exchange passages. |
