Experimental investigation of enhanced surfaces in rectangular ducts

The following thesis documents the development and comparison of surface enhancements within low aspect ratio rectangular ducts to be used in a liquid-side heat exchanger application. The working fluid is 50% by volume solution of anti-freeze and water flowing at Reynolds numbers of 1,600 to 6,800. Seven test sections representing different surface enhancement designs are designed and tested. Three dimpled surfaces are made as well as two ribbed surfaces and two ducts containing longitudinal vortex generators. A smooth test section is also tested for comparison of results to those found in literature. The experimental apparatus applies a constant wall heat flux to both top and bottom of the duct. Thermocouples measure local wall temperature and bulk inlet temperature. Pressure taps along the length of the duct allow the measure of local differential pressure drop. The above quantities allow the determination of local convective heat transfer coefficient and local Fanning friction factor throughout the Reynolds number range for each test section. Appropriate performance metrics based upon these results are evaluated and the relative merits of each surface enhancement are shown and discussed. Particularly, it is found that, as with rib-roughened geometries, intermittently roughened tubes are superior to fully-roughened tubes, As well, a friction similarity function is proposed directly from rib-roughened results that can be used to collapse friction factor curves for the dimpled roughness family. Finally, embossed-type longitudinal vortex generators are proven as effective as heat transfer augmentation in internal flow applications as more traditional air-side vortex generator designs --Abstract, page iii