Thermohydraulics of Laminar Flow Through Rectangular and Square Ducts With Axial Corrugation Roughness and Twisted Tapes With Oblique Teeth

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil $(175<Pr<538)$ through rectangular and square ducts with combined internal axial corrugations on all the surfaces of the ducts and with twisted-tape inserts with and without oblique teeth have been studied experimentally. The axial corrugations in combination with both twisted tapes with and without oblique teeth have been found to perform better than either axial corrugations or twisted-tape inserts acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow friction and thermal characteristics are governed by duct aspect ratio, corrugation angle, corrugation pitch, twist ratio, space ratio, length, tooth horizontal length and tooth angle of the twisted tapes, Reynolds number, and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that based on constant pumping power, up to 45% heat duty increase occurs for the combined axial corrugation and twisted-tape insert case compared with the individual axial corrugation and twisted-tape insert cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 30% for the combined enhancement geometry than the individual enhancement geometries.