Thermohydraulics of turbulent flow through rectangular and square ducts with axial corrugation roughness and twisted-tapes with and without oblique teeth

The heat transfer and the pressure drop characteristics of turbulent flow of air (10,000 < Re < 100,000) 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 twisted-tapes of all types with oblique teeth have been found to perform better than those without oblique teeth in combination with axial corrugations. 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-tape, 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 on the basis of constant pumping power, up to 55% heat duty increase occurs for the combined axial corrugation and regularly spaced twisted-tape elements inserts with oblique teeth case compared to without oblique teeth twisted-tape inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 47% for the combined enhancement geometry than the individual enhancement geometries. However, full-length and short-length twisted-tapes with oblique teeth in combination with axial corrugations show only marginal improvements over the twisted-tapes without oblique teeth.