The heat transfer and the pressure drop characteristics of turbulent flow of air 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 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. It has been found that up to fifty five per cent heat transfer coefficient 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. The friction factor has been reduced up to forty seven per cent 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.

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