| Heat transfer and friction in annuli with spirally fluted inner tubes were investigated in this study for the laminar, transition, and turbulent flow regimes. Fourteen fluted tubes were tested, with up to three outer smooth tubes for each fluted tube. Friction factors were calculated from measured values of pressure drop. Each fluted-tube annulus was also used as a water-water heat exchanger. Annulus Nusselt numbers were calculated from measured overall heat transfer coefficients and tube-side turbulent flow heat transfer coefficients available in the literature.;Flow patterns and transitions between flow regimes were qualitatively investigated through visualization tests and fluted-tube surface temperature profile measurements. The fluted inner tubes induced a significant degree of swirl in the flow. These tests also showed that the fluid does not flow exclusively in trough and crest zones near the wall and a purely axial flow away from the wall, but rather in a spiral pattern, alternately crossing over between the trough and the crest. Transition in these annuli occurred in the 310 $<$ Re $<$ 1000 range.;Friction factor enhancements in the laminar regime were typically between 1.1 to 2.0, while enhancement values of up to 10 were recorded in the turbulent regime. Nusselt numbers in the laminar regime were between 4 and 20 times the smooth annulus values, while turbulent enhancements were between 1.1 and 4.0. The friction factors and Nusselt numbers were found to be functions of the flute depth, pitch and angle, and the annulus radius ratio.;Friction factors were correlated in two regimes by developing augmentation functions with respect to smooth annulus values. The low Re correlation (Re $<$ 800) predicted 68% of the data within $pm$20%. The high Re correlation predicted 73% of the data within $pm$20%. One Nusselt number correlation was developed for the entire Re range (700 $<$ Re $<$ 40000) in terms of the fluted annulus friction factor correlation developed in this study and geometric parameters. This correlation predicted 84% of the data within $pm$20%. The most significant finding of this study is that in the low Re range (Re $<$ 3000), fluted inner tubes in annuli cause enhancements in heat transfer that are up to seven times the corresponding increase in friction factor. |
