This paper describes an experimental investigation of the effect of internal ribs and orthogonal-mode rotation on forced convection in circular tubes. Depending on the rib geometry, heat transfer is improved by up to threefold in comparison with a smooth-walled circular tube at zero rotation. Results are presented for three rib geometries which are typical of those used in turbine rotor blade cooling channels. With rotation Coriolis-induced secondary flows increase heat transfer on the trailing edge of the tube in comparison to the leading edge. The effect of centripetal buoyancy is also investigated and the combined effect of Coriolis forces and buoyancy is demonstrated.