Experimentally measured heat-transfer coefficients and visually observed flow phenomena for water flowing through an electrically heated rotating pipe with various inserts are reported. Convection heat transfer in flow through a rotating pipe or a rotating annulus was found to be an exceedingly complex mechanism which can not be treated adequately in terms of conventional parameters, but requires a detailed understanding of wave and vortex phenomena which were visually observed and photographed. Heat-transfer coefficients were measured in completely as well as partially filled flow channels. Three distinct flow regimes were observed in all cases; transition between regimes was found to depend on the rotational speed, but not on the axial flow velocity. Stationary vortexes and a counterrotational fluid motion were observed.