This paper investigates the convective heat transfer of a complex annular channel with an inner rotating wall. This configuration corresponds to the air gap of an open four-pole synchronous motor. The channel is constituted of a smooth static outer cylinder and a rotating inner cylinder with four slots such that the channel’s diameter is 15.8 mm. Experiments were performed for axial Reynolds numbers from 2140 to 6425 and rotational Reynolds numbers from 1750 to 35,000 (corresponding to Taylor numbers from 104 to 4. 106). Local heat transfer on both cylinders (rotor and stator) was measured using an infrared thermography device. PIV measurements were carried out in rotor slots. Results show a clear difference of heat transfer between slots sides and poles. In both cases, Nusselt number variation is comparable to the variation encountered in the entry region of a stationary channel. Both variation and level of heat transfer nonetheless depend on axial speed as well as rotational speed.