The heat transfer coefficient of fluid flow was experimentally investigated in an annulus with an inner rotating cylinder to estimate thermal fatigue of the inner and outer cylinders on the rotating machine. The following three conclusions were reached in the study. (1) Within the range of experimental conditions, the heat transfer coefficient did not depend on the axial flow rate, rather it showed a large dependence on the inner cylinder rotating speed. (2) The heat transfer coefficient at the top of the labyrinth was about three times an large as that at the bottom. (3) An empirical correlation equation considering the gap between the inner and outer cylinders was proposed, which predicted the heat transfer coefficient on the rotating machine within ±30%.
