Mixed convection in a semicircle enclosure of radius 0.15 m and length 0.75 m is studied experimentally. An insulated rotation cylinder is fixed at the center of the cavity to improve the heat transfer coefficient, the bottom wall of the enclosure is subjected to constant heat flux while the external curved wall is exposed to the ambient temperature. Cavity is placed horizontally and vertically and the angular velocities (62.8,83.73,125.6) rad/s. Results show an improvement of heat transfer coefficient with increasing angular velocity and with the horizontal and vertical cavity positioning.