Mixed convection in a square enclosure with a rotating flat plate

•Mixed convection arising from thermal buoyancy force and rotating flat plate inside an enclosure is studied.•Thermal oscillation occurs when the Rayleigh number exceeds some threshold.•Frequency of thermal oscillation depends on the Rayleigh number, rotor length, and rotating speed of the rotor.•There is a critical Rayleigh number. Below/beyond that the rotor enhances/suppresses the heat transfer.