Boundary-layer sweeping and heat-transport enhancing by a rotor driven by Rayleigh–Bénard convection

We investigate the close interaction between a free-rotating body and the turbulent flows of Rayleigh-Bénard convection in a horizontal cylindrical cell. The free body consists of eight Plexiglas fan blades, spoked to a horizontal shaft that is allowed to rotate about the central axis of the cylinder with negligible friction. The rotor is powered to spin by the convective flows and the tips of the blades invade and sweep the thermal boundary layers. Our heat transport measurements, over a large range of control parameters, show that the heat transport efficiency is notably enhanced and the cause of this effect is due to the intrusion of the previously forbidden thermal boundary layers. Flow visualization confirms that as the fan blades pass by, fluids parcels are forced to leave the thermal boundary layers, before they could become unstable and emit plumes as seen in the classical situations. We propose a phenomenological model in an effort to interpret the observed heat-transport enhancement.