Convection from an isolated heated horizontal cylinder rotating about its axis

The heat transfer by convection from an isolated heated horizontal cylinder rotating about its axis in air was measured for varying rotational speeds, cylinder temperatures and diameters; some measurements were also made in air at 4 atm pressure. The results show that the heat transfer is nearly constant at the free convection value for rotational speeds from zero to a critical value, and then increases in proportion to the 2/3 power of the rotational speed. The explanation of the constant heat transfer below the critical speed is shown to be that the heat transfer on the ascending side of the cylinder is higher, and that on the descending side lower, than the free convection at zero speed. The critical speed occurs when the circumferential speed of the rotating surface becomes approximately equal to the upward free convection velocity at the side of the heated stationary cylinder. Theory predicts this to occur at approximately R √(P/G) = 0.9, which agrees well with the experimental values. Photographs of the flow also confirm these conclusions. Above the critical speed the irregular turbulent motion near the cylinder is shown to be similar to that which occurs in free convection above a heated horizontal surface facing upwards. Starting with known values for the free convection in the latter case, the heat transfer from the rotating cylinder above the critical speed was deduced and agrees well with the experimental results.