Natural convection heat transfer from helicoidal pipes

Abstract

An experimental investigation is reported on natural convection heat transfer in air from the outer surface of constant heat flux helicoidal pipes with vertical and horizontal orientations. The temperatures along the flow direction and peripheral direction of the tube wall were measured. The test Rayleigh numbers range from 4 x 103 to 1 x 10 s for vertical coils and 5 x 103 to 1 X 10s for horizontal coils. The local and average Nusselt numbers are evaluated and correlated. For the vertical case, the results are compared with single horizontal cylinders. It is found that the heat transfer from the first turn is almost the same as that of the single horizontal cylinder. Because of the tube curvature, the heat transfer coefficient on the outer coil wall (i/r = 77/2) is higher than that on the inner coil wall (i/r = 377/2) in the middle turns of the coil. For the horizontal orientation, the results are well correlated with the tube diameter as the characteristic length. The local heat transfer characteristics are discussed as well. The overall average Nusselt number of the horizontal coil is higher than that of the vertical coil in the laminar region.