Natural convection heat transfer from two horizontal cylinders

Natural convection heat transfer from a single horizontal cylinder and a pair of vertically aligned horizontal cylinders is investigated. Surface heat transfer distributions around the circumference of the cylinders are presented for Rayleigh numbers of 2 × 106, 4 × 106 and 6 × 106 and a range of cylinder spacings of 1.5, 2 and 3 diameters. With a cylinder pairing the lower cylinder is unaffected by the presence of the second cylinder; the same is true of the upper cylinder if the lower one is not heated. However, when both cylinders are heated it has been found that a plume rising from the heated lower cylinder interacts with the upper cylinder and significantly affects the surface heat transfer distribution. Spectral analysis of surface heat transfer signals has established the influence of the plume oscillations on the heat transfer. Thus, when the plume from the lower cylinder oscillates out of phase with the flow around the upper cylinder it increases the mixing and results in enhanced heat transfer.