Natural convection heat transfer on a vertical cylinder submerged in fluids having high Prandtl number

Natural convection experiments on both vertical thin- and thick-cylinders were performed for Rayleigh numbers from 1.4 × 109 to 3.2 × 1013, the H/DO from 0.6 to 140 at high Prandtl numbers. A sulfuric acid-copper sulfate electroplating system based on analogy concepts were employed for the measurements of heat transfer rates. For vertical thin-cylinders, Nusselt numbers were in satisfactory agreements with the existing correlation on vertical plates. Especially, those measured at the high H/DO were higher even though the thermal boundary layer is much thinner than the cylinder diameter. This was due to the flow interactions, which promoted the development of momentum boundary layer and velocity fluctuations, leading to the enhancement of the Nusselt number. For vertical thick-cylinders, Nusselt numbers in laminar flows were in satisfactory with the existing correlations. However, those in turbulent flows decreased as Prandtl number increased in this study. Its phenomena are caused by the formation of very thin thermal boundary layer within viscous sublayer by very high Prandtl number. In this case, the velocity within thermal boundary layer becomes very low by no-slip condition near the heated wall and the heat is transferred by conduction rather than advection. Based upon the results, empirical correlations on vertical thick-cylinders were derived for laminar and turbulent flows.