Laminar natural convection heat transfer behavior in vertical cylinders with high Pr was experimentally and numerically performed by varying the diameters at diverse heights. Mass transfer experiments were conducted instead of heat transfer ones using an electroplating system based on the analogy concept. 3-D computational models using finite volume method have been developed and validated against experimental data. Both experimental and numerical results were in satisfactory agreement with the existing laminar natural convection correlation developed on a vertical plate. The dependency of the diameter at various Ra-H on Nu(H) was clearly observed; at the smallest diameter, heat transfer rates deteriorated due to stagnant flows, and then were enhanced due to the increased mass flow rates by flow-like chimney effects with increasing diameter, eventually coinciding with the existing laminar correlation. The influences of diameters on heat transfer rates and flows were analyzed from numerical simulations. (C) 2019 Elsevier Ltd. All rights reserved.