This study uses three-dimensional computational fluid dynamics commercial package along with experimental data and various flow models to investigate the natural convection heat transfer and fluid flow characteristics of a single-tube vertical plate fin and tube heat exchangers for various values of fin spacing and tube diameter. Temperature and velocity distributions of air between the two fins, fin temperature and heat transfer coefficient on the fins are determined using FLUENT along with various flow models. The inverse method in conjunction with the finite difference method and the experimental temperature data is applied to determine the fin temperature and heat transfer coefficient for the smaller tube. More accurate results can be obtained, if the heat transfer coefficient obtained is close to the inverse results and matches existing correlations. The numerical results of the fin temperature at the selected measurement locations also coincide with the experimental temperature data. The results show that RNG k-epsilon turbulence model is more suitable for this problem than laminar flow model. Three proposed new correlations between the Nusselt number and the Rayleigh number are in good agreement with the inverse results and numerical results obtained. (C) 2016 Elsevier Ltd. All rights reserved.