This research focused on the correlations associated with the physics of natural convection in circular fin-tube models. The limiting conditions are defined by two conditions. The lower limit ( D o / D → 1, s/D = finite value) corresponds to a horizontal circular tube, while the upper limit ( D o / D → ∞, s/D << 1) corresponds to vertical flat plates. In this paper, we proposed a corrected correlation based on empirical result. The circular fin-tube heat exchanger was divided into the A and B types, the categorizing criteria being D o / D = 1.2 , where D and D o refer to the diameter of the circular tube and the diameter of the circular fin, respectively. Moreover, with the computational fluid dynamics technique used to investigate the limiting conditions, the parametric range was extended substantially in this research for type B, namely 1.2 < D o / D ≤ 10. The complex correlation was also simplified to the form Nu L = C Ra s n , where C and n are the functions of the diameter ratio D o / D .
Keywords: circular fin-tube; correlations; heat exchanger; natural convection.