A parametric study on the dynamic natural convection from long horizontal fin arrays (L = 128, 254 and 380 mm) is made using a 3-D unsteady numerical analysis. The investigated range of height H is 6.4-38 mm and that of spacings is 6.4-20 mm. Increasing L tends to enhance the oscillating sliding-chimney flow, whereas increasing S weakens it. The time-averaged overall convection heat transfer coefficients (h) over bar are larger for high and short fin arrays, due to the stronger buoyancy and thinner boundary layers. The (h) over bar -S relation exhibits a steep drop when S is narrowed below a threshold, which is larger for lower and longer fins. The optimum fin spacing S-opt occurs near the threshold S, below which the benefit of increasing heat transfer area surrenders to the decrease of (h) over bar caused by excessive viscous drag. The heat transfer rate per unit base area Q/A(b) decreases with increasing L and decreasing H. The predicted dependence of S-opt on H and L agrees well with experimental results and is explained based on numerical results of flow and heat transfer characteristics. The present predictions of Nu agree well with the correlations in the literature which use S as the characteristic length. (C) 2013 Elsevier Ltd. All rights reserved.