This paper presents the results of a numerical study of unsteady natural convection heat transfer from a pair of isothermally heated horizontal cylinders in water. Particular attention is paid to the interaction between the two buoyant plumes and the relationship between unsteady fluid flow and heat transfer enhancement. When the cylinders are vertically aligned, the heat transfer effectiveness of the upper cylinder is affected by buoyancy-induced fluid flow induced by the lower cylinder. Moreover, strong oscillations appear in the local heat transfer rate and fluid flow field. A computational fluid dynamics model (CFD) is established and validated against experimental results for both averaged and local Nusselt number, for a range of Rayleigh numbers between 1.7 x 10(6) and 5.3 x 10(6) and a centre-to-centre cylinder spacing between 2 and 4 diameters. The validated CFD model is used to identify 3.5 diameters as the spacing which maximises the averaged Nusselt number on the upper cylinder among the values of Rayleigh number and spacing which were presently investigated. The transient CFD model is used to characterise the peak frequencies of the oscillating thermal plume, as well as analyse Fourier transforms and spectral coherence of the local Nusselt number and fluid velocity. This detailed numerical analysis has complemented previous experimental measurements, confirming some hypotheses on the mechanisms of heat transfer enhancement in this configuration. (C) 2015 Elsevier Ltd. All rights reserved.