Natural convection heat transfer phenomena for two vertically staggered cylinders were investigated experimentally and numerically for both laminar and turbulent flows. Experiments were performed for the Rayleigh numbers 1.5 x 10(8) and 2.5 x 10(10) and the Prandtl number 2014, varying the vertical and horizontal pitch-to-diameter ratios using a copper electroplating system. Numerical studies were carried out to investigate the effect of the Prandtl number. The heat transfer rates of the lower cylinders were in close agreement with those of single cylinders. However, the heat transfer rates of the upper cylinders were influenced by the plumes from the lower cylinders, exhibiting preheating, velocity, and side-flow effects. At a very small pitch, the preheating effect was dominant, whereas at a larger pitch, the velocity effect became dominant. Both effects weakened with increasing horizontal pitch-to-diameter ratio. Under turbulent flow conditions, it was visually and experimentally confirmed that laminarization occurs on the top part of the upper cylinder, due to buoyancy-aided mixed convective turbulent flow. (C) 2012 Elsevier Ltd. All rights reserved.