A numerical study of transient natural convection inside an air-filled horizontal cylinder with a coaxial inner triangular cylinder is performed with the Prandtl number and aspect ratio being fixed at 0.72 and 2.0, respectively. The effects of two different positions of the inner triangular cylinder and Grashof number are examined. The predicted development of the convective flow and heat transfer is presented by means of snapshots of streamlines and isotherms. It is shown that the flow development can be representatively classified into several stages based on time histories of the average Nusselt number over the outer circular cylinder. The time-averaged Nusselt number is scaled with Grashof number of power 0.21-0.24 and time duration for attaining a steady/quasi-steady state is also correlated to Grashof number. (C) 2010 Elsevier Ltd. All rights reserved.