A numerical investigation has been carried out to study the effect of a geometric perturbation on the three-dimensional buoyancy-driven flow and heat transfer in an annular cavity with impermeable end walls. The numerical scheme used in the present study is based on a Galerkin method of finite element formulation. The nature of the three-dimensional flow-field has been analyzed in detail. The local and average Nusselt numbers were obtained for a wide range of Rayleigh numbers, and the heat transfer performance was compared with that for the closed horizontal annulus without a perturbation. The effect of variation of a number of key geometric parameters of the perturbation on the overall heat transfer behavior has been studied. Some of the key features due to the introduction of the perturbation, and qualitative and quantitative effects of the perturbation within the annulus are discussed. It is shown that the introduction of the geometric perturbation can lead to a large increase in the overall heat transfer within the annulus.