This study numerically investigates the problem of natural convection flow ina vertical cylinder. The importance of this issue mainly stems from thermal systems (e.g. chimneys, hot air generators, solar collectors, and many others.). Despite the numerous studies on the modeling of free convection flow between two perpendicular plates, there are very few studies that have investigated regarding the convective flow between two cylindrical walls. Therefore, the study is limited to vertical cylinders with different heating modes.The first configuration includes heated walls of thermosiphon flow in a cylindrical channel.The second configuration introduces a circular heat source at the channel entrance, and the third configuration includes an ended vertical cylinder with a heat source centered in the cylinder (zs = 0.04). The flow comparison from the three configurations demonstrates how the hot disk affects the flow from a dynamic point of view.Furthermore, details about the flow and dynamical fields can be obtained from the solution equation of the conservation of momentum and energy, considering the differences between the boundary conditions of the studied configurations. The study covers Rayleigh numbers and focuses on the effect of the cylinders geometry on the characteristic of the flow as well as dynamical and thermal field characteristics and variations in Nusselt number. The studied flow is natural convection, laminar, steady, two-dimensional, and incompressible flow. Solutions were obtained using a numerical model based on the finite volume method and the Navier–Stokes equations. The velocity–pressure coupling was resolved using the SIMPLER algorithm. The temperature and vertical velocity profiles of the flow showed the existence of a boundary layer regime along the heated channel wall without a heat source. The layer continued along the cylindrical wall with the introduction of the heat source at the channel inlet. However, introducing the heat source caused a considerable change in the flow structure at the channels central part.Calculations were performed for the channel aspect ratio R* = 0.2 and different Rayleigh numbers (Ra = 105, 107, 108, 109 and 1010). Numerical results included velocity, temperature, and Nusselt number profiles.