An extensive numerical analysis is performed on a vertical cylindrical open cavity made of aluminum with varying thicknesses to study conjugate natural convection. The key purpose of this study is to evaluate the effect of cylinder wall thickness on natural convection at the outer wall of the cylinder, which is coupled to the inner cylinder wall that has a constant temperature of 450 K and the ambient air which temperature is 300 K. The Navier–Stokes equations, continuity equation, and energy equations are solved numerically. The finite volume method is applied in the computational domain that includes the vertical cylindrical open cavity by using ansys-fluent 18 to specify the properties of flow and exchange of thermal energy. A couple of pertinent parameters are used for the numerical investigation like Rayleigh number within laminar regime (within range 104–108), aspect ratio (undefined proportion of length to diameter of the cylinder (L/D)) of the vertical cylindrical open cavity (2, 4, 6, 8, 10), and cylinder wall thickness (0, 1, 2, 3, 4 mm). This study presents the velocity vector field and thermal plume contours. An approximate Nusselt number to Rayleigh number equation is developed for the vertical cylindrical cavity within the range of Rayleigh numbers addressed by this work.