Experimental and numerical investigation of turbulent natural convection between two horizontal concentric cylinders at different axis ratios and orientation angles

•Turbulent Natural convection heat transfer between two concentric cylinders with open ended was studied experimentally and numerically.•The angle of orientation for outer cylinder, ϕ, was varied from 0° to 90° with 15° increment.•The axis ratio of the outer cylinder was changed from 1.0 to 2.0.•Axial velocity and turbulent intensity were showed at different parameters.•The numerical results are in a good agreement with experimental results.•Maximum enhancement reaches 25% at axis ratio 2.0 and vertical position to the outer cylinder comparing to circular outer cylinder.