Calibration of radiation thermometers is one of the important research activities in the field of metrology. Many researchers in recent times have conducted numerical simulations on the calibration furnace to understand and overcome the experiment limitations. This paper presents a 2D numerical free convective study on the calibration furnace with the aperture using finite element method. The focused issues here are: aspect ratio effect on the flow pattern and temperature fields, heat transfer mechanism in the aperture zone as well as in hump regime. It is concluded that flow and temperature fields follow the same behavior in the hump regime as well as in the aperture zone. Also, it concluded that penetrative convection is more dominant for the enclosure of high aspect ratio.