In nuclear industry applications, from design and safety aspects, it is important to predict the flow and heat transfer. The passive cooling systems, one of the robust cooling systems in the nuclear design are based on natural convections. In the RANS approach, non-linear unknown heat flux term has to be closed by appropriate model for accurate predictions of flow and thermal fields. This thesis presents a numerical framework for simulating heat and flow transfer in natural convection flow regimes by employing algebraic turbulent heat flux model AHFM-NRG+, coupled with second order turbulence model. The turbulent heat flux model coefficients are calibrated and a correlation between Rayleigh and Prandtl numbers with model coefficient is defined. The model is employed for different test cases of Rayleigh Bénard convections and is validated by comparing the simulation results with reference DNS data.