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Two thermal adiabatic boundary conditions arise on the semitransparent window owing to the fact that whether semitransparent window allows the energy to leave the system by radiation mode of heat transfer. It is assumed that being low conductivity of semitransparent material, energy does not leave by conduction mode of heat transfer. This does mean that the semitransparent window may behave as only conductively adiabatic (q c = 0) or combinedly conductively and radiatively adiabatic (q c + q r = 0). In the present work, the above two thermal adiabatic boundary conditions have been investigated in natural convection problem for the Rayleigh number (Ra) 10 5 and Prandtl number(Pr) 0.71 in a cavity, whose left vertical wall has been divided into upper and lower parts in the ratio of 4:6. The upper section is semitransparent window, while lower section is isothermal wall at a temperature of 296K. A collimated beam is irradiated with different values (0, 100, 500 and 1000 W/m 2 ) on the semitransparent window at an angle of 45 0 . The cavity is heated from the bottom by convective heating with free stream temperature of 305K and heat transfer coef-

In the present work, the effects of diffuse and collimated radiation on the symmetrical cooling case of natural convection in a two-dimensional cavity heated from the bottom have been investigated, numerically. The collimated beam radiation feature has been developed in OpenFOAM framework and other libraries of fluid flow and heat transfer have been combined together for a coupled simulation of fluid flow and heat transfer. The cavity is convectively heated from the bottom with heat transfer coefficient of 50 W/m 2 K and free stream temperature 305 K, while both vertical walls of cavity are isothermal at temperature of 296 K. The top wall is adiabatic and all walls are opaque for radiation heat transfer. For collimated case, a small semitransparent window of nondimensional width of 0.05 at height of 0.7 has been created on the left wall and a collimated irradiation of value 1000 W/m 2 at an angle of 45 0 is applied on this semitransparent window. The study has been performed in two stages, first, the effect of diffuse radiation on the natural convection has been observed and then collimated beam is passed through the semitransparent window. The results reveal that the diffuse radiation has little effect on the dynamics of two rolls

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