Buoyancy driven thermal flow behavior inside an air saturated porous enclosure has been numerically studied for transient to steady state with the help of stream-function and isotherm profiles, Nusselt number, and entropy generation. Buoyancy has been induced from below, and the top wall is maintained at a relatively cold temperature while the periodic condition arises from left to right. Process parameters selected for this investigation are non-dimensional time (t, varied from 0.005 to 0.5), Rayleigh number (Ra, varied from 10⁴ to 10⁶) and Darcy number (Da, varied from 10^-3 to 10^-1). A typical formation of global circulation has been observed at lower periods of heating (λ/H ≤ 0.25) in cases of high Ra and high Da. Strength of the circulation rolls enhances 20 times from Ra = 10⁴ to Ra = 10⁵. The present investigation is commonly essential for thestability of high sensitive devices like chemical reactors.