In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. Results are presented in terms of wall temperature, mass flow rate and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles vs time show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but the best configuration during the transient heating at the first overshoot. Velocity profiles in the chimney allow for identification of some different fluid dynamic behaviors such as the vortex in lower corner and the cold inflow in the chimney. According to the temperature profiles, average Nusselt number profiles as a function of time show minimum and maximum values and oscillations before the steady state.

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