A combined experimental and numerical study of a Rayleigh-Bénard-Poiseuille air flow in a rectangular channel is presented. The aim of the present paper is to characterize a secondary instability, referred to as wavy instability and known to be a convective instability, with the objective to identify the best situation for an optimal homogenization of heat transfers in the system. A periodic mechanical excitation is introduced at channel inlet and the spatial and temporal evolution of the temperature fluctuations are analyzed, depending on the Rayleigh number, the frequency and the amplitude of the perturbation. The Reynolds number is fixed. As the saturated state is a priori the best situation to homogenize the transfers, the objective is to expand the saturation area and to generate a maximum saturation amplitude value. It is shown that the best choice is a high Rayleigh number or/and a large magnitude of perturbation associated with a specific low value of the forcing frequency.