The paper investigates the development of natural convection in air-filled square enclosure with a heat source flush mounted on the lower horizontal surface; the cavity is symmetrically cooled from the lateral walls. The experimental analysis has been carried out with the real time holographic interferometry that makes possible the investigation of the development of the convective phenomenon, and with the double exposure technique in order to obtain the temperature distribution inside the cavity at the steady-state. The experimental analysis allows the study of a limited range of Rayleigh numbers from about 1.63·104 to 2.8·105. On the contrary the numerical investigation makes possible to enlarge the analysis to a more wide range of Rayleigh numbers; thus the experimental analysis, has been integrated with the numerical results provided by the investigation with the commercial finite volume software Fluent 6.0. for Rayleigh number from 103 to 106. Different convective forms have been obtained depending on Ra, on the heat source length and on its position. The Nusselt number has been evaluated on the heater; graphs with relations between average Nu, Ra, the heat source length and its position are finally presented.

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