The objective of the present study is to gain some insight into fluid motion and heat transfer phenomena in the case of a square enclosure heated from below and symmetrically cooled from the sides; the effects of different values of the heat source is experimentally investigated. The localized heating is a centrally located heat source on the bottom wall; three different values of the heat source length (1/5, 2/5, 3/5 of the wall) are considered. The test cell is a square enclosure filled with air with isothermal side walls at equal temperatures Tc; the remaining vertical walls are realized with glass to allow optical access to the cavity. The top and bottom surfaces of the enclosure are made of plexiglass and, except for the heated section, are considered to be adiabatic. The located heat source is assumed to be isothermal at a temperature Th. The temperature distribution is experimentally measured by real-time and double-exposure holographic interferometry. The real-time technique is used in order to reveal the presence of plume oscillations while the double-exposure technique is used for steady-state measurements. Holographic interferometry shows the typical advantages over the classical optical techniques, such as high precision and sensitivity, very low noise level, and the possibility of displaying the temperature distribution across the whole investigated region. The objective of the heat transfer analysis is the experimental investigation of the Nusselt number distribution around the cavity at various Rayleigh numbers and several dimensions of the heat source. Different convection forms were obtained depending on Ra and on the heat source length. The Nusselt number was evaluated on the heat source surface and it showed a symmetrical form rising near the heat source borders.

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