Experimental investigation of free convection heat transfer past an isothermal horizontal plate at low Rayleigh numbers by holographic interferometry

This paper reports the results of an experimental investigation based on real time holographic interferometry aiming at the measure of thermal fields in free convection of air past an isothermal horizontal plate with a large aspect ratio of nearly 15. The considered range of the Rayleigh number extends from about 2300 to 9000. Measurements are characterized by a time resolution of 1/25 s, a value much lower than the plume oscillation period due to instability, whereas the largest spatial resolution for the experimental setup is nearly 5/100 mm. The local distribution of the surface heat flux has been calculated by estimating the air temperature gradient; thus, both the local and the average heat transfer coefficient have been determined. Finally, the average Nusselt number has been correlated to the Rayleigh number and the agreement with some leading empirical correlations for flat surfaces has been discussed.