THE PRANDTL NUMBER EFFECT ON THE OPTIMUM HEATING FREQUENCY OF AN ENCLOSURE FILLED WITH FLUID OR WITH A SATURATED POROUS-MEDIUM

This is a fundamental study of the Prandtl number effect on the natura l convection flow within an enclosure subjected to intermittent heatin g from the side. Previous studies, considering an enclosure fined with fluid or with a fluid saturated porous medium having a Prandtl number equal to seven, have demonstrated the existence of convection resonan ce when the heat pulsating frequency approaches the natural how freque ncy of the system. The natural frequency of the flow can be well estim ated using a general theory valid for fluid systems and for fully satu rated porous medium systems. The results presented here test the valid ity of the theory for estimating the resonance frequency of systems wi th different Prandtl number. Numerical simulations confirm the existen ce of a preferred (resonance) heat pulsating frequency that compares w ell with the theoretical estimates. They also indicate that natural co nvection resonance is smoothed (damped), for the same Rayleigh number and the same Darcy number (in the case of a porous medium), when the P randtl number increases or decreases from the value PI similar to 1. A nalysis of the theoretical model, in line with the numerical results, indicates that the resonance frequency of a fluid or a porous medium s ystem varies with pr(1/2). Copyright (C) 1996 Elsevier Science Ltd.