Transient natural convection in a square enclosure with horizontal walls submitted to periodic temperatures

Natural convection in a square cavity with its horizontal waifs submitted t o different heating models has been investigated by a finite difference pro cedure, The hot temperature of the bottom surface varies sinusoidally with time, while that of the opposite surface (cold temperature) is maintained c onstant or varied sinusoidally. The vertical walls are considered adiabatic . Parameters of the problem are the amplitude of the variable temperature(s ) (0 less than or equal to a less than or equal to 0.5), its (their) period (0.001 less than or equal to tau less than or equal to 1), the dephasing b etween hot and cold temperatures (phi = 0 and pi), the Rayleigh number (5 x 10(5) less than or equal to Ra less than or equal to 10(6)) and the Prandt l number (Pr = 0.72). The effect of these parameters on heat transfer and f luid flow within the cavity is studied The results obtained show that the h eat transfer can be enhanced or reduced, with respect to the case of consta nt temperatures, by proper choice of the variable heating mode, the paramet ers related to the periodic temperature(s), and the Rayleigh number. By var ying the two imposed temperatures, fundamental differences have been observ ed comparatively with the case of variable hot temperature and that of cons tant thermal boundary conditions.