FREE CONVECTION EXPERIMENTS OF ATMOSPHERIC AIR IN VERTICAL OPEN ANNULI

Experiments were conducted for free convection of atomospheric air in open annuli (annulus ratios of 1.155, 1.33, 1.63 and 12.0) with an isoflux inner wall and an adiabatic outer wall for surface temperatures up to 856 K. The experiments established that Rayleigh number increased as the film temperature of air increased, reaching a maximum at approximately 375 K, regardless of the annulus ratio. Beyond this temperature, the Rayleigh number decreased as the film temperature increased. For film temperatures of air less than about 375 K, the Nusselt number increased proportionally to the Rayleigh number raised to the one-fourth power. For higher film temperatures, however, the Nusselt number was inversely proportional to the Rayleigh number.