Effect of Thermal Rayleigh and Lewis Numbers on Double-Diffusive Natural Concection in a Closed Annulus

A numerical study for steady laminar double-diffusive natural convection within a vertical closed circular annulus is examined with a constant temperature and mass species (concentration) difference imposed across the vertical walls. The effects of thermal Rayleigh number (5000 ≤ Ra T ≤ 100,000) and Lewis number (1 ≤ Le ≤ 10) are investigated for buoyancy ratios of −2, − 1.2, − 1, and 2, representing conditions for mass species dominated opposing, transitional, thermal dominated opposing, and mass species dominated aiding flows, respectively. The relationships between the thermal Rayleigh number and the average Nusselt and Sherwood numbers for − 10 ≤ n ≤ 10 have been obtained for a Lewis number of 5. Also, the relationship between the Lewis number and the average Nussell and Sherwood numbers for the same buoyancy ratio range have been obtained for a constant thermal Rayleigh number of 50,000. The Lewis and thermal Rayleigh numbers are found to influence the buoyancy ratios at which flow transition and flow reversal occur.