A numerical study for steady laminar double-diffusive natural convecti
on within a vertical closed annulus is examined with constant temperat
ure and mass species (concentration) differences imposed across the ve
rtical walls. The annulus has an aspect ratio of 1 and a curvature rat
io of 2, while the fluid Prandtl number is 7. In this paper the proble
m is defined and the numerical solution procedure is validated. Moreov
er, the effect of buoyancy ratio on the flow structure and the resulti
ng heat and mass transfer rates is presented. It is determined that bu
oyancy ratio is the primary factor that defines flow structure, includ
ing concentration-dominated (buoyancy force) opposing flow, transition
al flow, thermal-dominated flow, or concentration-dominated aiding flo
w. The relationship for buoyancy ratios, in the range -10 less-than-or
-equal-to n less-than-or-equal-to 10, and the average Nusselt and Sher
wood numbers have been obtained for a thermal Rayleigh number of 50,00
0 and a Lewis number of 5. Future papers will include the effect of th
ermal Rayleigh number, Lewis number, and various geometric parameters
on the flow structure and heat and mass transfer.