| Summary: | The effects of opposing buoyancies on natural convection heat and mass transfer in the boundary layer over a vertical
cylinder immersed in a quiescent Newtonian fluid are presented in this paper. The surface of the cylinder is
maintained at a constant temperature and concentration. The homotopic transformation is proposed to transform the
physical domain into a flat plate. The boundary layer equations and the boundary conditions are solved numerically
using an implicit finite difference scheme and the Gauss-Seidel algorithm. The buoyancy ratio N, Prandtl number Pr
and Schmidt number Sc are important parameters for this problem. The numerical results for Pr=Sc and Pr≠ Sc,
including the velocity, temperature, concentration fields and the Nusselt number as well as the Sherwood number
along the surface of the cylinder are discussed for aiding and opposing buoyancies. Results show that the Nusselt
(Sherwood) number increases with positive or negative buoyancies ration N (N=Grc/Grt). Moreover, for opposing
flows with Sc<Pr , the flow is completely downward, the thickness of the concentration layer is larger than that of the
thermal layer . For Pr<Sc , the velocity are weak and the thermal layer thickness is much larger.
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