The transient processes of double-diffusive convection induced by buoyancy force in a vertical circular cylinder have been simulated numerically. Initially, the fluid in the cavity is homogeneous and the top and bottom walls are insulated, then constant temperature and concentration are imposed along the sidewall. The evolution of double-diffusive convection is investigated for specific parameters which are the Prandtl number Pr=7, the Lewis Number Le=5, the thermal Grashof number Gr_T=10~7, the buoyancy ratio N = 1, and the aspect ratio A = 2 for the enclosure. It is compared with nature convection driven by the temperature or solute buoyancy alone. The results show that the flow in the cavity is enhanced and the speeds of evolution of thermal and solutal field increase due to the thermal buoyancy and solutal buoyancy operating simultaneously. Contrasting to the thermal and solutal convection in the quasi-steady stratification state, the isothermal and isoconcentration lines aren't horizontal anymore for double-diffusive convection in that state. In the initial stage, the shape of time traces of temperature, concentration and velocity for double-diffusive convection are similar to those for thermal convection for sake of larger thermal diffusivity (Le=5). At the same time, the results show that the rates of heat and mass transfer at sidewall increase contrasting to those for thermal and solutal convection, respectively.
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