The double diffusive natural convection of hot block smoke was numerically studied at the Prandtl number Pr=0.71, the Rayleigh number Ra=10~9, the Lewis number Le=5 to 20, the ratio of initial hot block smoke volume against entire volume V_I=0.05 to 0.51% and the buoyancy ratio N=0 to 1.2. At N=0 and 0.5, the smoke plume reached the top wall. At .V=0.8, the block smoke moved upward owing to the thermal buoyancy, and then the plume separated into the upward plume due to thermal buoyancy and the downward plume due to solutal buoyancy. At N=1.0 and 1.2, the hot block smoke spread on the bottom and then a plume ascended owing to the thermal buoyancy with flow oscillation from the center of bottom.
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