OSCILLATORY DOUBLE-DIFFUSIVE CONVECTION WITH DIFFERENT BUOYANCY RATIOS IN A POROUS ENCLOSURE

Abstract

The transient behaviors of oscillatory double diffusive convection of a binary gas mixture in a porous enclosure subject to opposing thermal and compositional buoyancies are investigated numerically by employing the SIMPLE algorithm. Effects of two important parameters, the solutalto-thermal buoyancy ratio N and the solid-to-fluid heat capacity ratio σ, on the heat and mass transfer characteristics are studied. Steady oscillations occur for σ = 0.9 and N = 0.85~1.00, and σ = 1.0 and N = 0.75~0.95 as a result of the interaction between thermal and solutal buoyancy forces. For N 1.0, the flow is dominated by the solutal effect, and always circulates in the counter-clockwise direction. Therefore no oscillation occurs. For fixed σ, the frequency of oscillation decreases as N is increased due to reduced net buoyancy force. Time-averaged Nu and Sh also decrease as N is increased as a result of reduced convection effects. Nomenclature A enclosure aspect ratio, = L/W C dimensionless species concentration, = (c–cl)/(ch–cl) – 0.5 Da Darcy number, κ/W F inertia coefficient, -3/2 1.75e 150 N buoyancy ratio, βc(ch–cl) / βT(Th–Tl) U dimensionless x-component velocity, uW/α V dimensionless y-component velocity, vW/α Greek symbols e porosity η dimensionless time, αt/W θ dimensionless temperature, = (T–Tl)/(Th–Tl) – 0.5 κ permeability σ heat capacity ratio, = f pf f pf [ (1 ) ] s s c c c eρ e ρ ρ + − Subscripts h high l low