PLUME STRUCTURES IN NATURAL CONVECTION WITH TRANSPIRATION

Abstract

We present plume structures at high Rayleigh numbers (Ra_H ~ 10¹¹) and Schmidt numbers (Sc ~ 675), in convection driven by density differences across a horizontal membrane, when vertical transpiration velocities in the range of 0.002 cm/s ≤ V_o ≤ 0.065 cm/s are imposed. The density differences across the membrane are created by having brine with higher concentrations above the membrane and ammonium chloride with lower concentrations below it, with the bottom fluid being forced upward across the membrane by gravitational heads to create transpiration. Planar laser induced fluorescence (PLIF) visualizations in a horizontal plane grazing the top of the membrane, with refractive index matching, brings out the plume structures and their evolution with Ra_H and V_o. The plume structures show complex dendritic patterns of line plumes with plume-free patches; the plume-free patches decrease in area and number, with decrease in Ra_H and V_o. At low V_o, these plume-free patches have aligned plumes around them, while at larger V_o, such aligned plumes are not seen. We expect these patches to be due to the impingement of large-scale flow at low V_o, while they could be due to the modification of boundary layer instability at large V_o. These plumes become closer with increase in Ra_H, while they become more separated with increase in V_o. We quantify this variation by measuring the total plume length (L_p) by skeletonizing the plume structures and calculate the mean plume spacing (λ) from L_p; λ show nontrivial, non-power law dependence on Ra_H and V_o, different from those observed for the case of no throughflow and low throughflows.