Convection phenomena during the growth of sodium chlorate crystals from solution

Abstract

Schlieren observations of sodium chlorate crystals suspended in supersaturated solutions showed that, as supersaturation or crystal size increased, the boundary layer on the side faces remained stable while the top boundary layer and emerging plume changed from stable to partially oscillatory, with localized eddies or pulses rising periodically from portions on the top face. The transition was a function of the Grashof number Gr top (in which the characteristics length was defined as the area/perimeter of the top face) and the crystal height z, the critical Gr ∗ top being equal to 70 + (17 mm -1) z. Gr top correlated well with plume width, number of streamlines, eddy density, and plume velocity. The oscillation period, however, correlated best with the weight-increase growth rate. The mass transfer rate, as measured by the Sherwood number, correlated with Gr top according to the relation Sh=0.27 (Gr top) 1 4 . The linear growth rate varied as the inverse 1 4 power of the characteristic length and the 5 4 power of t he supersaturation. The vertical faces grew more rapidly than the horizontal faces, the ratio (1.1 to 1.6) having no apparent correlation with Gr top but increasing with decreasing supersaturation. A rough estimate of boundary layer flow versus growth rate for near-critical stable convection indicated that growth of the side faces depleted over half the excess solute in the boundary layer. External impulses initiated transient instabilities and/or oscillations in stable plumes, the threshold impulse energy being a function of Gr ∗ top minus Gr top. The duration of the transients increased with impulse energy until a maximum decay time was reached. Impulse energies greatly in excess of the threshold value caused transient separation of the boundary layer on the top face of in extreme cases over the entire crystal surface. Plume wavering, due to background-induced fluid motion, was observed only at the lowest supersaturations.