Mechanisms leading to the formation of double-diffusive layers during unidirectional solidification of aqueous NH4Cl solution

The phenomenon of unidirectional solidification of binary solutions leads to various flow patterns such as solute fingers, plumes, and double-diffusive layers (DDLs) due to density variations arising out of the coupled effects of thermal and solutal gradients. The present work reports experiments to elucidate the transport phenomena responsible for the formation of DDLs during unidirectional solidification of aqueous NH4Cl solution in a narrow solidification chamber. The spatiotemporally resolved flow and transported properties associated with the phenomenon have been determined through a combination of nonintrusive techniques. Dual-wavelength interferometry has been used for simultaneous measurement of temperature and salt concentration distributions. Various subprocesses such as convection transients, plume formation, and evolution of DDLs that result in the formation of the stepped structure of solutal-thermal gradients have been mapped through rainbow-schlieren deflectometry. Flow velocities have been quantified using PIV. Based on the whole-field measurements, plausible mechanisms responsible for the development and further evolution of DDLs have been identified. Direct experimental observations revealed the existence of counter-rotating rolls near the chamber sidewalls. Interaction of these convective rolls with the plumes originating from the mushy zone results in a downward inclination of DDLs (towards plume locations), thus giving DDLs a slanted shape instead of their horizontal orientations. The alternate sense of rotation of convective rolls in the adjacent DDLs has been explained through observations made using different experimental techniques. Experimental results are additionally supported with analytical scales of flow parameters such as plume velocity, diameter, the height of DDLs, the downward velocity of DDLs, etc.