An experimental investigation of jetting convection during Bridgman solidification of aqueous ammonium chloride

Jetting convection during vertical Bridgman solidification of 28 wt% NH₄Cl-H₂O is investigated using restrictions to flow in the two-phase mushy zone and viewed with the central dark ground optical method, particle tracking, and potassium permanganate (KMnO₄) dye to visualize the flow patterns. Thin plastic baffles are placed on the bottom of the quartz cuvette as restrictions to horizontal fluid flow within the mushy zone. Nine different baffle configurations using two baffle heights are utilized to determine the effect of varying the size of the region created by the baffles on the number and duration of convective jets that form during the solidification process. The goal was to reduce or, if possible, eliminate the jets. The results show that decreasing the size of the region created by the baffles will reduce the number of jets. However, this decreasing trend is balanced and sometimes dominated by an increase in the number of jets due to favorable jetting sites created by localized regions of reduced solid fraction created by and adjacent to the baffles. In addition, it is shown that adding the dye reduces the number of jets but does not significantly affect the starting times of the jets that did form. Finally, observations of dye movement from within the mushy zone into the bulk liquid support the view that jets are an evolved form of pluming convection and not independent of pluming.