Study of fluid flow during directional solidification of HℓO-NHℓCl using particle displacement tracking technique

The fluid flow during directional solidification of NH₄Cl_-H₂O was studied using a Particle Displacement Tracking (PDT) technique. The study applied orthogonal multiple- plane microscopic particle tracking and automated digital analysis to construct quantitative, three-dimensional velocity fields within regions of particular interest. The structure of jetting convection and fluid velocities were determined.

One experiment condition was selected: Temperature Gradient, G = 15°C/cm, Cooling Rate, CR = 60°C/hr. The duration of the experiments were 35 minutes. Two runs (designated Run 1 and Run 2) were analyzed using this technique.

Typical flow velocities during Run 1 ranged between 0.3 and 0.5 mm/s. Overall flow activity in the all-liquid region at time, t = 5 min, was observed to be minimal. The flow activity then sharply increased until t = 10 min. It continued to increase at a lesser rate reaching the peak at t = 20 min, the flow activity was seen to decrease gradually until the end of the run, t = 35 min.

Towards the end of Run 2, at approximately t = 35 min, a jet occurred. A jet is characterized by a channel of liquid, originating from the mushy zone, which travels upward and out of the mushy zone into the all-liquid region, with velocities within the channel being much greater than the velocities of the surrounding fluid. The surrounding liquid, in general, is found to travel downward, opposite in direction to the jet. The jet diameter was between 1.5 and 2 mm. Typical jet upward flow velocities ranged from 1 to 2.5 mm/s, with the central core of the jet exhibiting the highest velocity. The jet velocity radially decreased away from the core. The activity level at t = 35 min of the flow with the jet was observed to be greater than the activity of the flow without the jet.