CONVECTION AROUND A GROWING KDP CRYSTAL

Imaging of three dimensional convection patterns around the growing KDP crystal is discussed here. Four view angles are considered for tomographic reconstruction of the concentration field. In the experiments conducted, the crystal is not disturbed during the recording of the projection data. In addition, longer durations of crystal growth have been considered. Since finite time is required to turn the beaker and record projections, experiments have been conducted when the concentration field around the crystal is nominally steady. Occasionally, the plume above the crystal is marginally unsteady, and a time-averaged sequence of schlieren images has been used for analysis. The increment in the view angles is 45^o, covering the range of 0 to 180^o.

The crystal growth process has been initiated by inserting a spontaneously crystallized KDP seed into its supersaturated solution at an average temperature of 35^o. This step is followed by slow cooling of the aqueous solution. The cooling rate employed is 0.05^oC/hour. The seed thermally equilibrates with the solution in about 20 minutes. With the passage of time, the density differences within the solution are solely due to concentration differences. Adjacent to the crystal, the solute deposits on the crystal faces, and the solution goes from the supersaturated to the saturated state. When the crystal size is small, salt deposition from the solution on to the crystal occurs by gradient diffusion. Diffusive transport has been sustained for a longer duration of time (and larger crystal sizes as well) by maintaining a small degree of supersaturation in the solution.