Reconstruction of the concentration field around a growing KDP crystal with schlieren tomography.

Salt concentration distribution around a potassium dihydrogen phosphate (KDP) crystal growing from its aqueous solution has been experimentally determined using a laser schlieren technique. The growth process is initiated by inserting a KDP seed into its supersaturated solution, followed by slow cooling of the solution. Fluid convection leads to a distribution of concentration around the growing crystal. The pattern and strength of convection are important factors for the determination of the crystal growth rate and quality. Experiments have been conducted in a beaker with a diameter of 16.5 cm and a height of 23 cm. A monochrome schlieren technique has been employed to image the concentration field from four view angles, namely, 0 degrees, 45 degrees, 90 degrees, and 135 degrees. By interpreting the schlieren images as projection data of the solute concentration, the three-dimensional concentration field around the crystal has been determined using the convolution backprojection algorithm. The suitability of the overall approach has been validated using a simulated convective field in a circular differentially heated fluid layer, where full as well as partial data are available. Experiments have been conducted in the convection-dominated regime of crystal growth. The noncircular shape of the crystal is seen to affect axisymmetry of the concentration field close to the crystal surface. The reconstructed concentration fields reveal symmetry of the flow field away from the growing crystal. The solute concentration contours show large growth rates of the side faces of the crystal in comparison with the horizontal faces. In this respect, the concentration profiles are seen to correlate with the crystal geometry.