Flow Visualization and Modeling of Convection During Growth of KDP Crystals

Abstract

Buoyancy-driven convection is an important limiting factor during the growth of KDP crystals from their aqueous solution. Convection patterns have been visualised using a shadowgraph technique, and numerically analyzed. A numerical model is developed to obtain two-dimensional refractive index distribution averaged along the length of the growth chamber, followed by the tracing of light rays through the region of interest. The refractive index gradients around the crystal become sharper as the growth progresses, and manifest itself as high contrast plumes in the shadowgraph image. Estimates of the physical displacement of the light rays as they pass through the KDP solution have been found to be insignificant in comparison to the extent of disturbance, indicating that the higher order effects such as interference and scattering are negligible during shadowgraphic imaging of convection around a growing crystal. The experimental and numerical results show the utility of optical imaging of the convective field for monitoring crystal growth.