CONCLUSIONS

The characterization of convection patterns around a crystal growing from its aqueous solution is the subject of the present discussion. The study has been conducted using three laser measurement techniques that rely on the variation of refractive index with density, and hence the solutal concentration. These techniques have been validated in a thermal buoyancy experiment in a rectangular cavity. Subsequently, they have been used to study convection patterns around the growing crystal for varying process parameters. While images contain line-of-sight averaged information in a vertical plane, the possibility of reconstructing concentration contours on horizontal planes has been studied via tomography. Experiments reported in the study show that convection does influence crystal growth rate as well as its quality. The laser measurement techniques succeed in capturing the solutal concentration field (or its derivatives) around the crystal. Hence they can be used for online process control. Specific conclusions arrived at are given in the following paragraphs.

Comparison of optical techniques

Three refractive index-based optical techniques, namely interferometry, schlieren and shadowgraph have been applied to image the convective flow field in a differentially heated rectangular cavity. Air and water have been considered as working fluids. Experiments have been conducted over a wide range of temperature differences. The following conclusions have been arrived at in the present work.

1. In low temperature gradient experiments, all the three techniques correlate well with one another. Interferograms are limited by few fringes in air, and too many fringes in water.  The shadowgraph image does not show sufficient contrast for analysis. In this respect, the schlieren technique is most amenable to data reduction.
2. In high gradient experiments, both schlieren and shadowgraph yield clear images.  The interferograms are however corrupted by refraction errors. Schlieren and shadowgraph track the temporal response of the fluid medium in the form of the light intensity variation.
3. In high Rayleigh number experiments with water, the flow field is turbulent. Shadowgraph images are seen to be meaningful, as against interferograms and schlieren. They reveal a considerable amount of physical information, including boundary-layers, plumes and time scales.