OBJECTIVES OF THE PRESENT WORK

The importance of convection in determining crystal growth rates and crystal quality has been recognized by a variety of authors. Over the past decade, laser measurement techniques have become popular, though primarily as a flow visualization tool. Recent papers, however, have emphasized the possibility of quantitative measurements using optical methods. Against this background, results are discussed in the present article in the following sequence:

Convection in a rectangular cavity

The literature on refractive index based methods shows that of the three methods, interferometry has been predominantly applied for qualitative as well as quantitative analysis of the thermal fields in buoyancy-driven convection. Schlieren finds potential applications towards qualitative visualization of the flow field and to a limited extent, it has been applied for quantitative studies. Shadowgraph has been extensively used for qualitative imaging. The three techniques, however, have not been jointly compared against a benchmark experimental configuration. The present work compares interferometry, schlieren and shadowgraph techniques for the measurement of temperature distribution in buoyancy-driven convection in a rectangular cavity. The top and bottom walls of the cavity are maintained at uniform temperatures at all times in an unstably stratified configuration. Fluid media considered are air and water. Temperature differences of 5-50 K for air and 3-10 K for water have been employed in the experiments. Over the range of temperature differences considered, flow was seen to become progressively unsteady, finally becoming turbulent.

Comparison of optical techniques for a crystal growing from its aqueous solution

Interferometry, schlieren and shadowgraph are employed to visualize the convection field around a growing KDP crystal from its aqueous solution. Experiments have been conducted under practically identical conditions. The goal of the study is to explore the suitability of these measurement techniques to image, analyze and interpret the convective field around a growing crystal.

Schlieren study of convection around a crystal driven by buoyancy and rotation

The specific goals here are to understand (a) changes in convection pattern at various stages of the crystal growth process, (b) role of convection in creating regions of high/low concentration gradients, (c) possibility of controlling the size of these regions by providing crystal rotation, and (d) to demonstrate the suitability of the schlieren technique for quantitative mapping of solutal concentration around the growing crystal. The process parameters studied are cooling rate of the solution and rotation rate of the growing crystal. Crystal rotation is viewed in this study as a means of diminishing the impact of buoyancy. The crystal size plays an influential role in determining the relative importance of buoyancy and rotation. An independent study of crystal size has also been presented. Schlieren images have been analyzed to correlate the strength of convection currents with the concentration field and its gradients. The crystal quality has been gauged by examining the transparency of the crystal at the end of the experiment.