COMPUTER SIMULATION OF TRIGLYCINE SULFATE CRYSTAL GROWTH IN SPACE (FINITE-DIFFERENCE).

摘要

Triglycine sulfate crystal growth from an aqueous solution in space was modeled numerically using a Multiple Alternating Direction Implicit finite difference method. Experiments were carried out in NASA's Space Shuttle in Spacelab 3.;The temperature-time cycles planned for control of the sting and the wall temperatures of three growth cells were provided before the flight. These were arrived at by trial and error to meet all constraints as best as possible.;After the flight the crystal size and shape were calculated from the growth rate vs. time at each grid point and compared with the final shapes of the two space-grown crystals. Both theoretical and experimental crystals were dish-shaped. A diffusion coefficient of 2 x 10('-5) cm('2)/s brought the calculated size of one crystal into fair agreement with the experimental results.;Refractive index maps were produced from the calculated concentration and temperature fields. To help in interpretation of the space experimental results and in comparing theoretical results with experimental results, interferometric fringe patterns were calculated from the refractive index data. These theoretical fringe patterns agree well with experimental results.;Time-dependent two-dimensional convection-free calculations of the heat and mass transfer were performed in this study. The heat and mass transfer equations were coupled only on the moving boundary (crystal surface) via the solubility and interface kinetics relations.;In the absence of convection, the interface kinetics influences the growth rate more for a large diffusion coefficient than for a small diffusion coefficient. The interface kinetics has its largest influence when the crystal first starts to grow after dissolution.;Time-dependent three-dimensional free convection under earth's gravity and low gravity conditions also was calculated for a brief period. This free convection was calculated from previously developed concentration and temperature fields. After 14 sec the magnitude of the buoyancy-driven free convection due to both concentration and temperature is about 0.4 cm/s for earth's gravity and after 20 sec about 0.0005 cm/s for an acceleration of 10('-4)g. From prior literature it is estimated that at 10('-6)g convection would increase the growth rate by about 20% at steady state. However it is also estimated that the time required to achieve steady state convection exceeds the growth times used in Spacelab 3.