Modelling the solution growth of TGS crystals in low gravity

Abstract Triglycine sulphate crystals were grown from aqueous solutions on Spacelab 3 in 1985. We have modelled this experiment in two dimensions employing the finite volume code PHOENICS. Thermal and solutal convection were included in this model and the crystal growth rate was chosen as the sensitivity parameter for the response to convective transport. Simulations were carried out for steady, impulsive and periodic accelerations in order to determine tolerable acceleration levels. Longtime simulations of the experiment were conducted with steady background accelerations of 10-6g0 and 10-5g0, whereg0 is the terrestrial gravi tational acceleration. Impulsive and periodic disturbances of higher magnitudes were imposed at intermediate points. For steady accelerations and the cooling rate used in the space experiments, the growth rates at 10-6g0 are found to be governed by diffusion for the first five hours of the experiment, after which a small convective contribution develops. At 10-5g0, on the other hand, convective contributions become increasingly significant after about three and five hours, respectively, for an orientation of the residual gravity vector either parallel or perpendicular to the top face of the crystal. No steady-state transport conditions were found. The computed concentrations fields reflect the features of the concentration distributions found experimentally during the space flight. The sensitivity of the system to periodic acceleration disturbances is complex and increases with decreasing frequency, with 10-2g0 becoming intolerable at 10-1 Hz, and 10-4g0 around 10-3 Hz. An impulsive perturbation of 10-3g0 of 1 s duration was fo und to be of little consequence for the growth rate.