SCHOOL OF MECHANICAL ENGINEERING SEMINAR Monday, July 9, 2012 at 15:00 Wolfson Building of Mechanical Engineering, Room 206

Ph.D. student of Prof. Eliezer Kit and Prof. Alexander Gelfgat

School of Mechanical Engineering, Tel-Aviv University

This study is devoted to experimental measurements of primary and consequent bifurcations of flow driven by buoyancy, thermocapillarity (Marangoni convection) and rotation in Czochralski (CZ) configuration. Our primary efforts are focused on developing of an experimental facility for modeling of CZ melt flow enabling measurements of instabilities that can grow in such a model at various flow parameters. One of important objectives of this study is collecting experimental data for validation of computational codes that are developed for detecting the instability onset.

In the current study we implemented two setups that have essentially different geometrical scaling: the main (large) one and a scaled-double-down model. The characteristic size of the main experimental setup is chosen large enough to keep the critical temperature differences typical for instability onset around few degrees. This allows us to use distilled water and silicone oils as working liquids without accounting for their temperature-dependent physical properties in consequent computational modeling. On the other hand, the scaled-double-down model allows us to increase resolution of temperature scale. We employed an advance experimental technique by combining various measurement methods: the optical facilities such as Mach- Zehnder interferometry, Schlieren and Shadowgraph were used along with the thermocouples measurements, so that the thermocouples measurements are cross verified by independent and nonintrusive observations.

The sets of experiments were conducted with and without dummy rotation. The results, especially those without dummy rotation, show a good agreement between fundamental frequency and its harmonics measured by the interferometer and thermocouples located in the centre of the apparatus. The axial symmetry is conserved at low temperature differences up to the value ΔT = 3.5K (Gr =6.65×105). An increase in the temperature difference leads to the loss of axial symmetry and a transition from steady to time-periodic flow. We study how the transition point and the oscillations frequency depend on the geometry, crystal dummy rotation and Prandtl number. Basing on our results we confirm the theoretical prediction of destabilization of the Czochralski convective flow by weak rotation.

Experimental Study of Instabilities in Model of Czochralski Melt Flow Driven by Buoyancy, Thermocapillary Forces and Rotation

Vitaly Haslavsky