School of Mechanical Engineering Evgeny Miroshnichenko
School of Mechanical Engineering Seminar
Wednesday, June 13, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Scrutiny of Instabilities in Czochralski Crystal Growth Configuration Using Complementary Experimental Technologies
Mr. Evgeny Miroshnichenko
Mr. Evgeny Miroshnichenko
Ph.D Student of Prof. E. Kit
A parametric experimental study of cold plume instability that appears in the large-Prandtl-number Czochralski melt flows is reported. The critical temperature difference (the critical Grashof number) and the frequency of appearing oscillations were measured for varying Prandtl numbers, aspect ratios of the melt, and crystal/crucible radii ratio. The measurements were carried out by two independent and fully non-intrusive experimental techniques. The results are reported as dimensionless parametric dependencies, and then are joined into relatively simple empirical relations.
The parametric relations for the critical Grashof number and oscillations frequency are extended to include parameters of the capillary meniscus height for different Prandtl numbers, radii and aspect ratios. The results show that with increase of the meniscus height the critical temperature difference noticeably grows and sometimes doubles. The difference between results obtained for “short” and “tall” menisci is noticeable. This might explain the saturation of the dependence of the critical Grashof number on meniscus height. An additional qualitative experiment using PVC tube replaces the meniscus allows to observe even reverse the sense of the temperature dependence following the saturation.
Modeling of effects associated with the nonlinear crystal front shows different flow regimes distributed under the dummy. The results show that the presence of liquid filled cavity inside the crystal reduces dramatically the stability factor, even while axial symmetry preserved. Experiments preventing surface tension by covering of free surface estimated a contribution of Marangoni effect to instability. Therefore, a correction term considering these effects is required for the forgoing critical Grashof number and the frequency relations.
Two- and three dimensional flow patterns of supercritical regimes and different types of instability occurs in Czochralski model were visualized using Schlieren method.
Baroclinic instability caused by dummy rotation were also considered in purpose to examine Richardson/Reynolds numbers dependence over widened range of rotation rates.
