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.

בלוקצ'יין, מטבעות קריפטוגרפיים ועתיד הכסף 
 

עם מארק סמרגון  
 שותף מייסד וסמנכ"ל לענייני בלוק'ציין חברת קולו

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יום רביעי 09.05.18

התכנסות 18:30 

הרצאה (כולל שאלות ותשובות) 19:00-20:30

פרטים והרשמה בלינק המצורף 

 יש להרשם מראש על מנת להבטיח מקומך

בוגרים, סטודנטים, חברי סגל של הפקולטה להנדסה, כולל בני/בנות זוג- ללא תשלום

אורחים בתשלום סמלי בסך 40 ש"ח- לתשלום לחצו כאן

School of Mechanical Engineering Seminar
Wednesday, May 30, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Region of influence of SAOB actuator on a turbulent boundary layer with application to a symmetrical airfoil

Nimrod Shay

MSc Student of Prof. Avi Seifert

School of mechanaical engineering

It is common practice that aircraft control surfaces have symmetric airfoils with low drag and high lift. It is also common to find low aspect ratio wings as control surfaces. Improving the aerodynamic performance of the control surfaces can significantly expand the flight envelope of an aircraft, including shorter take-off and landing, better maneuvering and better stability in side winds. This study is focused on lift enhancement of an SA 14s 2D airfoil and finite span wing constructed from the same airfoil using active flow control (AFC) methods in the form of steady suction and oscillatory blowing (SaOB). The first part of the study deals with investigating the interaction of the SaOB actuator created perturbations with a turbulent flat plate boundary layer. The second part includes wind tunnel testing of a 2D airfoil model; baseline experiments, using suction only and using the combined effect of the SaOB actuator. The chord length of the model is 528mm (constant) and the experiments were performed at Reynolds numbers in the range of 600k-1.4M. The last and final part of the study includes wind tunnel testing of a finite wing model; again baseline experiments as well as SaOB experiments. The mean chord length is 475mm (trapezoidal shaped wing) and the experiments were performed at free-stream velocities of 22, 26 and 33 [m/s] which correspond to Reynolds numbers of 675K, 800K and 1.01M, respectively. It was found that applying AFC can lead to a reduction of up to 50% in the flat plate turbulent boundary layer's momentum thickness. When applied to the specific airfoil, a significant improvement in maximum lift and stall delay was created. Using suction alone proved to be a simple and robust solution with high energy efficiency, while suction combined with oscillatory blowing has a greater potential but require carful care in designing and incorporating the AFC system.