7 research outputs found
Linear stability of buoyant convection in a horizontal layer of an electrically conducting fluid in moderate and high vertical magnetic field
No full textLinear stability of buoyant convective flow in a horizontal layer of an electrically conducting fluid is considered with reference to horizontal Bridgman semiconductor crystal growth. The fluid flows owing to the horizontal temperature gradient in the presence of a vertical magnetic field. The main interest here is in the stability of the flow for a sufficiently strong magnetic field, for the Hartmann number Ha > 10, and increasing to high values, of the order of 103–104. The Prandtl number, Pr, has been fixed at Pr = 0.015. It is shown that besides the Hartmann number the instability strongly depends on the type of the thermal boundary conditions at the horizontal walls. For thermally conducting walls the basic temperature profile exhibits zones of unstable thermal stratification, which leads to instabilities owing to the Rayleigh-Bénard mechanism. However, the transitions between various, most unstable modes as Ha increases are not trivial. For sufficiently high values of Ha, the most unstable mode consists of transverse oscillatory rolls located in the region of unstable stratification. For thermally insulating walls, the transitions are simpler, and for sufficiently high Ha, the most unstable mode consists of longitudinal, steady, three-dimensional mode which is concentrated in the Hartmann layers at the horizontal boundaries. This mode has a combined dynamic-thermal origin and is owed to a strong shear in the Hartmann layers. The electrical boundary conditions do not qualitatively affect the picture of transitions between modes for both thermally conducting and thermally insulating walls.Publisher Statement: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Hudoba, A, Molokov, S, Aleksandrova, S & Pedcenko, A 2016, 'Linear stability of buoyant convection in a horizontal layer of an electrically conducting fluid in moderate and high vertical magnetic field' Physics of Fluid, vol 28, 094104 and may be found at https://dx.doi.org/10.1063/1.4962741 </p
Measurement technique for instant temperature distribution in the melt of large-scale Czochralski silicon growth
No full textIn the current work measurement method has been developed that allows to get instant temperature distributions in the melt of large diameter Czochralski silicon single crystal growth. Measurements of the velocity of the convective melt flows are also provided. Nowadays single-crystalline silicon is mainly used in electronic industry for the IC's production as a substrate material. Single crystals are pulled out from the melt by the Czochralski method (CZ). The quality of the single crystals is determined by various factors, but the most important among them are hydrodynamic flows due to non-stationary convection. The morphology of such flows determines radial and axial distribution of dopands in the growing crystal. The control of unstable convection is the most important problem in the modern Czochralski technology. The solution (that is state-of-art today) is the use of external magnetic fields (AC or DC) that allows to influence or damp the unstable motion of electrically conducting Si melt. These processes are very complex and for further optimisation of the CZ technology, especially in the context of transition to larger crystal diameters, it is necessary to conduct scientific investigations of the hydrodynamics and heat transfer phenomena in the CZ melt. Liquid silicon is a very aggressive material, therefore, temperature and velocity measurements in the melt are very expensive or practically impossible. The possibility of temperature field and distributions of the melt velocity investigation is provided in this work by means of a physical simulation technique and by a developed multi-channel acquisition system that allow to get instant temperature distributions in the whole cross-section of the melt and to study not only stationary temperature distributions in the melt, but, also to look after the temperature field evolution in time. That will allow to investigate various hydrodynamical instabilities in the melt. This work was carried out at the Institute of Physics of University of Latvia in the frame of grants of the Science Council of Latvia as well as in collaboration with one of the leader company in the field of the single silicon production - Wacker Sitronic AG (Germany).Abstract in English, Latvian, RussianAvailable from Latvian Academic Library / LAL - Latvian Academic LibrarySIGLELVLatvi
Velocity measurements in the liquid metal flow driven by a two-phase inductor
No full textWe present the results of velocity measurements obtained by ultrasonic Doppler velocimetry and local potential probes in the flow of GaInSn eutectic melt driven by a two-phase inductor in a cylindrical container. This type of flow is expected in a recent modification to the floating zone technique for the growth of small-diameter single intermetallic compound crystals. We show that the flow structure can be changed from the typical two toroidal vortices to a single vortex by increasing the phase shift between the currents in the two coils from 0 to 90. The latter configuration is thought to be favourable for the growth of single crystals. The flow is also computed numerically, and a reasonable agreement with the experimental results is found. The obtained results may be useful for the design of combined two-phase electromagnetic stirrers and induction heaters for metal or semiconductor melts
The Effect of “Wave Breakers” on the Magnetohydrodynamic Instability in Aluminum Reduction Cells
No full textWe report the results of the experiments on the suppression of the MHD instability in a model of the aluminium reduction cells. The idea behind the study is to introduce obstacles in the liquid metal to suppress the propagation of the rolling-pad instability wave. As a result, in some configurations with obstacles we detect lowering of the wave amplitude, reduction of its propagation speed and rise of the main parameters' thresholds, responsible for the instability onset.<br/
The onset of instabilities and finite amplitude waves in a model of aluminum reduction cells with nonuniform cathode current
No full textIn aluminum reduction cells, the electric current density at the cathode is seldom uniform. This may be due to a variety of reasons. One of the reasons is that a mass of undissolved alumina may get enrobed by the cryolitic bath, and this mix freezes due to the decrease in temperature below the liquidus point. Thus, an electrically resistive solid layer (“mud”) is formed at a part of the cathode. This leads to horizontal electric currents in the liquid aluminum layer, and their magnetohydrodynamic (MHD) interaction with the vertical magnetic field induces a rotating flow of aluminum. This may cause undesirable MHD instabilities. A quasi-two-dimensional model for the laboratory facility has been presented. The results show that the dominating feature of the flow is a depression of the free surface of the liquid metal above the mud spot. This is due to strong rotation of the liquid metal around the mud spot, which causes the centrifugal force. Other effects may include superimposed conventional MHD instability, which manifests itself in a rotating interface, modulated waves, reflection of the waves from the corners of the domain, sloshing, etc. It has been shown that small mud spots do not affect the cell stability, while the large ones may cause such deep depressions that the centrifugal force completely removes the liquid metal above the spot
Experimental study of interfacial instability in aluminium reduction cells
No full textPreliminary results of a laboratory study of magnetohydrodynamic instabilities in aluminium reduction cells are presented. The onset of instability, its frequency and amplitude have been measured with electric potential probes and with an optical camera. Reconstruction of electric
potential inside the liquid metal shows good qualitative agreement with the instabilities observed in industrial reduction cells
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Experimental model of the interfacial instability in aluminium reduction cells
No full textA solution has been found to the long-standing problem of experimental modelling of the interfacial instability in aluminium reduction cells. The idea is to replace the electrolyte overlaying molten aluminium with a mesh of thin rods supplying current down directly into the liquid metal layer. This eliminates electrolysis altogether and all the problems associated with it, such as high temperature, chemical aggressiveness of media, products of electrolysis, the necessity for electrolyte renewal, high power demands, etc. The result is a room temperature, versatile laboratory model which simulates Sele-type, rolling pad interfacial instability. Our new, safe laboratory model enables detailed experimental investigations to test the existing theoretical models for the first time
