Impact of the vertical potshell deformation on the MHD cell stability behavior of a 500 kA aluminium electrolysis cell

In previous publications [1,2], it was rationalized that a large vertical potshell deformation may have a negative impact on the operations of very high amperage cells. The MHD-Valdis non-linear Magneto-Hydro-Dynamic model was therefore extended to take into account the displacement of the potshell. The MHD cell stability behavior of a 500 kA cell with a 17.3 meters long potshell was then studied.

Models for magnetohydrodynamics of aluminium electrolysis cells

The electric current and the associated magnetic field in aluminium electrolysis cells create effects limiting the cell productivity and possibly cause instabilities: surface waving, ‘anode effects’, erosion of pot lining, feed material sedimentation, etc. The instructive analysis is presented via a step by step inclusion of different physical coupling factors affecting the magnetic field, electric current, velocity and wave development in the electrolysis cells. The full time dependent model couples the nonlinear turbulent fluid dynamics and the extended electromagnetic field in the cell, and the whole bus bar circuit with the ferromagnetic effects. Animated examples for the high amperage cells are presented. The theory and numerical model of the electrolysis cell is extended to the cases of variable cell bottom of aluminium layer and the variable thickness of the electrolyte due to the anode non-uniform burn-out process and the presence of the anode channels. The problem of the channel importance is well known Moreau-Evans model) for the stationary interface and the velocity field, and was validated against measurements in commercial cells, particularly with the recently published ‘benchmark’ test for the MHD models of aluminium cells [1]. The presence of electrolyte channels requires also to reconsider the previous magnetohydrodynamic instability theories and the dynamic wave development models. The results indicate the importance of a ‘sloshing’ parametrically excited MHD wave development in the aluminium production cells.

Thermoelectric MHD effects on equiaxed crystal morphology

The effects of a constant uniform magnetic field on a growing equiaxed crystal are investigated using a 3-dimensional enthalpy based numerical model. Two cases are considered: The first case looks at unconstrained growth, where the current density is generated through the thermo-electric effect and the current circulates between the tips and roots of the dendrite, the second represents an imposed potential difference across the domain. A jump in the electrical conductivity between the liquid and solid causes the current density to be non uniform. In both cases the resulting Lorentz force drives fluid flow in the liquid phase, this in turn causes advection of the thermal and solute field altering the free energy close to the interface and changing the morphology of the dendrite. In the first case the flow field is complex comprising of many circulations, the morphological changes are modelled using a 2D model with a quasi 3D approximation. The second case is comparable to classic problems involving a constant velocity boundary.

Thermoelectric MHD in dendritic solidification

The effects of a constant uniform magnetic field on dendritic solidification were investigated using an enthalpy based numerical model. The interaction between thermoelectric currents on a growing crystal and the magnetic field generates a Lorentz force that creates flow. The need for very high resolution at the liquid-solid boundary where the thermoelectric source originates plus the need to accommodate multiple grains for a realistic simulation, make this a very demanding computational problem. For practical simulations, a quasi 3-dimensional approximation is proposed which nevertheless retains essential elements of transport in the third dimension. A magnetic field normal to the plane of growth leads to general flow circulation around an equiaxed dendrite, with secondary recirculations between the arms. The heat/solute advection by the flow is shown to cause a change in the morphology of the dendrite; secondary growth is promoted preferentially on one side of the dendrite arm and the tip velocity of the primary arm is increased. The degree of approximation introduced is quantified by extending the model into 3-dimensions, where the full Navier-Stokes equation is solved, and compared against the 2-dimensional solution.

External circuit system effects on chlorine plasma instabilities

The effects of the external circuit on plasma instabilities in all inductive plasma source are investigated. The instabilities are found to be asymmetric with respect to the circuit input impedance. A simplified model of the antenna-plasnia coupling provides an explanation of the asymetry.

Parametric instabilities and localization of nonlinearly coupled electromagnetic modes in astrophysical dusty plasmas

The nonlinear coupling between the Alfven-Rao (AR) and dust-Alfven (DA) modes in a uniform magnetized dusty plasma is considered. For this purpose, multi- fluid equations (composed of the continuity and momentum equations), the laws of Faraday and Ampere and the quasi-neutrality condition are adopted to derive a set of equations, which show how the fields of the modes are nonlinearly coupled. The equations are then used to investigate decay and modulational instabilities in magnetized dusty plasmas. Stationary nonlinear solutions of the coupled AR and DA equations are presented. The relevance of the investigation to nonlinear phenomena (instabilities and localized structures) in interstellar molecular clouds is also discussed.

Phonons, Instabilities and Origin of Polarization in KDP Crystals

The -phonons of KH2PO4 (KDP) and its deuterated analog DKDP are studied via first-principles linear response calculations. The paraelectric phase shows two instabilities. One for a z-polarized mode, which leads to the spontaneous polarization Ps of the ferroelectric phase. The other corresponds to a two-fold degenerate xy-polarized mode. Other phonons are analyzed, which couple to the ferroelectric one at large amplitudes and are relevant for the ferroelectric transition. We show that Ps is mainly of electronic nature, since it arises mostly from an off-diagonal component of the Born effective charge tensor of H, with minor contribution from P atoms displacements.