László I. Kiss

Comparative performance of nongray gas modeling techniques

This study compares the relative efficiencies (accuracy and computation time) of five methods developed for characterizing radiative exchanges in enclosures, mono- and bi-dimensional, containing real gases: the correlated-k method (CK), three versions of the hybrid CK/statistical narrow-band method (SNB-CK), and the spectral line-based weighted sum of gray gases method (SLW). Attention is drawn to the hybrid SNB-CK method featuring band regrouping strategies. The version proposed in this article, which is characterized by a selective regrouping of bands, leads to good results, while the computation time is significantly decreased with respect to the original SNB-CK version. Mono- and bi-dimensional enclosures are analyzed. Diagrams illustrate the recommended fields of application of the various methods.

Mixing Characteristics of an Axial-Flow Rotor: Experimental and Numerical Study

In the metallurgical industry, various types of rotors are used for the injection and distribution of gas and for homogenizing molten metal. In the present work, the liquid-gas two-phase flow around an axial type impeller is studied in a water model, in order to analyze the bubble break-up and coalescence and metal mixing. Details like primary and secondary vortex structure, gas flooding between the blades and gas dispersion are recorded by using high speed photography.A mathematical model that takes into account the combined effect of bubble break-up and coalescence is implemented in the commercial computational fluid dynamics (CFD) software FLUENT. In the proposed work, the impeller is explicitly described in three dimensions using Multiple Reference Frame Model. Dispersed gas and bubbles dynamics in the turbulent water are modeled using an Eulerian-Eulerian approach with dispersed k-epsilon turbulent model. The model predicts spatial distribution of gas hold-up, average bubble size and flow structure. Good qualitative agreement between physical model and simulation is achieved when comparing the bubble size distribution, flow structure and mixing.

Experimental Study of the Morphology and Dynamics of Gas-Laden Layers Under the Anodes in an Air–Water Model of Aluminum Reduction Cells

The bubble layer formed under an anode and the bubble-induced flow play a significant role in the aluminum electrolysis process. The bubbles covering the anode bottom reduce the efficient surface that can carry current. In our experiments, we filmed and studied the bubble layer under the anode in a real-size air–water electrolysis cell model. Three different flow regimes were found depending on the gas generation rate. The covering factor was found to be proportional to the gas generation rate and inversely proportional to the angle of inclination. A correlation between the average height of the entire bubble layer and the position under the anode was determined. From this correlation and the measured contact sizes, the volume of the accumulated gas was calculated. The sweeping effect of large bubbles was observed. Moreover, the small bubbles under the inner edge of the anode were observed to move backward as a result of the escape of huge gas pockets, which means large momentum transport occurs in the bath.

Optimization of Friction Stir Welding Tool Advance Speed via Monte-Carlo Simulation of the Friction Stir Welding Process

Recognition of the friction stir welding process is growing in the aeronautical and aero-space industries. To make the process more available to the structural fabrication industry (buildings and bridges), being able to model the process to determine the highest speed of advance possible that will not cause unwanted welding defects is desirable. A numerical solution to the transient two-dimensional heat diffusion equation for the friction stir welding process is presented. A non-linear heat generation term based on an arbitrary piecewise linear model of friction as a function of temperature is used. The solution is used to solve for the temperature distribution in the Al 6061-T6 work pieces. The finite difference solution of the non-linear problem is used to perform a Monte-Carlo simulation (MCS). A polynomial response surface (maximum welding temperature as a function of advancing and rotational speed) is constructed from the MCS results. The response surface is used to determine the optimum tool speed of advance and rotational speed. The exterior penalty method is used to find the highest speed of advance and the associated rotational speed of the tool for the FSW process considered. We show that good agreement with experimental optimization work is possible with this simplified model. Using our approach an optimal weld pitch of 0.52 mm/rev is obtained for 3.18 mm thick AA6061-T6 plate. Our method provides an estimate of the optimal welding parameters in less than 30 min of calculation time.

Thermal conductivity of halide solid solutions: measurement and prediction.

The composition dependence of the lattice thermal conductivity in NaCl-KCl solid solutions has been measured as a function of composition and temperature. Samples with systematically varied compositions were prepared and the laser flash technique was used to determine the thermal diffusivity from 373 K to 823 K. A theoretical model, based on the Debye approximation of phonon density of state (which contains no adjustable parameters) was used to predict the thermal conductivity of both stoichiometric compounds and fully disordered solid solutions. The predictions obtained with the model agree very well with our measurement. A general method for predicting the thermal conductivity of different halide systems is discussed.

Modeling the Behavior of Alumina Agglomerate in the Hall‐Héroult Process

During the feeding of the Hall-Heroult cell, cold alumina comes into contact with the electrolyte bath and tends to agglomerate due to the formation of a frozen bath layer that holds physically the alumina together. This agglomeration, producing alumina agglomerates, called also lumps or sludge, affects both the rate of alumina dissolution and the stability of the cell. In this paper, all the physical phenomena (heat and mass transfer) between the formation and the complete dissolution of agglomerate will be described and are defined by a set of equations. This mathematical model allows observing the strong coupling between the mechanisms of heat and mass transfer e.g. the solidification with diffusion of chemical species, the infiltration of the bath in the porous agglomerate and the dissolution of sintered alumina. A parametric study using this model might identify the most important factors related to the lifespan and behavior of alumina agglomerates.

Experimental Determination of the Thermal Diffusivity of α-Cryolite up to 810 K and Comparison with First Principles Predictions

In aluminum electrolysis cells, a ledge of frozen electrolyte is formed on the sides. Controlling the side ledge thickness (a few centimeters) is essential to maintain a reasonable life span of the electrolysis cell, as the ledge acts as a protective layer against chemical attacks from the electrolyte bath used to dissolve alumina. The numerical modeling of the side ledge thickness, by using, for example, finite element analysis, requires some input data on the thermal transport properties of the side ledge. Unfortunately, there is a severe lack of experimental data, in particular, for the main constituent of the side ledge, the cryolite (Na3AlF6). The aim of this study is twofold. First, the thermal transport properties of cryolite, not available in the literature, were measured experimentally. Second, the experimental data were compared with previous theoretical predictions based on first principle calculations. This was carried out to evaluate the capability of first principle methods in predicting the thermal transport properties of complex insulating materials. The thermal diffusivity of a porous synthetic cryolite sample containing 0.9 wt % of alumina was measured over a wide range of temperature (473–810 K), using the monotone heating method. Because of limited computational resources, the first principle method can be used only to determine the thermal properties of single crystals. The dependence of thermal diffusivity of the Na3AlF6 + 0.9 wt % Al2O3 mixture on the microstructural parameters is discussed. A simple analytical function describing both thermal diffusivity and thermal conductivity of cryolite as a function of temperature is proposed.

Structural Characterisation and Thermophysical Properties of the Side Ledge in Hall-Héroult Cells

In the modern Hall-Heroult cells, a frozen bath layer — the side ledge — protects the sidewalls from the very corrosive liquid bath. This frozen bath layer has a significant impact on the heat balance of the cells as well as on the bath chemistry. For this reason, the geometry, the structure, the distribution of the chemical composition and certain physical properties of the side ledge must be studied. Those characteristics are important to the development of a mathematical model for the Hall-Heroult cells. Despite of all the research efforts invested, only a few results are available in the published literature. This paper presents a few results and observations, obtained by the analysis of side ledge samples, extracted from post-mortem cells. The results show a very inhomogeneous structure and a strong dependence of the thermophysical properties on material structure and temperature.

Developing a New Process Indicator Based On The Relationship Between an Electrolysis Cell Impurity Balance and its Incidents

Aluminerie Alouette recently studied the relationship between electrolysis cell operation and their impurities’ balance. It was observed in some cases that the concentration of impurities in the metal is lower than expected from raw material mass balance, mainly for iron, gallium, titanium and vanadium. These cells were inspected to find the cause of the imbalance; methodology and results of the inspection are presented in this paper. Analyses regarding cell performance were conducted with regard to cell impurities distribution (metal/crust/gas duct). The factors correlating these phenomena are discussed, with focus on operational results. It was found that anodic incidents are strongly related to very low impurity concentrations in the aluminium and it was possible to develop an indicator at Aluminerie Alouette to quickly detect anodic incidents using vanadium content in the aluminium. This process tool is presented and discussed in the article.

Effect of the Bubble Growth Mechanism on the Spectrum of Voltage Fluctuations in the Reduction Cell

The spectrum of the voltage oscillations in the aluminum reduction cell depends on the size of the anode. Small laboratory cells and industrial size cells exhibit different fluctuation patterns. The growth of bubbles can be divided into two periods controlled by different physical mechanisms. The detachment frequency of the bubbles from the nucleation sites depends on the current density and detachment size, the latter being influenced by the material, microstructure and shape of the anode bottom. After detachment the main factor causing growth of the traveling bubbles is coalescence. A mathematical model that keeps track of each and every individual gas bubbles generated under the anode was developed and used to analyze the character of the fluctuations of the cell voltage. It is shown that in the case of industrial size anodes the voltage oscillations are dominated by the dynamics of the bubble interactions (coalescence) in the two-phase layer, while in small size laboratory systems the frequency of nucleation of the individual bubbles can be observed.