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Abstract
The injection of nitrogen in molten aluminum through a static impeller in a tank degassing unit is studied. Using basic principles of fluid mechanics, it is analyzed the influence of the nozzle diameter on the bubble diameter and the mean residence time of the bubbles in the molten aluminum. By means of transient isothermal 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions and the tank degasser dynamics is studied. Finally, an adiabatic CFD simulation is carried out in order to elucidate the changes in the molten aluminum temperature due to the injection of nitrogen at ambient temperature. This simulation shows that molten aluminum does not suffer drastic temperature reductions given that, in spite that the nitrogen is fed at ambient temperature, the mass of nitrogen is relatively small compared with the mass of aluminum.
Highlights
Molten aluminum is very reactive chemically, and promptly reacts with water forming atomic hydrogen which is dissolved in it
Results of the Computational Fluid Dynamics (CFD) transient isothermal 2D simulations show the evolution of the phase distributions in the tank degasser, and these results are depicted in Figures 5-7 for values of the nitrogen flow rate of 1, 1.5 and 2 m3/h, respectively
It can be noted that the nitrogen flow rates accelerates the tank degasser dynamics, i.e. the time required to achieve the same level of stirring in the melt decreases as the nitrogen flow rate is increased
Summary
Molten aluminum is very reactive chemically, and promptly reacts with water forming atomic hydrogen which is dissolved in it. The worldwide most employed process for hydrogen removal from molten aluminum is the inert gas injection by rotating impeller (IGIRI). The IGIRI process has been extensively analyzed by plant trials, physical modeling and computer simulations in order to elucidate the most important parameters and variables which determine the efficiency of hydrogen removal from molten aluminum. In [10] it is analyzed the influence of four major parameters (gas flow rate, rotational speed, refining time, and stewing time) on the degassing rate of an aluminum alloy with the IGIRI process by using an orthogonal experiment methodology. The authors report that the effectiveness of the rotary degassing process is highly dependent on the combination of rotational speed and the gas flow rate, and that a wrong combination of these factors may result in no improvement or even degradation in the quality of castings. Using transient 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions, the system dynamics and the temperature distribution is studied
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