Design of a rotor for aluminum degassing assisted by physical and mathematical modeling
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Design of a rotor for aluminum degassing assisted by physical and mathematical modeling Ramírez-Argáez, M.1, Abreú López, D.1 and González Rivera, C.1 1
Departamento de Ingeniería Metalúrgica, Facultad de Química, UNAM, Circuito de los Institutos s/n, Cd. Universitaria, Del. Coyoacán, C.P. 04510, Cd. De México,
ABSTRACT
Recent studies on aluminum degassing [1, 2] show that although the impeller speed and the gas flow rate are important process variables in terms of the productivity and operational costs, the impeller design is also a key design parameter influencing the productivity and the quality of the aluminum in foundry shops. In this work, an improved design of an impeller is tested through a water physical model and mathematical modeling and its performance is compared against commercial designs of impellers. A full-scale water physical model of a batch aluminum degassing unit was used to test the impellers by using the same operating conditions (580 rpm and 40 liters per minute) and by performing deoxidation from water by purging nitrogen into the water saturated with oxygen (similar to the dehydrogenation). A mathematical model based on first principles of mass and momentum conservation equations was developed and solved numerically in the commercial CFD code ANSYS Fluent to describe the hydrodynamics of the system with the objective of explaining the deoxidation kinetics observed in the experiments. It has been found that the new impeller design shows a better performance than the commercial designs in terms of degassing kinetics for the conditions used in this study, which is explained since the new design promotes a flow dynamics that increases the pumping effect, creating a bigger pressure drop and fluid flow patterns which help to drag and distribute more evenly the bubbles in the entire ladle than the commercial designs.
INTRODUCTION The quality of aluminum castings highly depends on the refining processes where nonmetallic inclusions, alkali metals and hydrogen are removed from the melt. Removal of impurities is achieved by fluxing gases through a rotating impeller that breaks-down gas into fine bubbles with a big interfacial area, high residence time and preferentially evenly distributed in the entire ladle. Physical and mathematical modeling have been employed to understand the degassing process through the impeller-injector technique. Regarding mathematical models, Zhang et al. [3] published a review that describes chronologically the evolution of the mechanistic models for simulation of the degassing of aluminum. Two kinds of models are described. The first type of model is based on a simple mass balance of hydrogen for both batch and continuous reactors. This type of model can be subdivided into four types of models that predict hydrogen removal evolution in time during gas fluxing based. All of these models assume that degassing is controlled by mass transfer of hydrogen in the liquid side of the liquid-bubble interphase, where the key parameters are the interfacial area and the global mass
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