Modelamento matemático do escoamento de fluidos no processo RH para previsão da taxa de circulação do aço

Abstract

The RH process is a refining process used in the production of interstitial free steels. In this process, vacuum and inert gas injection promote the refining reactions (removal of C, H and N). The vacuum chamber is connected to the ladle containing liquid steel by two snorkels. The circulation of liquid steel between the vacuum chamber and the ladle is induced by gas injection through nozzles located at the upleg snorkel. The circulation rate has a significant effect on the decarburization and degassing rates and, consequently, on the productivity of the equipment. In the present work, a mathematical model to simulate the two-phase fluid flow in the RH degasser was developed. Modeling of turbulent two-phase flow is not a simple task, particularly under the conditions prevailing at metallurgical reactors. Different forces have to be considered and different models have been proposed to evaluate these forces. None of these models has been fully validated and they usually require the definition of coefficients whose values change according to the system being studied. In the case of the RH degasser, the variations of pressure and temperature inside the computational domain and the presence of a free surface add more difficulties to the development of an accurate mathematical model. The mathematical model for two-phase flow in a RH degasser was developed using the commercial software Ansys-CFX 11. In the present approach, the mathematical model was developed to simulate two phase flow in a physical model of the RH degasser, using water to simulate steel. In this physical model, the variations of pressure are much less significant and an isothermal domain can be considered. Different models for turbulence and also for the interphase drag and non-drag forces have been considered. An Eulerian-Eulerian approach has been adopted. The predictions of the model in terms of melt circulation rate were compared to experimental results obtained in a physical model of a RH degasser. In this model, water was used to simulate liquid steel and air was used to simulate argon. The melt circulation rate in the physical model was determined by the injection of a solution of potassium chloride at the upleg snorkel and measuring the variation of its concentration with time at the downleg snorkel. Images taken from the physical model at the upleg snorkel and of the vacuum chamber were also used to validate the predictions of the model. The predictions of melt circulation rates with the different versions of the mathematical model were close to the experimental results, but none of the models could exactly reproduce the variation of the melt circulation rate with the gas flow rate. These results indicate that some xv adjustments are still required to improve the quality of the predictions of the mathematical model.