Modeling the Interface Dynamics in Continuous Casting Molds employing ANSYS CFX

There are some mechanisms in continuous casting that could cause the entrapment of slag. One of them is the entrapment due to the high shear stress acting on the interface between the molten steel and the slag. There are few works in the literature about the behaviour of the interface between slag and steel. These works apply experimental and numerical approaches to this problem. However, instead of slag and steel others fluids are employed. In order to evaluate the steel-slag interface dynamics, simulations of water flow in continuous casting molds are carried out employing the software ANSYS CFX 11.0. The results are compared to experimental and numerical results published in the literature. ANSYS CFX results presented good agreement with the experimental data. 1. SLAG ENTRAPMENT IN CONTINUOUS CASTING INTRODUCTION In continuous casting molten steel is injected into the mold through a submerged entry nozzle (SEN). The molten steel flows from the SEN, reaches the narrow face of the mold and then flows towards the SEN again, causing shear stress on the interface. Depending on the superficial velocity of the molten steel near the slag-steel interface, slag can be entrained into the steel bath [4]. This entrapment of slag results in a defect in the final product and must be avoided. Therefore it is of fundamental importance to determine the critical value of the superficial velocity of steel beyond which slag is entrained. In the literature there are few works about the behaviour of the interface between slag and steel. These works apply experimental and numerical approaches to this problem. However, instead of slag and steel, other fluids are employed. For the case of silicon oil and water flow, it is related that the critical superficial velocity of water beyond which silicon oil is entrained in the water bath lies between 0.15 and 0.4 m/s [5,6]. One of such works employing different fluids was done by Gupta and Lahiri [3]. They studied experimentally the water flow inside casting molds with different aspect ratios. Employing different port diameters and angles, they analysed the effect of the casting-speed in the meniscus behaviour. In their paper there are five consecutive frames showing the interface position at different times, with an interval of 0.04 s among them. One of these pictures shows the entrapment of a bubble. Based on the work of Gupta and Lahiri, Dash and coworkers [2] performed twodimensional numerical simulations of the water flow in a mold. Their objective was to predict the shape of the free surface and the entrapment of air bubbles. They compared their numerical results with those obtained by Gupta and Lahiri in their experiments, and concluded that their results presented good agreement with the experimental ones. In this work, numerical simulations of the water flow in 1:8 aspect ratio mold of [3] were carried out employing the software ANSYS CFX 11.0. The results obtained in these simulations were compared to those obtained numerically by [2] and experimentally by [3], and will be presented further. 2. EXPERIMENTAL MODEL In their study, Gupta and Lahiri employed a 80 mm-thickness mold and a SEN with parallel ports. The submergence depth was kept at 150 mm. This mold is illustrated in Fig. 1, which is based on the figure presented in [2]. In this figure the shaded area indicates the portion of the domain occupied by water at time t=0. Fig. 1: The mold employed in the experiments by [3] and in the numerical simulations (Dimensions in mm). In the experiments the water flow rate was equal to 1.22 x 10 m/s.