Design of a physical model for fluid flow in steel melt

In steelmaking, fluid flow in steel melt controls mixing and mass transfer reactions in converter and ladle. Fluid flow in the tundish of a continuous caster is also important to evaluate the performance of the tundish with reference to its ability to distribute molten steel  in all molds at constant superheand to remove inclusions during the process of continuous casting. Experiment in full scale size of the steelmaking vessel with molten steel is very difficult and pose practical difficulties. Suitably designed models are very helpful to conduct large number of experiments to arrive at optimum results. These optimum results can be verified in the prototype selectively.

i) Selection of model vessel

The prototype vessels in steelmaking are converter, ladle and tundish. Converters and ladles are more or less cylindrical in shape. Whereas tundish is a rectangular with side walls inclined. Model vessel is designed by geometric similarity. A scale factor =1 represents full scale model. Full scale models may become difficult to handle since the dimensions involved would be large.

We select scale factor for the purpose of illustration. If the industrial ladle has a diameter of 4m, model vessel diameter would be 80cm, while the aspect ratio (bath height/bath diameter) for both vessels will be same. The aspect ratio of industrial ladle is 0.9. Therefore model bath height is 72cm. similarly we can design model converter and model tundish by selecting a suitable scale factor.

ii) Selection of model steel melt phase

In order to compare the results of two geometrically similar systems it is essential that transport mechanisms should be similar in both the systems. For example if flow is turbulent in the prototype then turbulent flow should also prevail in model liquid. Density and viscosity of the fluid are the two important fluid properties that govern fluid flow behavior. Density represents inertia of fluid against an applied force and viscosity is internal friction of fluid. The ratio of density to viscosity, that is

(11)

is called kinematic viscosity of fluid. Kinematic viscosity represents the diffusion of momentum flux into the liquid and governs the fluid flow behavior. Though absolute value of viscosity and density of steel melt may differ from the model liquid, similarity in kinematic viscosity in both the fluids ensures similar fluid flow behavior. In this connection water is the fluid whose kinematic viscosity is  which is very close to that of molten steel melt.  Thus water can be selected as model liquid. In fact water model has been very widely used to investigate the behavior of steel melt. Some references are given at the end of the lecture.