| Abstract | In continuous galvanizing line operations, the thickness and the uniformity of the zinc coating are governed by the gas jet wiping process which is a complex multi-phase, multi-scale phenomenon. In this process, a steel strip is passed continuously through a molten zinc bath at around 460°C. When the steel strip exits from the bath, it is coated with a layer of liquid zinc, whose thickness is controlled by a wiping gas jet impinging on both sides of the strip. The dimensional characteristics and stability of the coating depend on various factors such as the jet dimensions and speed, the viscosity of the liquid zinc, the speed of the strip and the height of the gas knifes. This makes the process optimization difficult and the right operating window is hard to achieve, especially when there are changes in the product type or bath operations. To advance the fundamental understanding of such intricate process, an efficient and accurate numerical model would be highly valuable. In this work, we present the development of a finite element method for free surface flows in the presence of surface pressure and shear forces such as those encountered during the wiping process. The method considers the effect of the gravity and the boundary conditions describing the moving strip. Solutions are obtained on a fixed mesh by using the level-set technique for the free surface and a discontinuous pressure gradient approach on elements cut by the liquid/gas interface. The effect of the wiping jet is incorporated through distributed normal and tangential forces on the liquid/gas interface reproducing the pressure and shear stress distribution from the turbulent gas flow. Those forces can be obtained either from experiments or by numerical simulation. The method is first validated for the liquid entrainment inside a cavity, the upper surface being subjected to shear forces. Then, the methodology is applied to solve the flow dynamics in the coating layer under the effect of wiping jets. It is shown that the method is able to tackle the extreme range of length scales encountered, from the one of the moving strip in the order of meters, to that of the coating thickness of only a few microns. The finite element stabilization method capable to handle the high aspect ratio meshes employed for this application is also discussed. Finally, a study of the effect of different parameters on the coating thickness will be presented. |
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