Aerodynamic investigation about the cause of check-mark stain on the galvanized steel surface

Abstract

When galvanized steel strip is produced through a continuous hot-dip galvanizing process, the thickness of the adhered zinc film is controlled by impinging a thin plane nitrogen gas jet. In such a gas wiping process frequently there appears stain of check-mark shape. The check-mark stain is caused by non-uniform zinc coating over the surface. In order to find the main reason of the appearance of the check-mark stain, the unsteady compressible flow field is numerically simulated by using LES turbulence model for two cases of L/d=6.67 in the Case 1, 11.54 in the Case 2 where d is the nozzle width and L is the lip-to-plate distance. It was found that there are alternating plane-wise vortices near the impinging stagnation region, and that such alternating vortices move almost periodically to the right and to the left sides on the stagnation line due to the jet flow instability and alternating pressure peaks along the stagnation line. Since higher stagnation pressure removes more molten zinc adhered on the surface, the zinc coating thickness is thinner there. In addition, since the strip moves upward at a constant speed, the non-uniform coating surface is formed with a variety of patterns like "W", "V" and "X". The angle of the check-mark was calculated by using both the moving speeds of the steel strip and the sidewise movement of the vortices along the stagnation line. It was favorably compared with the experimental measurement.