Development of Novel Air-knife System to Prevent Check-mark Stain on Galvanized Strip 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 normally impinging a thin plane nitrogen gas jet. In such a gas wiping process, frequently there appears stain of check-mark, hereinafter called “check mark stain”. The check mark stain is due to non-uniform zinc coating over the surface of the steel strip. Presence of such check-mark stain lowers the quality, productivity and profitability of the end products. From our proceeding research, it was found that there are alternating stream-wise vortices impinging on the steel strip that move almost periodically to the right and to the left along the stagnation line due to the jet flow instability. This instability is closely related to buckling of the center sheet of the plane jet. Since higher stagnation pressure removes more molten zinc adhered on the surface, the zinc coating thickness is thinner at the high pressure point. In addition, since the strip moves upward with a constant speed, the non-uniform coating surface is formed with a variety of patterns like “W”, “V” and “X”.
In the present study, in order to avoid the appearance of the check-mark stain, a new type of the air-knife system is proposed. This system consists of the main jet and a guide jet located beneath the main jet. The main jet removes the molten zinc and the guide jet prevents the formation of alternating impinging vortices on the steel strip so that it suppresses the cause of the check-mark stain. The design concept of the proposed air-knife system is verified by investigating the 3-D turbulent flow in the impingement jet region obtained numerically by using a commercial code FLUENT. Large eddy simulation (LES) technique is used to solve the governing conservation equations of mass, momentum and heat for 3-D compressible turbulent flow field.