Aerodynamic Investigation of Air Knife System to Find Out the Mechanism of the Check Mark in a Continuous Hot-Dip Galvanizing Process

When galvanized steel strip is produced through a continuous hot-dip galvanizing process, the thickness of the adhered zinc film is controlled by a gas wiping process. In the gas wiping process there is a technically serious problem which is called a “check mark problem”. The check mark is caused by non-uniform coating on the steel strip surface. Such a non-uniform zinc coating lowers the quality, productivity and profit of the end products. In the present study, to find out the causes of the check mark and technical methods to resolve the check mark problem, the flow field of the high speed rectangular nitrogen gas jet which is impinging on the moving steel strip in the continuous hot-dip galvanizing system has been investigated numerically by using a commercial 3-D flow analysis code, FLUENT. LES (Large Eddy Simulation) is used to obtain instantaneous flow field in the region under consideration. Numerical studies were conducted for two ratios of the plate distance (d) to the nozzle width (x) d/x = 6.7, 10.5 under the same jet Reynolds number of Re = 20000. It was found that the check mark is caused by the alternating vortices which are generated on the jet impinging line (stagnation line). The center of the alternating vortex has a relatively low pressure compared with the periphery of the vortex. The high impinging pressure removes the adhered molten zinc more than the low pressure. Hence the non-uniformity of the zinc coating appears on the strip surface. Such the alternating vortices move periodically to the right and to the left sides on the impinging line due to the jet flow instability and the pressure force balance. In addition since the strip moves upward at a constant speed, the non-uniform coating results in a variety of patterns like “W”, “V” and “X”. This pattern is collectively called as “check mark” in the production field. The angle of the check mark was calculated by using both the moving speeds of the steel strip and the vortices. It was favorably compared with the experimental measurement.