Numerical investigation of multiple-slot jets in air-knife wiping

Gas-jet wiping using an air knife is an effective hydrodynamic method to control the coating thickness of zinc on a moving steel substrate in the continuous hot-dip galvanizing process. The current generation of single-slot air knives is widely used in the galvanizing industry but has limitations in producing low coating weights at the higher line speeds desired for the current generation of automotive sheet steel products. In this work, a novel configuration of a multiple-slot jet (multijet) air knife is investigated through numerical simulations as an alternative to the traditional single-slot air knife. The aim of this study is to investigate the sensitivity of the coating weight to the pressure and shear stress profiles in order to determine whether there are operating regions that are more robust to air-knife geometry changes. A modified geometry for the multislot air knife is proposed based on computational fluid dynamics results obtained from a parametric study. The effects of different operating conditions such as the main jet Reynolds number (Rem), auxiliary jet Reynolds number (Rea), and jet-to-wall distances (Z/D) on the final coating thickness were investigated. The results of the modeling showed that by setting the auxiliary jet Reynolds number at a fraction (25%) of the main jet Reynolds number, lighter coating weights can be achieved for higher strip velocities and higher wall-to-jet distances as compared to the single-slot jet design. It is believed that this geometry will provide a robust operating window to enable the prototype design to be employed in the industrial setting.