Flow stabilization and coating thickness reduction through use of multi-slot air-knife in gas jet wiping

Abstract

This paper investigates the gas jet wiping process, which is widely employed for controlling the final zinc-alloy coating thickness of a moving steel strip in the continuous hot-dip galvanizing process. In this study, large eddy simulation (LES) was employed to determine the effect of lower velocity symmetric auxiliary jets on main jet stabilization and coating thickness reduction during the gas jet wiping process. For validation purposes, experimental measurements of coating thickness were also carried out for the prediction of final coating thickness via a novel prototype multi-slot air knife used as the wiping actuator. Good agreement was found between the experimental measurements and model predictions for the coating weight, which also confirmed the applicability of the Elsaadawy et al.¹ model for the prediction of final coating weight for the multi-slot air knife geometry used. It was found that the jet flapping observed during single jet wiping could be prevented through use of the multi-slot air knife operating with lower velocity symmetric auxiliary jets (Re_a/Re_m =  0.45) situated on both sides of the main jet. The auxiliary jets modified the flow field in the main jet shear layer and diminished the formation of alternating vortices, which are the main cause of jet flow oscillations. As a result, a stabilized impinging flow with a longer potential core was found for the multi-slot jet configuration. This led to increased pressure gradient and increased shear stresses in the vicinity of the wiping region and, consequently, lower coating weights were obtained through use of the multi-slot air knife compared to the conventional single slot jet.