Study Of Coanda Double Air-knife In Jet Wiping Process Of Continuous Hot-dip Galvanizing Lines

The rapid development of domestic construction and automotive industries, stimulate the demand for high quality steel strip. C urrently in the process of hot-dip galvanizing, the coating thickness is controlled by impinging gas jets scrape off excess the zinc liquid which adhere on the strip surface.At first, the numerical prediction model of steel strip surface coating thickness is obtained by combining the control equation for zinc liquid flows which is based on the theory of fluid mechanics with gas jet flow fields. The results of the model obtained the galvanized strip surface coating thickness agrees well with measured and the literature results.On industrial continuous hot-dip galvanized steel strip surface, uneven coating with visually observable quasi-periodic patterns has occurred, which we call zinc ripples. The large eddy simulation(LES) method is adopted to analyze the characteristics of the zinc ripples. Prediction model of galvanized coating thickness distribution and numerical calculation method is put forward based on the gas jet unsteady turbulent flow of large eddy simulation. O ur numerically predicted coating thickness distribution agrees well with that on a real galvanized steel strip. Parametric effects on the zinc ripples are then investigated numerically. It is found that with smaller impingement distance, smaller lip gap and larger air-knife pressure, the zinc ripples become less significant.Then we studied the single air-knife, Coanda single air-knife, double air-knife on effects of the steel strip surface coating thickness. Contrast with the single air-knife, the results show that Coanda single air-knife can make jet flow deflection so as to improve the effect of jet wiping. Double air-knife 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 zinc ripple s.Finally, combining the advantages of Coanda single air-knife and double air-knife studied a new type Coanda double air-knife. Three different structure forms of Coanda double air-knives has been studied, namely the second knife lip for Coanda knife lip, the third knife lip for Coanda knife lip, the second & third knife lip for Coanda knife lip. Next the surface of strip steel zinc coating thickness and the zinc ripples is studied. The results show that double air-knife with the third knife lip for Coanda knife lip can get more uniform distribution of the coating thickness.In this paper, the research results laid the important theoretical basis for improving the surface quality of the continuous hot-dip galvanized steel strip and increase its market share.