Numerical analysis for the coating thickness prediction in continuous hot-dip galvanizing

Abstract

Gas wiping is a decisive operation in the hot-dip galvanizing process. Especially, it has a crucial influence on the thickness and uniformity of coating film, but may be subsequently responsible for splashing. To date, the number of fundamental studies on the jet structure impinging on a vertical moving strip for various nozzle systems has not been sufficient to draw any meaningful conclusion. In this connection, at first, to confirm the validation of numerical analysis, the impinging jet pressure on the surface of a vertical strip by experiment is compared with the results by numerical analysis. Next, after confirming for the superiority of a constant expansion rate nozzle in splashing, the relationship between the stagnation pressure and the impinging jet pressure distribution issuing from the nozzle system of constant expansion rate is investigated. Finally, by using the calculated wall shear stress, the relationships among the coating thickness, strip speed and nozzle stagnation pressures are clarified. It is found that the impinging wall pressure for the case of constant expansion rate nozzle is more favorable for the problem of splashing to the case of the conventional one. Furthermore, from the point of view of energy conservation, it is advisable to use a constant expansion rate nozzle as a gas-wiping nozzle.