Experimental and Numerical Study of Coating Thickness Using Multi-slot Air Knives
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he steel industry, the continuous hot-dip galvanizing (CHDG) process is broadly used in order to protect the underlying steel substrate from the effects of environmental degradation. The CHDG process includes a molten zinc (Zn) bath, usually held at 733 K (460 C), in which the steel substrate is constantly submerged, resulting in deposition of the liquid Zn-AlFe alloy on the steel substrate.[2] Upon exiting the bath, the substrate is coated with a rather thick layer of liquid zinc because of viscous drag forces.[3] In order to control the coating layer thickness on the steel, a pair of impinging gas jets (referred to as air knives in the industry), generally in the single-slot configuration, is located above the bath (Figure 1). These air knives remove the excess zinc by applying a pressure gradient (dp/dx) and shear stress (s) to the coating layer and return the excess molten Zn liquid to the bath.[3] In this
A. YAHYAEE SOUFIANI and J.R. MCDERMID are with McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4L8, Canada. Contact e-mail: [email protected] A.N. HRYMAK is with Western University, 1151 Richmond Street North, London, ON, N6A 5B9, Canada. F.E. Goodwin is with the International Zinc Association, 2530 Meridian Parkway, Suite 115, Durham, NC, 27713. Manuscript submitted May 4, 2019.
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manner, the desired final film thickness (hf), which is a function of the nozzle pressure Ps, the strip velocity Vs, the nozzle-to-moving sheet distance Z, the jet width D, and the viscosity and density of the liquid zinc,[4] can be obtained after the wiping action of the air knives. Steel sheet products are generally used by the automotive industry for either structural members or closure panels. A recent trend within the automotive industry has been to reduce the Zn coating weight applied to the steel sheet in order to reduce the overall mass of the automotive body-in-white, thereby increasing fuel efficiency and reducing costs.[5] Furthermore, the industry is motivated to reduce the local variability of the zinc coating thickness in order to mitigate the practice of ‘‘over-coating,’’ also with the objective of reducing costs. There are several studies available in the literature on using a single-slot turbulent impinging gas jet for controlling the liquid zinc coating thickness on the metal strip during continuous galvanizing.[3,4,6–8] The early work on modeling the gas-jet wiping process to predict final coating thicknesses—usually expressed as coating weights (e.g., g/m2)—assumed a decoupled model—i.e., a thin liquid coating film with boundary conditions on the surface that related to the impinging flow field (i.e., pressure profile gradient and shear stress gradient). A fundamental analytical coating weight model has been presented by Thornton and Graff[3] to predict the coating film thickness after wiping by a single-slot air knife. They postulated that only the maximum wall
hf
Z x
D y
h(x,t) V s
Fig. 1—Schematic of the conventional single-slot gas-jet wiping process for co
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