CFD Analysis of Sag Line Formation on the Zinc-coated Steel Strip after the Gas-jet Wiping in the Continuous Hot-dip Galvanizing Process

Abstract

of the coating thickness for long wavelength perturbations. Ellen and Tu 3) have suggested a non-dimensional model that takes into account the surface shear stress and their model gives significantly more accurate prediction of the final coating thickness. Also, Tu and Wood 4) have experimentally demonstrated that the final coating thickness depends on both the pressure and shear stress distributions on the strip surface. Although Tuck and Vanden-Broeck.5) and Yoneda 6) suggested that the final thickness depends on the surface tension and the shear stress on the strip, recently Gosset and Buchlin 7) have shown that the surface tension does not play any role in determining the final coating thickness.There have appeared several numerical simulations of the gas wiping process to predict the final coating thickness by using CFD. One of the recent studies is that by Lacanette et al. 8,9) who numerically simulated the final coating thickness by using volume of fluid-large eddy simulation (VOF-LES) modeling. Their results were satisfactorily compared with their experimental data. They found that the coating thickness is independent of the nozzle-to-plate distance when it is smaller than 8 times of the nozzle slot width. Myrillas et al. 10) tried to validate a model for the gas jet-liquid film interaction in the gas wiping process. In their study, they found that two-phase numerical simulations using LES is more suited to model of the complex interaction than k-e turbulence model. Through these previous studies it can be concluded that the dominant factors affecting the final coating thickness are pressure gradient and shear stress on the strip.So far, most of the previous studies were focused on the average final coating thickness along longitudinal direction and their numerical and mathematical models are made in two-dimensional domain. In spite of its good productivity and easy control of the zinc coating thickness, however, the coated film surface after gas wiping has frequently three dimensional surface defects such as dents, blow lines, peculiar features and sag-lines.11) Therefore, in order to obtain a uniform coating on the strip, it is necessary to investigate in details the three dimensional character of the coated surface after the gas wiping process.Among these surface defects, the sag lines (or snaky coating) cause many problems such as irregularity in the electrical and thermal characteristics and the diffused reflection on the coated surface. The snaky coating is that with an oblique patterns appearing on the coated film surface after the gas wiping process. Depending on its seriousness, the snaky coating is usually classified into five grades as shown in Fig. 2. The arrows in Fig. 2 indicate the moving direction of steel strip. The first grade indicates that no sag line is observed on the surface. In the surface pattern of the 2nd grade, rather short sag lines appear irregularly, thus a pattern of sag lines is vaguely discernible. On the other hand, the 3rd-5th grade snaky coatings reveal oblique...

Numerical Verification of the Performance of Double Jet Air-knife System in Avoiding the Check-mark Stain on the Zinc Coated Surface

“…The mean coating thickness distribution along the longitudinal y-direction is computed by using the method of Ellen and Tu, 3) hereinafter called "Ellen and Tu's model". And in order to compute the coating thickness distribution along the transverse z-direction quickly, the mathematical perturbation method proposed in our recent paper 4) is used. Also, the relaxation time that was suggested in that paper is used to determine the longitudinal position where the thickness of the coated layer is settled.…”

Experimental verification of the feasibility of the CFD approach in an air-knife

“…Additionally, they are used to provide humidity to foods in the food sector [1][2][3]. Air knives are commonly used in the continuous hot dip galvanizing procedure, a coating technique in the steel industry [4][5][6][7][8][9][10][11]. The paper industry is one with effective use of air knives in the thermal paper production process where a very thin layer of chemicals is spread on paper [12].…”

“…Researchers made a variety of recommendations to prevent errors and control the coating thickness for zinc-coated surfaces in continuous hot dip galvanizing processes [4,6,8,10]. So et al [4], analyzed surface flaws (which called sag lines) on steel sheet surfaces and developed a new model to predict the variation in coating thickness. They performed 3-D CFD simulations.…”

Vortex Dynamics and Fluctuations of Impinging Planar Jet

“…It is envisaged that fluctuation of this wall pressure and wall shear stress cause variation in coating thickness. Recent studies [6][7][8][9][10] have shown the ripples on the coating surface are correlated to the fluctuating impinging pressure and shear stress which in turn is a consequence of the jet oscillation. Numerical studies of air-coating liquid interaction showed that jet oscillation is reflected on the final coating thickness 9,10) with the low frequency components of the impingement wall pressure fluctuation correlated to the waviness of the coating surface.…”