Investigation of the Gas‐Jet Wiping Process − Two‐Phase Large Eddy Simulations Elucidate Impingement Dynamics and Wave Formation on Zinc Coatings

Abstract

In addition to corrosion resistance and processing properties, high coating uniformity is a key quality criterion for galvanized steel sheets. Hydrodynamic gas jet wiping has proved to be an efficient method to control the coating thickness. However, the occurrence of nonuniformities is attributed to the unsteadiness of the impinging jet. For the first time, vertical surface non-uniformities resulting from the interaction of the impinging jet with the liquid coating are numerically predicted under industrial boundary conditions using the ANSYS Fluent 1 . The turbulent flow field of the compressible wiping gas is accomplished by the LES (Large-Eddy-Simulation) turbulence model, whereas the interphase between the wiping gas and the liquid coating is modeled by the VOF (Volume-of-Fluid) method. It is found that the liquid coating reacts relatively slowly to the high frequent flapping gas jet. Only, when the jet is deflected for a comparatively long period, significant waves are able to develop. The waviness predicted by the simulation model is in good agreement with experimental results. Thus, the model enables a careful study of process settings on the vertical coating uniformity characteristics. For the studied case, an increase of nozzle inclination is found to enhance the performance in terms of coating uniformity significantly.

Experimental Characterization of the Jet Wiping Process

“…Whether such trends can be justified on the basis of a liquid film instability requires further investigation, although several authors have shown, using standard linear stability analysis, that the flow in the final coat is neutrally stable [49,50]. On the other hand, on the basis of the average thickness curve in Fig.12, such trend is in line with the hypothesis that undulation in the final coat results from an instability of the impinging jet flow, which produces time variations of the pressure gradient [15][16][17][18]. The wiping curve in Fig.12 predicts, in fact, a saturation of the final thickness at the largest pressure gradients, which implies that thinner coats are less sensitive to variations in the wiping conditions than thicker ones.…”

“…The drawback of the method, on the other hand, is the limited surface quality achievable, which constitute a major concern for applications demanding high surface finish quality such as, for example, automotive industry. Understanding the mechanisms responsible for the coating nonuniformities requires a time-resolved characterization of the interaction between the gas jet flow and the liquid film, up to day only achieved by means of numerical simulations [13][14][15] or simplified laboratory models of the wiping process [16][17][18]. Most of the experimental works on the jet wiping, in fact, have been focused on average quantities to validate physical models [8,19,20] or in the definition of operational maps to avoid the liquid film break up, a phenomenon known as splashing [20,21].…”

Coating Film Profiles Generated by Fluctuating Location of the Wiping Pressure and Shear Stress

“…Free jet flows are well known to exhibit fluctuations [1][2][3][4][5][6][7] which occur over a broad frequency range and for the case of jets impinging on a surface this inherent unsteadiness remains a primary feature of the jet flow. [8][9][10][11][12] One characteristic of the impinging jet flow is the jet buckling mode observed in the case of asymmetric jet flows. 9) The jet buckling is directly associated with the presence and instantaneous locations of the large-scale vortex structures which evolve along the shear boundaries between the jet and the surrounding air due to the velocity gradients that exist across these boundaries, particularly over the early part of the jet flow.…”

“…Additionally, the spatiotemporal behaviour of the large-scale recirculating eddies located immediately outside the impingement zone of the jet are an additional likely cause of jet instability. 11,15,16) Owing to these aforementioned behavioural characteristics of the impinging jet flow, jet driven inconsistency in the thickness of the coating-and, concomitantly, the surface quality-produced on CGL may be anticipated, particularly at operating points (settings) at which the coating may be highly sensitive to perturbations at the frequencies inherent to the impinging jet.…”

Vortex Dynamics and Fluctuations of Impinging Planar Jet

“…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. 11) Furthermore, an analytical coating model 12,13) demonstrated the coating thickness variation is sensitive to the fluctuation frequencies of both the magnitudes and positions of pressure and shear stress profiles. Using non-dimensional frequency parameter, df jet /U, where f jet is the fluctuation frequency of the wall pressure and wall shear stress profile (either magnitude or position), it was shown that thickness variation is small (less than 1% of the average coating thickness) when df jet /U > 1 and, in addition, the thickness variation increases as df jet /U reduces with a peak at df jet /U ~ 0.05 for both magnitude and position fluctuation, respectively.…”