12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The variation of thin metallic coating thickness produced in continuous galvanizing lines (CGLs) is attributed partly to the fluctuations in both the spatial profile and gradient of the impingement wall pressure. A fluctuating pressure profile can significantly alter both the magnitude of the maximum pressure gradient doing the majority of the wiping work and its location. A wiping jet with more stable pressure profiles is expected to produce smoother coating than fluctuating ones. However, the dynamics or time variation characteristics of impingement wiping jet wall pressure has not been addressed adequately in the literature. This study experimentally investigated the dynamics of impingement wall pressure of a planar air jet. Both time-averaged and time-varying wall pressure were measured and analyzed. The effects of the ratio of jet-surface distance, H, to jet nozzle width, d, and Reynolds number, Re, based on d and mean jet velocity, Uo, have been elucidated. The measured time-averaged wall pressure profiles agree well with the published data (Tu and Wood, 1996) and LES Computational Fluid Dynamics (CFD) simulations results. It was found that the time-averaged pressure profiles are primarily influenced by H/d ratio. However, both H/d and Re affect the standard deviation of the time-varying wall pressure. The ratio of the largest standard deviation relative to time-averaged stagnation pressure increases linearly with H/d, whereas with the same H/d, this ratio decreases as Re increases. Additionally, the spectra of the time-varying pressure are distinguished between two scenarios of i) H/d < 6: the spectrum is dominated by high frequencies characterized by the jet or the frequencies from the fluctuation of supply pressure, whichever effect is dominant, and ii) H/d ≥ 6: the spectrum is independent of the jet characteristics and the supply pressure fluctuation and have a wide range of low frequencies. This phenomenon can be explained by the mixing of jet shear layers beyond the jet core for high H/d (≥ 6) cases. The key application of the present study outcomes together with the analytical model of coating (Johnstone et al., 2017, 2019) is a prediction of H/d and Re ranges where coating ripples are expected to occur.