Numerical Investigation of Multiple-Impinging Slot Jets in the Gas-Jet Wiping of Liquid Zinc Coatings

Abstract

<p>A turbulent impinging slot jet is a device which is used in various industrial applications such as glass tempering, heating of complex surfaces, cooling of turbine blades, cooling of electronic devices and in the continuous hot-dip galvanizing line, which is the focus of this study. An impinging slot jet is used to control the zinc film thickness on the sheet substrate to reach uniform product coating thickness by applying a pressure gradient and shear stress distribution on the moving substrate, after immersion in a bath of molten zinc. The impinging jet wipes the excess molten zinc from the steel strip through the combined effects of a pressure gradient and shear stress distribution on the steel strip.</p> <p>In this study, the fluid flow of three multiple-impinging slot jet configurations discharging air at high velocity on a moving substrate were investigated numerically. Computational fluid dynamics was used to determine the wall pressure results and wall shear stress distributions due to the multiple impinging slot jets, and these results were used as boundary conditions in an analytical model to estimate the final liquid zinc thickness on the substrate. The standard k - ε turbulence model with non-equilibrium wall treatments was used to capture the turbulence parameters in the flow field.</p> <p>The knowledge of using multiple-impinging slot jets in the hot-dip galvanizing line process as a wiping actuator is quite limited. There is not any systematic work available in using these devices as a wiping actuator. In this study, three models of multiple slot jets were developed numerically with the goal of estimating the coating weight on the moving sheet substrate. The conventional model of a single-impinging slot jet was used as a base case for comparing the wall pressure results, wall shear stress distributions and consequently the coating weight data on a moving substrate with different multiple-impinging slot jet configurations. Adjusting the various process parameters such as main slot jet Reynolds number (Re_m), auxiliary slot jet Reynolds number (Re_a), plate-to-nozzle ratio (z / d) and sheet substrate velocity (V_substrate) allows the producers to control the coating weight on a moving sheet substrate.</p> <p>For this study, the numerical simulations were solved using FLUENT commercial code. A comprehensive set of numerical modeling over a wide range of process variables was performed for all configurations in order to present a broad summary of the coating weight trends in the wiping process. A full analysis of the wall pressure distributions and wall shear stress results, as well as coating weight estimation generated under different impinging slot jets have been presented in this study.</p>