Simulation of flow in a continuous galvanizing bath: Part II. Transient aluminum distribution resulting from ingot addit
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1/17/04
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Simulation of Flow in a Continuous Galvanizing Bath: Part II. Transient Aluminum Distribution Resulting from Ingot Addition F. AJERSCH, F. ILINCA, and J.-F. HÉTU The coupled phenomena of momentum, heat, and mass transfer were simulated in order to predict and to better understand the generation and movement of intermetallic dross particles within certain regions of a typical galvanizing bath. Solutions for the temperature and aluminum concentration can be correlated with the solubility limits of aluminum (Al) and iron (Fe) to determine the amount of precipitated aluminum in the form of Fe2Al5 top dross. Software developed by the Industrial Materials Institute of the National Research Council of Canada (IMI-NRC), including k- turbulence modeling for heat and mass transfer, was adapted for the simulation of a sequence of operating parameters. Each case was modeled over a period of 1 hour, taking into account an ingot-melting period followed by a nonmelting period. The presence of an ingot significantly changes the temperature distribution and also results in important variations in the local aluminum concentration, since the makeup ingot has a higher aluminum concentration. The simulation showed that during the ingot melting, the total aluminum concentration is higher at the ingot side of the bath than at the strip exit side. The region below the ingot presents the highest aluminum concentration, whereas lower aluminum concentrations were found in the region above the sink roll, between the strip and the free surface. It was shown that precipitates form near the ingot surface because this region is surrounded by a solution at 420 °C, which is lower than the average bath temperature of 460 °C. When no ingot is present, the total aluminum concentration becomes much more uniform and decreases with time at a constant rate, depending on the coating thickness. This information is of major significance in the prediction of the formation of dross particles, which can cause defects on the coated product.
I. INTRODUCTION
AS a companion to the first part of this article,[1] this subsequent study takes into account the variation of the solubility of the aluminum and iron in a galvanizing bath as a result of local temperature variations. The previous article showed that the temperature variations are caused by the periodic additions of the cold zinc ingots, which take a finite time to melt, followed by a period when no additions are made to the bath. During ingot melting, the induction heaters operate at maximum capacity and heat input is reduced to a level of 20 pct of this value after melting, maintaining an average bath temperature of 460 °C. The previous article simulated the flow and temperature distribution at steady-state conditions for the case of no ingot present and for the melting-ingot case. Since the surface of the melting ingot is at 420 °C, this condition represents the most severe temperature variation in the bath, since the energy to melt the ingot needs to be supplied by induction
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