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TECHNICAL PAPERS

Jet Wiping in Hot-Dip Galvanization

[+] Author and Article Information
Anne Gosset1

 von Karman Institute for Fluid Dynamics (VKI), Chaussée de Waterloo 72, 1640 Rhode-St-Genèse, Belgiumgosset@vki.ac.be

Jean-Marie Buchlin

 von Karman Institute for Fluid Dynamics (VKI), Chaussée de Waterloo 72, 1640 Rhode-St-Genèse, Belgiumbuchlin@vki.ac.be

1

Corresponding author.

J. Fluids Eng 129(4), 466-475 (Dec 27, 2006) (10 pages) doi:10.1115/1.2436585 History: Received April 24, 2006; Revised December 27, 2006

This paper presents an analysis of the gas-jet wiping process in hot-dip galvanization. This technique consists of reducing the liquid film thickness on a moving substrate by applying gas slot jets. A theoretical development allows the computation of the film thickness evolution in the wiping zone. It is further simplified to an engineering model which predicts directly the final coating thickness, in good agreement with wiping experiments. The limit of applicability of jet wiping is due to the occurence of a violent film instability, called splashing, which takes the form of a liquid droplet emission just upstream the nozzle. An experimental investigation of this phenomenon is conducted on a water-model facility. Two nozzle designs are tested. The effect of process parameters such as the strip speed, the nozzle pressure, the standoff distance, and the tilt angle of the nozzle on splashing is emphasized. A dimensionless correlation is established to predict the operating conditions leading to splashing occurence. It is successfully confronted to observations made on galvanization lines.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 10

Evolution of normalized final film thickness with the nozzle tilting angle at constant Rej=5100 for different Z∕d values

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Figure 11

Dimensionless splashing curves for Z∕d=10, nozzle T1

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Figure 12

Dimensionless splashing curves for Z∕d=10, nozzle T2

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Figure 13

Dimensionless splashing curves for Z∕d values, for tilt angles α=0deg and 10deg

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Figure 14

Effect of surface tension in the prediction of final zinc coating thickness on galvanization lines at Z∕d=8 and Ca=0.0065(U=1.5m∕s)

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Figure 15

Measured and predicted coating thickness on galvanization lines

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Figure 16

Typical galvanization process points (stable and with splashing) with empirical correlation for splashing occurrence

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Figure 17

Parallel evolution of splashing and wiping at α=0deg and 30deg for a constant coating thickness of 20μm, in galvanization with liquid zinc

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Figure 18

Evolution of the estimated maximum substrate speed below splashing for the two nozzle geometries (Z∕d=10, hf=20μm) in galvanization with liquid zinc

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Figure 3

Typical film shape in jet wiping with pressure gradient and shear stress profiles due to the jet

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Figure 2

(a) Splashing phenomenon on a galvanization line; (b) fully developed splashing with water; the runback flow is completely separated along the strip width; and (c) sideview visualization of splashing with water

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Figure 1

Schematic of the jet wiping operation applied to the galvanization process

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Figure 4

Comparison of the dimensionless volumetric film flow rate predicted by the knife model and various models from literature as a function of the dimensionless jet pressure gradient for Ca=0.01

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Figure 5

Schematic of the water jet wiping facility

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Figure 7

Effect of surface tension in the prediction of final film thickness at Z∕d=10 and Ca=0.055

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Figure 8

Comparison of film thickness after wiping obtained experimentally, and predicted by the knife model for Rej=4500 and U=1.5m∕s

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Figure 9

Experimental wiping curves for the two nozzle geometries T1 and T2 at Z∕d=10 and U=1.5m∕s

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