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Self-Excited Oscillations of Wiping Air Knives, Part I: General Behavior and Fluid Loading on the Strip Sheet
Samir Ziada, Adam Ritcey, Jili Ma, Joseph McDermid

Last modified: 2018-04-14

Abstract


In the continuous hot dip galvanizing process, a high speed planar air jet is used at each side of the steel sheet substrate to control the thickness of zinc coating. The impingement of these jets on the moving steel sheet, after its immersion in a bath of molten zinc, forces the excess liquid zinc to be “wiped” back into the zinc bath through the combined effects of the pressure gradient and the shear stress distribution on the strip. In this first of a series of three papers, the air knives are shown to experience two different types of self-excited oscillations; one is due to the impingement of the planar jets on the steel strip while the second is associated with the impingement of the opposing planar jets on each other beyond the edges of the steel strip. Because of the difficulty involved in measuring the fluid forces on the strip, a large scale model of a planar jet impinging on a plate is designed with the ability to control the jet oscillation by means of synthetic jets focused on the jet exit. Measurements of the fluid forces on the impingement plate in the presence and in the absence of jet oscillation will be presented; including the steady and unsteady pressure profiles as well as the skin friction distributions. The other two parts of this series of papers address passive methods to suppress the excitation mechanisms caused by the jet impingement on the strip and the impingement of the opposing planar jets.

Keywords


Wiping Air Knives, Planar jet oscillations, opposing jets oscillation

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