Impinging planar jets are often employed on industrial Continuous Galvanising Lines (CGL) to wipe away or remove excess drag-out coating material from steel strip, in order to control the thickness of the applied metallic alloy coating. The coating material is initially applied in a continuous manner to both sides of the strip by passing the moving strip through a bath of the molten metallic alloy. This forms the first of numerous different coatings applied, and these collectively form the final coated product ready for market.

Ideally, a thin uniform coating thickness of the order of 20-30 microns is required.  However, under some conditions, the coating thickness may be uneven in the along-strip direction.

Andrew Johnstone, an Associate Research Fellow in Steel Research Hub at the University of Wollongong (UOW), is investigating the inherent unsteadiness of the wiping jets as a probable cause or contributor to surface unevenness, and surface roughness more generally.

In collaboration with BlueScope, Andrew and the UOW team have developed a numerical coating model to predict the coating thickness behaviour in order to assess its sensitivity to fluctuations in the wiping jet flow. Jet flows are known to exhibit fluctuations that occur over a broad frequency range. Collectively, several features of these flows result in unsteadiness in the effectiveness of jet wiping of the coating. Consequently, this has direct implications for the quality of the surface of the metallic alloy layer produced using the CGL process.

Using the numerical model developed by the team, the unsteadiness of the actual wiping jets was mimicked. The results show exceedingly strong frequency dependence in the degree of coating thickness variation. Through this low-cost, quick and flexible numerical approach, a general frequency dependence of the coating has been identified. This has highlighted a critical operating threshold that produces either a smooth or rough coating finish of the metallic alloy coating. This identified link between jet fluctuation frequency and coating thickness response has been employed in the planning of laboratory-scale experiments carried out at BlueScope facilities and can be used to assist selection of jet operational settings in industry and optimisation of jet design.

Andrew has said that it has been a privilege to play a role in this process and to engage with and learn from the Hub industry partners. “It is particularly pleasing to be part of the collaborative industry-university effort delivering tangible outcomes of both a practical and academic nature.”