12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The gas wiping is generally used for coating weight control of continuous galvanizing lines (CGL). The gas wiping is the method to scrape off the surplus plating liquid after immersing a steel sheet in the molten plating bath by a gas jet by a slim slot nozzle of one pair of steel sheet front and back. Extensive coating weight control is possible by coordinating gas jet pressure depending on line speed, and it is superior economically. However, several surface defects to be caused by the wiping gas jet such as “bath wrinkle” or “checkmark" being so-called turbulence may occur other than the quality defect that is easy to happen in the steel sheet edge such as edge splashing and the edge overcoat. Parameters to influence impinging jet properties of the two-dimensional jet such as the wiping gas jet include a nozzle shape, distance between gas nozzle and steel sheet, and gas jet velocity. The turbulence properties of the two-dimensional free jet are studied for a long time, it is known that the flow field is divided into “near field” where the jet velocity does not decrease near the nozzle injection port (potential core) exists, “transition region” where mixing with the surrounding fluid proceeds, and “far field” sufficiently downstream from the nozzle. In practice gas wiping is often performed in the transition region, but no studies have been conducted to investigate the effect of the nozzle internal shape in this region. In this report, jets with different initial jet flow velocity distributions were created depending on the internal shape of the nozzle, and the impinging jet behavior in the transition region was evaluated by the flow visualization. In the visualization experiment, two types of nozzles were used: a nozzle provided with a parallel part 25 times the nozzle gap in the flow path of the nozzle injection port, and a nozzle that is curvilinearly contracted toward the nozzle injection port. Water mist as tracer particles was introduced into the nozzle, the area from the nozzle injection port to the collision plate was illuminated with a laser sheet, and the flow field was photographed with a high-speed camera. The jets ejected from the two types of nozzles differed significantly in the vortex formation at the edges of the jet before the impingement, and there were also differences in the vibration state and frequency of the impinging jet and the state of jet separation on the wall. This suggests that the internal shape of the wiping nozzle greatly affects the plating surface quality.