A Noble Gas Wiping System to Prevent the Edge Overcoating in Continuous Hot-dip Galvanizing

Abstract

A noble method is proposed to prevent the edge overcoating (EOC) that may develop near the edge of the steel strip in the gas wiping process of continuous hot-dip galvanizing. In our past study (ISIJ International, Vol. 27 (2003), No. 10, pp. 1495No. 10, pp. -1501, it was found that the EOC is caused by the alternating vortices which are generated by the collision of two opposing jets in the region outside the steel strip. In the present study, the flow field around the gas wiping system has been analyzed numerically and it was found that when the two opposing jets collide at an angle much less than 180°, the alternating vortices disappear and the impinging pressure on the steel strip surface becomes nearly uniform. In order to deflect both jets downward by a certain angle, a cylinder with small diameter is installed tangentially to each exit of the lower lips of the two-dimensional opposing jets. The three dimensional flow field with the proposed device is analyzed numerically by using the commercial CFD software, STAR-CD. And the coating thickness is calculated by solving the boundary layer momentum equation with an integral analysis method. In order to compare the present noble method with the conventional edge baffle plate method to prevent the EOC, the flow field with edge baffle plates is also calculated. The calculation results show that the tangentially installed cylinder at the lower lip of the jet exit is significantly more effective than the edge baffle plate.KEY WORDS: continuous hot-dip galvanizing; edge overcoating; impinging jet; gas wiping.In the present study, the 3-D unsteady compressible turbulent flow field with the proposed configuration with a pair of cylinders in the continuous hot-dip galvanizing system is simulated by using a commercial CFD software, to find out how the cylinder affects the flow behavior of the wiping gas. The simulation results for the impinging pressure and the shear stress on the steel strip surface are used to calculate the thickness of the adhered zinc film by the noble integral analysis method of the boundary layer momentum equation. 7) In addition, the flow field generated by the edge baffle plate is also analyzed to compare its performance for reduction of the EOC with that of the cylinder installed at the lower lip of the air knife nozzle. Numerical Analysis Numerical Analysis of 3-D Unsteady CompressibleFlow In the continuous hot-dip galvanizing process, a heattreated steel strip is passed through a molten zinc bath and the molten zinc is adhered on the steel strip surfaces. Usually, the thickness of the adhered zinc film is nearly 10 times thicker than desired one, and the excessive zinc is removed by nitrogen gas jets. This wiping gas jet system is often called the air knife system. Usually, the Mach number of the nitrogen gas flow is in a range of 0.3-0.6. Therefore, in addition to the continuity and momentum equations, the equation of state and the energy equation are needed. Large Eddy Simulation (LES) is used to solve the turbulent equations, of which f...

Numerical Simulation of the Effects of Air Knives on Coating Thickness

“…This software can be used to simulate and analyze fluid flow and heat transfer in areas with complex geometries. Fluent can also simulate sophisticated flow fields ranging from incompressible flow to moderately and highly compressible flow by using a variety of methods, such as multi-grid acceleration and convergence technologies, to achieve optimal convergence accuracy [7] . The grid-based 2D model established through a numerical simulation is a quad mesh with meshes refined at the air knife outlet near the wall with a total of approximately 200,000 meshes.…”

Aerodynamic Investigation about the Cause of Check-mark Stain on the Galvanized Steel Surface

“…The principal cause of the EOC was found by Kim et al 7) They simulated numerically the turbulent flow field around the gas wiping region formed by the two impinging/opposing jets and revealed that due to the oscillation of the opposing jets in the outside region of the strip edge, the static pressure decreases significantly toward the strip edge, which dramatically reduces the zinc wiping force. After this research, Ahn et al 8 When galvanized steel strip is produced through a continuous hot-dip galvanizing process, the thickness of the adhered zinc film is controlled by impinging a thin plane nitrogen gas jet. In such a gas wiping process frequently there appears stain of check-mark shape.…”

Experimental verification of the feasibility of the CFD approach in an air-knife

“…Additionally, they are used to provide humidity to foods in the food sector [1][2][3]. Air knives are commonly used in the continuous hot dip galvanizing procedure, a coating technique in the steel industry [4][5][6][7][8][9][10][11]. The paper industry is one with effective use of air knives in the thermal paper production process where a very thin layer of chemicals is spread on paper [12].…”