Probleemstelling:

The galvanization process dates back to the 18th century; it has been used and continuously improved since then to give steel a corrosion resistant outer layer, as a far cheaper alternative to stainless steel. For that purpose, the steel is dipped in molten zinc which proceeds to form a protective oxide layer on cooling down. This product is used for many common applications (think lantern poles, screws, etc…), but ArcelorMittal takes it a step further: they have a high speed, continuous process where steel strips pass through a molten zinc bath at speeds of several meters per second. This produces high quality galvanized steel plate for use in e.g. the automotive industry. [1,2]

In the continuous hot dip galvanizing process (see video), a steel strip is passed through a bath of molten zinc as shown schematically in Figure 2. When the sheet is pulled vertically out of the bath, it is coated by a thin layer of molten zinc. To reduce the thickness of the coating further, opposing air knives remove excess material, a technique also called gas jet wiping. These air knives are high velocity planar jets of air (or sometimes nitrogen), with Mach numbers typically in the range of 0.3 - 0.8 [3]. This results in a complex process which is not yet completely understood at ArcelorMittal. Gaining a deeper understanding of the involved flow phenomena and improving the efficiency of the process calls for numerical modeling. This requires complex CFD simulations which can handle both compressible and multiphase flows.

Figure 1: Application in the automotive industry. [source]

Figure 2: The continuous galvanization process. [2]

Doelstelling:

The goal of the thesis is to provide insight in the flow mechanisms of the air knives and to investigate problems encountered at ArcelorMittal. This will require firstly an in-depth analysis of the available literature. Next 2D and 3D CFD models will be developed, which can be tested for several parameters (nozzle geometry, flow velocity, …). To validate the results, they can be compared to installations and results from in-house tools from ArcelorMittal.

At a later stage in the thesis, the problem of edge overcoating will be studied. This phenomenon is sketched in Figure 3: at the end of the steel sheet, the two opposing air knives interact, thereby changing the flow patterns and pressure distribution at this location, resulting in a thicker coating. This is naturally undesirable, and several possible solutions have been proposed in the literature, which can be compared using CFD simulations.

Figure 3: Schematic of edge overcoating. [1]

[1] Alibeigi S. Experimental Investigation of Air-Knife Geometry in Continuous Hot-Dip Galvanizing (Master dissertation).
[2] Yoon HG, Ahn GJ, Kim SJ, Chung MK. Aerodynamic investigation about the cause of check-mark stain on the galvanized steel surface. ISIJ international. 2009 Nov 15;49(11):1755-61.
[3] Yoon HG, Chung MK. Development of novel air-knife system to prevent check-mark stain on galvanized strip surface. ISIJ international. 2010 May 15;50(5):752-9.