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Jet stripping in galvanisation processes: heat transfer and fluid flow properties
2001 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In the galvanisation process at SSAB Tunnplåt in Borlänge the plate passes a vessel of molten zinc. Then it moves vertically upward and a thin film of molten zinc follows. To obtain the desired coating thickness, the plate passes a gas knife that peels off most of the zinc. On the final product there is an undesired wave pattern in the coating zinc layer. There are two parallel projects studying this phenomenon. This analyses the mean flow characteristics and the temperature distribution while the other considers the stability problem. The phenomenon has also been studied experimentally. The characteristics of the mean flow are very much dependant on the impact from the gas knife. The gas knife affects the coating zinc layer with a pressure as well as a shear stress. How these are distributed along the plate is determined by the distance between plate and gas knife, the supply pressure to the jet and the nozzle width. Once the shear stress and pressure distributions are known, the zinc flow can be analysed with the Navier-Stoke's equation. To solve the Navier-Stoke's equation for the velocity profile in the zinc it is necessary to know the boundary conditions. In this case there is a no slip condition at the zinc-plate interface. This means that the zinc has the same velocity as the plate. At the free surface the zinc is affected by pressure and shear stress from the gas knife. Integrating the velocity profile over the film thickness gives the zinc flux. Once the flux is derived, the coating thickness can be determined. The final coating thickness is most often in a range from 10 to 30 m, and what determines the thickness is jet supply pressure, jet-to-plate distance and plate velocity. A large pressure gives a small thickness while a large jet-to-plate distance and plate velocity gives a large thickness. The temperature distribution is also studied. The method used here is called the integral method and the purpose of this study was to estimate the point of solidification and to use this result in the analysis of the stability. To determine the temperature distribution the heat transfer coefficient has to be known. This coefficient determines the rate of convection, which is the main cooling mechanism in the hot dip galvanising process. The value of the heat transfer coefficient varies very much along the plate. Most of the time there is only free convection with a heat transfer coefficient of approximately 6W/m2K, but close to the gas knife the value can be more than 100 times larger. There are also so called cooling ramps that increase the cooling. These must also be considered in the analysis. Radiation is also an important cooling mechanism. The result of the analysis show that the point of solidification varies very much depending on the process parameters plate velocity, jet supply pressure and jet-to-plate distance. In some cases the zinc freezes very close to the jet. In others, it freezes several meters above.

Place, publisher, year, edition, pages
2001.
Keyword [en]
Technology, galvanistaion, jet stripping, fluid flow, heat transfer
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-53560ISRN: LTU-EX--01/095--SELocal ID: a922fb15-307e-48db-bc17-acde23afe4fcOAI: oai:DiVA.org:ltu-53560DiVA: diva2:1026935
Subject / course
Student thesis, at least 30 credits
Educational program
Mechanical Engineering, master's level
Examiners
Note
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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