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Wave-instabilities associated with galvanisation
2001 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Steel sheets are often galvanised to protect the steel from corrosion. In the galvanisation process at SSAB Tunnplåt in Borlänge the steel sheet 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 two so-called gas knives that peel 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 thesis considers the wave generation and the stability of the fluid flow while the other analyses the mean flow characteristics and the temperature distribution. The mean flow profile of the zinc depends on the pressure gradient and the surface shear stress, both imposed by the gas knives. It also depends on the strip speed. By studying small perturbations of the steady mean flow the local stability along the plate is determined. There are three parameters that can be varied to achieve the desired coating thickness, strip speed, supply pressure to the gas knives and the distance between the gas knives and the plate. When one of these parameters is varied, it strongly effects the stability. An increased supply pressure gives weaker instability since the coating thickness is smaller. A decreased jet-to-plate distance gives the same effect. An increased strip speed gives stronger instabilities since more fluid is carried up with the steel sheet. The coating thickness is very important for the stability of the flow. These results were clearly verified by the conducted experiments. The local stability analysis shows that disturbances grow strongly in the area close to the gas knives, before passing them. After passing the gas knives the both the pressure gradient and the surface shear stress stabilises the flow. Far from the gas knives, where there effect is negligible, the flow is weakly unstable caused by gravity. Depending on the input parameters it is possible to minimise the wave growth, based on the theory. It is interesting to compare different combinations of input parameters that give the same coating thickness. A large supply pressure and jet-to-plate is compared to a small pressure and distance. For the first case the wave growth close to the gas knives are stronger but the stabilising effect is both stronger and acts over a longer range, giving a smaller amplitude in the end. The results show that it probably requires some kind of additional force acting on the flow to get a stable flow. Two different methods have been considered, double gas knives and an applied magnetic field. By using the stabilising effect from the surface shear stress it would be interesting to see the effect from an extra gas knife. The other method is to apply a uniform high frequent magnetic field to the zinc coating. The analysis showed that it is possible to achieve a stabilising force on the flow, but it requires very high frequencies (~1-100GHz) and with very small magnetic density flux (~1-10T).

Place, publisher, year, edition, pages
2001.
Keywords [en]
Technology, galvanisation, jet stripping, fluid flow, stability, MHD
Keywords [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-51936ISRN: LTU-EX--01/096--SELocal ID: 91ae9cf5-6c82-4c4b-a0bc-e278d9207ab1OAI: oai:DiVA.org:ltu-51936DiVA, id: diva2:1025300
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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