Flow analysis of a four-strand steelmaking tundish using physical and numerical modelling

Cloete, Jan Hendrik (2014-04)

Thesis (MEng)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: In modern steelmaking a tundish serves as an important metallurgical reactor to remove inclusions and maintain thermal and chemical homogeneity in the product. In this study the flow behaviour in a four strand tundish was investigated by means of a 1/2 scale water model, based on Froude number similarity, as well as by using numerical modelling. Both the numerical study and physical model were used to characterise residence time distribution (RTD) in the vessel and to calculate properties pertaining to the tundish flow regime. The three different tundish configurations investigated in this study are: a bare tundish with no flow control devices, a tundish with a turbulence inhibitor and a tundish using a turbulence inhibitor with holes in combination with dams. Preliminary investigations focussed on the framework for obtaining an accurate numerical solution within reasonable computational times. The effect of assuming symmetry and dynamically steady flow in the numerical model was shown to be small relative to the effect of grid size and justifiable by the savings in computational time. The grid independence study indicated the importance of using a finer mesh in areas of high velocity gradients to obtain realistic results and also to limit the number of computational cells. A procedure using gradient adaptation was used to refine the mesh automatically in the required regions for different tundish geometries. Results also showed that the inlet boundary of the numerical model should be selected at the ladle outlet, since assuming a flat velocity profile at the nozzle port resulted in significant changes in the RTD response. Comparison of the results obtained using the numerical model with those from physical experiments yielded an average error of less than 10%. This was assumed to be a good prediction, considering the assumptions employed in the numerical model. Both the physical and numerical models showed that a tundish without flow control devices was prone to significant short circuiting. The addition of a turbulence inhibitor was shown to be successful in preventing short circuiting and provided surface directed flow, which is thought to aid inclusion removal in the slag. Additionally, the minimum, peak and mean residence times and plug flow volume fraction were increased significantly, while the dead volume fraction decreased. However, using a turbulence inhibitor with holes in combination with dams showed that this configuration may cause increased refractory wear together with increased risk of slag entrainment due to flow patterns with increased surface turbulence. It also showed that the short-circuiting might not be eliminated completely. This indicates that certain design changes to tundish flow control systems can introduce problems that outweigh the benefits of the altered flow patterns. Furthermore, the numerical method, which was based on the water model, was modified to simulate the high temperature steel process. A very good match was obtained between the results using the two different numerical models. This serves as additional evidence that tundish water modelling based on Froude number similarity provides a good representation of the actual industrial process. Using the numerical model based on the high temperature steel process the effect of turbulence inhibitor shape was studied for four different turbulence inhibitor designs. Results showed the best performance, based on flow characteristic properties and surface turbulence values, was achieved for the design using a rectangular box-like shape with flanges at the top. However, the comparison emphasized the effect of the turbulence inhibitor shape on the flow behaviour, as each design yielded completely different flow patterns. It was also observed that a good turbulence inhibitor provided an optimum amount of turbulent suppression. Insufficient suppression would cause fast flows, which will result in insufficient residence time for inclusion flotation and high surface turbulence values, which may cause slag entrainment. On the other hand, too much suppression may increase the variation between strands.

AFRIKAANSE OPSOMMING: Die verdeeltrog speel ‘n belangrike rol in die moderne staalvervaardigingsproses deur inklusies te verwyder en termiese en chemiese homogeniteit in die produk te verseker. In hierdie studie is die vloeigedrag in ‘n verdeeltrog met vier uitlate bestudeer deur middel van ‘n 1:2-skaal watermodel, gebaseer op Froude-getal gelykheid, sowel as deur numeriese modellering. Beide die numeriese en watermodel is gebruik om die verblyftyd-distribusie in die trog te karakteriseer en om waardes te bereken wat die vloeigedrag in die verdeeltrog beskryf. Drie verskillende verdeeltrog-opstellings is in hierdie studie bestudeer, naamlik: ‘n leë verdeeltrog met geen vloeibeheertoestelle nie, ‘n verdeeltrog met ‘n turbulensie- inhibeerder en ‘n verdeeltrog wat gebruik maak van ‘n turbulensie-inhibeerder met gate, gekombineer met lae damwalle. Vroeë ondersoeke het gefokus op ‘n metode om akkurate numeriese resultate binne aanvaarbare tye te verkry. Die invloed van die aannames van simmetrie en dinamiese gestadigde vloei op die resultate is bepaal om klein te wees teenoor dié van die roostergrootte. Die gebruik van die aannames is dus geregverdig deur die afname in berekeningstyd wat dit meebring. Die roosteronafhanklikheidstudie het getoon dat dit belangrik is om die vloeivergelykings oor klein volume-eenhede op te los in areas van hoë snelheidsgradiënte, beide om realistiese resultate te verseker en om die aantal volume-eenhede te beperk. ‘n Prosedure wat gebruik maak van gradiëntaanpassing is gebruik om die roostergrootte outomaties te verklein in die areas met hoë snelheidsgradiënte vir verskillende verdeeltrog-opstellings. Resultate het ook getoon dat dit belangrik is om die inlaatgrens van die numeriese model by die smeltpot se uitlaat te kies, aangesien noemenswaardige verskille in die verblyftyd-distribusie waargeneem is wanneer ‘n uniforme snelheidsprofiel aanvaar is waar die metaal die verdeeltrog binnegaan. Daar is gevind dat die resultate by die numeriese en watermodelle verskil met ‘n gemiddelde fout van ongeveer 10%. Dit word beskou as ‘n goeie ooreenstemming, as die aannames wat in die numeriese model gebruik word in ag geneem word. Beide die numeriese en watermodelle het getoon dat die metaalvloei in die leë verdeeltrog geneig is om die kortste roete na die uitlate te kies. Deur van ‘n turbulensie inhibeerder gebruik te maak word hierdie probleem opgelos deur die metaalvloei opwaarts te forseer. In die proses neem die minimum-, piek- en gemiddelde verblyftye, sowel as die propvloei volumefraksie, toe, terwyl die dooievloei fraksie afneem. Die turbulensie inhibeerder met die gate en die lae damwalle is egter meer geneig tot erosie van die vuurvaste materiaal en kan ook vloeipatrone veroorsaak wat metaalskuimdruppels in die metaal kan opneem. Verder volg ‘n klein fraksie van die vloei steeds ‘n kortpad na die uitlaat. Hierdie resultate toon dat sekere veranderings aan verdeeltrog vloeibeerheerstelsels nadelige gevolge kan hê wat enige positiewe gevolge uitkanselleer. Verder is die numeriese metode wat tot dusver op die watersisteem gebaseer is, aangepas om die vloei in die hoëtemperatuur staalproses te simuleer. ‘n Baie goeie vergelyking is verkry tussen die resultate wat die twee numeriese modelle voorspel. Hierdie dien as ‘n verdere bewys dat ‘n watermodel, wat op Froude-getal gelykheid gebaseer is, die industriële proses akkuraat kan simuleer. Deur gebruik te maak van die numeriese model van die hoë temperatuur staalproses, is die effek van die turbulensie-inhibeerder se vorm vir vier verskillende ontwerpe bestudeer. Die studie het getoon dat die beste resultate vir vloeikarakteriserende eienskappe en oppervlak- turbulensie verkry is met ‘n reghoekige ontwerp. Die resultate beklemtoon egter die belangrikheid van die invloed van die turbulensie-inhibeerder se vorm op die vloeigedrag in die verdeeltrog, aangesien elke vorm noemenswaardige verskillende vloeipatrone opgelewer het. Daar is opgemerk dat ‘n goeie turbulensie-inhibeerder ‘n optimale hoeveelheid turbulensie onderdrukking veroorsaak. In die geval van te min onderdrukking is die verblyftye te kort en die oppervlak turbulensie te hoog. Te veel onderdrukking kan egter lei tot groot variasie in die eienskappe van die metaal by die verskillende uitlate.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/86686
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