Fundamental Experimental and Numerical Investigation Focusing on the Initial Stage of a Top-Blown Converter Process

Detta är en avhandling från Stockholm : KTH

Sammanfattning: The aim of this thesis work is to increase the knowledge of phenomena taking place during the initial stage in a top blown converter. The work has been done in a few steps resulting in four different supplements. Water model experiments have been carried out using particle image velocimetry (PIV) technology. The system investigated was a fundamental top blown converter where an air jet was set to impinge on a water surface. The flow field of the combined blown case, where an air jet was introduced through a bottom nozzle, was also captured by the PIV. The work clearly showed that the flow field caused by an impinging top blown jet alone could not match that of the bottom blown case. The main re-circulation loop (or vortex) was investigated with respect to position and it was found that an increased flow rate pushes the center of the re-circulation loop downwards into the bath. However, for the top-blown case there is a point when the flow rate is too large to cause a distinguishable re-circulation loop since the jet becomes more plunging (i.e. penetrates deep into the bath) than impinging, with large surface agitation and splashing as a result.A numerical model with the same dimensions as the experimental system was then created. Three different turbulence models from the same family were tested: standard-, realizable- and a modified-(slight modification of one of the coefficients in order to produce less spreading of the air jet) k-? turbulence model. It could be shown that for the family of k-? turbulence models the difference in penetration depth was small and that the values corresponded well to literature data. However, when it comes to the position of the re-circulation loop it was shown that the realizable k-? model produced better results when comparing the results to the experimental data produced from the PIV measurements, mentioned earlier.It was then shown how the computational fluid dynamics (CFD) model could be coupled to thermodynamics databases in order to solve for both reactions and transport in the system. Instead of an air-water system, a gas-steel-slag system was created using the knowledge obtained in the previous simulation step described above. Reactions between gas-steel, gas-slag, steel-slag and gas-steel-slag were considered. Extrapolation of data from a few seconds of simulation was used for comparison to experimental data from the literature and showed reasonable agreement. The overall conclusion was that it is possible to make a coupling of the Thermo-Calc databases and a CFD software to make dynamic simulations of metallurgical processes such as a top-blown converter.A parametric study was then undertaken where two different steel grades were tested; one with high initial carbon content (3.85 mass-%) and one with lower carbon content (0.5 mass-%). The initial silicon content was held constant at 0.84 mass-%. Different initial temperatures were tested and also some variation in initial dissolved oxygen content was tried. It was found that the rate of decarburization/desiliconization was influenced by the temperature and carbon concentration in the melt, where a high temperature as well as a high carbon concentration favors decarburization over desiliconization. It was also seen that the region affected by a lower concentration of alloys (or impurities) was quite small close to the axis where the impinging jet hits the bath. Add the oscillating nature of the cavity and it was realized that sampling from this region during an experiment might be quite difficult.