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<p>The growth and removal of inclusions in stirred ladles hasbeen studied. First, the importance of different growthmechanisms suggested in the literature were studied. Simulationresults from a fundamental model of an induction-stirred ladlehave been used as input in the calculations. Based on thegrowth calculations it was concluded that four of the growthmechanisms need not to be considered since they contribute solittle: i) diffusion of oxygen and aluminum to the inclusionsurface, ii) diffusion coalescence, iii) Brown motioncollision, and iv) laminar shear collision. The majorcontributor to inclusion growth is turbulent collision. Growthdue to Stoke's collisions is also somewhat important if largedifferences among inclusion sizes exist.</p><p>Growth of inclusions in gas stirred ladles was studied usinga similar approach as the one for induction stirred ladles, butwith use of simulation results from a fundamental mathematicalmodel of a gas-stirred ladle. Similarly to what was found inthe case of induction stirring, it was found that turbulentcollisions and Stokes collisions appeared to be the majormechanisms for inclusion growth. The contribution of laminarshear collisions to growth was deemed negligible compared tothat of turbulent collisions.</p><p>For the gas stirred ladle different removal mechanisms werealso studied, based on input data from a mathematical model ofa gas-stirred ladle. It was found that different modelssuggested to predict the inclusion removal due to bubbleflotation gave very different results. Also, all models assumeda spherical shape of the gas bubbles, which was found to beless realistic. Therefore, a new model for inclusion removal byspherical cap bubble flotation was developed. In the newcalculations, the most important mechanisms of inclusionremoval were found to be removal to the top slag and removal bybubble flotation, assuming spherical-cap bubbles and planecontact. When the bubbles were assumed to be spherical,resulting removal rates were lower than when they were assumedto be spherical caps. Based on these results it is concludedthat more research is needed to obtain a better understandingof the importance of bubble flotation on inclusion removal.Experiments are clearly needed to determine which modelconcepts produce predictions in best agreement withcorresponding data from actual steelmaking processes.</p>

The growth and removal of inclusions in stirred ladles hasbeen studied. First, the importance of different growthmechanisms suggested in the literature were studied. Simulationresults from a fundamental model of an induction-stirred ladlehave been used as input in the calculations. Based on thegrowth calculations it was concluded that four of the growthmechanisms need not to be considered since they contribute solittle: i) diffusion of oxygen and aluminum to the inclusionsurface, ii) diffusion coalescence, iii) Brown motioncollision, and iv) laminar shear collision. The majorcontributor to inclusion growth is turbulent collision. Growthdue to Stoke's collisions is also somewhat important if largedifferences among inclusion sizes exist. Growth of inclusions in gas stirred ladles was studied usinga similar approach as the one for induction stirred ladles, butwith use of simulation results from a fundamental mathematicalmodel of a gas-stirred ladle. Similarly to what was found inthe case of induction stirring, it was found that turbulentcollisions and Stokes collisions appeared to be the majormechanisms for inclusion growth. The contribution of laminarshear collisions to growth was deemed negligible compared tothat of turbulent collisions. For the gas stirred ladle different removal mechanisms werealso studied, based on input data from a mathematical model ofa gas-stirred ladle. It was found that different modelssuggested to predict the inclusion removal due to bubbleflotation gave very different results. Also, all models assumeda spherical shape of the gas bubbles, which was found to beless realistic. Therefore, a new model for inclusion removal byspherical cap bubble flotation was developed. In the newcalculations, the most important mechanisms of inclusionremoval were found to be removal to the top slag and removal bybubble flotation, assuming spherical-cap bubbles and planecontact. When the bubbles were assumed to be spherical,resulting removal rates were lower than when they were assumedto be spherical caps. Based on these results it is concludedthat more research is needed to obtain a better understandingof the importance of bubble flotation on inclusion removal.Experiments are clearly needed to determine which modelconcepts produce predictions in best agreement withcorresponding data from actual steelmaking processes. / NR 20140805

PhD - Arts / Popular fiction columns have been among the most resilient and versatile of the newspaper sub-genres in Kenya. Since the 1970s, these columns have remained a permanent feature in the Kenyan newspapers. Among the most popular of these columns is Whispers, a satirical column written by one of Kenya’s most talented writers of the 1980s—90s decades, Wahome Mutahi. At a time when the state had all but monopolised public sites of expression in the country, Whispers kept the Kenyan popular media porous, opening up spaces for the discussion of social and political issues that could only be ‘whispered’. This study gives a detailed discussion of this column against the historical dynamics of post-independence Kenya. I examine how Whispers became a public space where Kenya’s postcolonial existence, in its many contradictory faces was constantly interrogated. I argue that this column provided its readers certain ‘moments of freedom’; it was a site where the limits of social and political taboos were boldly tested. In Whispers, people could heartily laugh at authority, and at themselves, but ultimately reflect on the reasons for their laughter. By providing such a space for self-reflection and for the critique of society, I argue that the Kenyan newspaper became an important site of cultural production especially in the 1980s through the 1990s. The introductory parts of this thesis attempt a theorisation of the ‘popular’ and later trace the emergence of popular fiction as a category of critical literary exegesis in Kenya. I examine the beginnings and growth of popular fiction, focusing mainly on the role of the popular press. The median chapters examine how the Kenyan newspaper provides the space within which popular fiction interfaces with journalism to constitute ‘publics’, by drawing on popular cultural resources to mediate contemporary and topical issues. The thesis gives a detailed reading of the cultural forms that offer subject populations interpretive frameworks within which to make sense of their world. The last part of the thesis continues this discussion with an analysis of how the ‘popular’ mediates questions of power in postcolonial Kenya.

<div>A numerical simulation was developed to capture the phenomena of electromagnetic stirring in a metallurgical ladle. Electromagnetic stirring requires an external magnetic field to be imposed on the molten steel bath, which is governed by the principles of magnetohydrodynamics. Electromagnetic stirring benefits over traditional stirring methods by offering non-invasive stirring, melt homogeneity, and ease of configuration alterations. Insight to the electromagnetic stirring phenomena is limited experimentally due to the high temperatures of the molten-steel bath. This investigation will include two numerical simulations, the first of which is to generate a magnetic field to properly stir the steel bath. The second incorporates the generated magnetic field and solves the fluid flow due to the magnetohydrodynamics interactions. The results of these numerical simulations will help to provide further understanding of the electromagnetic stirring method. This simulation was used to analyze the molten-steel bulk velocity, vortex formation, flow development time, slag-eye size, and wall shear stress in a metallurgical ladle.</div><div><br></div><div><div>The transient development of the bulk velocity in an EMS ladle was compared with the literature study completed by Sand et al. 2009. The comparison of the developed bulk velocity resulted in a percentage difference of 0.98% and an absolute difference of 0.007 [m/s]. Both numerical models, in the current work and the literature study, obtained a developed flow within 25 seconds of stirring. For the parametric studies, it was found that the addition of a circumferential taper angle to the geometry reduced the bulk velocity and slag-eye size formed compared to a cylindrical ladle. The electric current amperage of the external magnetic field coil system was determined to precisely adjust the bulk velocity. A 150 [A] reduction in amperage results in a ~20% loss in the bulk velocity magnitude. The locations of the high shear stress regions were determined which remained near the stirring unit.</div><div><br></div><div>From this study, it is recommended to use a magnetohydrodynamics package offered within a multiphysics numerical solver since the FLUENT® MHD module inherently under-predicts the velocity as well as the issue of the numerical instabilities of the Lorentz force calculations.</div></div><div><br></div>

QC 20101126

The term "Metal Matrix Nano-Composites (MMNCs)" broadly refers to a composite system that is based on metal or alloy substrate, combined with metallic or non-metallic nano-scale reinforcements. The main advantages of MMNCs include excellent mechanical performance, feasible to be used at elevated temperatures, good wear resistance, low creep rate, etc. In the recent past, MMNCs have been extensively studied, especially the method of fabrication as the processing of such composites is quite a challenge. Though a variety of processing methods have been explored and studied over the years, none have emerged as the optimum-processing route. The major issue that needs to be addressed is the tendency of nano-sized particles to cluster and also the challenge as to how to disperse them in the bulk melt. This work explored the feasibility of utilizing Lorentz forces to address both of these critical issues: clustering and dispersion. The work was carried out both theoretically as well as with accompanying validation experiments. The results indicate that Lorentz Forces may be viable and should be considered in the processing of MMNCs.

The LD-process, called after the cities Linz and Donawitz, is used to convert pig iron into crude steel by blowing oxygen on top of the pig iron. A LD-LBE converter, Lance Bubbling Equilibrium, also stirs the melt trough a bottom stirring system. The bottom stirring in a LD-LBE converter is believed to have a positive effect alone on the phosphorous removal. Previous studies have shown that the temperature and slag composition are the main factors affecting phosphorus removal. Phosphorus binds to the slag easier at low temperature and to slag with certain levels of dissolved calcium (a process additive). Different praxes were tested and a better dephosphorisation was reached. The bottom stirrings effect on the dissolution of calcium additives is a possible explanation to the results and mechanisms presented in this study. The study also aimed to investigate the use of nitrogen as stirring gas instead of argon. Nitrogen is removed from the steel during the formation of carbon oxide gases. Nitrogen was used in varying amounts as stirring gas during the first half of the oxygen blow. It proved to be safe to use as long as there was a high content of carbon in the melt. However using nitrogen beyond half of the blow showed to be risky for nitrogen sensible steels; even in small amounts since there is not enough carbon left to degas the steel from nitrogen. Slopping happens when formed gas from the LD-process is trapped in the slag. The slag level rises and sometimes it floods the converter resulting in yield losses. The influence of the bottom stirring on slopping was studied, which resulted in the conclusion that slopping cannot be avoided by simply improving the bottom stirring. Although some verification studies remains to be done, if the suggestions based on the results of this thesis were employed, savings in the oxygen and stirring gas economies could be made. Not least improvements on the iron yield. / En järnmalmsbaserad stålproduktion börjar med att järnmalm matas i en masugn tillsammans med koks, kalk och tillsatsämnen. Ut kommer råjärn med höga kol och svavelhalter. Råjärnet transporteras till stålverket i så kallade torpedvagnar. I vissa stålverk, t.ex. SSAB Special Steels i Oxelösund, renas råjärnet från svavel i torpedvagnen. I andra stålverk svavelrenar man i separata skänkar. Svavelreningen sker med bland annat kalciumkarbid som binder till svavlet. Det svavelfattiga råjärnet måste sedan renas från kol för att bli stål. Det görs i en LD-konvertern (Linz Donawitz). LD-konvertern laddas med flytande råjärn som har en kolhalt på 4,5 procent och som är runt 1350 grader varmt. Råjärnet kyls genom att cirka 20 procent skrot tillsätts. En syrgaslans sänks sedan in i konvertern ovanför smältan och reningen startar.  Syrgaslansen blåser syrgas i ultraljudsfart vilket oxiderar en del av järnet, så väl som kol, kisel, mangan, fosfor and andra föroreningsämnen i råjärnet. Kol försvinner ur konvertern i form av kolmonoxidgas. Andra oxiderade föroreningar och järnoxid bildar tillsammans en så kallad slagg som flyter ovanpå smältan. Det tillsätts även så kallade slaggbildare som förbättrar upptaget av föroreningar i slaggen. Processen varar i cirka 17 minuter och är mycket beroende av slaggen som bildas. Kol försvinner ur konvertern i form av kolmonoxidgas. Under processens gång rör man om smältan med hjälp av gaser som spolas genom botten av konvertern. Omröringen jämnar ut smältans sammansättning och temperatur. När man inte länge behöver avlägsna kol stoppas processen. Stålets temperatur är då cirka 1700 grader och kolhalten ligger nära 0,05 procent. Stålet överförs sedan till en skänk för att skilja det ur slaggen. Stålet förädlas vidare i olika processer där sammansättningen justeras så att det möter kundens krav. Sedan gjuts stålet i strängar för transport till valsverk eller kunder. Denna studie behandlar bottenomrörningen under LD-processen i SSAB Special Steels's stålverk i Oxelösund. Omrörningen sker genom åtta porösa stenar i botten av konvertern som blåser med argon eller kväve. Gasflödet genom stenarna justeras genom ett ventilsystem. Under blåsningen rör man om med hjälp av förinställda program. Omrörningens primära funktion är att avlasta syrgaslansen. I fallen där ingen bottenomrörning finns måste syrgaslansen blåsa ”hårdare” på stålet för att avlägsna kol. Avlastningen som bottenomrörningen bidrar med gör att processen även kallas för LD-LBE, där LBE står för Lans Bubbling Equilibrium. Bottenomrörningen tros ha en positiv effekt på stålets rening från fosfor. Sedan tidigare vet man att temperatur och slaggsammansättning är de största faktorerna som påverkar fosforreningen. Fosfor tas lättare upp i slaggen vid låga temperaturen samt i slagg med högre kalkhalter. Olika omrörningsprogram testades och en bättre fosforrening nåddes. Bottenomrörningen visade sig ha positiva effekter som är teoretisk kopplade till kalksmältning. Två möjliga förklaringsmekanismer hittades. Studien undersökte även användningen av kväve som omrörningsgas istället för argon, då kväve är ekonomisk fördelaktig gentemot argon. Kväve finns inlöst i råjärnet som sätts in i konvertern. Kvävgasen försvinner ur stålet under och med hjälp av kolreningen. Det visade sig vara säkert att använda kväve från start fram till halva syrgasblåset på kvävekänsliga stålsorter, var efter man sedan byte till argon. Kväve som används sent under blåset visade ge högre kvävehalter. Urkok är en kraftig volymökning av slaggen som sker när bildad gas från reningen av smältan fångas i slaggen och får slaggen att ”koka över”. Urkok resulterar i ekonomiska förluster då slaggen som lämnar konvertern vid urkok är rik på järn. Bottenomrörningens eventuella påverkan på urkok studerades. Det visade sig att urkok inte kan undvikas genom att enbart optimera bottenomrörningen.

In the oxidative refining of metallurgical grade silicon the loss of Si to the slag in the form of SiO2 is an economical concern. The purpose of this report is to investigate the possibility of using SiO2 and mechanical stirring to remove Ca and Al as a substitute to the oxidative refining. In the experiments graphite crucibles were used in a vertical resistance furnace and controlled argon atmosphere. The removal rate of Ca and Al are measured by X-ray fluorescence and the slag is examined in a scanning electron microscope. The slag formation kinetics are examined and a calculation of the activities of Ca, Al and their respective oxides in the slag phase is conducted. The driving forces of creating CaO and Al2O3 in this system is calculated to better understand the behavior of the Ca and Al removal. The results show that removal of dissolved Ca and Al by mechanical stirring is possible and in this setup a stirring time of 20 minutes is sufficient since no more refining can be obtained by increasing it.

The influence of electromagnetic stirring in an electric arc furnace (EAF) has been studied. Using numerical modeling the effect of electromagnetic stirring on the thermal stratification and fluid flow has been investigated. The finite element method (FEM) software was used to compute the electromagnetic forces, and the fluid flow and heat and mass transfer equations were solved using a finite volume method (FVM) software. The results show that electromagnetic stirring has a significant effect on temperature homogenization and mixing efficiency in the bath. The important part of this study was calculation of heat transfer coefficient. The results show, electromagnetic stirring improves the heat transfer from the melt to scrap which is dependent on the stirring direction and force magnitudes.