Thèse (M.Eng.) -- Université du Québec à Chicoutimi, 2004. / Bibliogr.: f. 234-242. Document électronique également accessible en format PDF. CaQCU

The primary focus of the present work is to model the entire casting process from filling stage to complete solidification. The model takes into consideration any phase change taking place during the filling process. An implicit volume of fluid (VOF) based algorithm has been employed for simulating free surface flows during the filling process and the model for solidification is based on a fixed-grid enthalpy-based control volume approach. Solidification modelling is coupled with VOF through User Defined Functions (UDF) developed in commercial fluid dynamics (CFD) code FLUENT 6.3.26. The developed model is applied for the simultaneous filling and solidification of pure metals and binary alloy systems to study the effects of filling process on the solidification characteristics, evolution of mushy zone and the final macrosegregation pattern in the casting. The numerical results of the present analysis are compared with the conventional analysis assuming the initial conditions to be a completely filled mould cavity with uniform temperature, solute concentration and quiescent melt inside the cavity. The effects of process parameters, namely the degree of superheat, cooling temperature and filling velocity etc. are also investigated. Results show significant differences on the evolution of mushy zone and macrosegregation between the present analysis and the conventional analysis. The application of present model to simulate three dimensional sand casting is also demonstrated. The three dimensional competetive effect of filling generated residual flow and the buoyancy-induced convective flow pattern cause significant difference in macrosegregation pattern in casting.

<p>Predmet izučavanja ove disertacije je ispitivanje mogućnosti imobilizacije&nbsp;toksičnih metala u sedimentu primenom imobilizacione tehnike&nbsp;solidifikacije/stabilizacije portland cementom, kalcijum-oksidom, prirodnim zeolitom,&nbsp;letećim pepelom, kaolinitom, montmorilonitom, i različitim sme&scaron;ama ovih agenasa&nbsp;kao i određivanje njihove efikasnosti u zavisnosti od brojnih faktora. Efikasnost&nbsp;imobilizacionih postupaka praćena je kori&scaron;ćenjem tzv. testova izluživanja, odnosno&nbsp;određivanjem koncentracije metala koji su u toku određenog vremena iz imobilisane&nbsp;faze pre&scaron;li u mobilnu fazu gde se smatraju potencijalno opasnim i biodostupnim.&nbsp;Krajnji cilj svakog testa izluživanja je mogućnost procene adekvatnosti primene&nbsp;određene imobilizacione, odnosno remedijacione tehnike. Efikasnost imobilizacionih&nbsp;postupaka zavisi i od određenih osobina: pH rastvora, dodatka imobilizacionog agensa&nbsp;&scaron;to je istraživanjem ispitano. Na osnovu ovog testa, odnosno &nbsp; odnosa kumulativne&nbsp;frakcije metala koja je oslobođena iz imobilizacione sme&scaron;e u mobilnu fazu i vremena&nbsp;određen je tip mehanizma koji omogućava transport metala &nbsp;(spiranje, difuzija,&nbsp;rastvaranje). Takođe su određeni i i parametri (koeficijenti difuzije, indeksi&nbsp;izlužljivosti) koji&nbsp; će poslužiti za ocenu efikasnosti prethodno primenjenih&nbsp;imobilizacionih tehnika. Primenjeni su i modifikovani testovi izluživanja sa ciljem &scaron;to&nbsp;bolje simulacije realnih uslova. U modifikovanim testovima izluživanja kori&scaron;ćen je&nbsp;rastvor sa pH 3,25 da bi se simulirali uslovi koji opona&scaron;aju kisele ki&scaron;e, odnosno uslovi&nbsp;u realnom sistemu. Takođe je kor&scaron;ćen i rastvor huminskih materija sa ciljem&nbsp;simulacije uslova velikog organskog opterećenja do kojeg bi moglo doći u slučaju&nbsp;akcidentnih situacija (na primer poplava) i generalno uslova koji se stvaraju u &nbsp;prirodi&nbsp;pri raspadanju organskog materijala (li&scaron;će, trava, itd.). Rezultati dobijeni simulacijom&nbsp;ovih uslova će dalje omogućiti modelovanje pona&scaron;anja metala u smislu dugoročnog&nbsp;&quot;izluživanja&quot; iz tretiranog sedimenta kao i procenu najefikasnijih agenasa &nbsp; za&nbsp;imobilizaciju različitih metala u sedimentu. Na osnovu dobijenih rezultata, zaključeno&nbsp;je da su optimalni agensi za imobilizaciju metala u sedimentu sme&scaron;a cementa i kreča&nbsp;(5% cementa i 10% kreča), sme&scaron;a montmorilonita i kreča (30% montmorilonita i 10%&nbsp;kreča), leteći pepeo (30%) i zeolit (30%). U ovim sme&scaron;ama dominantan mehanizam&nbsp;izluživanja je difuzija, a ove sme&scaron;e se mogu smatrati inertnim otpadom po svim&nbsp;ispitivanim kriterijumima. Ovi rezultati se mogu upotrebiti za projektovanje i&nbsp;izgradnju pilot postrojenja na kome bi se ispitala efikasnost ovih agenasa za&nbsp;remedijaciju sedimenta u realnim uslovima. Dobijeni podaci su neprocenjivi sa&nbsp;aspekta ekonomski i ekolo&scaron;ki prihvatljivog upravljanja sedimentom.</p> / <p>This work is concerned with exploring the possibilities of immobilization of toxic&nbsp;metals in sediments using solidification/stabilization as imobilization technique and&nbsp;using Portland cement, calcium oxide, natural zeolite, flying ash, kaolinite, montmorilonite, and various mixtures of these agents and determine their&nbsp;effectiveness depending on many factors. Performance of imobilization procedures&nbsp;was followed by the use of so-called leaching tests and determination of the metals&nbsp;concentration that are within a specified interval of time leached from the immobile phase and as such can be considered potentially hazardous and bioavailable. The ultimate goal of every leaching test is to assess the adequacy of the possibility of applying certain imobilization or remediation technique. Performance of imobilizaction procedures depends on certain characteristics: pH of solution, additon of imobilization agent, etc. Based on this test, ie relation between the cumulative fraction of metal leached from imobilization mixture and time, the type of leaching mechanism that allows the transport of metals (wash-off, diffusion, dissolution) was determined. Other parameters which will serve for evaluating the efficiency of the previously applied imobilization techniques (diffusion coefficients, leaching indices) were also determined. Modified leaching tests were applied with the aim of better simulation of real conditions on the field. In the modified leaching tests the solution with pH 3.25 was used to simulate conditions that mimic acid rain and conditions in the real system. The humic acid solution was also used with the aim to simulate high organic loads which could occur in the case accident situation (eg floods) and the general conditions that are created in naturewhen decaying organic material (leaves, grass, etc.). The results obtained by simulating these conditions will enable the modeling of behavior of metals in terms of long-term leaching period from the treated sediment and assess the most effective agents for the immobilization of various metals in the sediments. Based on these results, it was concluded that the optimal agents for immobilization of metals in the sediment mixture of cement and lime (5% cement and 10% lime), a mixture montmorilonite and&nbsp; lime (30% montmorilonite and 10% lime), fly ash (30%) and zeolite (30%). In these mixtures dominant leaching mechanism is diffusion, and these mixtures can be considered as inert waste by all tested criteria. These results can be used to design and builda pilot plant in which order to evaluate the effectiveness of these agents for remediation of sediment in real &nbsp;terms. The obtained data are invaluable from the aspect of economic and ecologically acceptable management of sediment.</p>

Manufacturing of light-weight materials is associated with several types of casting defects during solidification. Porosity defects are common, especially in aluminum and its alloys, which initiate crack propagation and thereby cause drastic deterioration in the mechanical properties. These defects, classified as micro and macro defects (based on their sizes), are mainly governed by release of hydrogen into the liquid at the solid-liquid interface, which triggers the nucleation and growth of hydrogen bubbles in the melt. Subsequently, these bubbles interact with solidifying interfaces such as dendritic arms and eutectic fronts, leading to the formation of pores. Macroscopic defects in the form of voids are created due to solidification shrinkage. The primary focus of the present work is to develop phenomenological models for the evolution of microporosity and microstructures during solidification. The issues outlined above typically occur in multi-phase environments comprising of solid, liquid and gaseous phases, and over a range of length and time scales. Any phenomenological prediction would, therefore, require a multi-phase-scale approach. Principles of volume averaging are applied to equations of conservation to obtain single-field formulations. These are then solved with appropriate interface tracking techniques such as Enthalpy, Level-set, Volume-of-fluid and Immersed-boundary methods. The framework is built up on a standard pressure based incompressible fluid flow solver (SIMPLER algorithm) and coupled modeling strategies are proposed to address the interfacial dynamics. A two-dimensional framework is considered with a fixed-grid Cartesian co-ordinate system. Scaling analyses are performed to bring out the relative effects of various competing parameters in order to obtain further insights into this complex phenomenon. The numerical results and scaling predictions are validated against experimental observations published in literature. In literature, numerical predictions of microporosity mainly include criteria based models based on empirical relations and deterministic/stochastic models based on diffusion driven growth assuming spherical bubbles. The dynamic evolution of non-spherical bubble-metal interface in a three-phase system is yet to be captured. Moreover, several in-situ experiments have shown elongated bubble shapes during the engulfment phase, therefore a criterion to define the dependence on cooling rates and the resulting bubble morphology can possibly deliver further practical insights. We propose a numerical model for hydrogen bubble growth, its movement and subsequent engulfment by a solidifying front, combining the features of level-set and enthalpy methods for tracking bubble-metal and solid-liquid interfaces, respectively. The influx of hydrogen into heterogeneously nucleated bubbles results in growth of bubbles to sizes up to a few hundreds of microns. In the first part of this numerical study, a methodology based on the level-set approach is developed to simultaneously capture hydrogen bubble growth and movement in liquid aluminum. The solidification is first assumed to occur outside the micro-domain providing a specified hydrogen influx to the bubble-in-liquid system. The level-set equation is formulated in such a way as to account for simultaneous growth and movement of the bubble. The growth of a bubble with continuous and fixed hydrogen levels in the melt is studied. The rates of growth of bubble-liquid and solidifying interfaces are compared using an order of magnitude analysis. This scaling analysis explains the thought experiment proposed in the literature, where difference in bubble shapes was attributed to the cooling rate. Moreover, it shows explicit dependence on bubble radius and cooling rate leading to a new criterion for bubble elongation proposed in this thesis. This also highlights the comparison between solidification and hydrogen diffusion time-scales which primarily govern the competitive growth behavior. The bubble-in-liquid model is coupled with microscopic enthalpy method to incorporate effects of solidification and study the interaction of solid-liquid and bubble-liquid interfaces. The phenomena of bubble engulfment and elongation are successfully captured by the proposed model. A parametric study is carried out to estimate the bubble elongation based on different initial bubble sizes and varying cooling rates encountered in typical sand, permanent mold and die casting processes. Although simulation of microstructures has been extensively studied in the literature, very few models address the phenomena of simultaneous growth and movement of equiaxed dendrites. The presence of different flow environments and multiple dendrites are known to alter the position and shape of the dendrites. The proposed model combines the features of the following methods, namely, the Enthalpy method for modeling growth; the Immersed Boundary Method (IBM) for handling the rigid solid-liquid interfaces; and the Volume of Fluid (VOF) method for tracking the advection of the dendrite. The algorithm also performs explicit-implicit coupling between the techniques used. Validation with available literature is performed and dendrite growth in presence of rotational and buoyancy driven flow fields is studied. The expected transformation into globular microstructure in presence of stirring induced flows is successfully simulated. A simple order estimate for time required for stirring is performed which agrees with numerical predictions. In buoyancy driven environment of a settling dendrite, the arm tip speeds show expected higher velocity of the upstream tip compared to its counterpart. The model is extended to study thermal and hydrodynamic interactions between multiple dendrites with appropriate considerations for different orientations and velocities of the dendritic solid entities. The present model can be used for the prediction of grain sizes and shapes and to simulate morphological transformations due to different melt flow scenarios. In the final part, the methodology presented for growth and engulfment of hydrogen bubbles is extended to study the phenomenon of diffusion driven bubble growth occurring in direct foaming of metals. The source of hydrogen is determined by the rate of decomposition of the blowing agent. This is accounted for by a source term in the hydrogen species conservation equation, and growth rate of hydrogen bubbles is calculated on the basis of diffusive flux at the interface. The level-set method is used for tracking the bubble-liquid interface growth, and the macroscopic enthalpy model is used for obtaining heat transfer and solid front position. The model is validated with analytical solution by comparing the front position and the solidification time. The variation of foam density with a transient hydrogen generation source is studied and qualitatively compared with results reported in literature. The modeling strategies proposed in this work are generic and therefore have potential in simulating a variety of complex multi-phase problems.