Alexandre José da Silva

Simulation of the solidification of pure nickel via the phase-field method

The Phase-Field method was applied to simulate the solidification of pure nickel dendrites and the results compared with those predicted by the solidification theory and with experimental data reported in the literature. The models behavior was tested with respect to some initial and boundary conditions. For an initial condition without supercooling, the smooth interface of the solid phase nucleated at the edges of the domain grew uniformly into the liquid region, without branching. In an initially supercooled melt, the interface became unstable under 260 K supercooling, generating ramifications into the liquid region. The phase-field results for dendrite tip velocity were close to experimental results reported in the literature for supercooling above 50 K, but they failed to describe correctly the nonlinear behavior predicted by the collision-limited growth theory and confirmed by experimental data for low supercooling levels.

Modelagem e simulação computacional da injeção de carvão pulverizado no canal adutor da “raceway” de altos-fornos

O alto-forno e um reator de fluxo contra corrente e tem como principal objetivo a producao de gusa. A injecao de materiais particulados e feita via ventaneiras objetivando diminuir a quantidade de coque carregado via topo do alto-forno. Esta tecnica abriu as opcoes de utilizacao de materias-primas de menor valor agregado que o coque, insumo que no Brasil e produzido com misturas de carvoes importados. Este trabalho propoe a simulacao da injecao de carvao pulverizado utilizando um modelo matematico 3D, multifasico. Como fases consideramse uma fase gasosa e duas fases solidas correspondendo a dois tipos de carvoes pulverizados, com alto e baixo teor de volateis respectivamente. Desta forma, as fases solidas, quando consideradas simultaneamente na injecao, sao representadas por diferentes campos de velocidade e temperatura, alem de sua propria composicao quimica. As taxas de cinetica quimica das reacoes foram consideradas atraves de modelos classicos da literatura e eventualmente tiveram suas constantes ajustadas para melhor representarem os carvoes individualmente no processo industrial. A solucao numerica foi obtida atraves de um codigo desenvolvido em Fortran 90/95, com base no metodo de volumes finitos utilizado na discretizacao do sistema de equacoes diferenciais nao lineares. A malha utilizada e nao ortogonal para melhor representar a geometria do equipamento experimental descrito na literatura e utilizado para validar o modelo e, posteriormente, a geometria das ventaneiras com diferentes pontos de injecao. Os resultados obtidos de temperatura e composicao do gas apresentam uma boa concordância com os resultados experimentais. O modelo tambem se mostra eficiente para a previsao do efeito de diferentes tipos de carvao injetados via ventaneiras. Palavras-chave: Alto-forno, reducao, ventaneiras, injecao de carvao, modelagem matematica, simulacao computacional.

Simulação transiente de um reator de leito fluidizado em 3D

Fluidized bed reactors reached became popular in the 80-90s. Since then, this equipment has become important for thermo electrical generation, because it permits fuel options for a high degree of conversion but with lower costs than those used in industrial processes. This technique does not play am important role in Brazil yet, where the use of optional electric generation technologies are now being considered. Thermal generation is one option in which that fluidized bed reactors can be used to burn materials like coal and biomasses. The mathematical models have been efficient in investigating the virtual conditions of the processes. In this work, a mathematical model aiming to simulate the fluidized bed reactor is proposed. The model uses the concept of a multiphase-multicomponent system to simulate the transient inner flowing conditions in 3D. The transport equations of mass, momentum and energy are solved by the finite volumes method. The phases are modeled using continuum mechanics principles, where a collection of identical particles is considered. The simulation results of the fluidized bed reactor showed good agreement with the experimental data.

Study of Iron Nanopowders into Fluids of Industrial Lubrication

The machines and equipment has required increasing performance of lubricating fluids and coolants which plays important role on reducing friction with the metal parts and heat extraction. Viscosity and thermal conductivity are the most important properties of lubricants, in relation to the friction between the fluid molecules. This paper presents two useful models to predict this properties and their relation with the particles volume fraction and temperature in the nanofluid formed by adition of iron or particles produced by friction. Nanofluids are innovative heat transfer fluids with superior potential for enhancing the heat transfer performance of conventional fluids. In this paper the Unit Cell Model (UCM) which considers the Brownian movement experienced by the nanoparticles are adapt to predict the increment of thermal conductivity of iron nanopowders and standard lubrication oil. The viscosity of the nanofluids was adapt from a model usually suitable for predict the effective viscosity of emulsions. Model results indicated a strong effect of the particle size and volume fractions on the increment of thermal conductivity.

Modelado del Proceso Convencional de Colada Continua de Aceros Libres de Interticios

The purpose of this work was to develop a mathematical model able to simulate the continuous casting process of IF steel (Interstitial Free).and to determine the optimum operating conditions. The model formulation involves the momentum equations of liquid and solidified materials coupled with the temperature evolution of the slab. The differential equations and boundary conditions are numerically solved using the finite volume technique with appropriated boundary conditions for each region of the casting machine. The model predictions were compared with industrial data for conventional continuous casting conditions and extended to IF steel.

Study of Hydrate Formation Kinetics in Petroleum Pipes by the Phase Field

We report the influence of phase field parameters on the modeling of gas hydrates formation. Also, the interface of the surface tension, super-cooling, and homogeneous and heterogeneous nucleation on the interface morphology and growing kinetics is evaluated. The mathematical model consists of simultaneous energy and phase field equations and is solved using the finite volume method. Results indicated that decreasing the surface tension leads to an increase of interface roughness and higher interface instability. It is also observed that an increase in the surface tension occurs together with an increase of surface thickness and lower growing kinetics. In this case, to promote the hydrate growth, it is necessary to impose a super-cooling of 2 K. Regarding homogeneous and heterogeneous nucleation, two conditions were simulated: a random distribution of nuclei, where the evolution of hydrate formation shows the occurrence of coalescence and the growing kinetics of coalesced was lightly decreased in comparison to the isolated portions of the hydrate; and heterogeneous nucleation along all the extension of the wall, where hydrate grows inward, the liquid region by mean of a homogeneous advance of the interface. In this paper, a model is reported that can be used to predict the hydrate growth process and asses parameters that are difficult to obtain experimentally.

Modelagem da cinética de formação de hidratos utilizando o Modelo do Campo de Fase em condições similares a dutos de petróleo

Natural hydrates are crystalline compounds that are ice-like formed under oil extraction transportation and processing. This paper deals with the kinetics of hydrate formation by using the phase field approach coupled with the transport equation of energy. The kinetic parameters of the hydrate formation were obtained by adjusting the proposed model to experimental results in similar conditions of oil extraction. The effect of thermal and nucleation conditions were investigated while the rate of formation and morphology were obtained by numerical computation. Model results of kinetics growth and morphology presented good agreement with the experimental ones. Simulation results indicated that super-cooling and pressure were decisive parameters for hydrates growth, morphology and interface thickness.

Simulação computacional da difusão molecular de poluentes líquidos de aterros em solos organo argilos – Aterro Metropolitano de Gramacho, RJ

Um dos principais problemas ambientais causados pelo acumulo de lixo e a liberacao de lixiviado no solo. O trabalho realizado compreende a utilizacao de dois instrumentos computacionais na avaliacao da difusao de ions dissolvidos na massa liquida proveniente de aterro sanitario. O software POLLUTE e o modelo computacional (MPHMTP) Multi Phase Heat and Mass Transfer Program, Castro (2000), inicialmente desenvolvido para simulacoes de processos siderurgicos e posteriormente adaptado ao calculo do escoamento e a interacao quimica de efluentes resultantes da decomposicao umida de rejeitos solidos urbanos e industriais em meios porosos (Pinto, 2004 e Forster, 2006). O modelo utiliza o metodo de volumes finitos para a realizacao das simulacoes de difusao molecular dos ions cloreto, sodio calcio, magnesio, potassio e amonio. Os ions analisados sao os que possuem maior concentracao no lixiviado e no solo orgânico argiloso do Aterro Metropolitano de Gramacho, localizado no municipio de Duque de Caxias, Rio de Janeiro. Os resultados mostram a validade e adequacao dos modelos POLLUTE e MPHMTP utilizados, quando foram comparados os resultados numericos dos modelos com os obtidos experimentalmente, atraves da escala de laboratorio proposta. Palavras-chave: Simulacao Computacional, Difusao Molecular, Meios Porosos e Volumes Finitos.