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Étude du comportement des fluides magnétorhéologiques (MR) en compression-cisaillement simultanéeLucking Bigué, Jean-Philippe January 2016 (has links)
Les fluides magnétorhéologiques (MR) sont des fluides intelligents dont la viscosité apparente peut être modifiée rapidement (<1ms) par l'application d'un champ magnétique externe. À l'aide de cette caractéristique unique, les embrayages MR permettent de moduler rapidement un couple entre deux surfaces sans contact mécanique direct. De construction simple et robuste, les embrayages MR offrent ainsi un potentiel remarquable d'innovation pour diverses applications robotiques nécessitant un contrôle rapide et une haute fiabilité, comme dans les domaines de l'automobile [10], de l'aéronautique [16] ou de l'interaction humaine [77].
À ce jour, les embrayages MR exploitent le fluide MR strictement en cisaillement pur. Dans de telles conditions, la densité de couple des embrayages est limitée par l'optimisation de la mécanique des embrayages (ex. : poids) [63] et les propriétés fondamentales des fluides MR (ex. : contrainte) [11]. Alors qu'en cisaillement pur, la contrainte des fluides MR est limitée à ∼100 kPa, des études récentes démontrent qu'elle peut être augmentée d'un ordre de grandeur (>1000 kPa) lorsque le fluide MR est soumis à une compression, avant d'être cisaillé [89]. La combinaison de la compression et du cisaillement du fluide MR pourrait ainsi décupler la densité de couple des embrayages MR, mais ce phénomène d'agrégation assistée par compression, aussi appelé squeeze-strengthening ou super-strong} (SS), est toujours très mal compris. Beaucoup d'incertitude persiste quant à l'origine exacte du phénomène [45], des conditions qui le favorisent [55] [75] et des effets qu'il engendre [31].
Dans le but ultime d'augmenter la densité de couple des embrayages MR à l'aide du phénomène SS, le présent projet de recherche vise à étudier le comportement rhéologique des fluides MR dans des conditions de compression et de cisaillement simultané afin d'en comprendre les conditions qui favorisent le phénomène d'augmentation des contraintes. Pour ce faire, un banc d'essai permettant la compression pure, le cisaillement pur et la compression-cisaillement simultanée des fluides MR est conçu et utilisé pour étudier le fluide MR lorsque soumis à des conditions de chargement typique des embrayages MR. Les résultats expérimentaux issus de cette vaste étude expérimentale permettront d'établir un lien direct entre ce phénomène et le celui de filtration, duquel un modèle prédictif sera proposé. À l'aide du modèle théorique, le phénomène SS sera étudié à l'aide de diverses compositions de fluide MR (ex. : concentration, taille des particules, viscosité) et différentes géométries de compression, ce qui permettra de valoriser le modèle pour la conception préliminaire d'embrayages MR qui exploitent le phénomène SS.
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Estudo da dispersão na secagem de frutos de café em secador de bandejas vibradasSfredo, Marilia Assunta 28 August 2006 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / To study the dispersion of the coffee fruits during the drying, a vibrated tray drier
with recycle was used. The dryer consists basically of four sections: drying vertical tunnel,
vibration system, system of warm air supply to the drying tunnel and recycle system of
coffee fruits. The drying tunnel contains four perforated trays through which the coffee
fruits and air flow out, in cross flow. The coffee fruits drying was carried out using two
experimental design, where the studied variables were, for the first design: coffee fruits
temperature (40; 45 and 50ºC); mass of coffee fruits (11.5; 12.5 and 13.5 kg) and air mass
rate (7; 8 and 9 kg air/min); the varieties of coffee fruits were: Acaiá, Catuaí and Mundo
Novo; and for the second design: mass of coffee fruits (10; 12 and 14 kg) and air mass rate
(7; 8 and 9 kg air/min), with coffee fruits temperature around 45ºC and the coffee variety
Mundo Novo. For the first experimental design, the coffee fruits temperature only
influenced significantly the drying time, where the largest temperature level reduces in
26.77 h the drying time. For the second experimental design, the studied variables were not
significant on drying time. Coffee fruit sphericity, density, sticky decreased with the
decrease of the moisture content. With reference to the quality of coffee grain, the best
operational conditions were obtained with greater coffee fruits mass and air mass rate. The
coffee fruits flow in the drying tunnel is promoted by vibration of the trays coupled to
electromagnetic vibrators. The vibration amplitude was determined by an accelerometer
connected to a signal conditioner and an analogical oscilloscope. The vibration amplitude
decreased with the reduction of the coffee fruits moisture content due to the shrinkage and
decrease of the mass, sticky, density and particle size coffee fruits. The coffee fruits mass
rate and the residence time distribution were determined (RTD), during the drying. At the
end of the drying, the flow occurs with easiness due to: absence of sticky of the coffee
fruits; decrease of the particle mass and particle vibration damping decrease, due to particle
rigidity acquired in the drying. The dispersion coefficient (Ez) was determined by Taylor
Dispersion Model, Free Dispersion Model and Modified Free Dispersion Model. For the
great majority of the experiments, the model that better fitted the experimental data (greater
coefficient of correlation) was the model of the Modified Free Dispersion. The dispersion
coefficient (Modified Free Dispersion) ranged from 1.31×10-4 to 68.67×10-4 m2/s. The
Péclet number ranged from 1.15 to 31.00. / Para estudar a dispersão dos frutos de café durante a secagem, utilizou-se um
secador de bandejas vibradas com reciclo. O secador consiste basicamente de quatro
seções: túnel vertical de secagem, sistema de vibração, sistema de injeção de ar aquecido no
túnel de secagem e sistema de reciclo dos frutos de café. O túnel de secagem contém quatro
bandejas perfuradas por onde escoam os frutos de café e o ar, em fluxo cruzado. A secagem
dos frutos de café foi realizada utilizando-se dois planejamentos experimentais, onde as
variáveis estudadas foram, para o primeiro planejamento de secagem: temperatura dos
frutos de café (40; 45 e 50ºC); massa de frutos de café alimentada (11,5; 12,5 e 13,5 kg) e
vazão de ar de secagem (7; 8 e 9 kg ar/min), a variedade dos frutos de café foram: Acaiá,
Catuaí e Mundo Novo; e para o segundo planejamento de secagem: massa de frutos de café
(10; 12 e 14 kg) e vazão de ar (7; 8 e 9 kg ar/min), mantendo-se a temperatura dos frutos
em 45ºC e a variedade Mundo Novo. Para o primeiro planejamento somente a temperatura
dos frutos influenciou significativamente o tempo de secagem, onde o maior nível de
temperatura reduz em 26,77 h o tempo de secagem. Para o segundo planejamento as
variáveis estudadas não foram significativas para o tempo total de secagem dos frutos de
café. Durante a secagem ocorre encolhimento dos frutos de café, diminuição da
esfericidade, da pegajosidade, da densidade aparente e aumento da área superficial
específica com a diminuição do conteúdo de umidade dos frutos de café. Em relação à
qualidade do grão de café, as melhores condições operacionais foram obtidas com maior
massa e maior vazão de ar de secagem. O escoamento do café no túnel de secagem é
promovido pela vibração das bandejas acopladas a vibradores eletromagnéticos. A
amplitude de vibração foi determinada por um acelerômetro acoplado a um condicionador
de sinal e a um osciloscópio analógico. A amplitude vibracional diminuiu com a
diminuição da umidade dos frutos de café devido ao encolhimento e à diminuição da
massa, da pegajosidade, da densidade e do diâmetro dos frutos de café. Durante a secagem
foram determinadas a vazão mássica dos frutos de café e a distribuição do tempo de
residência (DTR). Ao final da secagem, o escoamento dos frutos de café ocorre com maior
facilidade devido a: ausência de pegajosidade dos frutos de café; diminuição da massa das
partículas e redução do amortecimento da vibração dos frutos de café devido à rigidez
adquirida na secagem. O coeficiente de dispersão (Ez) foi determinado pelos modelos da
Dispersão de Taylor, da Dispersão Livre e da Dispersão Livre Modificado. Para a grande
maioria dos experimentos, o modelo que melhor se ajustou aos dados experimentais (maior
coeficiente de correlação) foi o Modelo da Dispersão Livre Modificado. O coeficiente de
dispersão dos frutos de café variou de 1,31×10-4 a 68,67×10-4 m2/s. O número de Péclet
variou de 1,15 a 31,00. / Doutor em Engenharia Química
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Nanoparticle Removal and Brownian Diffusion by Virus Removal Filters: Theoretical and Experimental StudyGustafsson, Olof January 2017 (has links)
This study aims to examine the throughput of nanoparticles through a Cladophora cellulose based virus removal filter. The effect of Brownian motion and flow velocity on the retention of 5 nm gold nanoparticles, 12.8 nm dextran nanoparticles and 28 nm ΦX174 bacteriophages was examined through MATLAB simulations and filtration experiments. Modeling of Brownian motion at different flow velocities was performed in MATLAB by solving the Langevin equation for particle position and velocity for all three types of particles. The motion of all three particle types was shown to be constrained at local flow velocities of 1∙10-2 m/s or greater. The constraint was greatest for ΦX174 bacteriophages, followed by dextran particles and then gold particles as a result of particle diameter. To verify the effect experimentally, virus removal filters were prepared with a peak pore width of 23 nm. Filtration experiments were performed at different flux values where gold and dextran particles did not exhibit any difference in retention between fluxes. However, a significant amount of gold and dextran particles were removed by the filter despite being smaller than the measured pore size. A decrease in retention with filtrated volume was observed for both particle types. Filtration of ΦX174 bacteriophages exhibited a difference in retention at different fluxes, where all bacteriophages where removed at a higher flux. The results from both simulations and experiments suggest that the retentive mechanism in filtering is more complex than what can be described only by size exclusion sieving, Brownian diffusion and hydrodynamic constraint of particles.
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