1,720,969 research outputs found
Modellazione numerica dei meccanismi di segregazione in materiali granulari sfusi
Full text linkI materiali granulari sono profondamente radicati nella lunga storia della scienza e della tecnologia. Essi vengono trattati quotidianamente da numerose industrie tra cui l’industria chimica, l’industria alimentare e l’industria farmaceutica. Tuttavia, la loro manipolazione e la loro lavorazione rimangono un importante problema in numerose applicazioni industriali. Uno dei problemi più rilevanti è quello della miscelazione di particelle aventi proprietà diverse dal momento che tendono a segregare spontaneamente. Pertanto, lo sviluppo di strumenti per prevedere la segregazione è essenziale per controllare e ridurre al minimo il fenomeno.
Questo progetto di ricerca riguarda la modellazione numerica dei meccanismi di segregazione in materiali sfusi, per miscele con vari gradi e tipi di dispersione particellare e applicati a diversi contesti industriali. Nello specifico, studieremo dapprima la segregazione per dimensione in miscele binarie diluite. Successivamente tratteremo la segregazione per taglia in sistemi di particelle multicomponenti e polidispersi. In questi casi, le equazioni di segregazione sono accoppiate con la reologia del flusso solido in modo bidirezionale. Poiché l’accoppiamento bidirezionale è piuttosto complesso, esistono solo pochi altri studi a riguardo. Proporremo poi un nuovo modello matematico per descrivere la segregazione per densità in miscele binarie. A differenza dei modelli precedenti, in questo caso utilizzeremo un accoppiamento unidirezionale. Inoltre, il campo di velocità sarà determinato direttamente da soluzioni analitiche piuttosto che risolvendo l’equazione di conservazione della quantità di moto. L’inclusione delle differenze di densità avrebbe portato a campi di velocità comprimibili e quindi, a modelli più complessi. Un ulteriore capitolo descrive un nuovo modello di segregazione per taglia che include la comprimibilità del campo di velocità. Ove possibile, i modelli saranno validati con esperimenti. Negli altri casi, la procedura di validazione si realizzerà con simulazioni DEM.
Tutte le teorie sono in grado di riprodurre sia qualitativamente che quantitativamente ciò che accade nella realtà. Pertanto, i modelli di segregazione proposti rappresentano un ulteriore passo avanti verso una descrizione completa e accurata della segregazione in una varietà di flussi granulari densi. Questi modelli possono inoltre aiutare gli ingegneri a sviluppare strategie di mitigazione, a progettare e dimensionare razionalmente apparecchiature e a sviluppare più efficaci sistemi di controllo di processo.Granular materials are deeply rooted in the long history of science and technology. Furthermore, several industries process granular materials routinely, including chemical, food and pharmaceutical industries. Handling and processing of these materials remain a major challenge in numerous industrial applications. The main difficulty regards the mixing of particles with different properties because of their tendency to segregate spontaneously. Thus, the development of tools for predicting segregation is essential in order to control and minimize the occurrence.
This research project is concerned with the numerical modelling of segregation mechanisms in bulk materials for mixtures with varying degrees and types of particle dispersity, in many industrial settings. More specifically, we first study size-driven segregation in diluted binary mixtures. We then tackle segregation due to size differences in multi-component and polydisperse particle systems. In these cases, the segregation equations are fully coupled with the solid flow rheology. Since the coupling is challenging, only a few other studies exist in this area. We then propose a new mathematical model for density-driven segregation in binary mixtures. Unlike the previous models, in this case, we employ a one-way coupling. Furthermore, the velocity field is determined directly from analytic solutions rather than by solving the momentum equation. The inclusion of density differences would have led to compressible velocity fields and hence, to more complex models. An additional chapter describes a new model for particle-size segregation that include the compressibility of the velocity field. Wherever possible, the models are validated in a two-way comparison among experiments and theory. In the other cases, the validation procedure is accomplished with DEM simulations.
All theories are capable of reproducing both qualitatively and quantitatively what happens in reality. Thus, the proposed segregation models represent a step towards a complete and accurate description of segregation in a variety of dense granular flows. Furthermore, the models can help engineers in developing mitigation strategies and in rationally designing and scaling equipment, processes, and process control
Characterising powder flowability at high shear rates by the ball indentation method
No full textUnreliable powder flow is a major problem during processing of powders. The shear cell is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions, with established methods for designing large bins and hoppers based on the measurement. However, this method is not suitable for measuring the flowability of dynamic systems, such as powder mixing. Here, the ball indentation method is investigated as a technique for evaluating powders in the intermediate and dynamic regime of flow. The method, which simply consists of dropping a ball onto a cylindrical bed of powder previously consolidated, directly measures hardness, which is related to the unconfined yield stress of the powder by the constrain factor (Hassanpour and Ghadiri, 2007). The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The shear rate is varied by using a range of indenter materials and sizes, and a range of drop heights. The hardness against the strain rate is considered for several materials. It was found that the indenter size does not influence the hardness results, which are consistent with the flowability evaluation achieved with the rheometer. Furthermore the hardness, which is independent of the strain rate in quasi-static conditions, becomes shear rate dependent in intermediate regime of flow. Further work is needed to evaluate hardness in the rapid granular flow regime
CFD-PBE coupled model for size-driven segregation in polydisperse granular flows
No full textMixtures of granular materials made of different-sized particles may segregate when subjected to vibration or shear rate and in the presence of a gravitational field. This leads to highly inhomogeneous mixtures, which are undesirable in many industrial processes. This work focuses on size-driven segregation in polydisperse mixtures. We described the evolution of the particle size distribution through a Population Balance Equation (PBE), which we solved numerically with the Direct Quadrature Method of Moments. To allow segregation and micromixing to occur, we closed the size-conditioned velocity of the particles with a segregation-remixing model. The PBE was then included in an Eulerian-Eulerian model and solved in a commercial Computational Fluid Dynamics (CFD) code. We used the model to study granular flows down inclined planes. The numerical results were then compared with those obtained from Discrete Element Method simulations. The CFD-PBE model predicts well segregation and micromixing in packed beds of polydisperse powders
Characterising powder flowability at high shear rates by the ball indentation method
Full text linkThe Ball Indentation Method has been experimentally investigated as a new technique to characterize powder flow at high shear rates. The results indicate that the hardness is independent on the strain rate in quasi-static conditions, whilst the flow resistance can be related to the strain rate in intermediate flow regime, where the rheological behaviour of the powders becomes liquid-like. Despite the results agree with the simulations reported in literature, the method has some limitations.openEmbargo temporaneo per motivi editorial
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Characterising powder flowability at high shear rates by the ball indentation method
Full text linkThe Ball Indentation Method has been experimentally investigated as a new technique to characterize powder flow at high shear rates. The results indicate that the hardness is independent on the strain rate in quasi-static conditions, whilst the flow resistance can be related to the strain rate in intermediate flow regime, where the rheological behaviour of the powders becomes liquid-like. Despite the results agree with the simulations reported in literature, the method has some limitations
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