1,721,036 research outputs found
Measurement of gas hold-up distribution in stirred vessels equipped with pitched blade turbines by means of electrical resistance tomography
No full textGas dispersion is a widespread operation in the process industry. The effectiveness of the dispersion affects fluid mixing, heat and mass transfer rates and, as a consequence, the chemical or biochemical reactions involved. In this work, the gas-liquid dispersion in stirred tanks equipped with pitched blade turbines (PBT) is investigated by means of Electrical Resistance Tomography (ERT). The main goal of this study is the assessment of gas-distribution in the vessel. Measurements are taken in case of different fluid dynamic regimes, which occurrence depends on the operating conditions. The effect of impeller rotational speed on gas-liquid dispersion is investigated in vessels having different sizes. The adoption of ERT allows the assessment of gas distribution through the vessel volume without any particular limitation about the maximum gas volume fraction, so that experimental conditions close to that adopted in industrial operation can be effectively managed. The transition between flooding and loading regime is investigated, in which the impeller starts to effectively distribute the gas throughout the liquid phase. The vessel size is found to play a role in determining the condition at which loading regime occurs. The data could be either used to set up simplified correlations for the transition between flooding/loading regimes or as a valuable benchmark for CFD simulation, given the detailed information available about the spatial distribution the dispersed gas
Experimental analysis of gas-liquid dispersion in mechanically stirred tank by means of quantitative Electrical Resistance Tomography
No full textElectrical Resistance Tomography (ERT) is adopted extensively to measure fundamental dispersion properties in dense multiphase systems, where other techniques are either impossible to use (e.g. optical techniques), or very expensive (CT, PEPT, especially at large scales). Quite satisfying qualitative results can be obtained by ERT (sufficient for the accurate assessment of regime transitions or blending times), but the ability to reconstruct quantitative data has still to be fully addressed.
In this paper, a practical method for obtaining quantitative data from ERT measurements is proposed and validated through
experimental data. Several aspects are addressed: the effect of iterative vs non-iterative conductivity tomogram algorithms, determination of the optimal iteration number for iterative procedures and the choice of the most suitable conductivity-holdup relation. The general method proposed is applied to the dispersion of gas in a mechanically stirred tank agitated by a pitched blade turbine, finally resulting in affordable data for CFD models validation
Modeling of Magnetic-Field-Assisted Fluidization: Model Development and CFD Simulation of Magnetically Stabilized Fluidized Beds
Full text linkMagnetic-field-assisted fluidization is starting to be considered as a viable alternative to standard fluidized beds for those operations (such as particle separations, filtration, adsorption) in which the solid phase can be made of magnetic particles or, alternatively, the fluidizing agent is a ferro-fluid; thus the fluid bed responds to the action of magnetic fields, and stabilized fluidization regimes can be generated.
One of the major difficulties to be tackled is the development of a predictive model capable of estimating the stabilized-to-bubbling transition velocity for a given magnetic field or, on the other hand, the magnetic field intensity required to stabilize the bed to a quiescent condition. The fluid dynamics prediction of a stabilized bed is also a challenging task at the moment.
On this basis, a very simple model for the description of MSFB was derived in this contribution starting from basic fluid dynamics and magnetodynamics equations. The model was implemented in a commercial CFD code in order to simulate the effect of the magnetic field onset on a freely bubbling fluidized bed
Solid distribution and mixing time in stirred tanks: The case of floating particles
No full textThis work concerns floating particle distribution and liquid mixing dynamics in a solid-liquid stirred tank. Measurements of local solids concentration distribution at steady-state conditions and of liquid homogenization in the presence of dispersed particles at transient conditions are collected with up-pumping and down-pumping pitched blade turbines. Electrical resistance tomography is the selected experimental technique for the data acquisition in the opaque solid-liquid mixture. For the time dependent conductivity data processing, the same method commonly applied to the mixing time determination from planar laser induced fluorescence measurements is considered. The method is successfully extended to the characterization of liquid mixing with floating particles. The local data obtained by the ERT technique over the whole vessel volume are fully exploited. The robustness of the method for tackling conductivity variations close to the liquid free surface and due to the particles movement is assessed
Solid distribution and mixing time in stirred tanks. The case of floating particles.
No full textThe dispersion of solid particles and the liquid homogenization dynamics in a slurry stirred tank equipped with different impellers is investigated by Electrical Resistance Tomography (ERT). The selected experimental technique allows to overcome the typical limitations of optical methods and to gain insight into the complex behaviour of two-phase systems, without restriction on the upper value of the average volumetric fraction of the dispersed phase, this conditions being of interest for several chemical and biochemical processes.
A laboratory scale baffled stirred tank equipped with different impellers, namely a Rushton Turbine, a Pitched Blade Turbine and a Lightnin A310 impellers, is investigated, in order to evaluate the dispersion features under radial, axial and mixed prevailing flow fields. The analysis concerns dense solid-liquid systems (mass of solid to liquid ratio up to 0.43) under fully or partially suspended solids. The dispersed phase distribution data are based on the measurement of the conductivity of the mixture as compared with the conductivity of the pure liquid, while the mixing time is estimated from the time evolution of the averaged conductivity on each measurement plane after the injection of a small amount of salt aqueous solution.
The analysis of the experimental results allows to suggest simple correlations for the interpretation of both the concentration distribution and the dimensionless mixing time.
Beside, the experimental data can be adopted as a benchmark for advanced modelling techniques based on Computational Fluid Dynamics, whose scant validation out of dilute conditions is often due to limited information on the local dispersion features
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
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