322,906 research outputs found
A comparison between single- and multi-objective optimization to fit spectral induced polarization data from laboratory measurements on alluvial sediments
No full textSpectral induced polarization measurements on unconsolidated and saturated alluvial samples, sand-clay mixtures and well sorted sandy samples, are modelled with the generalized Cole–Cole phenomenological model and two simplified models: the standard Cole–Cole and the Cole–Davidson model. The goodness of fit is evaluated, as a first step, through the root mean square error, weighted on the data errors of the real and the imaginary component. At a later stage a multi-objective optimization is proposed, based on two different indicators for the resistivity amplitude and phase misfit. The analysis of the misfits variations among all the tested parameters associations is conducted to identify the Pareto set of optimal solutions. Both procedures lead to model parameter estimates comparable with literature values. However, the multi-objective approach provides information about the uncertainty of the parameter estimates and highlights the presence of more than one characteristic value for the relaxation time and the frequency exponent in many samples, thus suggesting the possible occurrence of different polarization processes in the investigated frequency range
Implementazione della realizzabilità nel modello k-e di Launder e Spalding in un codice commerciale CFD
No full textEstimation of sediment texture from spectral induced polarisation data using cluster and principal component analysis
No full textSpectral induced polarisation data are usually interpreted with simple models in order to derive petrophysical relationships between electrical and sedimentological properties, such as texture, clay content, and permeability. The aim of this work is to explore the value of spectral induced polarisation in addition to conventional direct-current resistivity measurements for determining textural properties of saturated samples collected from alluvial deposits. For this, an advanced data processing approach that combines cluster and principal component analysis was developed and applied to integral parameters derived from Debye decomposition of spectral induced polarisation data. This data processing procedure allowed identifying groups of samples with a similar spectral induced polarisation response and to derive a characteristic grain-size distribution for each group of samples.The method to estimate the grain-size distribution from spectral induced polarisation data was successfully validated using independent sediment samples. The remaining uncertainty in the estimation of sediment texture from spectral induced polarisation data was attributed to the effect of pore size distribution and mineralogy, which were not considered in the present work but can be added in the future within the same conceptual workflow
The effect of electrolyte concentration on counter-current gas–liquid bubble column fluid dynamics: Gas holdup, flow regime transition and bubble size distributions
No full textElectrolytes are well-known to suppress the coalescence, stabilize the homogeneous flow regime and increase the gas holdup; however, there is a lack of studies concerning the influence of electrolyte concentration on counter-current bubble column fluid dynamics. In this paper, we contribute to the existing discussion by experimentally investigating the effect of electrolyte concentration (sodium chloride, NaCl) on gas holdup, flow regime transition and bubble size distributions in a large-diameter and large-scale counter-current bubble column (5.3 m in height and 0.24 m inner diameter). We considered gas superficial velocities in the range of 0.004–0.20 m/s and liquid superficial velocities up to −0.06 m/s. Air was used as the dispersed phase and various aqueous solutions of NaCl—up to the critical concentration—were used as the liquid phase. The gas holdup measurements were used to investigate the bubble column fluid dynamics and to analyze the flow regime transition. The image analysis was applied, near the sparger and in the developed region, to study the bubble size distributions in batch and in counter-current modes. The presence of NaCl—up to a critical concentration—stabilizes the homogeneous flow regime, increases the gas holdup and shifts the bubble size distribution toward lower bubble diameters. The counter-current mode destabilizes the homogeneous flow regime when using tap water and stabilizes the homogeneous flow regime when using aqueous solution of NaCl. The results suggest that the changes in the bubble size distributions stabilize/destabilize the homogeneous flow regime and, thus, increase/decrease the gas holdup. © 2016 Institution of Chemical Engineer
Holdup and local flow properties in a counter-current bubble column
No full textGas-liquid bubble column reactors are met in several industrial plants in the chemical and oil industry fields. The understanding of the global hydrodynamics, the local flow phenomena and the bubble properties are of fundamental importance in the comprehension of the flow dynamics as well as the mass transfer phenomena. This paper describes the experimental results obtained in a counter-current circular column of 240 mm in terms of the fluid dynamic behavior. The counter-current flow studied concerns an upward flow of air and a downward flow of liquid at ambient temperature and pressure. Air is introduced by means of a spider sparger distributor. Different liquids have been used as working fluids: water, water with NaCl (at different concentrations) and mixtures of water and ethanol. The following range of operating conditions was analyzed: superficial air velocities up to 20 cm/s and superficial liquid velocities up to - 11 cm/s, corresponding to a global air volume fraction (the holdup) up to 26%. The experimental investigations concerns (i) flow visualization, (ii) local data from a double fiber optical probe and (iii) holdup measurements. The images obtained from an optical camera were used to observe the general flow patterns and obtain information concerning the bubble shape. The data obtained from the double fibre optical probe were used to study the local flow characteristics at different radial position for different operating conditions. In particular, the local void fractions, the bubble velocities, the bubble mean diameters and the bubble diameter distributions and are presented and discussed. The bed expansion technique was used to obtain the gas holdup measurements for every operating condition. The gas holdup measurements are discussed, compared with existing correlations and used for investigating the flow regime transitions. Finally, the gas holdup and the local void fraction measurements data are compared and used for understanding the local hydrodynamics
Bubble sizes and shapes in a counter-current bubble column with pure and binary liquid phases
No full textIt is generally admitted that the “global-scale” behavior of bubble columns is imposed by the “local-scale” phenomena. For this reason, understanding the fluid dynamics in bubble columns relies on the precise knowledge of the so-called “birth and life” of bubbles. A-priori knowledge of the bubble sizes and shapes is required to characterize the “local-scale”, to understand the “global-scale”, to set-up and validate numerical models, as well as to support scaling-up methods towards the “industrial-scale”. This paper contributes to the present-day discussion by proposing an experimental research devoted to clarify the relationships between the bubble sizes and shapes, the integral flow parameters, and the liquid phase properties. The experimental study, based on a bubble-identification methods, was performed in a “large-scale” bubble column (inner diameter equal to 0.24 m, height equal to 5.3 m) operated in the batch and in the counter-current modes with pure (deionized water) and binary (mixture of ethanol and deionized water) liquid phases. The system was operated in the pseudo-homogeneous flow regime with superficial gas velocities in the range of 0.0037–0.0188 m/s and superficial liquid velocity, in the counter-current mode, equal to −0.066 m/s. In the different experimental runs, bubble size distributions and shapes were obtained at different radial and axial locations. The experimental observations have been presented, compared with literature correlations, used to develop novel correlations (to be applied in practical applications), compared with previously obtained experimental data and interpreted in a multi-scale point of view. The comprehensive dataset obtained within this research may be used to improve the validation of numerical approaches and, in particular, to tackle the unsolved issue of developing break-up and coalescence kernels
Influence of electrolyte concentration on holdup, flow regime transition and local flow properties in a large scale bubble column
No full textWe experimentally investigate the influence of the electrolyte concentration on holdup, flow regime transition and local flow properties in a large scale bubble column, with air and water as working fluids. The column is 0.24 m inner diameter, 5.3 m height and the air is introduced by a spider sparger up to a superficial gas velocity of 0.2 m/s. The influence of five NaCl concentrations are investigated by using gas holdup and optical probe measurements. The gas holdup measurements are used for analysing the flow regime transition between the homogeneous and the transition regime and the optical probe is used for studying the local flow characteristics at different radial positions. The presence of NaCl - up to a critical concentration - increases the gas holdup. The increase in the gas holdup is due to the inhibition of the coalescence between the bubbles and, thus, the extension of the homogeneous regime. The results are in agreement with the previous literature on smaller bubble columns
Comprehensive experimental investigation of counter-current bubble column hydrodynamics: Holdup, flow regime transition, bubble size distributions and local flow properties
No full textIn this paper, we apply a variety of experimental techniques to investigate the influence of the counter-current mode on bubble column hydrodynamics. We study an air-water bubble column, which is 5.3 m in height and has an inner diameter of 0.24 m, and we consider gas superficial velocities in the range of 0.004-0.20 m/s and liquid superficial velocities up to -0.09 m/s. The experimental investigation consists of holdup, gas disengagement, image analysis and optical probe measurements. The holdup measurements are compared with the literature and are used to investigate the flow regime transition. The gas disengagement measurements are used to further investigate the flow regime transition and study the structure of the holdup curve. The image analysis is used to study the bubble shapes and size distributions near the sparger and in the developed region of the column; in particular, the image analysis is applied to different gas velocities in the homogeneous regime in both the batch and counter-current modes. The optical probe is used to acquire radial profiles of the local properties (i.e., local void fraction and bubble rise velocity) to study the flow properties and further investigate the flow regime transition. Comparing the results from the different techniques, the influence of the gas superficial velocity and the liquid superficial velocity is discussed considering all main aspects of the two-phase flow, from the local flow properties to the global flow features. The counter-current mode is found to increase the holdup, reduce the bubble rise velocity, destabilize the homogeneous regime and change the local flow properties. © 2016 Elsevier Ltd
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