1,721,025 research outputs found
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
The influence of Variable Geometry Control on a R290 Ejector Refrigeration System
Full text linkThe large-scale deployment of ejector refrigeration systems (i.e., solar-based ejector refrigeration systems), although representing a promising alternative compared with mechanical compressor ones, is hindered due to limitations regarding ejector control modes. Indeed, ejectors are fluid-dynamics controlled devices and, because of their fixed geometry, they operate at their highest efficiency in a narrow range of operating conditions, which is in contrast with the dynamic pressure and temperature levels characterizing real applications. In this context, variable geometry ejectors (VGE) represent a promising solution to increase the flexibility and operation range of this component. The present study aims to extend the present body of knowledge regarding VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the performance of the refrigeration system. The analysis has been carried out using an integrated lumped parameter/CFD approach, thus linking the local flow properties and global performances. Different spindle positions have been tested to assess how the different nozzle area ratios affect both the entrainment ratio and the critical pressure. Results showed that increasing primary nozzle area ratio the system can effectively reduce the thermal input, increasing the average COP at the expanse of a lower critical pressure. In conclusion, using a moving spindle control system might ensure an improvement of the ejector performance
Prediction of bubble size distributions in large-scale bubble columns using a population balance model
Full text linkA precise estimation of the bubble size distribution (BSD) is required to understand the fluid dynamics in gas-liquid bubble columns at the "bubble scale," evaluate the heat and mass transfer rate, and support scale-up approaches. In this paper, we have formulated a population balance model, and we have validated it against a previously published experimental dataset. The experimental dataset consists of BSDs obtained in the "pseudo-homogeneous" flow regime, in a large-diameter and large-scale bubble column. The aim of the population balance model is to predict the BSD in the developed region of the bubble column using as input the BSD at the sparger. The proposed approach has been able to estimate the BSD correctly and is a promising approach for future studies and to estimate bubble size in large-scale gas-liquid bubble columns
Scale-resolving CFD modeling of a thick wind turbine airfoil with application of vortex generators: Validation and sensitivity analyses
No full textTwo-phase bubble columns: A comprehensive review
Full text linkWe present a comprehensive literature review on the two-phase bubble column; in this review we deeply analyze the flow regimes, the flow regime transitions, the local and global fluid dynamics parameters, and the mass transfer phenomena. First, we discuss the flow regimes, the flow regime transitions, the local and global fluid dynamics parameters, and the mass transfer. We also discuss how the operating parameters (i.e., pressure, temperature, and gas and liquid flow rates), the operating modes (i.e., the co-current, the counter-current and the batch modes), the liquid and gas phase properties, and the design parameters (i.e., gas sparger design, column diameter and aspect ratio) influence the flow regime transitions and the fluid dynamics parameters. Secondly, we present the experimental techniques for studying the global and local fluid dynamic properties. Finally, we present the modeling approaches to study the global and local bubble column fluid dynamics, and we outline the major issues to be solved in future studies
On the scale-up criteria for bubble columns
Full text linkIt is generally admitted that experimental data obtained in “laboratory-scale” bubble columns are representative of “industrial-scale” reactors if the well-known three “Wilkinson et al. scale-up criteria” are satisfied: (a) the diameter of the bubble column is larger than 0.15 m, (b) the sparger openings are larger than 1-2 mm and (c) the aspect ratio is larger than 5. In this paper, we contribute to the existing discussion and we have experimentally studied the combined effect of the aspect ratio (within the range of 1-15) and the sparger design (considering both “coarse” and “fine” spargers) on the gas holdup in a large-diameter and large-scale gas-liquid bubble column. The bubble column has been operated both in the batch mode and in the counter-current mode. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included deionized water and different aqueous solutions of organic (i.e., ethanol) and inorganic (i.e., sodium chloride, NaCl) active agents. It is found that the “Wilkinson et al. scale-up criteria” are valid for the air-water case in the batch mode for “very-coarse” spargers. Conversely, they are no more valid when considering different liquid velocity, and/or aqueous solutions of active agents, and other sparger openings
On the scale-up criteria for bubble columns
Full text linkIt is generally admitted that experimental data obtained in “laboratory-scale” bubble columns are representative of “industrial-scale” reactors if the well-known three “Wilkinson et al. scale-up criteria” are satisfied: (a) the diameter of the bubble column is larger than 0.15 m, (b) the sparger openings are larger than 1–2 mm and (c) the aspect ratio is larger than 5. The aim of this communication is to contribute to the existing discussion. To this end, this communication collects relevant experimental investigation and include new experimental data: in particular, we have experimentally studied the combined effect of the aspect ratio (within the range of 1–15) and the sparger design (considering both “coarse” and “fine” spargers) on the gas holdup in a large-diameter and large-scale gas-liquid bubble column. The bubble column has been operated both in the batch mode and in the counter-current mode. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included deionized water and different aqueous solutions of organic (i.e., ethanol) and inorganic (i.e., sodium chloride, NaCl) active agents. It is found that the “Wilkinson et al. scale-up criteria” are valid for the air-water case in the batch mode for “very-coarse” spargers. Conversely, they are no more valid when considering different liquid velocity, and/or aqueous solutions of active agents, and other sparger openings
Role of Methanogenesis and Sulfate Reduction in Lifetime Performance of Hydrogen Storage in Depleted Gas Reservoirs
No full textGeological formations potentially provide suitable options for underground hydrogen storage (UHS). For seasonal operations of UHS, working gas capacity cannot coincide with the total amount of hydrogen stored in a geological formation where key aims include (i) maintaining reservoir pressure, and (ii) avoiding productions of cushion gas (e.g., CO2, CH4, N2) during withdrawal cycles. Otherwise, when considering long-term (or lifetime) UHS scenarios, the kinetics of the chemical reactions associated with mixtures of hydrogen with the fluids residing in the porous formation can evolve to attain equilibrium conditions.
Here, we consider lifetime behavior of UHS scenarios and assess uncertainties associated with hydrogen losses due to its conversion into other chemical species. Given the time scales involved, we disregard kinetic behavior of hydrogen consuming reactions while evaluating the loss of underground stored hydrogen due to its conversion to other chemical products at reservoir thermodynamic equilibrium conditions. Our results are tied to (i) shallow, (ii) intermediate, and (iii) deep reservoirs. Our modeling study suggests that hydrogen losses at equilibrium conditions in shallow reservoirs (low temperature/pressure conditions) is around 5% more (on average) than the corresponding losses associated with deep reservoirs (high temperature/pressure conditions)
Computational fluid dynamics simulation of the homogeneous flow regime in a large-scale bubble column
Full text linkBubble column reactors are widely used in various industrial applications, yet their characterisation at both global and local scales remains highly complex. This study presents a CFD Eulerian multi-fluid approach to model the hydrodynamics of large-scale bubble columns, focusing on the homogeneous flow regime. Transient 3D simulations were performed using the commercial software ANSYS Fluent 2023 R2, assuming a fixed mono- or polydispersed bubble size distribution to represent the dispersed gas phase. The results were thoroughly analysed and the discrepancies between the numerical results and the experimental data were discussed, highlighting areas for future improvement
Experimental study of the liquid velocity and turbulence in a large-scale air-water counter-current bubble column
Full text linkMeasuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55%) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns
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