1,720,964 research outputs found
Computational fluid dynamics simulation of the heterogeneous regime in a large-scale bubble column
Full text linkBubble columns are used in many industrial applications, but the complex fluid dynamics phenomena has limited their design and optimization processes. Computational Fluid Dynamics (CFD) is a promising tool to investigate the complex multi-scale flow physics characterising multiphase reactors. In this work, a CFD Eulerian-Eulerian modelling approach is developed to describe the hydrodynamics of a large-scale bubble column operated over a wide range of superficial gas velocities (0.0188 – 0.20 m/s). Available experimental results were used for the model validation. A drag law for oblate bubbles was considered and coupled with a drag modification function to include the effects of bubble–bubble interactions. The numerical approach was tested considering a mono-dispersed approximation and including coalescence and breakup by using a Population Balance Model (PBM). The role played by the lift force was investigated and, for the reactor configuration considered, it turned out to be essential in the description of the local flow properties
Bubble Column Reactors: the Variables Influencing the Mono-Dispersed Homogeneous Flow Regime
No full textThe fluid dynamics of large-diameter bubble columns explicates in six flow regimes emergin upon an increase in the gas flow rate and can be described and predicted via correct modelling of the flow regime transition coordinates. This study focuses on the transition between the mono-dispersed and poly-dispersed homogeneous flow regime and defines a statistical approach to determine the significative variables influencing the transition. The analysis is performed by coupling: (a) the Ordinary Least Squares method (OLS), to determine the relationship between the variables, (b) the Variance Inflation Factor (VIF), to check for multicollinearity issues, and (c) the Least Absolute Shrinkage and Selection Operator (LASSO), to select suitable variables. Subsequently, the Classification and Regression Tree (CART) approach has been applied to generate homogenous clusters of bubble columns in terms of flow regime transition
A Lumped Parameter Approach for Determining the Pressure Gradient in Gas-Liquid Annular Flows
Full text linkThis study defines a statistic-derived lumped parameter approach to determine the pressure gradient in two-phase annular flows. The statistical model was defined by coupling: (1) the ordinary least squares method (OLS) to determine the relationship between the variables, (2) the variance inflation factor (VIF) to check for multicollinearity issues, and (3) the least absolute shrinkage and selector operator (LASSO) to select the relevant predictors. Finally, a lumped parameter approach is derived based on the classification and regression tree (CART) approach. The model identifies the liquid and gas Reynolds numbers, the liquid phase properties, the pipe diameter, and the surface tension as significant variables influencing the two-phase pressure gradient
Numerical investigation of film thickness and wave statistics in gas-liquid downwards annular flows
Full text linkTwo-phase gas-liquid annular flows are observed in a broad range of industrial processes, such as production and pipeline systems for oil and gas distribution, steam generators, boiling water reactors, and emergency core cooling facilities to protect nuclear reactors.
Although the global flow characteristics of annular gas-liquid flows have been studied experimentally for more than 50 years, their numerical modelling is still immature.
We present a computational fluid dynamics model based on the volume of fluid method for simulating annular gas-liquid flows, focusing on the regular wave flow regime. We performed transient simulations on a 3-D domain using a commercial code (ANSYS Fluent 2021 R1). The mesh sensitivity analysis indicates that a very fine mesh must be used near the pipe wall to capture the liquid-gas interface correctly (Fig. 1).
The code is validated through available experimental data [1] regarding topological flow properties. In particular, we considered mean film thickness, film roughness, base film thickness, and wave film thickness. We studied two operating conditions. The first is characterized by liquid and gas Reynolds numbers of 1 250 and 25 000, respectively. The second has the same liquid Reynolds number as the first, but the gas Reynolds number is increased to 30 000.
A post-processing procedure is implemented to obtain the time traces of film thickness at 12 circumferential positions to capture the asymmetries in the flow.
The numerical values of the quantities analyzed are in good agreement with the experimental findings, with a maximum error of 21.02% concerning the wave film thickness. The errors regarding the mean film thickness and film roughness are less than 10% for both the case studies.
Considering the film thickness of time traces at different circumferential positions, we calculated the cross-correlation coefficients between them. The high values of the cross-correlation coefficients indicate that waves are coherent over the circumference of the pipe, following the experimental findings.
Finally, to better understand wave activities, we generated the power spectral density functions for the two cases studied. They are characterized by a quasi-linear power decay, similar to that of the Kolmogorov spectrum for homogeneous and isotropic turbulence, which becomes slightly steeper for the case characterized by a higher gas Reynolds number, in accordance with the experimental data
Computational Fluid Dynamics Modelling of Two-Phase Bubble Columns: A Comprehensive Review
Full text linkBubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies
Computational fluid dynamics modelling of the regular wave flow regime in air-water downwards annular flows
Full text linkAlthough the global flow characteristics of annular gas-liquid flows have been studied experimentally for more than 50 years, the spatiotemporally-resolved details of these flows have remained relatively unexplored until recently, with data provided via advanced experimental methods based, e.g., on optical techniques. Similarly, the numerical modelling of annular flows is still an immature process. The present work aims to provide a computational fluid dynamics (CFD) model based on the volume of fluid (VOF) method for simulating annular gas-liquid flows, setting the stage for a deeper investigation of these flows at global and local scales. The work focuses on the most common downwards annular flow (DAF) flow pattern: the regular wave regime. 3-D and 2-D axisymmetric transient simulations have been performed using a commercial code (ANSYS Fluent 2021 R1). The code is validated through available experimental data regarding topological flow properties, mainly film thickness and wave statistics. The validation results suggest that 3-D simulations are needed to provide predictions that agree with the experimental data, highlighting strong 3-D features in the flow
Computational fluid dynamics modelling of large-scale bubble columns: From the mono-dispersed to the pure heterogeneous flow regime
Full text linkThis study proposes a CFD model to simulate large-scale bubble columns operating in different flow regimes. Transient 3-D simulations were performed employing a commercial code (ANSYS Fluent), and the numerical results were compared with available experimental data. The superficial gas velocity ranges between 0.0037 m/s and 0.2 m/s, covering both the mono-dispersed and pure-heterogenous flow regimes, where bubbles coalescence and breakup were modelled. The results have been critically analysed, and the discrepancies between the numerical and experimental results have been deeply commented on, setting the stage for future improvements
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
Two Phase Bubble Columns: the Determinants of the Flow Regime Transitions
Full text linkThe fluid dynamics in large-diameter bubble columns can be described by an analytical relation between two global flow parameters, the drift flux and the gas holdup. This relation, named bubble column operating curve, builds on five flow regime transitions. In order to determine the variables influencing the flow regime transitions, a statistical approach was derived by coupling: (1) the ordinary least squares method (OLS) to determine the relationship between the variables, (2) the variance inflation factor (VIF) to check for multicollinearity issues, and (3) the least absolute shrinkage and selection operator (LASSO), to select suitable variables. It was found that the geometrical characteristics of the sparger strongly influence the flow regime transitions, and uniform aeration is essential for all the regimes to exist. Increasing the superficial liquid velocity in the counter-current mode destabilises the mono-dispersed and poly-dispersed homogeneous flow regimes. As for the aspect ratio, an increase in the column aspect ratio slightly destabilises the existing flow regimes. The statistical method identifies viscosity as the only significative variable concerning the liquid phase properties
Influence of the gas phase on a large-scale bubble column fluid dynamics: Gas holdup, flow regime transitions, and bubble size distributions
No full textWe present the results of an experimental study of a large-diameter bubble column operating in batch mode with air or CO2 as the gas phase and water as the liquid phase. The bubble column has an inner diameter of 0.24 m and is 5.3 m in height. The superficial gas velocity varied between 0.0037 m/s and 0.0233 m/s. The experimental investigation consists of gas holdup measurements and image analysis. Flow regime transitions were detected by gas holdup measurements, and image analysis was used to investigate the bubble size distributions. The results suggest that the homogeneous flow regime is stabilized when CO2 is used as the gas phase. The gas holdup is higher for the CO2-water system at fixed superficial gas velocity, especially in the heterogeneous flow regime. In the homogeneous flow regime, the mean bubble diameter and the fraction of large bubbles are lower for the CO2-water system, explaining the higher gas holdup
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