Computational Fluid Dynamics Modelling of Two-Phase Bubble Columns: A Comprehensive Review

Bubble 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 simulation of the heterogeneous regime in a large-scale bubble column

Bubble 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

Numerical investigation of film thickness and wave statistics in gas-liquid downwards annular flows

Two-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

Promotion of proliferation and metastasis of hepatocellular carcinoma by LncRNA00673 based on the targeted-regulation of notch signaling pathway

we read with great interest the paper by Dr. Chen et al1, recently published in European Review for Medical and Pharmacological Sciences and titled ‘‘Promotion of proliferation and metastasis of hepatocellular carcinoma by LncRNA00673 based on the targeted-regulation of notch signaling pathway’’. Authors concluded that lncRNA00673 is highly expressed and may be a potential target for the treatment of Hepatocellular Carcinoma (HCC). Moreover, according to authors, it can promote the proliferation and metastasis of HCC by the regulation of Notch signaling pathway. We congratulate the authors for their interesting work

Two Phase Bubble Columns: the Determinants of the Flow Regime Transitions

The 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

Computational fluid dynamics modelling of the regular wave flow regime in air-water downwards annular flows

Although 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

Laparoscopic Excision of Endometriosis May Require Unilateral Parametrectomy

Nerve-sparing complete excision of endometriosis may not be possible. In these patients, unilateral parametrectomy may be a reasonable alternative management strategy

Capability of non liner eddy viscosity model in predicting complex flows

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.The research field of the study is related with turbulence modeling. The general objective is the implementation in a commercial code, of two equations Non Linear Eddy Viscosity Model (NLEVM) which removes Boussinesq linear approximation for the Reynolds stress tensor. The work described in the paper implements a second order k-ε model based over Shih, Zhu and Lumley (1993) [1] and Craft, Launder and Suga (1996) [2] in the finite volume commercial code ANSYS-FLUENT v. 6.3.26, by writing additional subroutines. The model has been validated through experimental and DNS data available in the literature. The benchmarks shown in this paper are the straight Square Duct [8] and the Backward-Facing Step [9, 10]. After the validation, the model has been used for predicting the flow behavior for complex industrial applications. The geometry used is similar to the bowl-shape downcomer of nuclear reactor. This is an application field of interest still under study by the same research group and an international consortium.vk201

Islands as Time Capsules for Genetic Diversity Conservation: The Case of the Giglio Island Mouflon

The use of multidisciplinary approaches of investigation including biological, biogeographical, historical, morphological, and genetic analysis, can be useful in identifying and preserving biodiversity. The present study focuses on the characterisation and conservation of a mouflon population (Ovis gmelini musimon) from the Mediterranean island of Giglio. Here we provide the first molecular data on the Giglio population and compare it with mouflons from Sardinia, Elba, and Corsica using both nuclear and mitochondrial markers. Our results suggest that the Giglio mouflon harbours genetic variability likely of Sardinian origin but not represented in the current Sardinian mouflon diversity. Although not presenting the typical characteristics of an invasive alien species, the Giglio mouflon is being subjected to eradication through culling or trapping and surgical sterilization. The molecular evidence we report highlights that such actions are causing the irremediable loss of ancestral genetic variants of the genus Ovis. Finally, we highlight how a multidisciplinary approach is necessary to aid the conservation and management of the anthropochorous populations of Mediterranean mammals