Flow In Bubble Column Research Articles

Euler-Euler simulation of a bubble column flow up to high gas fraction

This study investigates homogeneous flow in a bubble column up to 50% gas holdup. For low to medium gas holdup below ∼20%, the good performance of an established baseline model is confirmed. In this range, the mixture pressure gradient is decisive in determining the relative velocity, resulting in good predictions without considering swarm effects. However, beyond a gas holdup of ∼20%, a swarm corrector to the drag force becomes necessary, for which several proposals from the literature are evaluated. In addition, the lift force influences the shape of the gas fraction profile depending on the bubble size, which has a significant impact on the liquid flow inside the column. For wall-peaked profiles, the liquid flow remains moderate, while center-peaked profiles strongly boost the liquid velocity. Finally, several mechanisms proposed in the literature for inducing unstable flow based on the lift force, bubble-induced turbulence or flooding are investigated. Of these only the first gave qualitative agreement with the observed gas holdup.

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.

Effect of superhydrophobic surfaces on bubble column flow dynamics

Bubbly flow in bubble column reactors promotes mixing necessary for many chemical processes. We show that if superhydrophobic-coated material is introduced into a bubble column, there can be a substantial difference in gas holdup and earlier initiation of churn-turbulent flow which can alter larger-scale mixing without a need to change the superficial gas velocity. Addition of superhydrophobic surface can also cause bubbles to (directly or indirectly assisted by the surface) escape faster to the free surface resulting in a reduced void fraction (i.e., reduced gas holdup). As the flow becomes optically opaque at few percent gas phase volume fraction, we utilize two dual plane wire mesh sensors to obtain velocity profiles and bubble size distributions, in addition to the traditional pressure and level based gas holdup measurements to calculate average phase fraction. Additionally, a custom build photon-counting dual energy threshold X-ray computed tomography system is employed to get a higher resolution measurement of the time average phase fraction non-intrusively. We report satisfactory agreement between these techniques with differences arising for understood reasons, and use the insight thus yielded to discuss the effect of superhydrophobic surfaces on bubble column flow dynamics.

Drag models coupled with CFD–PBM method for simulation in bubble columns

AbstractIn this study, three‐dimensional numerical simulation of gas–liquid flow in bubble columns was realized by using the computation fluid dynamics (CFD)–population balance model (PBM). The new drag model improves the stability‐constrained multi‐fluid (SCMF‐C) model because of the consideration of the wake accelerating and the hindering effects for calculating the drag correction factor. The gas holdup, axial liquid velocity, and bubble size distribution (BSD) predicted by four drag models at 0.02 and 0.1 m/s were compared. The results revealed that the proposed drag model can provide excellent predictions for both bubbly and heterogeneous flows. Because the wake accelerating and the hindering effects were considered, reliable predictions were achieved for the gas holdup, and the problem of uniform gas holdup distribution was mitigated. Therefore, the SCMF‐C model can be extended for nonuniform BSD. The gas holdup and liquid velocity increased, and the nonuniformity of radial results became pronounced at 0.1 m/s. The profiles of four drag models were similar at a low height, whereas the difference between the simulations of the four models became obvious with the variation of heights. The results of the four models were accurate, and the BSD was wide at 0.1 m/s. Subsequently, the feasibility of the four drag models was evaluated at 0.2 and 0.4 m/s. The results of the comparison revealed that the proposed drag model exhibited excellent feasibility at higher gas velocities and was powerful for the simulation of bubble columns.

Euler-Euler simulation of absorption and desorption in co- and counter-current bubble column flows

Mass transfer in bubbly flows is important in many engineering applications. Simulation of such processes on technical scales is feasible by the Euler-Euler two-fluid model, which relies on suitable closure relations describing interfacial exchange processes. In comparison with the pure fluid dynamics of bubbly flows however, modeling and simulation of bubbly flows including mass transfer is significantly less developed. In particular, previous studies have focused entirely on absorption in upward vertical flows, whereas the present study considers a larger variety of conditions including desorption and counter-current (downward) flow. Suitable experimental data for comparison are available from the classic work of Deckwer et al. [Canadian Journal of Chemical Engineering 56 (1978) 43–55]. In line with previous studies on the co-current absorption cases from that work, a monodisperse approximation is made. In addition, a class method to treat bubble shrinkage and growth is implemented in the OpenFOAM code and tested by showing the crossover between two monodisperse cases.

Numerical simulation of gas–liquid flow in the bubble column using Wray–Agarwal turbulence model coupled with population balance model

In this paper, an improved computational fluid dynamic (CFD) model for gas–liquid flow in bubble column was developed using the one-equation Wary–Agarwal (WA) turbulence model coupled with the population balance model (PBM). Through 18 orthogonal test cases, the optimal combination of interfacial force models, including drag force, lift force, turbulent dispersion force. The modified wall lubrication force model was proposed to improve the predictive ability for hydrodynamic behavior near the wall of the bubble column. The values simulated by optimized CFD model were in agreement with experimental data, and the errors were within ± 20%. In addition, the axial velocity, turbulent kinetic energy, bubble size distribution, and the dynamic characteristic of bubble plume were analyzed at different superficial gas velocities. This research work could provide a theoretical basis for the extension of the CFD–PBM coupled model to other multiphase reactors..

Exploration on the stability conditions in bubble columns by noncooperative game theory

• Different pathways of compromise of dominant mechanisms in bubbly flow. • Formulation of different stability conditions for gas–iquid bubbly flow. • Game theory and non-inferior solution to unravel the compromising mechanisms. • Understand regime transition and structure of bubble classes via stability condition. The energy-minimization multiscale (EMMS) model, originally proposed for gas–solid fluidization, features a stability condition to close the simplified conservation equations. It was put forward to physically reflect the compromise of two dominant mechanisms, i.e. , the particle-dominated with minimal potential energy of particles, and the gas-dominated with the least resistance for gas to penetrate through the particle bed. The stability condition was then formulated as the minimization of the ratio of these two physical quantities. Analogously, the EMMS approach was later extended to the gas–liquid flow in bubble columns, termed dual-bubble-size model. It considers the compromise of two dominant mechanisms, i.e ., the liquid-dominated regime with small bubbles, and the gas-dominated regime with large bubbles. The stability condition was then formulated as the minimization of the sum of these two physical quantities. Obviously, the two stability conditions were expressed in different manner, though gas–solid and gas–liquid systems bear some analogy. In addition, both the conditions transform the original multi-objective variational problem into a single-objective problem. The mathematical formulation of stability condition remains therefore an open question. This study utilizes noncooperative game theory and noninferior solutions to directly solve the multi-objective variational problem, aiming to explore the different pathways of compromise of dominant mechanisms. The results show that only keeping the single dominant mechanism cannot capture the jump change of gas holdup, which is associated with flow regime transition. Hybrid of dominant mechanisms, noninferior solutions and noncooperative game theory can predict the flow regime transition. However, the game between the two mechanisms makes the two-bubble structure degenerate and reduce to the single-bubble structure. The game of the three mechanisms restores the two-bubble structure. The exploration on the formulation of stability conditions may help to understand the roles and interactions of different dominant mechanisms in the origin of complexity in multiphase flow systems.

Microbubbles, oscillating flow, and mass transfer coefficients in air-water bubble columns

Mass transfer coefficient kLa determines the flux of dissolved oxygen DO in bubble columns. The kLa values can be determined experimentally or predicted using a correlation. Common correlations link kLa with superficial air velocity, air hold-up volume, bubble contact time, bubble rising velocity, or bubble Sauter diameter.We compared experimentally found and predicted kLa values to evidence that none of the above correlations is applicable for microbubbles flow. The only way to accurately predict the flow is by using mass transport models needed to calculate kL. The transition from kL to kLa requires a redefinition of the specific interfacial area a for microbubbles.Microbubbles distribute DO quicker than macrobubbles. Oscillating flow can raise the DO level to saturation. The generation of oscillating flow does not require extra energy turning the subsurface aeration more energy-efficient than the surface aeration.

Euler/Euler large eddy simulation of bubbly flow in bubble columns under CO2 chemisorption conditions

It has been generally recognised that the turbulent dispersion force plays an important role in interphase momentum transfer. However, the effect of the added mass stress on the momentum and mass transfer in bubble column bubbly flow has not been addressed appropriately. As the turbulent eddies in the surroundings of bubbles interact strongly with the rising bubbles in bubble column bubbly flow, such interactions will bring out the change of interfacial areas between the bubbles and carrier fluid, consequently leading to changes in the interfacial mass transfer. When employing large eddy simulation for modelling bubbly flow coupled with the chemisorption process, the SGS filtered velocity fluctuations of liquid phase can be interpreted as the turbulent eddies that continuously hit the surfaces of bubbles, causing bubble deformation and the variation of bubble interfacial areas, which give rise to the turbulent dispersion and added mass stress forces. The present study will demonstrate through Euler/Euler large-eddy simulations (LES) modelling that by considering the turbulent dispersion force (SGS-TDF) and added mass stress (SGS-AMS), the bubble dynamics and mass transfer under the chemisorption conditions can be better indicated, which leads to remarkable improvements in the prediction of bubble lateral dispersion and the interfacial mass transfer. The turbulent dispersion and added mass stress related to the spatially filtering were proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble-induced turbulence. A comprehensive assessment of the effects of these additional filtered stress terms on the time-averaged velocity and bubble volume fraction profiles, flow patterns, mass transfer and the pH variation during CO2 chemisorption and the turbulent kinetic energy and species concentration spectra was conducted.

On Inter-bubble distances and bubble clustering in bubbly Flows: An experimental study

• Positional relationships of bubbles were studied experimentally in the dense swarms. • Dimensionless inter-bubble distances were extracted from X-ray measurement data. • The distance of nearest neighbor bubbles is close to that of FCC structure. • Vertical clustering is observed for Eo ≥ 3.82. • Clustering orientation was found to be independent of the gas holdup in this study. Swarm effects in bubble columns result from the interaction of the bubbles at higher gas holdup. The flow around individual bubbles in a swarm influences the movement of the following bubbles and this has a feedback on the flow regime on a larger scale. That is, hydrodynamics that can no longer be described with models for single rising bubbles. The experimental investigation of the multiscale hydrodynamics in three-dimensional bubble columns was so far limited by the available measurement techniques. In this study, ultrafast X-ray tomography (UFXCT) was applied on bubbly flows in a lab scale bubble column to determine inter-bubble distances and clustering characteristics in the vicinity of individual bubbles for different gas holdups and bubble diameters. Applying the pair correlation function reveals a pronounced vertical clustering for large bubbles with Eo ≥ 3.82, whereas no clustering has been observed for bubbles of lower Eo . It was found that clustering orientation mainly depends on deformability of the bubbles but not on gas holdup.

LES modelling of multiphase turbulent flows in bubble columns using an adaptive-mesh finite element method

Turbulence drives mixing in multiphase flow reactors such as bubble columns, which are widely used in chemical and bioprocess industry. The understanding of turbulence in these two-phase gas–liquid columns is critical to understanding the dynamics of these flows for better equipment design. The present work focuses on the finite-element computational fluid dynamics modelling of turbulent flows in bubble columns with the use of the large eddy simulation (LES) model, which allows for resolving the dynamics of turbulent flows such as the oscillating plume. Two-dimensional fixed- and (optimized) adaptive-mesh simulations were performed and the results are validated against published experiments. The global flow parameters, including time-averaged gas holdup and plume oscillation period, showed good agreement with experiments. At a superficial gas velocity of 0.6cms−1, a plume oscillation period of 6.2s was predicted with a validation error of 9.6% and a gas holdup of 1.4% with a validation error of 7.5%. The novelty of the work lies in the use of a finite element framework with adaptive mesh refinement and the LES turbulence model to simulate bubble column reactors.

Microbubbles, Oscillating Flow, and Mass Transfer Coefficients in Air-Water Bubble Columns

Microbubbles, Oscillating Flow, and Mass Transfer Coefficients in Air-Water Bubble Columns

Numerical investigations into two‐phase turbulent modalities for bubble column flows

AbstractUsing the software Fluent, computational fluid dynamic simulations of bubble column flows were carried out in a 3D, unsteady, Euler‐Euler framework, combined with a population balance model (PBM), and discussed in connection with laboratory‐scale bubble columns, with special attention to two‐phase turbulent modalities (TPTMs) representing gas‐liquid turbulent interaction. Based on both time‐averaged and instantaneous characteristics of the flow fields, including four typical modalities (single‐phase, single‐phase and bubble‐induced turbulence (BIT), mixture, and per‐phase) and two rare modalities (mixture‐and‐BIT, and per‐phase‐and‐BIT), overall, six different TPTMs were comprehensively investigated. The results reveal that the single‐phase‐and‐BIT modality is suitable for simulating bubble column flows. However, the utilities of the single‐phase and per‐phase modalities are highly consistent, and it is recommended to use them with BIT modification. Besides, the mixture modality is unique compared with the three other typical modalities, and can be used as an acceptable simplified approach.

Turbulent flow in a square cross-sectioned bubble column computed by a scale-resolving Reynolds-stress model

The present study focuses on the computations of turbulent flow in a square cross-sectioned bubble column by utilizing the two-fluid model (TFM) in conjunction with advanced Reynolds-stress models (RSMs) within the Unsteady Reynolds-averaged Navier–Stokes (URANS) framework. The use of such an advanced modeling approach in combination with the TFM, rarely employed for two-phase flow computations, is motivated by its inherent capability of resolving both Reynolds-stress anisotropy and stress-dissipation anisotropy, also of the corresponding residual turbulence. The presently adopted RSMs are based on the formulation proposed initially by Jakirlić and Hanjalić (2002) for incompressible single-phase flows. Two different RSM versions (Jakirlić and Maduta (2015)), both based on the homogeneous dissipation (εh) concept that employs the specific dissipation rate (ωh=εh/k) as the length-scale-determining variable, are applied in the present work. The baseline model version is formulated within the conventional RANS framework, whereas the advanced model represents an instability-sensitized, eddy-resolving RSM variant, capable of adequately capturing the fluctuating turbulent motion in accordance with the SAS methodology (Scale-Adaptive Simulation) proposed by Menter and Egorov (2010). The results obtained by both RSMs are discussed along with the corresponding experimental database made available by Deen and co-workers (Deen et al., 2000, 2001; Deen, 2001). Additionally, the most-widely used modeling approach for two-phase flows is followed by utilizing the Standard high-Reynolds number k-ε model for the purpose of a comparative assessment. Furthermore, both RANS models are extended by two different proposals for a model term accounting for the so-called bubble-induced turbulence (BIT). The model equations are implemented into the open source software OpenFOAM® based on the finite-volume method on unstructured meshes. The results obtained exhibit high level of agreement with the experimental reference demonstrating high potential of both Reynolds-stress models in computing the bubbly flows. This relates in particular to the correspondingly captured resolved turbulence intensity in this bubble-plume-induced unstable flow event, leading subsequently to a correctly returned velocity field. The mean flow asymmetry, characterizing the baseline Eddy-Viscosity Model (EVM) performance was remedied only after introducing the BIT-related source term contributing appropriately to intensified turbulence production.

Large-eddy simulation of gas–liquid two-phase flow in a bubble column reactor using a modified sub-grid scale model with the consideration of bubble-eddy interaction

The Eulerian–Eulerian Large-eddy simulations (LES) of gas–liquid two-phase flow in a cylindrical bubble column reactor have been conducted. When considering the turbulent eddy viscosity in LES, apart from the well-accepted contributions from shear turbulence and bubble induced turbulence (BIT), the effect of the interaction between entrained bubbles and eddies with a similar turbulence length scale to the sub-grid scale (SGS) cannot be neglected. With the consideration of the bubble response to the eddies on the induced sub-grid stresses, a modified SGS model, which incorporates the Stokes number, St, was proposed. The results of LES clearly indicate that the use of the modified SGS model can effectively capture the transient bubbly flow in the cylindrical bubble column. The power turbulent kinetic energy spectrum obtained in LES indicates that a slope similar to Komogorov -5/3 scaling law and the -3 scaling law can still be identified for a critical frequency f=10.70 Hz.

A bubble-induced turbulence model for gas-liquid bubbly flows in airlift columns, pipes and bubble columns

The presence of bubbles dramatically changes the liquid turbulence in bubbly flows. Modifications to liquid turbulent transport equations by adding source terms representing the effect of bubble-induced turbulence (BIT) are usually made to promote the reliability of Computational Fluid Dynamics simulations. However, existing closures for BIT fail to give reliable predictions of both mean flow properties and turbulence kinetic energy. In this work, a new BIT model, which embeds both the bubble swarm effects and the physics of bubble-induced vortices, is proposed to modify the Reynold stress model for bubbly flows. The anisotropy nature of BIT is also considered. The new BIT model can predict both mean flows and turbulence kinetic energy in airlift column systems, while investigated existing BIT models from the literature fail. Three simulation test cases for pipes and one for a bubble column have been conducted to show the validity of the proposed BIT model.

Experimental investigation on pair bubble columns under high voltage DC electric filed

The behavior of a pair bubble column rising in an insulating liquid in the presence of a high voltage electric field was investigated. The purpose of this study was to investigate the bubble column flow regime and the possibility of transferring it without changing the basic parameters of the bubble column. The use of the electric field was the main factor in applying this change to the bubble column. The results show that the employment of an external electric field significantly changes the size, detachment frequency and the pattern of the bubble column. With an increase in electrical potential, the bubble growth time and detachment volume (bubble size) decline continuously due to electrical stress on the bubble surface. Moreover, at sufficiently high electric potential, the electric field intensifies the bubble collision and coalescence, which subsequently leads to more enhancements of the heat and mass transfer in the bubble column reactor. The use of the electric field increased the bubble detachment for Db=1.12mm by 250%, and for the largest bubble in range Db=3.32mm, this increase was about 80%.

On the prediction of gas hold‐up in two‐phase flow systems using an Euler–Euler model

AbstractWe quantify the ability of the two‐fluid Euler–Euler model to predict the overall gas hold‐up during two‐phase flow in vertical columns using a combination of experiments and simulations. Gas hold‐up in a bubble column and gas hold‐up in the less‐frequently studied co‐current flow are investigated. For homogeneous flow characterized by nearly uniform bubble size, Euler–Euler model predictions are within 10% of the experimental values for both modes of operation, if the bubble diameter supplied as input to the model is the average bubble diameter in the physical system. This also holds true for heterogeneous flow in bubble columns despite the presence of a broad distribution of bubble sizes, if turbulence and bubble swarm effects on momentum exchange between phases are properly accounted for. Swarm corrections adequate for bubble columns, are less successful for co‐current heterogeneous flow, for which gas hold‐up predictions are least accurate (average error of 22%).

Computation of gas-liquid flow in a square bubble column with Wray-Agarwal one-equation turbulence model

The newly proposed one-equation Wray-Agarwal (WA) turbulence model coupled with Euler-Euler (E-E) approach was used to simulate the gas-liquid flow in a square bubble column, which further validated the accuracy and expanded the application of the WA model comparing to a preliminary study (Ouyang et al., 2019). All simulation results of the WA model were compared with two widely used turbulence models of standard k-ε and SST k-ω and literature data. The numerical results of the time-averaged axial liquid and gas velocity were validated with experimental data, and it was found that the WA model showed overall the best agreement. Profiles of turbulent kinetic energy, turbulent dissipation rate, turbulent viscosity and gas holdup were investigated at three different heights. Results for the first time validate the WA model based on the turbulent characteristic values of gas-liquid flow in the bubble column, which shows its promising prospective in multiphase flow simulation.

Influence of the bubble size distribution on the bubble column flow regime

The role of the bubble size dependent lateral lift force on the flow regime is experimentally investigated in a tall bubble column. In the experiments, only the initial bubble size distribution was modified by using a gas sparger with different injection needles and varying the flow through specific injection needles. The gas flow rate was kept unchanged in all experiments. Depending on the bubble size distribution homogeneous, transitional or heterogeneous flow regimes were observed. The flow regime is in good agreement with the predictions from the stability criterion obtained by a previously conducted linear stability analysis for poly-dispersed flow. It was found that as a rule of thumb the following criterion can be used.If the majority of the gas volume is transported by bubbles smaller than the critical diameter at which the lift force changes its sign, the flow is stabilized leading to a homogeneous flow regime. If most of the gas volume is transported by bubbles larger than that diameter the flow is de-stabilized leading a heterogeneous flow regime. If the fraction of gas transported by small and large bubbles is about the same, the initial conditions remain dominant throughout the column height.