Stability Modeling of Water-Based Surfactant Covered Micro-bubble Fluids

Currently, due to the decrease in easy access to crude oil reservoirs, oil and gas industries have focused on production from heavy oil and depleted reservoirs. In recent years, micro‐bubble fluids with surfactant and polymer layers around the bubbles are investigated as a part of drilling fluids and their positive effect on the formation damage is proven. Stability of the bubbles in the fluid is very important, and the optimum surfactant and polymer types should be chosen at optimum concentrations. In this work, an attempt is made to analyze the stability of bubbles of the drilling fluid, based on the diffusive mass transfer concept. Mass transfer and interfacial mass transfer coefficients become more important when surfactant concentration gradient exists in micro‐bubble layers. Interfacial mass transfer coefficients have an important effect on mass transfer phenomena in Aphron fluids system; so, the precise selection of these coefficients results in the conformity of modeling and experiments. We can say that reducing the mass transfer rate from bubble layers will result in stable bubbles in the fluid and, thus, the efficiency of the fluid during drilling will not decrease. It is shown that the interfacial mass transfer coefficient decreases with an increase in surfactant concentration.     

Mass transfer and shear in an airlift bioreactor: Using a mathematical model to improve reactor design and performance

Several studies have shown a strong relationship between morphology and agitation ( [Cui et al., 1997] and [Berzins et al., 2001] ). The shear stress distribution and mass transfer are the important parameters which can improve the performance of bioreactor. In this work, a mathematical model using computational fluid dynamics (CFD) techniques is used to study the gas–liquid dispersion in an airlift reactor. Multiple rotating frame (MRF) technique is used to approximate the movement of the impeller in the stationary reactor. Population balance modeling (PBM) is used to describe the dynamics of the time and space variation of bubble sizes in the reactor. The PBM equation is solved using an approximate method known as the class method (CM) and the bubble sizes are approximated through a discrete number of size ‘bins’, including transport, and different bubble phenomena. These equations of the CM are then written as scalar transport equations and added to the multiphase fluid mechanical equations describing the dynamics of the flow. All these equations are solved using control volume formulation through the use of an open-source CFD package OpenFOAM. The model is used to analyze an existing geometry of an airlift bioreactor and validate the modification on the initial design. The new design of airlift gives a clear performance by the increase of the global and local mass transfer and the decrease of the shear stress.     

Computational fluid dynamics for dense gas–solid fluidized beds: a multi-scale modeling strategy

Dense gas–particle flows are encountered in a variety of industrially important processes for large scale production of fuels, fertilizers and base chemicals. The scale-up of these processes is often problematic, which can be related to the intrinsic complexities of these flows which are unfortunately not yet fully understood despite significant efforts made in both academic and industrial research laboratories. In dense gas–particle flows both (effective) fluid–particle and (dissipative) particle–particle interactions need to be accounted for because these phenomena, to a large extent, govern the prevailing flow phenomena, i.e. the formation and evolution of heterogeneous structures. These structures have significant impact on the quality of the gas–solid contact and as a direct consequence thereof strongly affect the performance of the process.Due to the inherent complexity of dense gas-particles flows, we have adopted a multi-scale modeling approach in which both fluid–particle and particle–particle interactions can be properly accounted for. The idea is essentially that fundamental models, taking into account the relevant details of fluid–particle (lattice Boltzmann model (LBM)) and particle–particle (discrete particle model (DPM)) interactions, are used to develop closure laws to feed continuum models which can be used to compute the flow structures on a much larger (industrial) scale. Our multi-scale approach (see Fig. 1) involves the LBM, the DPM, the continuum model based on the kinetic theory of granular flow, and the discrete bubble model. In this paper we give an overview of the multi-scale modeling strategy, accompanied by illustrative computational results for bubble formation. In addition, areas which need substantial further attention will be highlighted.     

Verhalten von Blasen in Blasensäulen mit Bodenschwinger

Bubble columns equipped with an vibration exciter, which pulsates with a frequency between 40 and 400 Hz, show significantly improved mass transfer rates. This results from a reduction of bubble size by cleavage, the associated increase in surface area and, additionally, an intensive movement at the bubble interface. The observed phenomena are explained based on flow simulations. The energy required for a oscillating bubble column is less than for a gassed stirred tank.     

Bubble size modeling approach for the simulation of bubble columns

The constant bubble size modeling approach (CBSM) and variable bubble size modeling approach (VBSM) are frequently employed in Eulerian–Eulerian simulation of bubble columns. However, the accuracy of CBSM is limited while the computational efficiency of VBSM needs to be improved. This work aims to develop method for bubble size modeling which has high computational efficiency and accuracy in the simulation of bubble columns. The distribution of bubble sizes is represented by a series of discrete points, and the percentage of bubbles with various sizes at gas inlet is determined by the results of computational fluid dynamics (CFD)–population balance model (PBM) simulations, whereas the influence of bubble coalescence and breakup is neglected. The simulated results of a 0.15 m diameter bubble column suggest that the developed method has high computational speed and can achieve similar accuracy as CFD–PBM modeling. Furthermore, the convergence issues caused by solving population balance equations are addressed.     

Models for the Numerical Simulation of Bubble Columns: A Review

This work reviews the state‐of‐the‐art models for the simulation of bubble columns and focuses on methods coupled with computational fluid dynamics (CFD) where the potential and deficits of the models are evaluated. Particular attention is paid to different approaches in multiphase fluid dynamics including the population balance to determine bubble size distributions and the modeling of turbulence where the authors refer to numerous published examples. Additional models for reactive systems are presented as well as a special chapter regarding the extension of the models for the simulation of bubble columns with a present solid particle phase, i.e., slurry bubble columns.     

STRESSES ARISING FROM TRAILING INTERFACE RETRACTION IN BUBBLES LEAVING SIEVE PLATE SPARGERS

Interfacial phenomena frequently give rise to energetic fluid motions in the culture of plant and animals cells, and these motions may result in loss of cellular viability. Bubble disengagement processes have often been the center of attention in this regard. Recent work has shown, however, that surprisingly large interfacial velocities can be generated in aqueous solutions of glycerol by an elongation-retraction mechanism occurring just above the orifice following bubble formation at sieve-plate spargers. While it is evident that this process can occur under a variety of conditions, it is now clear that certain combinations of hole size, gas rate, and medium viscosity result in unusually energetic interfacial motions. An extensive investigation of this phenomenon has been completed, involving both the collection of experimental data and hydrodynamic modeling; this work has shown that shear stresses can be generated by this process that may be as large as 2000 to 3000 dynes/cm² in 50% glycerol solutions.     

Experimental studies on the instabilities of viscous fingering in a Hele-Shaw cell

When air is injected into silicone oil contained in a horizontal Hele-Shaw cell, a single air bubble forms and grows showing various interesting phenomena. In this study the effects of the bubble front velocity, air injection velocity at a nozzle, fluid properties and cell depth on the stability of the growing bubble were investigated experimentally. By using the modified capillary number involving the aspect ratio, we obtained the onset conditions of the unstable bubble. Also, the bubble width was analyzed both quantitatively and qualitatively. Before the bubble experiences splitting, the bubble front velocity is almost proportional to the air injection velocity. Therefore the latter velocity may be used in a practical sense.     

Eindimensionale Modellierung und Simulation von Blasensäulenreaktoren

Reactor models that feature a practical way to design bubble columns on the (semi‐)industrial scale have been published only rarely in the scientific literature. Creating a one‐dimensional model in the equation‐oriented simulation software ASPEN Custom Modeler^TM, a compromise between model precision and modeling can be reached. The model quantitatively describes the processes in a bubble column reactor with sufficient accuracy.     

Quality of mixing in a downflow bubble column based on information entropy theory

Quality of mixing in a modified downflow bubble column has been analyzed by using information entropy theory. Mass transfer efficiency based on quality of mixing has also been enunciated in this work. Empirical models have been developed for downflow system with the parameters which affect the quality of mixing and mass transfer efficiency. The developed correlation for quality of mixedness in the downflow bubble column was interpreted by the mass transfer phenomena. The present analysis on the quality of mixing in downward two-phase flow in bubble column may give insight into a further understanding and modeling of multiphase reactors in industrial applications.     

加热上升管内过冷流动沸腾数值模拟

采用计算流体动力学（CFD）程序CFX4.4对加热上升管内过冷流动沸腾工况下气水两相流动局部两相流参数（空泡份额和汽泡尺寸）进行了数值模拟。对数值差分方法、相关模型（界面力和气泡诱导的紊流）和汽泡尺寸进行了敏感性分析。空泡份额分布计算结果与实验结果比较表明，在低空泡份额工况下，两者符合较好，在高空泡份额工况下两者存在一定偏差，并且气相速度和汽泡尺寸的计算结果不理想。计算结果与实验结果之间的差异说明程序模型对于加热上升管内过冷流动沸腾模拟并不完善，建立更为合理的汽泡尺寸模型，考虑汽泡的合并和撕裂是必要的。     

DISENGAGEMENT ENERGETICS AT THE SURFACE OF AERATED REACTORS

Aerated reactors are widely used in cell culture processes. The bubbles that are introduced into such reactors provide essential mixing and oxygen, but the disengagement of those bubbles can result in the imposition of undesirable stresses upon fluid-borne cells. This occurs because some of the energy associated with interfacial tension is converted to high-velocity motions in the form of film rupture, droplet ejection and bubble cavity collapse. Obviously these effects would be amplified if there was an affinity between the cells and the gas-liquid interface. The present work was carried out in an effort to better characterize the energy, frequency of occurrence, and periodicities of these phenomena. This investigation examined the effects of fluid (medium) properties, bubble size, and gas rate upon both film and jet droplet ejection processes, as well as upon pressure disturbances measured at the reactor wall just beneath the free surface     

Gekoppeltes Berechnen von Blasengrößenverteilungen und Strömungsfeldern in Blasensäulen

Coupled Calculation of Bubble Size Distribution and Flow Fields in Bubble Columns In this paper the use of computational fluid dynamics (CFD) for the calculation of flow fields in bubble columns is explained. The local bubble size distribution is considered with the aid of a simplified balance equation for the average bubble volume in bubbly flow. Models are developed for the rate of bubble break‐up and coalescence based on physical principals. The flow fields in cylindrical bubble columns without internals are calculated using the Euler‐Euler method. The small and large bubble fraction are considered as pseudo‐continuous phases in addition to the liquid phase. The calculated flow fields are characterised by several large scale vortices. The local volume fractions of gas and liquid are very inhomogeneous and highly time dependent. The calculated volume fractions, velocities and bubble size distributions agree well with experimental results for bubble columns up to 0.3 m in diameter.     

3D Single Bubble Breakup Tracking in a Stirred Tank

A detailed understanding of turbulent fluid particle breakup mechanisms is essential for the accurate modeling of gas/liquid and liquid/liquid dispersions. The design of a fully automated setup for the three‐dimensional serial examination of the single bubble breakup process in a stirred tank, ensuring high repetition rates necessary for the additionally automated statistical analysis, is described. The implementation of a three‐dimensional automatic bubble breakup tracking tool is illustrated. At last, exemplary bubble breakup trajectories that show the benefits and limitations of the developed system and method are discussed.     

气泡羽流气液两相流动特性的研究进展

气泡羽流是一种复杂的气液两相流,广泛应用于废水处理、石油加工、环保等工业领域。气泡羽流的流动特性对气液两相间质量、动量传递及工业应用至关重要。本工作总结了理论与实验研究等方面气泡羽流流动特性的研究进展。详细讨论了气泡羽流气液两相流体水力学特性、羽流运动行为的影响因素。根据气含率、气泡直径等水力学参数的预测模型和经验公式,归纳了不同液相物性和结构参数下羽流模型的适用范围,揭示了流动对传质的作用。总结了分层流体中气泡羽流流型变化规律、羽流去分层效果以及引起流型变化的影响因素。阐释了横向流动环境下羽流的偏移行为呈线性变化,该变化与横向流速及表观气速等因素有关。最后讨论了气泡羽流气液两相流动特性研究手段和理论方法的局限性,展望了气泡羽流运动规律多尺度研究的方向。     

A generalized approach to model oxygen transfer in bioreactors using population balances and computational fluid dynamics

In many biological processes, increasing the rate of transport of a limiting nutrient can enhance the rate of product formation. In aerobic fermentation systems, the rate of oxygen transfer to the cells is usually the limiting factor. A key factor that influences oxygen transfer is bubble size distribution. The bubble sizes dictate the available interfacial area for gas-liquid mass transfer. Scale-up and design of bioreactors must meet oxygen transfer requirements while maintaining low shear rates and a controlled flow pattern. This is the motivation for the current work that captures multiphase hydrodynamics and simultaneously predicts the bubble size distribution.Bubbles break up and coalesce due to interactions with turbulent eddies, giving rise to a distribution of bubble sizes. These effects are included in the modeling approach by solving a population balance model with bubble breakage and coalescence. The population balance model was coupled to multiphase flow equations and solved using a commercial computational fluid mechanics code FLUENT 6. Gas holdup and volumetric mass transfer coefficients were predicted for different superficial velocities and compared to the experimental results of Kawase and Hashimoto (1996). The modeling results showed good agreement with experiment.     

Numerical and experimental studies of bubble growth during the microcellular foaming process

A new experimental technique for studying the dynamics of bubble growth in thermoplastics using scanning electron microscopy is developed. The influence of temperature, saturation pressure, molecular weight, and the nature of physical blowing agent are investigated. The experimental results show that, the above, process variables control the growth of foams during processing. The existing Newtonian model for the growth of a single bubble in an infinite amount of polymer has been modified to account for the non-Newtonian effects by modeling the polymer as a power law fluid. The experimental data has been compared with the appropriate viscoelastic cell model which considers the growth of closely spaced spherical bubbles during the foaming process. The simulation results indicate that the predictions of the cell model are in qualitative agreement with the trends of the experimental data and the quantitative agreement is reasonable. The cell model also gives an equilibrium radius which agrees with the experimental data. Other viscous models do not predict the equilibrium radius of the bubble and underpredict the experimental data.     

Prediction of gas density effects on bubbly flow hydrodynamics: New insights through an approach combining population balance models and computational fluid dynamics

Detailed measurements and computational fluid dynamics (CFD) investigation of the hydrodynamics in a bubble column containing internal features causing flow disturbances are presented for both air and helium gases. An optical needle probe has been used to measure profiles of bubble size, bubble velocity, and gas holdup at different locations across the cross section of the column. An approach combining CFD with population balances is able to represent observed multiphase flow phenomena such as the effect of the pipes to remix and redistribute the gas as well as the tendency of the gas to channel through a slit in the pipes rather than go around the pipes. The comparison of CFD simulation to experimental measurements reveal that the overall decrease in gas holdup observed when switching from air to helium gas can be explained by swarm effects, whereas the steeper decrease in the gas holdup profile across the column is due to coalescence effects. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3764–3774, 2018     

Non-isothermal gas-assisted displacement of viscous Newtonian fluids at high capillary number in gas-assisted injection molding

Gas-assisted injection molding is a polymer processing technology in which a penetrating gas bubble hollows out a plastic part as it cools and solidifies within a mold. In this study, non-isothermal gas injection experiments at high capillary number illustrate the effects of delay time in gas injection, tube diameter, capillary number, and temperature-sensitive fluid viscosity and flow activation energy on coating thickness. Experiments with polybutene H-300 and Dow Corning silicon oil (DC-200) in stainless steel tubing (1.27 and 0.635 cm) demonstrated fractional coverage increasing from 0.6 to a maximum in the range of 0.63–0.83 at short delay times, then decaying toward 0.6 at long delay times upon approaching the cooled isothermal state. Further analysis is drawn from simulations based on a simple theoretical model incorporating one-dimensional heat transfer with convection at the outer surface of the mold, non-isothermal behavior of the viscous fluid, and radial velocity profiles in the one-phase fluid flow region. Quantitative agreement is found between experimental and simulated results. Two-dimensional modeling and simulation methods extend the prior results to illustrate transient axial and radial heat transfer as well as flow behavior with respect to the penetrating gas bubble within the fluid flow region.