Dispersed Bubbles Research Articles

Dissuasão de peixes associada à dispersão de bolhas em turbina tipo

During the maintenance procedures of generating units, the operational maneuver of shutting down the turbine causes low water flow, which may lead to the accumulation of fish inside the draft tube. Considering the principles of ichthyofauna protection, several repulsion systems have been developed to minimize the confinement and the possible risks to the ichthyofauna. The main objective of this study was to investigate the dissuasive effectiveness on the ichthyofauna caused by the dispersion of bubbles along the draft tube during the shutdown of the generating unit. The monitoring of the ichthyofauna in the draft tube was carried out with the aid of a hydroacoustic system. The bubble dispersion procedure was executed using the compressed air injection system built into the draft tube of the Generating Units at the Jirau Hydropower Plant, on the Madeira River, in Rondônia, Brazil. 9 machine shutdowns were tracked. They were carried out in two stages; the first stage was the conventional shutdown and the second was the shutdown of the generating unit with bubble dispersion. During both stages, the movement of the confined ichthyofauna was continuously monitored by a hydroacoustic real-time monitoring system installed in the fixed-wheel gates. The results show a reduction of about 42% in the ichthyofauna movement after the bubble dispersion procedure, making it possible to infer a considerable reduction in ichthyofauna confinement in the draft tube. This observed effectiveness, in an unfavorable environment with the high turbidity of the Madeira River, shows the considerable potential of this robust and low-cost technology for the protection of ichthyofauna in the hydroelectric sector.

Advanced control of microstructure in ice cream by agitation and mixing during freezing process

Agitation and mixing play an important role in ensuring the appropriate dispersion of ice crystals, bubbles, and fat globules during the freezing process in ice cream manufacturing. The dispersion determines the internal structure of ice creams and hence, the ice cream quality. To achieve advanced control of the internal structure of ice creams, in this study, the effect of agitation on air entrainment, micronisation of air bubbles, and fat globule agglomeration was investigated using a batch-type freezer with an anchor impeller. Besides, rheological properties of an ice cream sample that was stored in the refrigerator for 24 h after freezing were analysed. In addition to steady agitation, unsteady agitation with a periodical change in the rotation direction was employed. Temperature measurement and flow observation suggested more effective scraping of frozen phase from the cooling surface for unsteady agitation than for steady agitation. Furthermore, air entrainment and fat globule agglomeration during freezing were promoted under unsteady agitation. Consequently, the sample manufactured under unsteady agitation exhibited enhanced thermal robustness compared to that manufactured under steady agitation. Thus, unsteady agitation can be an effective strategy to control the formation of microstructures in ice cream.

Air entrainment by a hydrofoil in a high-speed water flow for design of bubbly drag reduction ships

We have investigated the flow behavior around a hydrofoil bubble generator in a high-speed channel flow up to 10 m/s for the first time to understand the air entrainment characteristics of this hydrofoil device caused purely by the inertia effect of water flow. This study is motivated by our previous success in achieving ship drag reduction for full-scale vessels (Kumagai et al., 2015). Depending on the water flow velocity U and the angle of attack of the hydrofoil α, three different major patterns were identified: continuous generation of dispersed bubbles, intermittent formation of an air cavity, and steady formation of a super air cavity that stretched behind the hydrofoil. The transition from the dispersed bubble state to the air cavity formation regime is explained theoretically using the equation of bubble motion coupled with a potential flow analysis. The air volume flow rate increased linearly with U, and its slope was steeper in the dispersed bubble state than in the air cavity formation regime. The highest air flow rate was realized at U = 9 m/s, which corresponds to the nominal air layer thickness at ta = 4.1 mm, and reaches the feasible range for realization of bubbly drag reduction for a long ship hull.

An experimental study on air-oil flow patterns in horizontal pipes using two synthetic oils

Investigations on the gas-oil mixture flow of highly viscous oils have become increasingly significant in various industries for the past couple of years. This study examines the characteristics of air-oil flow patterns to understand the effect of the oil phase's thermophysical properties and flow rates on flow patterns. An experimental study was conducted to visualize the air-oil mixture flow in horizontal pipes. The diameters of the horizontal pipes were selected as 20 and 40 mm, respectively. Two different oils having substantially different viscosities were employed as the liquid phase. Based on the visualization results, air-oil mixture flow is categorized into seven flow patterns: stratified smooth, stratified wavy, annular, plug, slug, bubbly, and dispersed bubble flows. Along with representative images, this study demonstrates the essential characteristics of individual flow patterns. Furthermore, it discusses the influence of each phase's flow rate, oil viscosity, and surface tension on flow patterns. The processes of liquid entrainment in stratified wavy flow, bubble tip breakup in plug flow, and liquid slug aeration in slug flow are also illustrated, respectively, with the corresponding sequential representative images and schematic diagrams.

Numerical simulation of two-phase gas-liquid flowthrough horizontal annulus pipe

Chemical, petroleum and nuclear systems are only a few of the industrial processes that utilize gas-liquid flow in annular closed channels. However, concentric horizontal annuli flow patterns have received little attention. The ability to precisely characterize two-phase flow patterns using computational techniques is crucial for the production, transportation, and optimization of designs. This current research aims to establish the accuracy of the computational fluid dynamics (CFD) model in predicting the gas-liquid flow pattern in the concentric annulus pipe and validating the flow pattern of liquid holdup with experimental results from the literature. The simulations were done on a test section of a 12.8 m length pipe with a hydraulic diameter of 0.0168 m using air and water as the working fluids. The volume of fluid (VOF) model in Ansys Fluent based on the Eulerian- Eulerian approach in conjunction with the realizable k-ε turbulence model was used to model the gas-liquid flow pattern, i.e. dispersed bubble, elongated bubble, and slug in a horizontal annulus. A comparison of the model with the experimental high-speed video images shows a reasonable agreement for the flow pattern and liquid holdup data.

Thermodynamic Modeling of Aqueous Nanobubble Dispersion

Summary The amount of gaseous species in water or brine can be greatly enhanced in the form of nanobubble (NB) dispersion. Aqueous NB dispersion has vast industrial applications, potentially in enhanced oil recovery (EOR) and carbon dioxide (CO2) sequestration to control the mobility of gaseous species and the geochemistry associated with CO2 dissolution. Development of such NB technologies depends on a proper understanding of thermodynamic properties of aqueous NB dispersion. The objectives of this research are to analyze the thermodynamic stability of aqueous NB dispersion and to apply a thermodynamic equilibrium model to analyze experimental data. We first present a thermodynamic formulation for modeling aqueous NB dispersion, which clarifies that aqueous NB dispersion occurs in the aqueous phase that is supersaturated by the gaseous species in the system. That is, the gaseous species are present in two modes—dispersion of gas bubbles under capillary pressure and molecule dispersion (supersaturation) in the external aqueous phase. Such a thermodynamic system is referred to as aqueous NB fluid in this research and specified by (NC + 3) variables (e.g., temperature, total volume, components’ mole numbers, and capillary pressure), in which NC is the number of components. We then present a novel implementation of the GERG-2008 equation of state (EOS) in minimization of the Helmholtz free energy to solve for equilibrium properties of aqueous NB fluid. GERG-2008 was used in this research because it is suitable for modeling an aqueous phase that is supersaturated by gaseous species. The thermodynamic equilibrium model was applied to experimental data of aqueous NB fluid with nitrogen (N2) at pressures up to 277 bara (4,019 psia) and 295.15 K (71.6°F). Application of the model to experimental data indicates that a large fraction (0.8–0.9) of the total amount of N2 is in the form of molecule dispersion, but such supersaturation of the aqueous phase is possible because of the presence of NB dispersion with capillary pressure. That is, NB dispersion can increase the gas content in aqueous NB fluid by enabling gas supersaturation in the aqueous phase as a thermodynamic system.

CFD evaluation of hydrophobic feedstock bench-scale fermenters for efficient high agitation volumetric mass transfer.

A new biomanufacturing platform combining intracellular metabolic engineering of the oleaginous yeast Yarrowia lipolytica and extracellular bioreaction engineering provides efficient bioconversion of plant oils/animal fats into high-value products. However, predicting the hydrodynamics and mass transfer parameters is difficult due to the high agitation and sparging required to create dispersed oil droplets in an aqueous medium for efficient yeast fermentation. In the current study, commercial computational fluid dynamic (CFD) solver Ansys CFX coupled with the MUSIG model first predicts two-phase system (oil/water and air/water) mixing dynamics and their particle size distributions. Then, a three-phase model (oil, air, and water) utilizing dispersed air bubbles and a polydispersed oil phase was implemented to explore fermenter mixing, gas dispersion efficiency, and volumetric mass transfer coefficient estimations (kL a). The study analyzed the effect of the impeller type, agitation speed, and power input on the tank's flow field and revealed that upward-pumping pitched blade impellers (PBI) in the top two positions (compared to Rushton-type) provided advantageous oil phase homogeneity and similar estimated kL a values with reduced power. These results show good agreement with the experimental mixing and kL a data.

Study of a Novel Fluidized Bed Flotation Column with Enhanced Bubble Dispersion

Flotation machines and flotation columns are widely used as separation equipment for coal sludge. In order to obtain better flotation results for different particle sizes, the bubble distribution and size in the flow field need to be studied. In this paper, a novel three-phase fluidized bed flotation column (TFC) was designed, and the shear effect of liquid velocity (0.198 m/s, 0.226 m/s, and 0.254 m/s) and static bed height (0.1 m, 0.2 m, and 0.3 m) on the bubbles in the mineralized flow field region was investigated and evaluated for the formation of bubbles by shear in laboratory and semi-industrial experiments. The results show that the increase in filling bed height has a very obvious strengthening effect on the reduction of bubble diameter, and after the filling bed height reaches a certain value, the filling bed height will weaken the effect of apparent gas velocity on bubble diameter. The apparent gas velocity has different influencing effects on bubble diameter, and under conditions of low water velocity, the increase in the apparent gas velocity contributes to the reduction of bubble diameter. The conclusions of this study are expected to optimize the operating parameters of the flotation mineralization process and enrich the study of TFC, which can provide a reference for the design of future TFC studies.

Frothers in flotation: A review of performance and function in the context of chemical classification

The significance of frothers in the context of mineral flotation is widely recognized, particularly in relation to their impact on bubble size and dispersion, coalescence inhibition, as well as the structure and stability of the froth. All these parameters hold substantial importance in determining the feasibility of the process including the overall recovery of valuable minerals and concentrate grade in flotation process. This review aims to offer a comprehensive overview of the importance of frothers in the flotation process focusing on their performance and function in the context of their chemical classification. The characterisation of flotation frothers is discussed, focusing on the most often used parameters including molecular weight, hydrophilic-lipophilic balance (HLB), dynamic frothability, and interfacial tension measurement. Additionally, the importance of advanced characterisation techniques such as single rising bubble experiments, high-speed video microscopy and infrared reflection absorption spectroscopy (IRRAS) is highlighted. Finally, the application of biodegradable and environmentally friendly biosurfactants such as rhamnolipids as “green frothers” in enhancing flotation rates is outlined.

An experimental study on pressure drop of air-oil flow in horizontal pipes using two synthetic oils

This study focuses on the frictional pressure drop in gas-liquid two-phase flow, characterized by nonlinear behavior under various flow conditions due to complex two-phase phenomena. The experimental investigation involves an air-oil mixture flowing through horizontal pipes with diameters of 20 mm and 40 mm, aiming to observe the flow patterns and quantify the pressure drop. Two oils with different thermophysical properties are used as the liquid phase. The operating temperature and pressure of the test pipes are controlled within the range of 24–44°C and 1–3 bar, respectively. Seven air-oil flow patterns are identified based on visualization results: stratified smooth, stratified wavy, annular, plug, slug, bubbly, and dispersed bubble flows. On analyzing 1043 data points from the experiment, this study explores the effects of total mass velocity, flow quality, pipe diameter, operating temperature, operating pressure, and oil type on the frictional pressure gradient. Particular attention is given to the variation of the frictional pressure gradient concerning operating pressure, and its relationship with total mass velocity, flow quality, and flow pattern is discussed. Additionally, the study examines predictions from 55 previous correlations against the current pressure drop data. The predictive capability of these correlations is evaluated for the entire dataset and its subsets, categorized by oil type, pipe diameter, operating pressure, flow condition, total mass velocity, flow quality, and flow pattern. Evaluation metrics include mean absolute errors and the proportions of data points within 30% and 50% of absolute error. In addition, average and standard deviation values are calculated for each dataset to provide a comprehensive assessment.

Optimization of flotation processes by bubble size selection

It is known that the size of the bubbles plays a decisive role in the flotation of particles. The paper presents the calculation of the optimal sizes of floating bubbles depending on the size of the particles of pollution. The latter allows you to adjust (select) the dispersion of bubbles, which increases the efficiency of the equipment: an increase in hydraulic loads, a decrease in capital and metal consumption.

Measurement of Gas Fraction in Gas-Liquid Dispersed Bubbly Flow With EMAT Array and EIT Sensor

Measurement of Gas Fraction in Gas-Liquid Dispersed Bubbly Flow With EMAT Array and EIT Sensor

Experimental and numerical investigation on the bubble breakup and coalescence characteristics in the highly efficient static mixers

CFD-PBM and experiment methods were applied to study the bubble dispersion characteristics in the static mixers with three twisted blades (TKSM). The gas-liquid velocity ratio was selected as drag correction factors. The simulation data was compared with experimental values at different superficial gas velocity and the results revealed that the modified model could effectively enhance the calculation accuracy of gas holdup. Furthermore, bubble size distribution (BSD) was well predicted by Luo-Prince model and the mean relative error (MRE) of axial d32 between the prediction of Luo-Prince model and experimental value was 2.86 %. Meanwhile, the influences of bubble collision mechanisms and coalescence correctors on the numerical prediction were quantitatively analyzed. The effects of turbulent fluctuation, viscous shear, wake entrainment and buoyancy driven were considered for the prediction of bubble collision frequency in TKSM. The prediction precision of cumulative probability distribution (CPD) was promoted by 17.26 % when the correctors obtained from Energy-Minimization Multi-Scale (EMMS) method were used. In addition, empirical parameter CDas was evaluated and the results showed that the Das coalescence model could accurately describe the BSD when CDas was 0.01–0.1. The backmixing performance of the gas phase in the TKSM exceeds that of the liquid phase, which is distinct from that in vortex unit.

Significant suppression of helium bubbles in oxide dispersion strengthened FeCrAl alloys irradiated by high concentration of helium

Oxide dispersion strengthened (ODS) FeCrAl alloys were successfully fabricated by various mechanical alloying time (10h and 40h), combined with spark plasma sintering (SPS) technique. To understand their helium irradiation resistance, two kinds of ODS FeCrAl alloys were implanted with helium ions at 400 keV in the fluence of 1 × 1021/m2 at 350 °C, 400 °C, and 450 °C to investigate the effect of high concentration helium on the materials in advanced nuclear reactors. The distributions of helium bubbles associated with the microstructural features were characterized by transmission electron microscope (TEM). The experimental results showed that both the helium bubble sizes and volume fractions increased with increasing the irradiation temperatures. The high sink strength of nano-oxides and small inter-oxides distance contributed to the good suppression of helium bubbles growth. In particular, we identified that the high-density nano-oxides with the small sizes (<10 nm) could sequester the helium into several (1–3) tiny bubbles to inhibit bubbles accumulating and coarsening both in elevated temperatures and high helium concentration. Additionally, grain boundaries demonstrated the weaker ability than nano-oxides to disperse helium bubbles, when compared helium irradiation resistance in ODS FeCrAl alloys and FeCrAl alloys. Eventually, ODS FeCrAl alloys in this study revealed the superior helium irradiation resistance than other studied alloys, making it promising to be one of the potential structural materials applicated in advanced nuclear reactors.

Analysis of the influence of swirling flow on the boiling heat transfer characteristics of two-phase flow

The Eulerian-Eulerian (EE) simulation approach + GENeralized TwO Phase (GENTOP) concept and two-fluid three-flow field with droplet were used to simulate the heat and mass transfer. Analysis encompassed the examination of steam and liquid phase distributions, as well as flow pattern transitions. Heat transfer enhancements were quantified using a comprehensive heat transfer factor. The disturbance intensity of the secondary flow was characterized by the absolute vortex flux. Results revealed a correlation between the intensity of the secondary flow and axial length, indicative of progressive fluid disturbance enhancement. Additionally, the maximal motion of the steam phase vortex was observed both near the wall and channel center. The EE+GENTOP concept exhibited superior capability in reproducing multi-scale flow including small-scale dispersed bubbles and large-scale continuous steam (coalescence and break-up). For investigations into the entrainment and deposition of liquid droplets in annular flow, the selection of a two-fluid three-flow field with the droplet model is recommended.

Modeling the distribution characteristics of vapor bubbles in cavitating flows

Dispersed vapor bubbles are the dominant rheology in cloud cavitation, and their size distribution is directly associated with cavitation noise and erosion. However, the numerical resolution of large numbers of dispersed bubbles remains a challenge. In this work, we establish a new cavitation model based on the population balance equation (PBE) that can predict the size distribution and spatiotemporal evolution of bubbles within cloud cavitation under different cavitation numbers. An expression for the phase transition source term without empirical parameters is derived based on the bubble size distribution (BSD) function, enabling the coupling of mass transfer in the governing equations with the PBE cavitation model. The cavitation model is solved alongside the Eulerian homogeneous mixture flow. The mass transfer between water and vapor, and the bubble coalescence and breakup under turbulent flows, are modeled to determine the BSD. The numerical model is carefully validated through comparisons with experimental results for cavitation flows on a wedge-shaped flat plate, and good agreement is achieved with respect to the pressure distribution, void fraction, and BSD. This confirms that our proposed cavitation model can accurately predict the void fraction and BSD within the cloud cavitation region.

Numerical modelling of two-phase flow hydrodynamics of column flotation - validation against ERT data

ABSTRACT In mineral processing, column flotation is being used extensively for fine particle beneficiation. Although a few attempts were made to develop computational fluid dynamics (CFD) models for column flotation in the past, validation against comprehensive experimental data is yet to be demonstrated. This work employs an Eulerian-Eulerian model coupled with the standard k-ε turbulence model for modeling the hydrodynamics in the column flotation. The two-phase simulations were performed on both laboratory and industrial-scale column flotations. The two-fluid model was further modified with the appropriate interphase forces (Ishii-Zuber drag force and Tomiyama lift force) and population balance method (PBM) to accurately simulate and predict the bubble dynamics. The axial and radial gas holdup variations along with the axial liquid velocity are predicted at different air and liquid superficial velocities. The numerical predictions were then validated against the Electrical Resistance Tomography (ERT) data in a 4-inch laboratory column flotation and conductivity probe data in an industrial column flotation. The modified two-fluid model was able to predict accurate hydrodynamics close to ERT data. The gas holdup was found to increase with the superficial air velocity, liquid height, and liquid velocity. The gas holdup found uniformly dispersed radially at low superficial air velocities and heterogenous dispersed bubble flow at higher superficial velocities. A parabolic profile of liquid flow observed at higher superficial air velocities.

Experimental investigation of flow orientation effects on cryogenic flow boiling

Cryogenic flow boiling physics is of paramount importance for future cryogenic space applications employing Cryogenic Fluid Management (CFM) technologies. This experimental study investigates the two-phase flow and heat transfer performance of LN2 flow boiling across five distinct flow orientations: vertical upflow, vertical downflow, horizontal flow, 45° inclined upflow, and 45° inclined downflow. The study employed the steady-state heating method within a circular heated tube featuring an 8.5-mm inner diameter and a 680-mm heated length. High-speed video recordings were utilized to capture two-phase flow patterns and interfacial behaviors across various flow orientations. The experiments covered a wide range of operating conditions, including mass velocities ranging from 351.80 to 1572.77 kg/m²s and inlet pressures from 297.14 to 1032.97 kPa, primarily with near-saturated inlet subcooling. Distinct two-phase flow patterns and regime transitions were identified for each flow orientation. Symmetrical flow patterns were evident in vertical orientations, whereas non-vertical orientations exhibited asymmetric flow stratifications, primarily influenced by the buoyancy force in a terrestrial gravity environment. Bubble dynamic parameters were quantified, and bubble collision and dispersion phenomena were visualized. Analyzing heat transfer performance based on local flow boiling curves and variations in heat transfer coefficients (HTCs), it was observed that vertical upflow demonstrated the most enhanced heat transfer performance, while vertical downflow exhibited the lowest. As mass velocity exceeded 830 kg/m²s, the differences in heat transfer among orientations became less distinct, emphasizing the role of flow inertia in mitigating the influence of flow orientation. A direct comparison of microgravity HTC data from a recent study by the authors against the 1-ge HTC data indicated enhanced heat transfer performance in microgravity for flow orientation configurations in terrestrial gravity. However, this enhancement progressively diminished as the heat flux increased. Distinct HTC trends were observed as mass velocity increased for different flow orientations and gravity levels.

Effects of Swirl on Bubble Flow and Its Development in a Horizontal Pipe

Effective length of swirl flow is important for its application in the industry, however, the length of the swirl flow in a horizontal pipe is largely unexplored. In this article, effects of swirl on a horizontal bubbly flow and its development in a 25 mm horizontal pipe with a length of 4 m were studied by an experimental and theoretical method. In the experiments, the superficial gas velocities ranged from 0.068 to 0.566 m/s and the liquid velocities ranged from 0.283 to 1.132 m/s. The results show that: instead of dispersed bubbles in the non-swirl bubbly flow, a gas column flow is transformed downstream of the swirler; then, it breaks up into separated gas slugs, and finally, transforms to a non-swirl bubble flow. The pressure drop in the swirl flow gradually decreases and approximates to that in the non-swirl flow along the streamwise direction. The transition mechanism for the gas column flow was proposed here, and the transition criteria of the gas column flow were developed based on Kelvin–Helmholtz instability, then, the effective length of swirl on the gas column flow can be predicted.

Euler-Euler LES of bubble column bubbly flows by considering sub-grid scale turbulent dispersion effect on modulating bubble transport

It has now been recognised and generally accepted that turbulent dispersion may be modelled using the time average of the fluctuating part of the interphase momentum, employing the drag the Favre averaged drag model for turbulent dispersion in Eulerian multi-phase flows. As the turbulent eddies in the surrounding of bubbles interact strongly with the bubbles in bubbly flow, the bubble trajectories and bubble oscillation take place accordingly as the consequence of continuous deformation of the bubble surfaces. When using large eddy simulation for modelling bubbly flow, the sub-grid scale (SGS) filtered velocity fluctuations of liquid phase can be interpreted as many small eddies that may act on the surface of bubbles, consequently giving rise to bubble shape variations and the dispersion of bubbles. This study employs Euler/Euler large-eddy simulation (LES) modelling to demonstrate that the turbulent dispersion force model can be used to effectively indicate the influence of turbulent eddies on bubble dynamics, in particular the bubble cluster oscillations, which leads to remarkable improvements in the prediction of bubble lateral dispersion behaviour. The use of spatial filtering to model the SGS bubble dispersion is proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble induced turbulence. The results of the time-averaged LES modelled bubble velocities and bubble volume fraction profiles are in good agreement with the experimental data while the turbulent kinetic energy spectrum obtained at different locations on the centreline of the bubble column still exhibits the conventional −5/3 scaling for shear induced turbulence and a −3 scaling for bubble induced turbulence.