Velocity Of Air Bubbles Research Articles

Continuous rise velocity of air bubbles in non-Newtonian biopolymer solutions

We have studied the free rise of small air bubbles in aqueous solutions of xanthan gum and carboxymethylcellulose. The rheological behavior of the biopolymer solutions used in our experiments was thoroughly characterized. The terminal rise velocity of the bubbles was measured by imaging their motion with a high-speed video camera. The rise velocity increased as a power law in bubble volume for the smallest bubbles studied, as expected in the Stokes flow regime, but became approximately constant for larger bubbles. We observed no discontinuity in the rise velocity over the range of bubble volumes studied, from 1μl to 4000μl. The shape of the bubbles changed from spherical to cusped to spherical cap as the volume increased. While the smaller bubbles rose vertically through the polymer solutions, the larger bubbles rose in a spiral or zig-zag path.

Effective medium approximation model of sea foam layer microwave emissivity of a vertical profile

A sea foam layer produced by wave breaking consists of seawater-coated air bubbles, fluid water, and air. The non-uniformity and microstructure of the air–water mixture in a foam layer can cause some important effects on microwave or optical properties. Considering the vertical non-uniformity of the air–water volume, the air-volume fraction is derived as a function of the foam depth variable, coated air bubble velocity, and foam temperature using the gas convection–diffusion equation. For a vertical graded profile of the air volume fraction, we discuss the effects of the coated air bubble velocity parameter and the air–sea temperature difference on the air volume fraction. The results show that the coated air bubble velocity is a key parameter that widely modulates the air volume fraction. Furthermore, an effective medium approximation (EMA) of spherical shell microstructures is proposed to investigate the graded foam effective permittivity and the emissivity of the sea surface covered with the foam layer of the graded air volume fraction in a vertical profile, and good agreement is obtained on comparing the EMA results with the experimental data of foam layer microwave emissivities at frequencies 1.4, 10.8, and 36.5 GHz. In our EMA model, the depth average of graded foam permittivity is adopted. This model can produce reasonable results by only tuning the air bubble velocity. It indicates that the EMA model can combine the effects of graded foam permittivity and foam microstructures into the coated air bubble velocity parameter. Meanwhile, we have qualitatively discussed the influence of air–sea temperature difference on the brightness temperature of the foam layer. It is shown that a negative (or positive) air–sea temperature difference enhances (or reduces) the sea surface brightness temperature, and the brightness temperature difference increases with an increase in microwave frequency.

환형 이젝터 루프 내부의 이상유동특성 파악을 위한 수치해석 및 유동가시화 연구

A water driven ejector loop was designed and constructed for air absorption. The used ejector was horizontally installed in the loop and annular water jet at the throat entrained air through the circular pipe placed at the center of the ejector. Wide range of water flow rate was provided using two kinds of pumps in the loop. The tested range of water flow rate was 100<TEX>${\ell}$</TEX> /min to 1,000 <TEX>${\ell}$</TEX>/min. Two-phase flow inside the ejector loop was simulated by CFD analysis. Homogeneous particle model was used for void fraction prediction. Water and air flow rates and pressure drop through the ejector were automatically recorded by using the LabView based data acquisition system. Flow characteristics and air bubble velocity field downstream of the ejector were investigated by two-phase flow visualization and PIV measurement based on bubble shadow images. Overall performance of the two-phase ejector predicted by the CFD simulation agrees well with that of the experiment.

Effect of disjoining pressure on terminal velocity of a bubble sliding along an inclined wall

The influence of salt concentration on the terminal velocities of gravity-driven single bubbles sliding along an inclined glass wall has been investigated, in an effort to establish whether surface forces acting between the wall and the bubble influence the latter’s mobility. A simple sliding bubble apparatus was employed to measure the terminal velocities of air bubbles with radii ranging from 0.3 to 1.5mm sliding along the interior wall of an inclined Pyrex glass cylinder with inclination angles between 0.6 and 40.1°. Experiments were performed in pure water, 10mM and 100mM KCl solutions. We compared our experimental results with a theory by Hodges et al. [1] which considers hydrodynamic forces only, and with a theory developed by two of us [2] which considers surface forces to play a significant role. Our experimental results demonstrate that the terminal velocity of the bubble not only varies with the angle of inclination and the bubble size but also with the salt concentration, particularly at low inclination angles of ∼1–5°, indicating that double-layer forces between the bubble and the wall influence the sliding behavior. This is the first demonstration that terminal velocities of sliding bubbles are affected by disjoining pressure.

Ultrasound monitoring of bubble size and velocity in a fluid model using phased array transducer

Experimental studies on composite materials highlighted the existence of gas bubbles (voids) at different scales (micro and macro). These voids, resulting from the fabrication process, are sources of weakness for the end-user material. Therefore, several studies focus on the evaluation and the minimization of void rate. During injection of resin into the fibrous matrix, bubbles appear. Some are dispersed while others persist and diminish the overall quality of the finished product. This study consists in detecting bubbles during the resin transfer molding (RTM) process using an original method based on ultrasound. However, in practice we have to face with several problems due to the heterogeneity of the environment, such as the differentiation of a bubble from a strand or fiber. Due to the complexity of the problem, an experimental setup was built in order to detect bubbles using an ultrasonic phased array transducer in a flow of viscous fluid only, without the presence of fiber matrix. The study deals with the flow of a model fluid, which simulates the injected resin, with presence of air bubbles trapped in a channel production. Since the ultrasonic characteristics of the experimental setup are well known and characterized, the number, velocity and size of air bubbles could be evaluated.

Hydrodynamic study of an internal airlift reactor for microalgae culture

Internal airlift reactors are closed systems considered today for microalgae cultivation. Several works have studied their hydrodynamics but based on important solid concentrations, not with biomass concentrations usually found in microalgae cultures. In this study, an internal airlift reactor has been built and tested in order to clarify the hydrodynamics of this system, based on microalgae typical concentrations. A model is proposed taking into account the variation of air bubble velocity according to volumetric air flow rate injected into the system. A relationship between riser and downcomer gas holdups is established, which varied slightly with solids concentrations. The repartition of solids along the reactor resulted to be homogenous for the range of concentrations and volumetric air flow rate studied here. Liquid velocities increase with volumetric air flow rate, and they vary slightly when solids are added to the system. Finally, liquid circulation time found in each section of the reactor is in concordance with those employed in microalgae culture.

ICONE19-44083 WHOLE-FIELD VELOCITY MEASUREMENTS OF ISOTHERMAL BUBBLE PLUME USING PTV

The current work focuses on obtaining bubble dynamics properties in a plume by the shadowgraphy technique. Bubble parameters such as the bubbles diameter and velocities were estimated using an in-house developed algorithm. Two gases were independently used in this study, namely Sulfur-Hexafluoride (SF6) and Air. Three separate cases were observed: small (0.01 L/min), medium (0.15 L/min), and large (0.30 L/min) flow rates. The bubble plume was created by injecting the gas from the bottom of a glass tank. The tank was illuminated with two halogen lamps. High speed cameras were situated in the front of the tank. The average axial velocity of Air bubbles was calculated to be 13.25 cm/s, 13.92 cm/s, 28.17 cm/s for small, medium, and large flow rates, respectively. The average axial velocity of SF6 bubbles was calculated to be 8.05 cm/s, 8.02 cm/s, and 24.64 cm/s for small, medium, and large flow rates, respectively. It was found that as the bubbles position increased vertically, they were more buoyant-driven than flow-driven. The present study constitutes part of a larger project devoted to the analysis of thermal-hydraulic characteristics of bubble plumes. Results from these series of experiments are intended to provide new and accurate information for the development and validation of correlations used in two phase flow models.

Motor oil removal from water by continuous froth flotation using extended surfactant: Effects of air bubble parameters and surfactant concentration

The objective of this study was to correlate the air bubble parameters in a flotation column and the surfactant concentration to the efficiency of motor oil removal from water by continuous froth flotation. C 14–15(PO) 5SO 4Na (branched alcohol propoxylate sulfate, sodium salt) was used to form microemulsions with motor oil. From the microemulsion phase diagram, the minimum surfactant concentration, known as the critical microemulsion concentration (CμC), to form a Winsor Type III microemulsion, was preliminarily selected for the froth flotation experiments. An increase in surfactant concentration was found to reduce the size and rising velocity of air bubbles in the froth flotation column, whereas the specific surface area, bubble surface area flux, bubble number flux, and residence time of the air bubbles increased with increasing surfactant concentration. The maximum removal of both motor oil and surfactant was found to correspond to the CμC. Beyond the CμC, the oil removal decreased with increasing the surfactant concentration because increasing the micelle concentration results in increasing oil stabilization.

Bubble Movement in Downward-Inclined Pipes

An experimental investigation with large-scale model tests of bubbles moving upward and downward in downward-inclined pipes is presented. The shape, velocity, and drag coefficient of single nonspherical air bubbles in continuous air-water flows are discussed. The bubble height depends mainly on the approach flow water velocity and the pipe slope. For stagnant bubbles, the bubble height is determined depending on these two parameters. Equilibrium of the drag and buoyancy forces is applied on single air bubbles in downward-inclined pipes. In pipes with pipe slope ranging from 0.052–0.087, the bubble drag coefficient is independent of the bubble Reynolds number. However, the bubble drag coefficient depends on the pipe slope and the approach flow water velocity. Using the approach of the equilibrium of the main forces the volume of stagnant bubbles can be predicted.

Measurement of two-dimensional bubble velocity by Using tri-fiber-optical Probe

In this study, an advanced measuring system with a tri-single-fiber-optical-probe has been developed to measure two-dimensional vapor/gas bubble velocity. The use of beam splitting devices instead of beam splitting lens simplifies the optical system, so the system becomes more compact and economic, and more easy to adjust. Corresponding to using triple-optical probe for measuring two-dimensional bubble velocity, a data processing method has been developed, including processing of bubble signals, cancelling of unrelated signals, determining of bubble velocity with cross correlation technique and so on. Using the developed two-dimensional bubble velocity measuring method, the rising velocity of air bubbles in gravitational field was measured. The measured bubble velocities were compared with the empirical correlation available. Deviation was in the range of ±30%. The bubble diameter obtained by data processing is in good accordance with that observed with a synchro-scope and a camera. This shows that the method developed here is reliable.

Study of Air‐Liquid Flow Patterns in Hydrocyclone Enhanced by Air Bubbles

AbstractIn order to improve the oil‐water separation efficiency of a hydrocyclone, a new process utilizing air bubbles has been developed to enhance separation performance. Using the two‐component phase Doppler particle analyzer (PDPA) technique, the velocities of two phases, air and liquid, and air bubble diameter were measured in a hydrocyclone. The air‐liquid mixing pump can produce 15 to 60 μm‐diameter air bubbles in water. There is an optimum air‐liquid ratio for oil‐water separation of a hydrocyclone enhanced by air bubbles. An air core occurs in the hydrocyclone when the air‐liquid ratio is more than 1 %. The velocities of air bubbles have a similar flow pattern to the water phase. The axial and tangential velocity differences of the air bubbles at different air‐liquid ratio are greater near the wall and near the core of the hydrocyclone. The measured results show that the size distribution of the air bubbles produced by the air‐liquid mixing pump is beneficial to the process where air bubbles capture oil droplets in the hydrocyclone. These studies are helpful to understand the separation mechanism of a hydrocyclone enhanced by air bubbles.

Motion and shape of bubbles rising through a yield-stress fluid

We study the velocity and shape of air bubbles rising through a transparent yield-stress fluid. The bubbles are small enough compared to the experimental vessel that effects of walls are weak. We find that the terminal rise velocity of the bubbles increases approximately linearly with bubble radius over the range of volumes accessible in our experiments. We observe bubble motion only when the bubbles are larger than a certain critical radius. In terms of a dimensionless yield parameter Y, the ratio between the force due to the yield stress and the buoyant force, we observe bubble motion only for Y ≲ 0.50 ± 0.04 . The bubbles are non-spherical, having the shape of an inverted teardrop with a rounded head and a cusp-like tail. The cusps may be an indication that elasticity plays a significant role in this system. By fitting the cross-sectional radius of the bubble as a function of the axial coordinate to an empirical function, we study the dependence of the bubble shape on volume and the yield stress of the material.

Effect of compressibility on the rise velocity of an air bubble in porous media

The objective of this study is to develop a theoretical model to analyze the effect of air compressibility on air bubble migration in porous media. The model is obtained by combining the Newton's second law of motion and the ideal gas law assuming that the air phase in the bubble behaves as an ideal gas. Numerical and analytical solutions are presented for various cases of interest. The model results compare favorably with both experimental data and analytical solutions reported in the literature obtained for an incompressible air bubble migration. The results show that travel velocity of a compressible air bubble in porous media strongly depends on the depth of air phase injection. A bubble released from greater depths travels with a slower velocity than a bubble with an equal volume injected at shallower depths. As an air bubble rises up, it expands with decreasing bubble pressure with depth. The volume of a bubble injected at a 1‐m depth increases 10% as the bubble reaches the water table. However, bubble volume increases almost twofold when it reaches to the surface from a depth of 10 m. The vertical rise velocity of a compressible bubble approaches that of an incompressible one regardless of the injection depth and volume as it reaches the water table. The compressible bubble velocity does not exceed 18.8 cm/s regardless of the injection depth and bubble volume. The results demonstrate that the effect of air compressibility on the motion of a bubble cannot be neglected except when the air is injected at very shallow depths.

Flux enhancement and cake formation in air-sparged cross-flow microfiltration

Abstract The flux enhancement in cross-flow microfiltration of submicron particles by sparged air-bubble is studied. The effects of operating conditions, such as air-bubble velocity, suspension velocity and filtration pressure, on the cake properties and filtration flux are discussed thoroughly. The results show that the pseudo-steady filtration flux increases as the air-bubble velocity and filtration pressure increase. The sparged air-bubble can significantly improve filtration flux, but the flux enhancement is more remarkable in the lower air-bubble velocity region. A gas–liquid two-phase flow model is adopted for estimating the shear stress acting on the membrane surface under various operating conditions. The cake mass can be significantly reduced by increasing the shear stress acting on the membrane surface. However, the SEM analysis illustrates that the particle packing structure becomes more compact as the air-bubble velocity increases. This results in a slightly higher average specific cake filtration resistance under higher air-bubble velocity. Consequently, a minimum flux occurs at the critical shear stress, e.g., τ w = 1.1 N/m 2 in this study, when these effects are both taken into consideration. As the shear stress increases by increasing the suspension or gas-bubble velocity, the filtration flux decreases in the low shear stress region but, on the contrary, quickly increases in the high shear stress region. Furthermore, a force balance model is derived for understanding the particle deposition on the membrane surface. The relationship among filtration flux, shear stress and overall filtration resistance is obtained and verified by experimental data.

Modeling of Particle Fouling and Membrane Blocking in Submerged Membrane Filtration

The models of particle fouling and membrane blocking in a submerged membrane filtration are developed in this study. The effects of operating conditions, such as aeration intensity (air flow rate) and filtration pressure, on the filtration flux, membrane blocking, and cake formation are discussed thoroughly. The experimental results show that the filtration resistances due to cake formation and membrane blocking play significant roles in determining the overall filtration resistance, but the latter one is more dominant. An increase in aeration intensity leads the filtration flux to increase due to the reduction of cake formation on the membrane surface. However, a higher filtration pressure causes more severe membrane internal blocking and then to lower filtration flux. The cake properties and the filtration resistance due to membrane blocking are analyzed and can be regressed to empirical functions of filtration pressure. A force balance model for particle deposition on the membrane surface is also derived. In order to estimate the shear stress acting on the membrane surface, the diameter, shape, and rising velocity of air bubbles are analyzed based on hydrodynamics. Once the model coefficients are obtained, the pseudo‐steady filtration flux under various conditions can be estimated by the proposed model and the basic filtration equation. The calculated results agree fairly well with the available experimental data.

The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles. The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U - U mf . The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U - U mf , the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.

Rise velocity of an air bubble in porous media: Theoretical studies

The rise velocity of injected air phase from the injection point toward the vadose zone is a critical factor in in‐situ air sparging operations. It has been reported in the literature that air injected into saturated gravel rises as discrete air bubbles in bubbly flow of air phase. The objective of this study is to develop a quantitative technique to estimate the rise velocity of an air bubble in coarse porous media. The model is based on the macroscopic balance equation for forces acting on a bubble rising in a porous medium. The governing equation incorporates inertial force, added mass force, buoyant force, surface tension and drag force that results from the momentum transfer between the phases. The momentum transfer terms take into account the viscous as well as the kinetic energy losses at high velocities. Analytical solutions are obtained for steady, quasi‐steady, and accelerated bubble rise velocities. Results show that air bubbles moving up through a porous medium equilibrate after a short travel time and very short distances of rise. It is determined that the terminal rise velocity of a single air bubble in an otherwise water saturated porous medium cannot exceed 18.5 cm/s. The theoretical model results compared favorably with the experimental data reported in the literature. A dimensional analysis conducted to study the effect of individual forces indicates that the buoyant force is largely balanced by the drag force for bubbles with an equivalent radius of 0.2–0.5 cm. With increasing bubble radius, the dimensionless number representing the effect of the surface tension force decreases rapidly. Since the total inertial force is quite small, the accelerated bubble rise velocity can be approximated by the terminal velocity.

Shapes and Rising Velocities of Single Bubbles rising through an Inner Subchannel

Shapes and velocities of single air bubbles rising through stagnant and flowing waters in an inner subchannel are measured by making use of fluorocarbon tubes. It is confirmed that (1) bubble shapes and motions in the subchannel are by far different from those in simple geometry, and they depend on the ratio λ of the bubble diameter to the subchannel hydraulic diameter, (2) when λ>0.9, a part of a bubble intrudes into neighboring subchannels, and thereby a kind of void drift takes place even with a single bubble, (3) the terminal velocity VT of a small bubble (λ<0.9) is accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of VT for cell-Taylor bubbles (λ>0.9) is presented, and (5) the rising velocity VB in laminar and turbulent flow conditions are well evaluated by substituting the proposed VT models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

Shapes and Rising Velocities of Single Bubbles rising through an Inner Subchannel

Shapes and velocities of single air bubbles rising through stagnant and flowing waters in an inner subchannel are measured by making use of fluorocarbon tubes. It is confirmed that (1) bubble shapes and motions in the subchannel are by far different from those in simple geometry, and they depend on the ratio λ of the bubble diameter to the subchannel hydraulic diameter, (2) when λ>0.9, a part of a bubble intrudes into neighboring subchannels, and thereby a kind of void drift takes place even with a single bubble, (3) the terminal velocity VT of a small bubble (λ<0.9) is accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of VT for cell-Taylor bubbles (λ>0.9) is presented, and (5) the rising velocity VB in laminar and turbulent flow conditions are well evaluated by substituting the proposed VT models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

Air bubble-induced detachment of polystyrene particles with different sizes from collector surfaces in a parallel plate flow chamber

Particle size was found to be an important factor in air bubble-induced detachment of colloidal particles from collector surfaces in a parallel plate flow chamber and generally polystyrene particles with a diameter of 806 nm detached less than particles with a diameter of 1400 nm. Particle detachment increased linearly with decreasing air bubble velocity and increasing liquid–air interfacial tension, regardless of particle diameter. The influence of air bubble velocity and liquid–air interfacial tension upon particle detachment was stronger on a hydrophilic collector surface than on a hydrophobic collector surface, whereas detachment was also less on positively-charged collector surfaces. In general, polystyrene particles with a diameter of 806 nm were less sensitive to the air bubble velocity than particles with a diameter of 1400 nm. However the larger particles were less sensitive to variations in liquid–air interfacial tension than the smaller ones. Hence, it is proposed that for larger particles, the liquid film in between the air bubble and the particle takes more time to thin down and form the three-phase contact, necessary to induce a detachment force.