Bubble Velocity Research Articles

Dynamics of double bubbles under the driving of burst ultrasound

This paper investigates the pulsations and translation of bubbles in a double-bubble system driven by burst ultrasound. Results illustrate that for two identical bubbles, decreasing the frequency of burst or increasing its amplitude can enhance the pulsations and improve the translation velocities of bubbles. In a certain scope, large bubble brings about fast translation velocity, but the velocity will fall down for too large bubble, such as the bubble with ambient radius over about its resonance radius. When the ambient radii of two bubbles are different, translation of the large bubble is smaller than that of the small bubble. In addition, the effect of initial distance between bubbles is described as well. If burst serials are used, shortening the time interval between each burst and improving the acoustic amplitude of bursts are beneficial for the translations of bubbles.

Understanding the effect of surfactants on two-phase flow using computer vision

The effect of surfactants on vertical gas-liquid flow is experimentally investigated in a 12.7 mm diameter tube at conditions relevant to an ammonia-water bubble absorber. The characteristics of two-phase flow are studied using an air-water mixture, both with and without the addition of 1-octanol as the surfactant. High-speed videography is used to study the flow patterns and quantify interfacial areas and bubble velocities. Novel computer vision-based methods are used to analyze and quantify these flow parameters. The addition of 1-octanol results in enhancement in interfacial area due to the prevention of bubble coalescence leading to many small diameter bubbles. Measured values of interfacial area are compared with predictions from correlations in the literature, and agreement and differences are interpreted and discussed. The bubble velocity is measured by object tracking using the optical flow method. Surfactants lead to a decrease in bubble velocity and increase in the residence time. These are surmised to be due to the shear stresses caused by the non-uniform concentration distribution of surfactant along the bubble surface. Overall, the addition of surfactants can lead to appreciable enhancement in heat and mass transfer rates due to their effect on interfacial areas and residence times.

Coronary artery cavitation as a trigger for atherosclerotic plaque progression: a simplified numerical and computational fluid dynamic demonstration.

Coronary cavitation is supposed to be generated by both concentric and eccentric coronary artery stenosis which propagates downstream the vessel, creating microbubbles which exploded when the fluid pressure was lower than the vapor pressure at a local thermodynamic state. To assess, using numerical and computational fluid dynamic analysis (CFD), the potential of cavitation to both induce damage to coronary artery endothelium and to promote atherosclerotic plaque progression. We retrospectively reviewed the data 12 consecutive patients evaluated between 1st January 2013 and 1st January 2014 with an isolated hemodynamically significant Left Main (LM) disease. The patient specific geometries have been reconstructed. Bubble velocity has been calculated in accordance with Newton's second law. Both the forces arising from the bubbles' interaction with the continuous phase and impact with the endothelium have been evaluated. The impact of turbulence on the motion of bubbles have been modelled with a dispersion model. Among the 12 patients retrospectively analysed [8 males, mean age 68.2 ± 12.8 years old], the mean LM stenosis was 72.3 ± 3.6%. As expected, in all subjects, LM stenoses induced cavitation which propagates downstream the vessel creating microbubbles. The higher concentration of vapor region was detected before the carina (within 0.8 to 1.3 cm from the stenosis). Due to the pressure gradient generated by the stenosis, formation of a re-entry jet which penetrates each bubble generated a shock wave. Before the carina, the mean bubbles radius observed was 4.2 ± 1.4 μm, which generated a mean peak pressure of 3.9 ± 0.5 MPa when they explode. The cavitation phenomenon is effectively generated in a model of LM bifurcation and instantaneous pressure-peaks due to collapses of vapor bubbles resulted in a measurable dynamic load on vessel wall potentially able to induce endothelial damage.

The rise of bubbles in shear thinning viscoelastic fluids

Bubbles in a liquid rise under gravity and separate to the top. Bubbly liquids exist commonly in nature and play a significant role in energy-conversion, oil and chemical industries. Therefore, understanding how bubbles rise is of great importance. Rheological properties of the fluid have a strong impact on single bubble rise and have been shown to change collective bubble rise at low gas volume fractions significantly. We expect that a viscoelastic fluid can strongly modify the rise of bubbles in more concentrated suspensions.We generate bubbly liquids up to gas fractions of 30 %. We measure the bubble size and the rise velocity in micellar solutions made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), a common system to create shear-thinning solutions.We show that when the NaSal concentration is small and the solutions are Newtonian, the bubble rise velocity decreases with increasing volume fraction of bubbles and the relationship between the two follows the Richardson-Zaki prediction. For the shear thinning viscoelastic solutions, the Richardson-Zaki relation no longer applies. Bubble clustering leads to faster rise velocities and a weaker dependence on the bubble volume fraction. At the largest concentration two rise regimes are observed. A fast one similar to that in the other shear thinning samples, followed by a very slow bubble rise. The slow rise velocity is attributed to the smallest bubbles rising so slowly, that at the shear rates around them, the fluid behaves as a Newtonian fluid. Therefore, bubble rise becomes again comparable to Stokes expectations. We also show that the peculiar dependence of the rise velocity with volume fraction of bubbles in the shear thinning viscoelastic solutions can have important implications in flotation as the area flux changes strongly with bubble volume fraction.

On the concentration models in electrical capacitance tomography for gas-fluidized bed measurements

Electrical capacitance tomography (ECT) has been widely applied in gas-fluidized bed measurements, which however is constrained by the sensor designs, image reconstruction algorithms and concentration models owing to the “soft-field” nature. Among the three factors, the concentration models that map the permittivity to solid concentration distribution are less understood. In this paper we present an experimental investigation on the effects of concentration models for the ECT measurements of gas-fluidized beds containing Geldart’s group A particles. The results show that the bubble velocity, bubbling frequency and fluidization regime transition velocity are intrinsic whatever the concentration model employed. However, the time-averaged solid concentrations measured by ECT with the conventional concentration models are usually higher than the corresponding results obtained by the bed expansion experiments in homogeneous fluidization. It has been further found that the concentration models can significantly influence the threshold selection in identifying the bubbles and surrounding emulsion phase in gas-fluidized beds. Thus, a new concentration model given in the formula of a power function (ϕ=Gα) has been proposed, with the optimal exponent α obtained via the least-square fitting of solid concentration in bed expansion during homogeneous fluidization. The model is robust and the sensitivity analysis shows that the variation of time-averaged solid concentration is negligible (≤ 3%) suppose that α changes by ± 20%.

Bubbles rising through a layer of Carbopol capped with water

Air bubbles are injected into a Carbopol column capped with water to mimic ebullition from lake sediments. The first bubble sets an invisible path as it rises through the Carbopol. Subsequent bubbles rise along this path and, unlike the first bubble, break up into two separate bubbles. The leading bubble rises through the water column, while the smaller breakup bubble separates and remains trapped within the Carbopol. A conduit that forms above this breakup bubble gradually fills with water from the upper layer. After a period of growth this conduit reaches an equilibrium depth, and then starts to shrink. Inside the conduit, the rising bubbles resemble Taylor bubbles, but have velocities approximately 5–7 times the maximum velocity of standard Taylor bubbles. The variation in rise velocity depends on the interaction of the rising bubble with previous breakup bubbles. In addition, rising bubbles expel water from the conduit and water flows back after bubbles escape. This process can enhance the transport of heat and contaminants to and from aquatic sediments during ebullition events. • A damaged pathway forms inside Carbopol due to the passage of bubbles. • Bubbles break up as they cross the Carbopol–water interface. • Upper portion of the damaged pathway evolves into a water-filled conduit. • Bubbles rise much faster inside the conduit than standard Taylor bubbles. • Water flows in and out of the conduit, enhancing mixing in aquatic sediments.

Numerical analysis of shock interaction with a spherical bubble

Two-dimensional and three-dimensional computational fluid dynamics studies of a spherical bubble impacted by a supersonic shock wave (Mach 1.25) have been performed to fully understand the complex process involved in shock–bubble interaction (SBI). The unsteady Reynolds-averaged Navier–Stokes computational approach with a coupled level set and volume of fluid method has been employed in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex ring agree very well with the relevant available experimental data. The predicted non-dimensional bubble and vortex velocities are also in much better agreement with the experiment data than values computed from a simple model of shock-induced Rayleigh–Taylor instability (the Richtmyer–Meshkov instability). Comprehensive flow visualization has been presented and analyzed to elucidate the SBI process from the beginning of bubble compression (continuous reflection and refraction of the acoustic wave fronts as well as the location of the incident, refracted and transmitted waves at the bubble compression stage) up to the formation of vortex rings as well as the production and distribution of vorticity. Furthermore, it is demonstrated that turbulence is generated with some small flow structures formed and more intensive mixing, i.e., turbulent mixing of helium with air starts to develop at the later stage of SBI.

Air‐Water Mixing in Vegetated Supercritical Flow: Effects of Vegetation Roughness and Water Temperature on Flow Self‐Aeration

AbstractVegetated flow is rarely investigated for supercritical flow regime, in which self‐aeration occurs and modifies the flow dynamics, energy dissipation, and mass transfer between the water body and ambient air environment. In this study, natural river water was used at two flow rates in combination with six bottom vegetation conditions to investigate the air‐water flow development in an 18‐m‐long sloping chute. The vegetation configurations included two artificial turfs with different grass lengths and three flexible plant covers with different canopy densities, in addition to a reference case of smooth solid boundary, providing flow resistance and turbulent disturbance with different orders of equivalent roughness heights. Physical measurements were focused on the air concentration, free‐surface splashing, turbulent velocity, bubble count frequency, and bubble size distributions from the upstream non‐aerated flow to the far‐field fully developed aeration region. Compared to the velocity developing to uniform equilibrium shortly downstream the inception of aeration, the flow self‐aeration reached air‐concentration equilibrium at further downstream positions, while the bubble count and air‐water interfacial area kept increasing. The vegetation roughness conditions affected the inception location, equivalent bubble rise velocity, superfacial splashing height, velocity gradient, and maximum bubble count near the free surface, whereas the equilibrium air concentration in the fully developed region depended only upon the slope. The vegetation type influenced the percentage of submillimetre bubbles. Little differences were shown in the air‐water flow characteristics by repeating the summer experiments in winter with a 25.5°C drop in water temperature, despite the variation in fluid viscosity thus the Reynolds number under identical discharge.

Effect of solid particles on the slug frequency, bubble velocity and bubble length of intermittent gas–liquid two-phase flows in horizontal pipelines

New experimental data are presented in this study for intermittent two and three-phase flows through horizontal pipelines involving air, water and polypropylene pellets of sizes ranging between 1 and 2 millimeters. Flow visualization and pressure measurements were performed and are explored in this work for intermittent flows. An automated image processing algorithm is developed to determine slug parameters of 59 flow conditions and compared with other measurements techniques. Correlations available in the literature for predicting slug frequency are tested with the new experimental data and curves of non-dimensional parameters are fitted to the data, the best fitting is obtained for the gas based Strouhal number against the input liquid or gas fraction. The influence of solid particles over the slug frequency is analyzed and reported. A decrease on the slug frequency is found when increasing solid concentrations. Finally, the influence of solid particles on the bubble length and velocity is investigated. A stronger influence of the gas flow rate on the velocity of the nose of the slug is observed for increasing solid concentration, together with more variability on the mean velocity. Also, longer bubbles are obtained with higher solid concentrations.

Bubble dynamics and mass transfer enhancement in split–and–recombine (SAR) microreactor with rapid chemical reaction

In this study, a split–and–recombine (SAR) microreactor which is composed of convergent–divergent arc channel and rectangular obstacles was designed, and its enhancement characteristics in gas–liquid rapid chemical absorption system were explored. In the SAR microreactor, the deformation, splitting, stretching and breakup of bubbles are monitored by a high–speed camera. Accordingly, the bubble velocity pulsation caused by SAR structure is analyzed. The squeezing mechanism for bubble breakup located on the front surface of the obstacle is revealed. Two breakup patterns, namely complete and partial breakup, are observed, and the mass transfer performance is highly related to them. The mass transfer enhancement factor (Eka) of SAR structure increases with gas/liquid flow rate in complete breakup pattern while decreases in partial breakup pattern. The increase of gas–liquid flow rates and the number of SAR units, or slower reaction rate strengthens the velocity fluctuation and bubble breakup, resulting in an increase in Eka, with a maximum value of 2.74. Overall, the SAR microreactor performs comparable to Corning Advanced Flow Reactor in term of mass transfer energy efficiency. This work guides the application of SAR microreactor in rapid gas–liquid chemical reaction system.

Drag coefficient of bubbles sliding beneath a towed model ship with variable tilt angles

The motion of bubbles sliding beneath a tilted model ship was experimentally studied to explore the fundamentals of a hydrodynamic force acting on bubbles inside the liquid-phase boundary layer. The contribution of bubble buoyancy to bubble motion in the main direction of flow was obtained when the angle of the ship was changed positively and negatively. The boundary layer profiles were measured through particle tracking velocimetry. The migration velocity of bubbles was measured through a particle tracking velocimetry type of image processing for various bubble sizes ranging from 2 to 20 mm in the streamwise chord length. The image processing revealed that the velocity of large bubbles decreased under the negative slope condition and increased under the positive slope condition because of the buoyancy effect. The velocity of small bubbles, however, was mainly affected by liquid flow due to the weakening of the buoyancy effect. From measured advection velocity of the bubble, the drag coefficient was estimated using the force balance equation. Finally, a constitutive equation was proposed to predict the advection velocity of the bubbles under the ship bottom surface for both positive and negative conditions.

Gas–Liquid Slug Flow Studies in Microreactors: Effect of Nanoparticle Addition on Flow Pattern and Pressure Drop

Colloidal suspensions of nanoparticles (e.g., metals and oxides) have been considered as a promising working fluid in microreactors for achieving significant process intensification. Existing examples include their uses in microflow as catalysts for enhancing the reaction efficiency, or as additives to mix with the base fluid (i.e., to form the so-called nanofluids) for heat/mass transfer intensification. Thus, hydrodynamic characterization of such suspension flow in microreactors is of high importance for a rational design and operation of the system. In this work, experiments have been conducted to investigate the flow pattern and pressure drop characteristics under slug flow between N2 gas and colloidal suspensions in the presence of TiO2 or Al2O3 nanoparticles through polytetrafluoroethylene (PTFE) capillary microreactors. The base fluid consisted of water or its mixture with ethylene glycol. The slug flow pattern with nanoparticle addition was characterized by the presence of a lubricating liquid film around N2 bubbles, in contrast to the absence of liquid film in the case of N2-water slug flow. This shows that the addition of nanoparticles has changed the wall wetting property to be more hydrophilic. Furthermore, the measured pressure drop under N2-nanoparticle suspension slug flow is well described by the model of Kreutzer et al. (AIChE J 51(9):2428–2440, 2005) at the mixture Reynolds numbers ca. above 100 and is better predicted by the model of Warnier et al. (Microfluidics and Nanofluidics 8(1):33–45, 2010) at lower Reynolds numbers given a better consideration of the effect of film thickness and bubble velocity under such conditions in the latter model. Therefore, the employed nanoparticle suspension can be considered as a stable and pseudo single phase with proper fluid properties (e.g., viscosity and density) when it comes to the pressure drop estimation.

The shape and drag coefficient for a single bubble rising in stagnant shear-thinning viscoelastic liquids

The motion of a single bubble rising in stagnant shear-thinning viscoelastic liquids was investigated using a high-speed camera. The recorded images were analyzed using MATLAB with a self-written code in order to obtain the bubble volume and velocity. Using the experimental observations, a simple shape map was proposed based on the Reynolds number and the Deborah number. The bubble velocity was also investigated, and the phenomenon of velocity jump was observed in the Polyacrylamide (PAA) solutions. Furthermore, an empirical correlation was proposed to predict the drag coefficient of bubbles rising instagnant shear-thinning viscoelasticliquids. The predicted results of the correlation showed good conformity with the experimental values corresponding to the PAA solutions of different concentrations and 95 wt% glycerin solution.

Experimental study of interfacial area of bubble plume based on bubble tracking by wire-mesh sensor

The fundamental information of bubble plume such as interfacial area, three-dimensional velocity, and bubble diameter are necessary to develop a kinematic pool scrubbing model for uncertainty reduction of severe accident analysis. In this study, an algorithm was introduced to polygonize the cross-sectional void fraction of the wire-mesh sensor and reconstruct it in 3D to track the individual bubbles detected. Based on the result, the interfacial area and three-dimensional velocity of individual bubbles in a bubble plume were obtained. The results show that the interfacial area from MELCOR model tends to overestimate the analyzed results due to remaining of large bubbles in the downstream. In addition, the effect of subcooling temperature and steam fraction on bubble formation was evaluated by applying the developed method into condensable gas experiments. The interfacial area increased significantly under high subcooling temperature and steam fraction due to atomization of bubbles caused by steam condensation.

AN INVESTIGATION OF THE EFFECT OF INITIAL BUBBLE DIAMETER ON THE BUBBLE TRAJECTORY IN THE FLOTATION COLUMN CELL USING CFD SIMULATION

The effect of initial bubble diameter on the bubble motion pattern in a flotation column has been studied by the twophase computational fluid dynamics (CFD) method. The two-phase simulations have been done using the volume of a fluid (VOF) model in ANSYS® Fluent® software. The computational field was a square cross-section column with a width of 0.1 m and a height of 1 m into which air was interred as a single bubble from the lower part of the column by an internal sparger. An experimental test has been also performed and the simulated results have been validated using the values obtained for the bubble rise velocity. A comparison of the simulation and the experimental results has confirmed that CFD can predict the bubble rise velocity profile and its value in the flotation column less than 5% relative to the experimental values. Then the simulations have been repeated with a 20% decrease and increase in the initial bubble diameter to investigate the effect of bubble diameter on the bubble flow pattern. The investigations have shown that as the bubble diameter increases, the velocity decreases and the bubble rises in a more zigzag direction as a result of two counter-rotating trailing vortices behind the bubble increasing.

Velocity Profiling of a Gas–Solid Fluidized Bed Using Electrical Capacitance Volume Tomography

In this work, a method of producing velocity profile maps from electrical capacitance volume tomography (ECVT) measurements by reconstructing displacement from measured changes in capacitance is developed and applied to fluidized bed systems. The mapping of the reconstruction leverages the gradient of the sensitivity distribution of the ECVT sensor to circumvent the need for image cross-correlation techniques. Experimental data of both bubbling and slugging fluidized beds are collected in a cold flow model. Adaptation of the technique is discussed in detail and velocity profiles are obtained for a range of gas flow rates. The produced velocity maps are compared against the established methods of cross-correlation and against empirical correlations from literature and are found to agree well in tracking slug and bubble velocity. The exception is when the tracked object is large relative to the ECVT sensor dimensions, a scenario that can be avoided through proper sensor design. The quantities of average velocity, momentum, and solids and gas volume fraction are derived from the image and velocity profiles. The results demonstrate and extend the power of ECVT as a measurement tool for the study and monitoring of gas-solid fluidized beds by providing a computationally cheaper alternative to 3-D cross-correlation for deriving velocity profiles.

Experimental study of hydrodynamic parameters regarding on geyser boiling phenomenon in glass thermosyphon using wire-mesh sensor

The thermosyphon is a type of heat exchanger that has been widely used in many applications. The use of thermosyphons has been intensified in recent years, mainly in the manufacture of solar collectors and various industrial activities. A thermosyphon is a vertical sealed tube filled with a working fluid, consisting of, from bottom to top, by an evaporator, an adiabatic section, and a condenser. The study of geyser-boiling phenomena, which occurs inside the thermosyphon is of extreme importance, therefore the experimental analysis of the parameters related to the two-phase flow (liquid-steam), such as void fraction, bubble frequency, bubble velocity, and bubble length are necessary, since these parameters have a significant influence on heat transfer. In this work, a pair of wire mesh sensors was used, a relative innovative technology to obtain experimental values of the reported quantities for measuring these parameters of slug flow in thermosyphons. An experimental set-up is assembled and the sensors are coupled to the thermosyphon enabling the development of the experimental procedure. Here is presented an experimental study of a glass thermosyphon instrumented with two wire-mesh sensors, in which the aforementioned slug flow hydrodynamic parameters inherent to the geyser type boiling process are measured. It was measured successfully, as a function of the heat load (110, 120, 130, 140, and 150 W), the void fraction (instantly and average), liquid film thickness, translation velocity of the elongated bubbles, lengths of the bubbles, and the liquid slug (displaced by the bubble rise up). It was observed that the higher the heat load, the lower is the bubble translation velocity. For all heat loads, based on the measured length of liquid slug (consequent displacement of liquid volume), caused by bubbles rise from evaporator to condenser, it could be affirmed to some extent that both boiling regime (pool and film) exist in the evaporator. The measured average void fraction (80%) and liquid film thickness (around 2.5 mm) during the elongated bubble passages were approximately constant and independent of the heat load.

Lattice Boltzmann simulation of three-dimensional fluid interfacial instability coupled with surface tension

In this paper, the development of three-dimensional fluid interfacial Rayleigh-Taylor (RT) instability coupled with the surface tension was numerically studied using the mesoscopic lattice Boltzmann method. We mainly analyzed the influence of surface tension on fluid interfacial dynamics and spike/bubble late-time growth. The numerical experiments show that there exists the critical surface tension (<inline-formula><tex-math id="M3">\begin{document}$\sigma_{\rm{c}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20212061_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20212061_M3.png"/></alternatives></inline-formula>) in the three-dimensional RT instability, above which the RT phenomenon does not appear and below which it would take place. It is found that the critical surface tension increases with the fluid Atwood number and the corresponding numerical predictions show good agreements with those of the theoretical analysis <inline-formula><tex-math id="M4">\begin{document}${\sigma_{\rm{c}}}= {{({{\rho_{\rm{h}}}-{\rho_{\rm{l}}}})g}}/{{{k^2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20212061_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20212061_M4.png"/></alternatives></inline-formula>. In addition, we can find that increasing surface tension reduces the roll-up of the interface and the complexity of interfacial structure, also preventing the breakup of the interface into the individual droplets. The late-time dynamics of phase interface change from the asymmetric development to the symmetry with respect to the middle axis. When the surface tension is sufficiently low, the spike and bubble amplitudes almost no longer change with it, and further increasing the surface tension can slow down the growth of the spike and bubble amplitudes. Furthermore, we can observe that the development of the high-Reynolds-number RT instability under different surface tensions can also be divided into four distinct stages, including the linear growth, saturated velocity growth, reacceleration, and chaotic mixing. The spike and bubble grow with approximately constant velocities at the saturated stage and their asymptotic values are consistent with those of the modified potential flow theory. In the following, the spike and bubble driven by the increasing Kelvin-Helmholtz vortices are accelerated such that their evolutional velocities exceed the solutions of the potential flow model at the reacceleration stage. The reacceleration stage cannot last infinitely and the spike and bubble velocities at the late time fluctuate with time, implying that the growth of the RT instability enters into the chaotic mixing stage. By numerical analysis, we demonstrate that the three-dimensional RT instability at the chaotic mixing stage has a quadratic growth and also report that the spike and bubble growth rates decrease with the surface tension in general.

Dynamics of a gas bubble penetrating through porous media

Porous media have been widely used for liquid-gas separation, benefiting from the strong capillary force generated from the micro/nanoscale pores. Understanding the flow characteristics in pore scale is significant for the design of porous structure. In this study, a numerical model was established to investigate the dynamics of a bubble penetrating through porous media at the pore scale. The two-phase interface was captured using the diffuse interface method. The influence of pore shape, width, and height on the bubble deformation, velocity, and critical pressure was investigated. For the same pore size, the largest bubble centroid velocity and the highest critical pressure exist in the quadrilateral pores compared rather than in the circular or triangular pores. As the pore width decreases, both the average velocity of the bubble centroid and the critical pressure increase. However, the critical pressure is independent of the pore height. As the pore height increases, the average velocity of the bubble centroid increases. A new correlation of the critical pressure for bubble penetration has been proposed, which is a function of the shape factor, the pore width, and the bubble diameter. The findings of this work can contribute to improving the design of porous media for two-phase separation.

Multiple Bubble Dynamics and Velocity Selection in Laplacian Growth Without Surface Tension

Multiple Bubble Dynamics and Velocity Selection in Laplacian Growth Without Surface Tension