Bubble Clusters Research Articles

Ganglia dynamics during imbibition and drainage processes in nanoporous systems

We study the gas–water transient imbibition and drainage processes in two-dimensional nanoporous media using our recently developed lattice Boltzmann model. To describe the microscopic molecular interactions, the model employs a pseudopotential that correlates the local density and interaction strength to perform simulation at a mesoscopic scale. The primary interest is ganglia dynamics in the nanoporous media affected by fluid and geometrical properties of the porous structure. We performed sensitivity analyses on the fluid and rock characteristics such as the Euler number, gas–water interfacial area, water film area, capillary pressure, pore size distribution, specific surface area, and wettability. The simulation results revealed the fingering nature of the nonwetting phase. In the imbibition process, the flow pathway of water results in isolated and trapped gas bubble clusters because of the strong attraction between water and solid surfaces. In the drainage process, the pressure difference between the gas phase and the water phase depends on both the capillary pressure and the disjoining pressure due to the presence of water film. Pore topography and specific surface area control the continuity of the fluid phases in the imbibition process. In nonwet systems, the water phase starts fingering in the nanoporous system. The present work elucidates the microscopic ganglia dynamics of gas–water two-phase flow in nanoporous media. The microscopic scale details will help establish the macroscopic flow equation to accurately predict two-phase flow in shale gas, tight oil, and caprock seals.

Numerical simulation of multi-size bubbly flow in a continuous casting mold using an inhomogeneous multiple size group model

Multi-size bubbly flow in a one-quarter-scale water model of a continuous casting mold was simulated using the inhomogeneous multiple size group (IMUSIG) model. The Sauter mean bubble diameter, gas volume fraction, interfacial area density, turbulence kinetic energy, and velocity field predicted by the IMUSIG and multiple size group (MUSIG) models were compared. The IMUSIG model separated the motions of the small and large bubbles on the basis of two velocity groups, and both models adopted 15 size groups. The results showed that the IMUSIG model better reflected the bubble size distribution near the nozzle. Small bubbles were further transported by the jet flow, which enlarged the bubble cluster range and decreased the gas volume fraction and interfacial area density in the upper part of the submerged entry nozzle, likely diminishing the coalescence rate in this region; consequently, the overestimation of Sauter mean bubble diameter outside the nozzle was corrected. The liquid-phase flow fields predicted by the IMUSIG and MUSIG models were very similar, and the IMUSIG-predicted magnitudes of velocity and turbulence kinetic energy were only slightly lower than the MUSIG predictions; this slight difference is attributable to the expanded bubble cluster range and higher interfacial area density, which weakened the upper recirculation in the IMUSIG model.

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

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

An Estimation of the Backscattering Strength of Artificial Bubbles Using an Acoustic Doppler Current Profiler.

Acoustic Doppler current profilers (ADCPs) were developed to acquire water current velocities, as well as depth-dependent echo intensities. As the backscattering strength of an underwater object can be estimated from the measured echo intensity, the ADCP can be used to estimate plankton populations and distributions. In this study, the backscattering strength of bubble clusters in a water tank was estimated using the commercial ADCP as a proof-of-concept. Specifically, the temporal variations in the backscattering strength and the duration of bubble existence were quantitatively evaluated. Additionally, the PDSL (population density spectrum level) and VF (void fraction) of the artificial bubbles were characterized based on the obtained distribution characteristics using a PDPA (phase Doppler particle analyzer).

Fabrication of a Tool Electrode with Hydrophobic Features and Its Stray-Corrosion Suppression Performance for Micro-electrochemical Machining.

Micro-electrochemical machining (micro-ECM) can machine microstructures with excellent surface integrity on difficult-to-cut alloys. During micro-ECM, stray corrosion results in tapered sidewalls of machined structures. To suppress the stray corrosion, a novel tool electrode with the sidewall insulation of a gas film is proposed by fabricating the metal tool sidewall into a hydrophobic surface. The sidewall surface is designed to be characterized with spherical array cavities (radius of 500 nm) acting as the hydrophobic features, enhancing the gas-shielding effect of the gas film under electrolysis. The fabrication process of the hydrophobic sidewall is described in detail, including the self-assembly procedure of a monolayer template of Φ1μm polystyrene microspheres, the copper-electroforming procedure for filling the microspheres' gaps, and the removal procedure for forming spherical array cavities. The fabricated tool electrode (Φ500μm) obtains the hydrophobic features of a contact angle of up to 138°. As a result, bubbles generated on the tool surface can form an air-electrolyte interface instead of a dispersed bubble cluster. Micro-ECM experiments of microstructures verify that the novel tool electrode can improve machining accuracy by suppressing sidewall stray corrosion.

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.

Bubble clustering in cosmological first order phase transitions

False vacuum decay in quantum mechanical first order phase transitions is a phenomenon with wide implications in cosmology and presents interesting theoretical challenges. In the standard approach, it is assumed that false vacuum decay proceeds through the formation of bubbles that nucleate at random positions in spacetime and subsequently expand. In this paper, we investigate the presence of correlations between bubble nucleation sites using a recently proposed semiclassical stochastic description of vacuum decay. This procedure samples vacuum fluctuations, which are then evolved using classical lattice simulations. We compute the two-point function for bubble nucleation sites from an ensemble of simulations, demonstrating that nucleation sites cluster in a way that is qualitatively similar to peaks in random Gaussian fields. We qualitatively assess the phenomenological implications of bubble clustering in early Universe phase transitions, which include features in the power spectrum of stochastic gravitational waves and an enhancement or suppression of the probability of observing bubble collisions in the eternal inflation scenario.

Observation of the bubble-like morphology of atmospheric air plasma in two-phase flow of vaporizing water

The splitting and/or coalescing branched streamer network patterns are the most common settings of the atmospheric plasma. In this work, it is reported that a volumetric bubble cluster pattern can be realized in the interaction process of an atmospheric pressure diffuse plasma with a two-phase jet flow of superheated water. A new method using electric heating of a metallic tube filled with water is proposed to generate such a kind of vaporizing fluid. The plasma characteristics during the hydrodynamic process have been investigated by means of the high-speed Mach–Zehnder laser interferometry. The crucial feature of the morphology of plasmas, marked by the shifted fringes, has been enveloped with high contrast bright boundaries in both schlieren and shadowgraph photography, suggesting a critical phenomenon may exist in the heat and mass transfer process. The formation of the bubble-like morphology has induced higher electron density, and it is also found that the increase in the temperature during the jet flow and the increase in the discharge current are accompanied. The methodology exhibits the potential of applications relevant in the context of atmospheric diffuse plasma sources of high electron density and tunable chemistry of the neutral species.

The influence of pressure on the acoustic cavitation in saturated CO2-expanded N, N-dimethylformamide

CO2-expanded organic solvent is a kind of important fluid medium and has broad applications in chemical industry, environmental protection and other fields. Ultrasonic cavitation in gas expanded liquids (GXLs) is conducive to enhancing mass transfer and producing many exciting phenomena. In this paper, the ultrasonic cavitations and streaming in the saturated CO2-expanded liquid N, N-dimethylformamide (DMF) at 4.2 MPa and 5.2 MPa are observed by a high-speed camera. The cavitation intensity and time trace of pressure pulses are recorded using a PZT hydrophone. The influences of gas–liquid equilibrium pressure and ultrasonic power on the cluster dynamics of transient and stable cavitation are examined. The excess molar enthalpies required for CO2 dissociation from DMF are calculated by Peng-Robinson equations of state and the change of surface free energy of CO2-expanded DMF is predicted. The results show that the excess enthalpy of the mixture is one of the key factors to control ultrasonic cavitation at high pressurized conditions, while the surface tension is the key factor for low pressure. As the increase of applied ultrasonic power, the formation and collapsing frequency of bubble clusters increases, and the amplitude and cyclic frequency of pressure pulse are enhanced. The transient cavitation intensity increases as it reaches a maximum value at a certain ultrasonic power and then decreases. The change trends of stable cavitation intensity under different pressures are basically same. It can be concluded from the evidence that ultrasonic cavitation in CO2-expanded DMF is affected by the combined effect of compression and substitution: compression depresses the nucleation and growth of bubbles, while the high solubility of CO2 in DMF is conducive to the generation of bubbles in cavitation.

Numerical investigation on the coupled mechanisms of bubble breakup in a venturi-type bubble generator

ABSTRACT The bubble breakup mechanism in a venturi-type bubble generator is a hot research topic, and the bubble breakup mechanisms induced by the axial regional flow field characteristics has long been studied and debated. A coupled volume of fluid and large eddy simulation numerical simulation method was conducted within the OpenFOAM® framework to track the two-phase interfaces and transient turbulence characteristics of the flow field in a venturi-type bubble generator. A turbulence inlet database was established to ensure the reliability of the numerical solution and the economy of the calculation. The results indicated that the bubble breakup induced by the axial regional flow field characteristics of the divergent section can be divided into a steady-state disturbed breakup pattern and an unsteady-state disturbed breakup pattern macroscopically, the latter playing a dominant role in generating fine bubbles. Furthermore, a series of typical bubble breakup processes were captured via a Lagrangian approach that showed that interfacial instability played an important role at each stage of bubble transport: Kelvin–Helmholtz instability caused bubbles to detach from the gas film during the steady-state disturbed breakup pattern, and Rayleigh–Taylor instability induced the deformation of detached bubbles in the unsteady-state disturbed region. Additionally, viscous shear force strengthened the interfacial instability in the binary breakup process, and turbulent fluctuations and collisions contributed to the shattering of defective bubbles. Moreover, a shearing-off process tended to occur in regions with small vorticity fluctuations, therefore playing a small role in creating large clusters of bubbles.

The effect of the cavity formation on the energy consumption characteristics of the agitated gas–liquid bioreactor

Detailed power consumption characteristics from experiments and simulations of gas–liquid stirred tanks with shear-thinning power-law liquids are presented. The motion of bubbles was investigated based on the Euler–Euler approach with a bubble cluster concept. The drag coefficient of bubbles with a constant bubble diameter was modeled as the Schiller and Naumann function. The predicted power consumption of a gas–liquid mixture shows reasonable agreement with experimental data with a maximum deviation of 8.9%. For the studied aerated systems, a qualitatively new power reduction correction equation was derived and demonstrated a reasonable agreement with experimental results compared with the literature-reported equations. The effect of the concentrations and gas flow rates on power consumption was presented. It was found that the power consumption of different mixtures is related to the change in the critical generalized Reynolds number, which was Reg = 440, 230, 100, 30, and 25 for 0.2%, 0.4%, 0.62%, 0.85%, and 1.25%, respectively.

Propagation of a Pressure Wave in a Channel Filled with Liquid Containing a Toroidal Bubble Cluster

Propagation of a Pressure Wave in a Channel Filled with Liquid Containing a Toroidal Bubble Cluster

Lattice Boltzmann method based simulation of two dimensional bubble group ripening process

Ostwald ripening refers to a process of a particle/droplet/bubble system under local thermal equilibrium state adjusting the size distribution spontaneously to reduce the total surface energy. A lattice Boltzmann approach is used to simulate the ripening process of a two dimensional vapor bubble cluster dominated by phase transition kinetics. By comparing the numerical results with the theoretical prediction derived in two-dimensional space, it is shown that the lattice Boltzmann method is accurate in the simulations. The results also indicate that the mass transfer in liquid phase is driven by hydrodynamic pressure distribution and the hydrodynamic collapse of the bubbles influences the size distribution function in a small size region. The influence of the parameters in the equation of state of the material is studied further. A positive relation between phase transition speed and specific internal energy is proposed, which enhances the thermal fundamental of phase transition.

Lattice Boltzmann Simulation on Two Dimensional Vapour Bubble Ripening

Ostwald ripening refers to a process of a particle/droplet/bubble system under thermal equilibrium state adjusting the size distribution spontaneously to reduce the total surface energy. With knowing lack in numerical studies, a lattice Boltzmann approach is used to simulate the ripening process of two dimensional vapour bubble cluster dominated by phase transition kinetics. By comparing the numerical results with the theoretical prediction derived in two dimensional space, it is shown that the lattice Boltzmann method is accurate in the simulations. The results also indicate that the mass transfer in liquid phase is driven by hydrodynamic pressure distribution and the hydrodynamic collapse of bubbles influences the size distribution function in small size region. The impact of the parameters of the equation of state of the material is studied further. A positive relation between phase transition speed and specific internal energy is proposed, which enhances the thermal fundamental of phase transition.

Flow aeration by an offset aerator: Air entrainment, bubbly flow turbulence, and adaptive-window cross-correlation processing

Chute aerators are used to induce flow aeration to prevent cavitation damages to flood discharge structures. The aerated chute flows are traditionally studied in terms of the air concentration distributions and air entrainment rate, whereas further detailed characterization of the bubbly flow associated with the turbulence modification has been missing. Herein the air–water flow turbulence was studied experimentally for a simple offset aerator model in a sloping chute. Advanced interfacial turbulence analyses were applied to the air–water flow measurement results using a dual-tip phase-detection probe. The new adaptive-window cross-correlation approach was first applied to the turbulence intensity estimate in both regions upstream and downstream the impact point. The overall spatial distributions of basic air–water flow properties agreed favorably with literature, and highlighted different patterns of air–water flow development in different sections of the flow. The turbulence level of the bubbly flow, and the bubble-turbulence interactions, were quantitatively described with the interfacial turbulence intensity, correlation turbulent length scale, and some measures of the bubble clustering behaviors. The interfacial turbulence intensity was obtained in the same order of magnitude as the common water-phase turbulence intensity observations, except for the impact zone. The results revealed the shear flow development and turbulence dissipation in the bottom layer of the aerated chute flow, which affected the local average bubble counts, the relative motions of neighboring bubbles, the diffusive transport of bubbly eddy structures, and consequently the near-wall bubble distributions critical for the protection of chute surface from cavitation erosion.

Dynamics of an acoustically driven cavitation bubble cluster in the vicinity of a solid surface

The dynamics of a cavitation bubble cluster under the influence of an acoustic field is a complex multiphase system that can be observed in acoustic cavitation. In the present study, a three-dimensional computational technique based on the multiphase lattice Boltzmann method (LBM) with multiple relaxation time (MRT) is adopted to investigate the acoustically driven cavitation bubble cluster dynamics near a solid wall at different wetting conditions. Herein, the Peng–Robinson–Stryjek–Vera equation of state with an acentric factor is incorporated in the LBM to accurately impose the physical properties of actual fluids. The validity and capability of the adopted MRT-LBM are confirmed by the excellent agreement of the present results compared to the computed data based on the Rayleigh–Plesset equation for a heterogeneous cavitation phenomenon. The obtained results for the acoustically driven cavitation bubble cluster dynamics demonstrate that the shielding effect of top bubbles prevents the pressure pulse from reaching the lower bubbles. Therefore, the cluster core and the bubbles near the solid surface are more affected by the destruction of the upper layer bubbles than the acoustic field. Also, it is found that the wettability of the solid wall significantly affects the irradiated impulsive pressure waves around the cluster. To justify this result from the physical point of view, the magnitude of the primary and secondary Bjerknes forces is measured and, accordingly, the growth and collapse of bubbles in the cluster under the influence of the acoustic field are discussed in detail.

Dynamics of evaporating thin liquid films in microchannel

Aiming at flow boiling in a micro circular tube with the inner diameter of 0.5 mm, experimental investigation on the flow and heat transfer characteristics of the thin liquid film has been carried out. Water was used as working fluid, and it was heated by a transparent ITO heater. The thickness of the thin liquid film was measured by a laser focus displacement meter, and the flow boiling visualization was performed simultaneously by using a high-speed camera. The result of synchronized flow visualization and flow boiling measurement showed that the temperature and pressure fluctuation was strictly consistent with the bubble cluster generation. The mechanism of flow boiling instability was analyzed. The void fraction based characteristics such as the bubble and liquid slug duration, and pair generation period were obtained from the dynamic distribution diagram of liquid film thickness. The experimental data of void fraction showed a good agreement with the homogeneous model. The temporary thin liquid film variation provides an opportunity to probing the physics of bubble dynamics and instability during flow boiling.

Effect of ruptured cavitated bubble cluster on the extent of the cell deformation by ultrasound

In this paper, the bubble-cell model is presented. The effects of the spacing between the bubble population and the cell, the radius of the bubble and the bubble medium on the degree of cell deformation were investigated by solving the Helmholtz equation and the equilibrium of motion equation using COMSOL Multiphysis@ software. The ultrasonic transducer is applied in a round bottom flask with the bubble-cell model on the side of the ultrasonic transducer. When the distance between the bubble cluster and the cell gradually increases, the extent of deformation of the cell is reflected as first increasing and then decreasing, reaching the maximum deformation at D = 2. When the radius of the bubble is changed, there is a “constant frequency” at low frequency ultrasound in any distance case, at which the cell deformation will be violent. However, when the bubble medium is changed, there is no significant change in the degree of deformation of the cells. In other words, changes in the structure of the bubble-cell model affect the degree of cell deformation, but without structural changes, the degree of cell deformation changes very little.

Application of macrolayer dryout model for the critical current density of water electrolysis

Electrochemical hydrogen evolving system such as the water electrolysis has the upper operation limit called critical current density (CCD) when the hydrogen film was formed on the surface as the vapor film in the critical heat flux (CHF) condition. This paper tried to apply one of the well-accepted CHF models, macrolayer dryout model to elucidate CCD phenomenon, which is a novel attempt to predict the CCD using the CHF model based upon the hydrodynamic nature of the phenomena. Hydrogen bubble behaviors at around the CCD were observed and compared with those of the CHF. At high nucleate bubble regime, presence of the macrolayer in the hydrogen evolving system was confirmed. Above this layer, hydrogen bubble cluster is generated and detached from the surface. At the CCD point, the nucleate bubbles adjacent to the surface started to coalescence and subsequently formed a hydrogen film. Based on the photographic analyses, the thickness of the macrolayer and the hovering time of the hydrogen bubble cluster were estimated. Application of these parameters to the existing macrolayer dryout model predicts the CCD well within 4.37%. It is concluded that the hydrodynamics of the CCD and the CHF are quite analogous.

Sign-changing blow-up for the Moser–Trudinger equation

Given a sufficiently symmetric domain Ω⋐R2, for any k∈N∖{0} and β>4πk we construct blowing-up solutions (uε)⊂H01(Ω) to the Moser–Trudinger equation such that as ε↓0, we have ‖∇uε‖L22→β, uε⇀u0 in H01 where u0 is a sign-changing solution of the Moser–Trudinger equation and uε develops k positive spherical bubbles, all concentrating at 0∈Ω. These 3 features (lack of quantization, non-zero weak limit and bubble clustering) stand in sharp contrast to the positive case (uε>0) studied by the second author and Druet [8].