Bubble Mass Research Articles

Numerical simulation using the bubble size distribution model of cryogenic cavitation around an axial helical inducer

A mathematical model using a bubble size distribution was developed by the author for a cryogenic high-speed cavitating flow in turbomachinery. In this model, bubble growth/decay is solved for each class of the bubble mass (as an accurate expression of bubble size) when bubbles with various masses mix in the same spatially discretized calculation region. The bubble growth/decay calculations employ a combination of two rigorous methods: a Rayleigh–Plesset equation for the bubbles’ oscillation, and a heat conduction equation in a thermal boundary layer around a bubble to evaluate the mass rate of evaporation or condensation. This mathematical model was coded using the OpenFOAM C++ toolbox, and a numerical simulation was conducted for a 3D rotational inducer in a cryogenic fluid, the results of which showed that bubbles of a certain specified size were distributed at certain regions on the inducer. This study demonstrated that the mathematical model of cavitation exhibits potential to contribute to fidelity in simulation for turbomachinery considering a fluid with strong thermodynamic effect.

Ethylene/ethane absorption with AgNO3 solutions in ultrasonic microreactors

Ultrasound is an effective method to intensify gas-liquid processes as the oscillation energy is easily focused at the gas-liquid interface, which can induce strong interface oscillation and acoustic streaming. The combination of ultrasound and microreactor provides ideal control over the ultrasound field, making the energy efficiently utilized. In this study, characteristics of chemical absorption of C2H4 from the mixture of C2H6/C2H4 into AgNO3 solutions under Taylor flow are investigated in a high-power ultrasonic microreactor. The effect of ultrasound on the bubble size reduction, absorption and mass transfer coefficient is presented and discussed. It is demonstrated that very large mass transfer coefficient with kLa and kL in the range of 7–42 s−1 and 0.00169–0.0225 m s−1 respectively are obtained, presenting significant intensification compared to absorption without ultrasound in the same reactor.

Bubble size distribution and mass transfer on a three-phase electroflotation column

This work aims to experimentally study the bubble size distribution and the oxygen transfer on the electroflotation process. The distribution of bubbles was measured using a high-speed camera. The measurements were conducted in a three-phase electroflotaion column (water- gas-olive stone) equipped with insoluble electrodes, stainless steel as cathode and titanium, covered with ruthenium oxide, as anode. The volumetric mass transfer coefficient kla was determined for some operating parameters such as current density, solid concentrations and sizes. In order to calculate the global coefficient of mass transfer kl, the specific interfacial area, a, was determined. It was chiefly found that bubble size distribution depends on current density and solid concentration, and the wide range of bubble sizes was found to be affected by the phenomenon of break up and coalescence. kla tended to decrease with the increase of solid concentrations. kl exhibited the same behavior as the volumetric mass transfer coefficient. The experimental results were also fitted with the theoretical models, relating ‘kla’, ‘kl’ and ‘a’ with Reynolds number, Schmidt number and operating conditions.

On suitability of phase-field and algebraic volume-of-fluid OpenFOAM® solvers for gas–liquid microfluidic applications

Microfluidic devices often contain several phases. Their design can be supported by interface-resolving numerical simulations, requiring accurate methods and validated computer codes. Especially challenging are submillimetre air bubbles in water due to their large density contrast and dominance of surface tension. Here, we evaluate two numerical methods implemented in OpenFOAM®, namely the standard solver interFoam with an algebraic volume-of-fluid method relying on a sharp interface representation and phaseFieldFoam relying on the phase-field method with diffuse interface representation. For a circular bubble in static equilibrium, we explore the impacts of uniform grid resolution and bubble size on bubble shape, mass conservation, pressure jump and spurious currents. While the standard interFoam solver exhibits excellent mass conservation with errors below 0.1% on fine grids, it lacks the accuracy to predict reasonable physics for a bubble in microfluidic systems. At higher resolution, large spurious currents significantly displace and deform the bubble, which is oscillating with resolution dependent mode and frequency. Furthermore, the pressure jump is consistently underestimated by more than 10%. The solver phaseFieldFoam suffers from much larger mass losses of up to 2%, which decrease as the ratio between interface thickness and bubble diameter decreases provided the diffuse interface region is adequately resolved. Spurious currents are very low and the bubble remains circular preserving its initial position with an error in pressure jump below 1%.

Evaluation of oxygen transfer from bubble and free surface in bubble reactors using CFD

This research uses the newly-developed Computational Fluid Dynamics (CFD) model which employs free surface mass transfer and bubble mass transfer using the small eddy model (SEM), and investigates the effects of multiple factors, including SEM coefficient, airflow rate, and bubble diameter. The study reveals that (1) two-domain simulations supports experimental data better than one-domain simulations, (2) free surface mass transfer occupies a small portion of overall mass transfer, irrespective of the SEM coefficient and (3) flow rate and bubble size is not a critical factor on free surface mass transfer in low airflow rate condition. Thus, to enhance free surface mass transfer, high flow rate condition first of all should be applied. This research demonstrates a new approach for evaluating mass transfer using the CFD model, making it helpful for deriving optimal bubble reactor design and operation.

Mass transfer characteristics for CO2 absorption in nanoabsorbents

In this work, an improved VOF-PLIC model is developed to simulate 3D bubble behavior and to analyze the mass transfer characteristics for CO2 absorption in pure methanol and methanol with Al2O3 nanoparticles. The lower bound boundary value of CO2 volume fraction(αg) is redefined according to the influence of αg on bubble shape and diameter as well as the interfacial area between gas and liquid. In addition, a reasonable drag force is added into the momentum equation using UDF code based on the assumption that the drag force only exists in liquid phase. It is found that the level lines of bubbles with the αg value of 0.05 agree well with the real case, and the drag force has significant influence on the bubble behavior and mass transfer characteristics. The velocity contours and pressure gradients around the bubble are proposed to analyze the influence of drag force on the mass transfer characteristics. Also, the influence of the Al2O3 nanoparticles on the mass transfer enhancement is studied, and the results show that the dispersed particles can significantly enhance the mass transfer by increasing the gas–liquid interfacial surface renewal frequency.

Enhanced photo bio-reaction by multiscale bubbles

Based on experimental and computational fluid dynamics simulations, the scale effect between the bubbles and the algae for different sizes in the culture process were investigated in a photobioreactor column under single and combined gas intake conditions, including factors such as the bubble distribution, gas holdup and mass transfer coefficient of different scale bubbles (500 μm microbubbles and 5 mm bubbles). In general, microbubbles could improve mass transfer and photo bio-reaction. For microalgae cultures with large particle sizes or high biomass concentration, the mixing ability of microbubbles was weakened, and therefore, the culture efficiency decreased. However, this can be improved by using millimeter bubbles with the strong turbulent characteristic. Therefore, a multiscale bubble combination intake method was proposed. This method can significantly strengthen both the multiphase mixture and mass transfer processes and was beneficial to improving the growth efficiency of Chlorella.

On single bubble mass transfer in a volatile liquid

We consider single bubble mass transfer of an non-condensible gas into a volatile liquid phase in industrial conditions, as observed for example in hydrocarbons liquid phase oxidation processes. Instantaneous bubble size, shape and velocity are measured using image processing with a particle tracking method. The mass transfer rate nitrogen into hot and pressurized liquid cyclohexane is deduced from the bubble volume decrease rate and is compared to literature correlations valid under isothermal conditions. Experiments are performed in a pressurized reactor for P=20bar, 30°⩽T⩽150°C and bubble Reynolds number Re=O(10–100). The analysis of bubble rise dynamics shows that the gas-liquid system studied can be considered as a clean system. The mass transfer results are found to follow isothermal correlations predictions excepted for ambient temperature for which liquid evaporation in bubbles is shown to be coupled with mass transfer. This phenomena seems to be a consequence of having a high Lewis number.

Study of Bubble Size, Void Fraction, and Mass Transport in a Bubble Column under High Amplitude Vibration

Vertical vibration is known to cause bubble breakup, clustering and retardation in gas-liquid systems. In a bubble column, vibration increases the mass transfer ratio by increasing the residence time and phase interfacial area through introducing kinetic buoyancy force (Bjerknes effect) and bubble breakup. Previous studies have explored the effect of vibration frequency (f), but minimal effort has focused on the effect of amplitude (A) on mass transfer intensification. Thus, the current work experimentally examines bubble size, void fraction, and mass transfer in a bubble column under relatively high amplitude vibration (1.5 mm < A <9.5 mm) over a frequency range of 7.5–22.5 Hz. Results of the present work were compared with past studies. The maximum stable bubble size under vibration was scaled using Hinze theory for breakage. Results of this work indicate that vibration frequency exhibits local maxima in both mass transfer and void fraction. Moreover, an optimum amplitude that is independent of vibration frequency was found for mass transfer enhancements. Finally, this work suggests physics-based models to predict void fraction and mass transfer in a vibrating bubble column.

Gas nucleus growth in high-viscosity liquid under strongly non-equilibrium conditions

The present paper describes the growth dynamics of a gas bubble formed as a result of nucleation in a highly-viscous gas-saturated liquid under rapid decompression. The mathematical model presented combines both dynamical and diffusion problems. The half-analytical solution for the dissolved gas concentration profile, bubble growth rate and the rate of bubble mass change is found. In contrast to previously reported solutions, it can be applied at all stages of the process in a wide range of supersaturation. The half-analytical solution is in good agreement with the results of numerical simulations and with known solutions for limiting cases.

Magnetic skyrmion bubble motion driven by surface acoustic waves

We study the dynamical control of a magnetic skyrmion bubble by using counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we determine the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele's method. The force that pushes the bubble is proportional to the strain gradient for the major strain component. We then study the dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two resonant frequencies.

An approach to couple velocity/pressure/void fraction in two-phase flows with incompressible liquid and compressible bubbles

A numerical approach to incorporate the compressibility of bubbles to a two-phase solver with incompressible liquid is presented. The scheme introduces a bubble mass density equation and the ideal gas law (though other thermodynamic relations can also be used) to compute the void fraction for use in the continuous phase equations. Pressure and velocity are strongly coupled on a collocated grid scheme by obtaining face velocities through nonlinear interpolation from nodal values using an algebraic equation. Tight coupling is achieved for pressure and void fraction by careful treatment of the bubble transport and compressibility terms in the resulting pressure Poisson equation. The novel proposed implicit strategy prevents numerical instabilities even at high void fractions and highly compressible bubbles. In addition, the proposed method can capture acoustic waves for void fractions between 0.1% and 90% with errors in speed of sound of less than 2% respect to the speed of sound of isothermal compressible bubbles and water mixtures. 1D, 2D and 3D numerical tests, including the bubbly flow around a surface ship, are performed to demonstrate robustness and range of applications of the proposed scheme. The approach is shown to be stable at high void fractions when other schemes fail.

Dynamics of the vapor cavity in the vertical tube of a small diameter after boiling-up in aqueous lithium bromide solutions

Dynamics of a single vapor cavity, moving under gravity in a vertical tube of a small diameter, has been studied. The experiment was carried out in water and aqueous LiBr solution in a tube with the length of 400 mm and diameter of 16 mm. Experiments have shown that dynamics of the rising vapor cavity after-the-fact boiling-up of aqueous LiBr solution in a vertical tube of a small diameter for some initial conditions differs qualitatively from dynamics of a gas bubble rising in a pool. Dynamics of the vapor cavity in aqueous LiBr solution and in water demonstrates quantitative but not qualitative differences. During bubble rising in a small diameter tube to time dependence for volume and mass of the vapor bubble, the regimes with pulsations were detected.

Mass transfer analysis for CO2 bubble absorption in methanol/Al2O3 nanoabsorbents

In this paper computational fluid dynamics (CFD) analysis is carried out to investigate CO2 bubble absorption characteristics in methanol/Al₂O3 nanoabsorbents. Bubble size, rising velocity and mass transfer rate are compared to the previous experimental results for validation. It is found that the distance traveled for each CO2 bubble increases as the concentration of Al2O3 increases, which, in consequence, increases the residence time between liquid and gas phases resulting in higher interfacial mass transfer rates. For the case of a bubble rising in the gap between walls, the wall shear stress has a major effect on the bubble diameter and rising velocity which in consequence affects the mass transfer coefficient. It is concluded that the mass transfer coefficient enhances by about 40% by adding Al₂O3 nanoparticles (0.01vol%) compared with pure methanol absorbent from the experimental and simulation results. It is also concluded that the use of nanoparticles has a higher impact on mass transfer rate than it does on mass transfer amount, which depends on the residence time and travel distance of CO2 bubbles.

Experimental analysis of Taylor bubble behavior and mass transfer during lateral oscillation of a vertical milli-channel

In this paper, we report on an experimental study on the influence of low-frequency horizontal vibration of a vertical millimeter-size channel with Taylor bubbles. We investigated the motion, shape and dissolution rate of individual elongated Taylor bubbles of air and CO2, which were freely rising in stationary water. Bubble size and dissolution rate were determined from microfocus X-ray radiographs. From the shrinking rate we calculated the liquid-side mass transfer coefficient. The rise velocity of bubbles and surface wave motion were analyzed using a videometric technique. The comparison of the results for the stationary and the oscillating channel showed that mechanical vibration of the channel is able to enhance the mass transfer coefficient from gas to the liquid phase by 80%–186%, depending on the frequency and amplitude of vibration. It was found that channel oscillation causes the increase of free rise velocity of bubbles which is mainly attributed to the development of propelling interfacial waves and increase of liquid film flow rate. Furthermore, analyzing the surface wave motion of bubbles revealed that the enlargement of contact area between the phases and the increased mixing enhances the mass transfer additionally up to 30% compared to non-agitated bubbles of similar Peclet number.

CFD-PBM simulations of a bubble column with different liquid properties

The CFD-PBM coupled model was validated in a bubble column with organic liquids under industrial conditions. Experimental data of the gas holdup, bubble size distribution and mass transfer rate were collected from the literature. The liquid viscosity and surface tension were two important parameters affecting the hydrodynamics. Low liquid viscosity and surface tension enhanced the bubble breakup but hindered the bubble coalescence. Compared with water, organic liquids led to a higher gas holdup, smaller bubble size and larger mass transfer rate. The elevated temperature decreased the liquid viscosity and surface tension. The effects of temperature on the hydrodynamic and gas-liquid mass transfer behaviors were well predicted using the corresponding liquid properties. The CFD-PBM coupled model gave good predictions because it quantitatively described the effect of liquid properties on the bubble size, interphase forces, turbulence parameters, and bubble breakup and coalescence behaviors. The simulations with different bubble breakup models showed that the accuracy of the bubble breakup model was crucial for reliable predictions of the CFD-PBM coupled model.

Dual-scale nanoripple/nanoparticle-covered microspikes on silicon by femtosecond double pulse train irradiation in water

Novel dual-scale structures were obtained by femtosecond double pulse train (subpulse delay Δt>0ps) one-step irradiating silicon in water. The dual-scale structures consist of microspikes of ∼2μm width and ∼0.5μm height, and nanoripples with a mean period of 146nm or nanoparticles with a mean diameter of 90nm which entirely cover on the microspikes, for linearly polarized or circularly polarized femtosecond laser respectively. The formation of dual-scale structures involves the following processes: (1) Continuously laser energy deposited at femtosecond to picosecond timescales within silicon surfaces and central regions, will result in enhanced capillary waves and thinner melted silicon layers. Hence, the microspikes can be induced at laser fluences below ablation threshold; (2) Later (>500–800 pulses), a mass of debris and bubbles produced will lead to the remarkably and uniformly scattering or shielding of subsequent incident laser energy. Hence, the nanostructures can be induced. The novel structures exhibit high-sensitive surface enhanced Raman scattering with an enhancement factor of 108 for Rhodamine 6G detecting. Besides, the novel structures have application potentials in improving the silicon hydrophobicity, antireflection, etc.

Experimental investigation of the effect of ethyl alcohol surfactant on oxygen mass transfer and bubble size distribution in an air-water multiphase Taylor-Couette vortex bioreactor

Volumetric liquid side mass transfer, bubble size distribution, and bubble shape were measured in a vertically oriented semi-batch gas-liquid Taylor-Couette vortex reactor with an aspect ratio of Γ=h/(ro−ri)=40 and radius ratio of η=ri/ro=0.75. Azimuthal Reynolds number, Axial Reynolds number and Capillary number were varied between 0 to 3.8×104, 7 to 99, and 6.1×10−6 to 76.0×10−6, respectively. Power-law correlations based on these data were presented for dimensionless Sauter mean diameter and Sherwood number in terms of the dimensionless parameters. Presence of ethanol as surfactant in the liquid was shown to inhibit bubble coalescence, which in addition to lowering interfacial surface tension causes a generally smaller bubble size at higher concentrations of surfactant. It was also shown that the mass transfer coefficient generally increases with higher concentrations of surfactant. Both the bubble diameter and mass transfer are influenced more at lower concentrations of surfactant, and the introduction of higher amount of ethanol causes a decrease in both parameters.

Simulating Spiraling Bubble Movement in the EL Approach

Simulating the detailed movement of a rising bubble can be challenging, especially when it comes to bubble path instabilities. A solution based on the Euler Lagrange (EL) approach is presented, where the bubbles show oscillating shape and/or instable paths while computational cost are at a far lower level than in DNS. The model calculates direction, shape and rotation of the bubbles. A lateral force based on rotation and direction is modeled to finally create typical instable path lines. This is embedded in an EL simulation, which can resolve bubble size distribution, mass transfer and chemical reactions. A parameter study was used to choose appropriate model constants for a mean bubble size of 3 mm. To ensure realistic solution, validation against experimental data of single rising bubbles and bubble swarms are presented. References with 2D and also 3D analysis are taken into account to compare simulative data in terms of typical geometrical parameters and average field values.

Preparation of Phase-Change Nanodroplets and an Experimental Study

To investigate the vaporization threshold of perfluoropentane encapsulated lipids nanodroplets and their feasibilities in assessing radio-frequency ablation margins in vitro. The nanodroplets with lipids shell and perfluoropentane (PFP) kemel was prepared by a Homogenization/emulsion method of which the size and zeta potential were measured by dynamic light scattering instrument (DLS). The phase change of perfluoropentane (PFP) nanodroplets (PFPNDs) stimulated by temperature rise was observed with the inverted optical microscope and ultrasonography. Echo intensity of bubbles generation from nanodroplets was calculated at different temperature points to plot the temperature-intensity curve. The temperature at the margins of hyperechogenicity zones generated from radiofrequency ablation (RFA) under ultrasonic guidance in tissue-mimicking phantom were measured with a thermocouple probe. The mean diameter of PFPNDs was 479.9±18.8nm. A few bubbles were observed at 50°C for PFPNDs with the inverted optical microscope, while a mass of bubbles generated rapidly from PFPNDs at 60°C, partly coalesced and ruptured ,and most of them ruptured at a higher temperature. No bubbles were observed in the Control group. With the ultrasonography, weak echo intensity of bubbles generated from PFPND at 50°C was detected. And when temperature rose to 60°C for PFPNDs or higher, the echo intensity reached peak value. Echo intensity in the Control group was not enhanced. The hyperechogenicity zones in the tissue-mimicking phantom dispersed with PFPNDs presented as elliptic shape along with the RFA needle, and the mean temperature at margin was 60.4°C. Little hyperechogenicity zones irregularly occurred in the Control group. The prepared PFPNDs can vaporize at given temperature. PFPNDs, with the vaporization threshold closed to the temperature threshold of tissues coagulative necrosis caused by RFA, is potential to assess the radiofrequency ablation margins in real time.