Bubble Trajectory Research Articles

Euler–Lagrange study of bubble breakup and coalescence in a turbulent boundary layer for bubble drag reduction

By injecting bubbles on the surface of ships or underwater vehicles, friction resistance can be effectively reduced. This technique is known as bubble drag reduction (BDR). The Euler–Lagrange method has been proved to be an effective numerical method in the study of the BDR mechanism. Bubble diameters were uniform in previous Euler–Lagrange simulations. However, many experimental results indicated that there is a nonuniform bubble size distribution under the action of turbulence, which affects the drag reduction significantly. In this paper, the authors developed a Euler–Lagrange code with the ability to simulate bubble breakup and bubble coalescence. The process of BDR on a flat plate is simulated by injecting bubbles into a turbulent boundary layer flow. Diameters of the bubbles are the same when they are injected into the flow field. During the simulation, bubbles of varying sizes can be clearly simulated in the evolution process due to breakup and coalescence. The drag reduction effect and bubble size distribution are validated by comparison with previous experimental data. Numerical results with and without breakup and coalescence models are discussed in detail to illustrate the advantage of the present algorithm. Moreover, bubble size distribution, bubble trajectory, and bubble induced turbulent modulation are analyzed in detail to explain the mechanism and its relationship with bubble size distribution.

Gas bubble dynamics in airlift photo-bioreactors for microalgae cultivation by level set methods

Coupled level set and CFD (computational fluid dynamics) methods are adopted in this work to track the moving gas–liquid interfaces in the riser of an external loop airlift photobioreactor (ALR) in which microalgae are used to produce biofuels and capture CO2 from flue-gas. Modeling the behavior of gas bubbles is a crucial aspect for the fine-tuning of the operation of the reactor when inserted into a closed-loop biorefinery at the pilot-scale. The experimental data used for simulation were completely acquired or calculated from hydrodynamic experimental campaigns carried out on the ALRs. The rise, coalescence, and shape dynamics of the bubbles of the flue-gas are simulated in a rectangular domain representing the vertical section of the ALR riser. Different correction approaches, such as the conservative level set method (CLSM), are proposed to face the volume loss characteristic of LSM. Computational results evidenced strong agreement with the experimental data (bubble shapes and trajectories). The physically-based CLSM model was then effectively used for the fine-tuning of the multiphase flow regime inside the ALRs, suggesting operating conditions for the outdoor cultivation with Reynolds number = 10000 – 11000, Sherwood number between 1400 and 1800, and spherical-caps bubbles in the upper half of the riser, mildly churning the microalgae while avoiding damages to their cells.

Compound analysis of gas bubble trajectories with help of multifractal algorithm

Three-dimensional trajectories of gas bubbles were the subject of study in this article. Authors carried out experiments in two glass tanks with dimensions of 300 × 300 × 1500 mm and 300 × 150 × 500 mm, respectively, filled with distilled water with a temperature equal to T = 21 ± 1 °C. The tests were carried out for an air volume flow rate, values from 0.00492 l/min to 0.0645 l/min. Basic characteristics of recorded measurement data like: analysis of the bubble detachment frequency from the nozzle, gas bubble size analysis have been shown. Based on the conclusions from those characteristics the multifractal algorithm analysis has been presented as one that allows comprehensive analysis of such a process. The results as a comparison of the main points on the air volume flow rate showed that: the trajectories of gas bubbles in liquid are multifractal and the width of the multifractal spectrum, and thus the complexity of the process being examined, depends on the air flow, the diameter of the bubble generating nozzle and the height of the liquid column in which the phenomenon is examined. The results of the analysis show that the key impact on the overall observed phenomenon is the deformation of the bubble surface, oscillation of the bubble column and observation of the two-phase flow spatially (three-dimensional).

Dynamics of single hydrogen bubbles at Pt microelectrodes in microgravity.

The dynamics of single hydrogen bubbles electrogenerated in acidic electrolytes at a Pt microelectrode under potentiostatic conditions is investigated in microgravity during parabolic flights. Three bubble evolution scenarios have been identified depending on the electric potential applied and the acid concentration. The dominant scenario, characterized by lateral detachment of the grown bubble, is studied in detail. For that purpose, the evolution of the bubble radius, electric current and bubble trajectories, as well as the bubble lifetime are comprehensively addressed for different potentials and electrolyte concentrations. We focus particularly on analyzing bubble-bubble coalescence events which are responsible for reversals of the direction of bubble motion. Finally, as parabolic flights also permit hypergravity conditions, a detailed comparison of the characteristic bubble phenomena at various levels of gravity is drawn.

Observation of Swirling-flow Behavior through Elbow and Orifice by Bubble Trajectory Visualization

Observation of Swirling-flow Behavior through Elbow and Orifice by Bubble Trajectory Visualization

Numerical study on the interaction of two bubbles rising side-by-side in viscous liquids

The hydrodynamic interaction of two bubbles released from submerged orifices was simulated using a 3D front-tracking type technique called the Local Front Reconstruction Method (LFRM). The effects of fluid properties and orifice spacing on the bubble trajectory, the interaction between two bubbles, and the vortex shedding pattern were investigated. Different types of interaction behaviors, which were observed in experiments by Kong et al. (2019), could be replicated by varying the liquid viscosity. The simulation results revealed that bubbles at low Reynolds number repel each other because the diffusion of vorticity at the bubble surface is blocked and becomes asymmetric due to the presence of a neighboring bubble. In contrast, at medium and high Reynolds numbers, the bubbles attract each other after their detachment from the orifices, and their subsequent possible bouncing depends on the orifice spacing. The bubble pair will feature a zigzagging motion while rising when double-threaded vortices are generated behind the bubble.

Transition of bubble motion from spiralling to zigzagging: A wake-controlled mechanism with a transverse magnetic field

We investigate the dynamics of a millimetre-sized air bubble rising in a liquid due to buoyancy under the influence of a magnetic field applied in a direction transverse to the bubble motion by conducting three-dimensional numerical simulations. The path and trajectory of the air bubble are examined by varying the strength of the transverse magnetic field. In the absence of a magnetic field, it is well known that under certain conditions a tiny air bubble undergoes spiralling motion due to its shape deformation and vortex shedding in the wake region. It is shown here that the spiralling motion of an air bubble observed without magnetic field transforms into a purely zigzagging motion under an applied transverse magnetic field. To understand the mechanism, we analyse the evolution of the wake vortices and forces experienced by the bubble in the presence of a transverse magnetic field. It is found that the twisted double-threaded vortices observed in a spiralling bubble in the absence of a magnetic field appear to rotate perpendicularly to the applied magnetic field across the vertical axis, thereby making the bubble shift its trajectory from spiralling to zigzagging. By calculating the lift and drag forces acting on the bubble, it is shown that how these forces acting on the bubble contribute to the vortex shedding patterns. Thus, the present study demonstrates the mechanism to control the trajectory of an air bubble under the application of a magnetic field.

Bubble Sweep-Down of Research Vessels Based on the Coupled Eulerian-Lagrangian Method

To explore the reason for the bubble sweep-down phenomenon of research vessels and its effect on the position of the stern sonar of a research vessel, the use of a fairing was investigated as a defoaming appendage. The separation vortex turbulence model was selected for simulation, and the coupled Eulerian-Lagrangian method was adopted to study the characteristics of the bubble sweep-down motion, captured using a discrete element model. The interaction between the bubbles, water, air, and hull was defined via a multiphase interaction method. The bubble point position and bubble layer were calculated separately. The spatial movement characteristics of the bubbles were extracted from bubble trajectories. It was demonstrated that the bubble sweep-down phenomenon is closely related to the distribution of the bow pressure field and that the bubble motion characteristics is related to the speed and initial bubble position. When the initial bubble position is between the water surface and the ship bottom, the impact on the middle of the ship bottom is greater and increases further with increasing speed. A deflector forces the bubbles to both sides through physical shielding, strengthening the local vortex structure and keeping bubbles away from the middle of the ship bottom.

Investigation of single bubble behavior under rolling motions using multiphase MPS method on GPU

Study of single bubble behavior under rolling motions can prove useful for fundamental understanding of flow field inside the modern small modular nuclear reactors. The objective of the present study is to simulate the influence of rolling conditions on single rising bubble in a liquid using multiphase Moving Particle Semi-implicit (MPS) method. Rolling force term was added to 2D Navier-Stokes equations and a computer program was written using C language employing OpenACC to port the code to GPU. Computational results obtained were found to be in good agreement with the results available in literature. The impact of rolling parameters on trajectory and velocity of the rising bubble has been studied. It has been found that bubble rise velocity increases with rolling amplitude due to modification of flow field around the bubble. It has also been concluded that the oscillations of free surface, caused by rolling, influence the bubble trajectory. Furthermore, it has been discovered that smaller vessel width reduces the impact of rolling motions on the rising bubble. The effect of liquid viscosity on bubble rising under rolling was also investigated and it was found that effects of rolling became more pronounced with the increase of liquid viscosity.

Experimental investigation on bubble coalescence regimes under non-uniform electric field

In this study, the characteristics of interactions between bubbles in ethanol were examined using a needle-ring electrode rig in the presence of a direct current electric field, using high-speed photography and with a focus on the effect of the electric field strength on the bubble coalescence. The results show bubble coalescence associated with four regimes in the manipulated parameter regions-namely, the coalescence of growing bubbles, single coalescence of rising bubbles, secondary coalescence of rising bubbles, and multiple coalescences of bubbles. The bubble interaction dynamics-which are characterized by the bubble trajectories, height positions, inclination angles, and bubble sizes-show that the bubble approach, collision, and coalescence in the electric field proceed in ways different from those reported in the literature. Furthermore, the cross-sectional average bubble diameter values corresponded to the bubble coalescence regimes. In addition, the increase in the Weber number (We) and electrical Bond number (BoE) gives rise to the bubble coalescence frequency. The We works to strengthen the wake induction, while the BoE works to accelerate the generation of smaller bubbles.

High-speed photography and image analysis of orbital motion of gas bubbles in ultrasonic field

Bubbles in the fluid have a great influence on the macroscopic physical properties and flow state of the fluid. The study of bubble motion in fluid is of great significance to ultrasonic cleaning, sonochemistry, flood discharge and energy dissipation, aeration and cavitation reduction. Bubbles in the fluid may also exhibit a special translational motion in an ultrasonic field—orbital motion. The orbital motions of gas bubbles attached to a boundary in ultrasonic field are investigated experimentally by high-speed photography and image analysis. The present study focuses on the trajectories of wandering bubbles, the arrangement of gas bubble array, the source of gas bubbles and the surface fluctuation details of gas bubbles. It is found that the circular trajectory of the wandering gas bubble is unstable, irregular and unsmooth. The holding gas bubbles in the bubble array originate from wandering bubbles. The transformation of wandering bubbles into array bubbles is mainly realized by merging and increasing the volume, thereby reducing the Bjerknes force. The wandering bubbles are produced by the merging of a large number of cavitation bubbles in the ALF (acoustic lichtenberg figure) structure, which makes the radial vibration bubbles gradually transform into the surface fluctuating bubbles. Under the action of Bjerknes force, the array bubbles show regular surface fluctuations, while the smaller ones are completely unstable, showing extremely irregular morphology, and have an influence on the surface fluctuation of nearby array bubbles. The array bubbles show a very regular arrangement, and the adjacent array bubbles have opposite vibration phases and repel each other. The orbital motion of gas bubbles attached to a boundary is significantly different from that of the bubbles suspended in liquid. First of all, the attached bubbles move along the solid wall, while the suspended bubbles move completely away from the vessel wall in the liquid. Secondly, the attached bubbles move around a regular array of bubbles, while the suspended bubbles move orbitally alone. In addition, the attached bubble is nearly hemispherical, and its surface fluctuates violently, and its motion track is extremely unsmooth, which is different from the circular motion of spherical suspended bubble. Finally, there is a strong interaction between the attached wandering bubble and the array bubble, which has a great influence on the vibration and trajectory of the attached bubble. In contrast, the circular motion of the suspended bubble does not have such a complex effect.

Preliminary numerical study of single bubble dynamics in swirl flow using volume of fluid method

Spacer grid with mixing vane had been widely used in nuclear reactor core. One of the main feather of spacer grid with mixing vane was that strong swirl flow was formed after the spacer grid. The swirl flow not only changed the bubble generation in the near wall field, but also affected the bubble behaviors in the center region of the subchannel. The interaction between bubble and the swirl flow was one of the basic phenomena for the two phase flow modeling in fuel assembly. To obatin better understanding on the bubble behaviors in swirl flow, full three dimension numerical simulations were conducted in the present paper. The swirl flow was assumed in the cylindral calculation domain. The bubble interface was captured by Volume Of Fluid (VOF) method. The properties of saturated water and steam at different pressure were applied in the simulation. The bubble trajectory, motion, shape and force were obtained based on the bubble parameters captured by VOF. The simulation cases in the present study included single bubble with different size, at different angular velocity conditions and at different pressure conditions. The results indicated that bubble migrated to the center in swirl flow with spiral motion type. The lateral migration was mainly related to shear stress magnitude and bubble size. The bubble moved toward the center with high velocity when the swirl magnitude was high. The largest bubble had the highest lateral migration velocity in the present study range. The effect of pressure was small when bubble size was the same. The prelimenery simulation result would be beneficial for better understanding complex two phase flow phenomena in fuel assembly with spacer grid.

Motion and mass transfer models for single bubble in an aluminum melt under a compound field of ultrasonic and rotating flow

A mathematical model has been developed for single bubble motion and mass transfer for aluminum degassing under a compound field of ultrasonic and rotating flow, and it showed good agreement with experimental data. The effects of the ultrasonic frequency and acoustic pressure on the bubble size, bubble trajectory, and mass transfer rate between the bubble and melt were investigated. The ultrasonic frequency and acoustic pressure had little effect on the bubble trajectory, and its curve radius gradually increased with increasing distance. The retention time of the bubble in the compound field was much greater than that in a single rotating flow field, which is conducive for improving the purification efficiency. Under the experimental conditions of 450 rpm rotation speed and 1.1 m/s gas flow rate, the optimal parameters are a frequency of 50 kHz and an acoustic pressure of 100 kPa.

Computational modeling of microbubble coalescence and breakup using large eddy simulation and Lagrangian tracking

AbstractThe flow of dispersed microbubbles was studied with an Eulerian–Lagrangian technique using large eddy simulation to predict the continuous liquid flow and Lagrangian tracking to compute bubble trajectories. The model fully accounts for bubble coalescence and breakup and was applied to horizontal and vertical channel flows. With low levels of turbulence, gravity in horizontal, and lift in vertical, channel flows govern the bubble spatial and collision distribution. When turbulence is sufficiently high to, at least partially, oppose bubble preferential concentration, more uniform collision and coalescence distributions are found, although these remain peaked near the wall in both configurations. Almost 100% coalescence efficiency was always found, due to bubbles colliding along similar trajectories, with breakup only recorded in a flow of low surface tension refrigerant R134a. Models like this can provide the required quantitative understanding of the microbubbles complex behavior, as well as supporting the development of more macroscopic modeling closures.

Combined effects of temperature field and ultrasonic vibration on bubble motion in laser joining of plastic to metal

A previous study has found that applying ultrasonic vibration in laser joining of Polyethylene terephthalate (PET) plastic to Ti metal can greatly reduce bubbles formed in the joint. In this study, the theoretical basis on the combined effects of pressure and temperature fields on the trajectory of bubbles was analyzed using finite element simulation. A non-uniform temperature field influences the evaporation momentum pressure acting on a bubble, causing it to travel from a low to a high temperature region. A properly designed ultrasonic transducer tool, however, can create an unbalanced pressure field around a bubble. Under the combined effects of these elements, bubbles in the molten pool can be made to escape from the joint. The predicted results and obtained experimental outcomes matched well, which were also supported by the evidence of captured high-speed camera images.

Effect of bubble flow on partial discharge under non-uniform AC electric fields in FC-72

The focus of this study is on thermal bubbles which limits the application of phase-change cooling technology in high electric fields. In order to investigate the impact of the boil-induced bubble flow on the partial discharge (PD), a PD test chamber is set up which is modified to adjust the intensity of the bubble flow. PD signals and characteristics of the bubble flow are recorded under different heating conditions and electric fields. It is found that thermal bubbles have negligible impact on the PD characteristics under a non-uniform field, but the discharge frequency decreases as the boiling intensity increases. A deviation occurs in the trajectory of bubbles, which makes discharge in bubbles impossible. It is also found that the heat induced fluid affects the density of dielectric liquid and ionization coefficient of the discharge environment, which reduces PD frequency.

Evaporation Momentum Force and Its Relevance to Boiling Heat Transfer

Abstract As a liquid evaporates into its vapor, the vapor phase leaves at a higher velocity than the approaching liquid and exerts a net momentum force on the evaporating interface. This force is especially relevant in the contact line region where liquid temperature is higher than the bulk liquid, and local saturation temperature is reduced due to curvature effects. These factors result in an increased evaporative flux resulting in higher evaporation momentum force that can influence the interface motion and bubble trajectory. This force provides a new mechanism for enhancing boiling heat transfer by altering the individual bubble trajectory. In microchannels, it can lead to flow instability. These effects are critically evaluated in this paper and their relevance to bubble growth and heat transfer phenomena during pool and flow boiling is presented. Two nondimensional groups K1 and K2, respectively, representing the ratio of evaporation momentum force to inertia and surface tension forces, have been used in modeling heat transfer and interface motion. Evaporation momentum force has been successfully applied in modeling critical heat flux (CHF) in pool and flow boiling, analyzing instability during flow boiling in microchannels, controlling individual bubble motion, and enhancing CHF and heat transfer coefficient (HTC) during boiling on flat surfaces as well as tubular geometries.

Measurement of multifractal character of bubble paths using image analysis

The trajectory of the virtual mass centers of sequentially departing gas bubbles in water flow has been analyzed in this study. The bubbles were generated from a brass nozzle of 1 mm inner diameter in a glass tank (300 × 150 × 500 mm3), filled with distilled water maintained at 21 ± 1 °C. A three-mirror system was used to obtain two frames of the workspace in a single shot of a Phantom v1610 high-speed camera. A three-dimensional computer reconstruction algorithm was developed, which consisted of a blur filter, canny edge filter, region of interest (ROI), and Kalman filter. This algorithm was used to obtain the coordinates of the center of mass of each individual gas bubble. We analyzed the bubble trajectories obtained for three different air volumetric flow rates. A multifractal analysis was used to evaluate the gas bubble dynamics. The wavelet transform modulus maxima method was used to obtain the singularity spectra of the bubble paths. The local Hölder exponent value proved that the gas bubble motion trajectories tended to behave in a deterministic and chaotic manner. In addition, the sensitivity of the chaotic bubble paths to small- and large-scale fluctuations of the lateral bubble displacement was investigated. In conclusion, the proposed approach allows us to distinguish between deterministic chaotic dynamics and the stochastic evolution of bubble paths.

Argon bubble flow in liquid gallium in external magnetic field

This paper presents detailed results of neutron imaging of argon bubble flows in a rectangular liquid gallium vessel with and without the application of external horizontal magnetic field. The developed image processing algorithm is presented and its capability to extract physical information from images of low signal-to-noise ratio is demonstrated. Bubble parameters, velocity components, trajectories and relevant statistics were computed and analysed. A simpler version of the code was applied to the output of computational fluid dynamics simulations that reproduced the experiment. This work serves to further validate the neutron radiography as a suitable method for monitoring gas bubble flow in liquid metals, as well as to outline procedures that might help others to extract data from neutron radiography images with a low signal-to-noise ratio resulting from high frame rate acquisitions required to resolve rapid bubble motion.

Effect of cavitation and free-gas entrainment on the hydraulic performance of a centrifugal pump

An experimental study on gas–liquid two-phase flow characteristics in a low-specific-speed centrifugal pump is presented via employing multiple investigation techniques, such as visualization observation, measurements of acoustic emission and vibration, etc. Specially, three different flow conditions were inspected, namely gas locking initiation, critical cavitation with/without free-gas presence, etc. For gas locking, the drastic deterioration of the pump performance and the disrupted balance of shaft were observed. Especially, at low rotational speeds, the gas locking accompanied with intermittent or churn flow can be triggered by even lower inlet gas volumetric fractions. When it came to the cavitation flow, a small amount of gas entrainment could induce the rapid deterioration of cavitation and stimulate much higher amplitude in low-frequency band of shaft rotation. The relationship between the gas bubble trajectory and the vibration level under the backflow is discussed. The results reveal that the combined effect of the free-gas entrainment and cavitation on the pump instability is much stronger than that under natural cavitation or free-gas entrainment flow, whose fault diagnosis can be determined by the data manifested in different spectral segments.