Bubble Trajectory Research Articles

Effect of Wall Proximity and Surface Tension on a Single Bubble Rising near a Vertical Wall

Path instability of a rising bubble is a complex phenomenon. In many industrial applications, bubbles encounter walls, and the interactions between the bubbles and the wall have a significant impact on flow physics. A single bubble rising near a vertical wall was experimentally observed to follow a bouncing trajectory. To investigate the near-wall dynamics of rising bubbles, 3D numerical simulations were performed based on the volume of fluid (VOF) method using the open source solver OpenFOAM. The effect of wall proximity and surface tension on the bubble trajectory was investigated. Previous studies have focused on the near-wall rising dynamics of bubbles for higher Eotvos numbers (Eo) and varied the Galilei number (Ga). The physical properties of the flow were chosen such that the free-rising bubble lies in the rectilinear regime. The Ga number was fixed and the Eo number was varied to analyze its effect on the bubble’s rising trajectory. It was found that the presence of the wall increases the drag experienced by the bubble and induces an early transition from rectilinear to a planar zigzagging regime. We identify the maximum wall distance and the critical Eo number for the bubble to follow a bouncing trajectory. The amplitude, frequency and wavelength of the bouncing motion are independent of the initial wall distance, but they decrease with decreasing surface tension.

Motion Characteristics of Gas–Liquid Two-Phase Flow of Microbubbles in a Labyrinth Channel Used for Aerated Drip Irrigation

The indefinite characteristics of gas–liquid two-phase flow limit the usage of aerated drip irrigation. Gas–liquid two-phase flow in a labyrinth channel was observed using a particle tracking velocimetry (PTV) technique in this study. The motion trajectory and velocity vector of large numbers of microbubbles were characterized and analyzed at 0.01, 0.02, 0.04 MPa inlet pressure and in three labyrinth channels with different geometries. The results indicated that bubbly flow was the typical flow pattern in a labyrinth channel, with slug flow occurring occasionally. Smooth and gliding motion trajectories of bubbles were observed in the mainstream zone, while twisted trajectories were seen in the vortex zone. Increasing the inlet pressure increased the number of bubbles and the trajectory length in the vortex zone. When the inlet pressure increased from 0.02 to 0.04 MPa, the 25th percentile of Rc-t (the Ratio of Circular path length in the vortex zone to the Total trajectory length for a single bubble) increased from 0 to 12.3%, 0 to 6.1%, and 0 to 5.2% for channels A, B, and C, respectively; the 75th percentile increased from 31.3% to 43.9%, 27.5% to 31.9%, and 18.7% to 22.3%. The velocity vectors of the bubbles showed position dependence. Bubbles with high speed were found in the mainstream zone with their directions parallel to the water flow direction. Bubbles with low speed were seen in the vortex zone, moving in all directions. With inlet pressure increased from 0.01 to 0.04 MPa, the mean instantaneous velocities of bubbles in channels A, B, and C are increased by 106.2%, 107.6%, and 116.6%, respectively. At 0.04 MPa, channel A has the longest path length and the highest instantaneous velocity of bubbles in the vortex zone among three channels, exhibiting the highest anti-clogging performance of the three channels. This study will help in the comprehensive understanding of gas–liquid two-phase flow in a labyrinth channel used for aerated drip irrigation.

Experimental Study on Three-Dimensional Bubble Rising Behaviours by Virtual Stereo Vision

This study addresses the measurement of three-dimensional (3D) bubble rising behaviour in still water with bubble equivalent diameters ranging from 2.61 mm to 5.11 mm using high-speed imaging and virtual stereo vision technology. The bubble shape, 3D trajectory/velocity, displacement angular frequency and terminal velocity of bubbles are analysed. The bubble equivalent diameter is obtained by the elliptic volume method. The bubbles are divided into small and large bubbles with a critical equivalent diameter of 4.49 mm, according to whether they are accompanied by deformation. The small bubbles (deq<4.49 mm) are spherical or ellipsoid, while the large bubbles (deq≥4.49 mm) exhibit ellipsoid, mushroom and hat shapes. The 3D trajectory is obtained by 3D reconstruction of bubble centroid coordinates. The rising trajectory of small bubbles shows 3D spiral motion, while the pitch increases gradually with the increase in the equivalent diameter. The trajectory of large bubbles is a two-dimensional (2D) zigzag. The bubble displacement curves in x- and z-directions are evaluated with third-order Fourier fitting. The results show that the bubble displacement frequency in the x and z-directions decreases with the increasing bubble diameter, and the displacement frequency in the xdirection is larger than that in the z-direction. The relative proportions of the viscous force, buoyancy, surface tension and inertial force on bubbles with different equivalent diameters are different, which leads to three trends in the vertical velocity of bubbles within the diameter range of this study. Finally, the bubble terminal velocity in still water is investigated. The terminal velocity first decreases and then increases with the increase in the equivalent diameter. The minimum value is 16.17 cm/s when the diameter of the bubble equivalent diameter is 4.49 mm. Moreover, the applicability of some classical prediction models is discussed.

Effect of heterogeneity size on three-phase contact line formation kinetics

Flotation relies on the differences in surface wettability. Although solids with the same hydrophobicity are generally regarded as having the same capture probability, the heterogeneities found on particle surfaces could complicate bubble–solid interactions. For surfaces with the same hydrophobicity and different sizes of inclusions, the heterogeneity size is expected to affect bubble–solid interactions. In this work, Microelectromechanical system techniques were used to fabricate chemically heterogeneous samples with the same hydrophobicity on which discrete hydrophilic dots with different diameters were distributed. The collision and adhesion processes between free-rising bubbles and these chemically heterogeneous surfaces were studied using a high-speed video camera. For all surfaces, the time intervals between successive collisions and the bubble trajectories were similar before bubble rupture. In contrast, the time elapsed between maximum air bubble deformation and rupture varied, with the rupture time appearing random. Nevertheless, the terminal diameter and average expansion velocity after bubble rupture were negatively correlated with the hydrophilic dot diameter. Thus, for heterogeneous surface with discrete hydrophilic dots, the bouncing behaviour of air bubbles before rupture is independent of heterogeneity size, whereas the three-phase contact line expansion behaviour after rupture is related to heterogeneity size. These insights can provide guiding principles for improving the flotation.

Influence of bubble surface loading on particle-laden bubble rising dynamics in a fluid flow system

Prediction of the rising behaviour of particle-laden bubbles in turbulent flow systems is fundamental to the design and operation of industrial separation processes. However, due to the complexities of such systems, the effect of fluid flow on the rising behaviour of particle-laden bubbles is often ignored. This study examines the combined effect of bubble surface loading (BSL) (particle diameter: 115 μm and 316 μm) and fluid flow on the rising behaviour of a particle-laden bubble (bubble diameter: 2.10 ± 0.08 mm) in a flow system with a pair of counter-rotating vortices, at Reynolds numbers (Re) ranging from 188 to 769. Specifically. the rising trajectory, rise velocity, and aspect ratio of the particle-laden bubbles were determined using the shadowgraphy imaging technique. The results showed that the rise velocity reduces as the BSL increases, reaching a plateau at BSL values > 50%. Over the range of tested conditions, the critical BSL value was appeared to be insensitive to particle size but increased as Re increased. The diminishing bubble surface mobility is believed to be responsible for the observed trend. Further, a drag modification factor as a function of BSL and Reynolds number has been developed. Noticeably, the drag coefficient curves peaked at the corresponding critical BSL values. The drag modification factor may prove to be useful in modelling of the particle separation processes.

Interface Capturing Flow Boiling Simulations in a Compact Heat Exchanger

Abstract High-fidelity flow boiling simulations are conducted in a vertical mini channel with offset strip fins (OSF) using R113 as a working fluid. Finite-element code PHASTA coupled with level set method for interface capturing is employed to model multiple sequential bubble nucleation using transient three-dimensional approach. The code performance is validated against experiments for a single nucleation site in a vertical rectangular channel. To assess code performance, a study on the bubble departure from the wall in a mini channel with OSF is carried out first. Contributions from the microlayer are not considered due to low heat flux values applied to the channel (1 kW/m2). The influence of surface characteristics, such as contact angle and liquid superheat on bubble dynamics, is also analyzed as well as the local two-phase heat transfer coefficient. For higher void fractions, two conical nucleation cavities are introduced in the same channel with OSF. Observed bubble characteristics (departure diameter, bubble departure frequency) are evaluated and bubble trajectories are presented and analyzed. The local heat transfer coefficient is then evaluated for each simulation. The results show approximately a 2.5 time increase in the local heat transfer coefficient when the individual bubbles approach the wall. With smaller bubble nucleation diameters, the heat transfer coefficient can increase by up to a factor of two. Thus, the current work demonstrates the flow modeling capability of the boiling phenomena in complex geometry with OSF.

Highly Efficient All-3D-Printed Electrolyzer toward Ultrastable Water Electrolysis.

The practical application of electrochemical water splitting has been plagued by the sluggish kinetics of bubble generation and the slow escape of bubbles which block reaction surfaces at high current densities. Here, 3D-printed Ni (3DP Ni) electrodes with a rationally designed periodic structure and surface chemistry are reported, where the macroscopic ordered pores allow fast bubble evolution and emission, while the microporosity ensures a high electrochemically active surface area (ECSA). When they are further loaded with MoNi4 and NiFe layered double hydroxide active materials, the 3D electrodes deliver 500 mA cm-2 at an overpotential of 104 mV for the hydrogen evolution reaction (HER) and 310 mV for the oxygen evolution reaction (OER), respectively. An all-3D-printed alkaline electrolyzer (including electrodes, membrane, and cell) delivers 500 mA cm-2 at a remarkable voltage of 1.63 V with no noticeable performance decay after 1000 h. Such a tailored bubble trajectory demonstrates feasible solutions for future large-scale clean energy production.

The movement and shape change characteristics of a bubble passing through a liquid-liquid interface

In order to study the movement and shape change characteristic of bubble when passing through the interface of two kinds of liquids with different viscosity, the free rising process of a single bubble in static stratified liquids was numerically simulated with the volume-of-fluid method. The results show that, when the initial height of bubble rising is the same, the rising velocity, deformation increase with the increase of bubble radius. When the maximum intensity of the vortex in the bubble is distributed at the top of the bubble, the top of the left and right sides and the bottom of the left and right sides, the bubble shape is spherical, ellipsoid and spherical cap shape respectively. At different initial heights, the bubble trajectory shows three different shapes ? linear, spiral, and C-shaped. The relation-ship between the bubble aspect ratio and rising height is predicted when different radius bubble passing through the interface. The amount of liquid B (lower layer) carried by the bubble increases with the increase of the bubble?s initial radius, and the amount of liquid carried by bubbles in C-shaped trajectory is higher than that in spiral trajectory.

Magnetohydrodynamics Simulation of the Nonlinear Behavior of Single Rising Bubbles in Liquid Metals in the Presence of a Horizontal Magnetic Field

Rising bubbles in liquid metals in the presence of magnetic fields is an important phenomenon in many engineering processes. The nonlinear behavior of the terminal rise velocities of the bubbles as a function of increasing field strength has been observed experimentally, but it remains poorly understood. We offer an explanation of the phenomenon through numerical calculations. A single rising bubble in stagnant liquid metal in the presence of an applied horizontal magnetic field is simulated. The observed nonlinear behavior is successfully reproduced; the terminal velocity increases with the increase in the magnetic field strength in the lower magnetic field regions but decreases in higher regions. It is shown that, in the lower region, the increase in the average bubble rise velocity results from the suppression of the fluctuations in the bubble trajectory in the vertical plane perpendicular to the magnetic field, as a consequence of the Lorentz force resulting from the component of induced electric current due to the magnetic field, which (approximately) acts in the opposite direction to that of the flow velocity. For higher magnetic field strengths, the Lorentz force induces a broadened wake in the vertical plane parallel to the applied magnetic field, resulting in a decrease in the rise velocity.

Quantification of motion characteristics of vertically ascending bubbles in NaCl solution via image processing

Abstract In this study, the motion characteristics of bubbles in a vertical rectangular glass groove were investigated using image-processing techniques. A new metric (bubble offset) to evaluate the level of solution disturbance was proposed. The motion characteristics, such as the trajectory, bubble diameter, rise velocity, and coalescence, during the bubble rise were studied using different NaCl concentrations and airflow rates. The functional relationship of the bubble diameter and velocity with height was obtained by least-squares fitting (correlation coefficient R2 ≥ 95%), and fits the power-law distribution. The results showed that the bubble motion characteristics were positively correlated with the airflow rate and negatively correlated with the NaCl concentration in the solution. The solution disturbance was closely related to the bubble offset, and the size of the bubble offset can be used to characterize the degree of the solution disturbance. The disturbance of the solution affects the bubble trajectory, rising velocity and coalescence. The bubble offset changed periodically with height, and its functional relationship was obtained. The accuracy of the functional relationship was verified using experimental data. The study of bubble motion characteristics is crucial for understanding phenomena such as heat and mass transfer and multiphase cooling.

Shape deformations and instabilities of single bubble rising in liquid metals

In this study, a computational fluid dynamics simulation was used to study single bubble flow in liquid metal. Until now, bubble trajectory and shape [Mougin, G. and Magnaudet, J., “Path instability of a rising bubble,” Phys. Rev. Lett. 88, 014502 (2002)] stability problems in liquid metal have only been insufficiently analyzed in the literature. Because of the difficulty of such an experimental validation, no universal correlations on terminal velocity, shape aspect ratio, and drag force coefficient have been produced to date. The existing bubble shape parameter and terminal velocity correlations with dimensionless numbers are still debatable, mostly because experimental validation is very challenging. The objective of this study was to develop new correlations between bubble stability and bubble deformation in liquid metals. An in-house code, PSI-BOIL, has been used for the simulations. A single bubble rising in a quiescent liquid has been simulated for three different sets of materials (nitrogen+mercury, argon+GaInSn, and argon+steel). The obtained results suggest that shape instability phenomena take place in the bubble dynamics in liquid metals for Eötvös numbers >1.7. Small bubbles (Eo < 1.7) maintain a stable ellipsoidal shape, while the shape and velocity of larger bubbles (Eo > 1.7) tend to oscillate with bubbles rising via non-rectilinear trajectories. The inviscid approximation works well for bubbles in liquid metals. It has been confirmed that the dynamics and the shape of small bubbles (Eo < 1.7) in liquid metals are only controlled by the Weber number.

Vorticity dynamics using PIV: Quantitative analysis of water perturbation by bubble rise

Bubble rise behavior and its disturbance of surrounding water are widespread in nuclear power plants and have a significant impact on the safety and efficiency of the installations. In this work, the dynamic parameters of six different sizes of bubbles and the perturbation of water are investigated, with the help of particle image velocimetry (PIV) and numerical image post-processing techniques. The morphology, trajectory, instantaneous velocity, velocity vector fields, and vorticity fields as well as their interactions are analyzed, by extracting data from the images. The results show that the shape of the rising bubbles is elliptical with small eccentricity most of the time, but not a standard round. In the initial stage of bubble generation, the eccentricity is large and the Weber number (We) of the bubble is small due to the surface tension. In addition, a standard annular flow field and two symmetric vorticities are found around the bubble in the initial stage, by calculating the velocity vector field and vorticity field of water. However, the equilibrium between the two vorticities can only be maintained for tens of milliseconds, and once the vorticity completely occupies the tail of the bubble, the bubble turns in the direction of the vorticity. This phenomenon occurs four to five times in the observed region and manifests itself macroscopically as a swing of the bubble trajectory. Further, the trajectory swing frequency and the bubble average velocity are found to decrease with the size of the bubble, which indicates that larger bubbles are more stable. However, the average velocity of the water increases with the increase of the bubble size, indicating that the water perturbation is more sensitive to the size of the bubble compared to the trajectory and morphology. Finally, a correlation is proposed for calculating the water velocity based on Reynolds number (Re), Eötvös number (Eo), and bubble velocity, which shows a good adaptation after comparing with the experimental data.

Research on the bubble sweep-down mechanism and suppression scheme of the research vessel

Vessels will encounter the phenomenon of bubble sweep-down when sailing in the ocean, which seriously affects the accuracy of underwater detection and resource exploration. In addition, the installation of defoaming attachments will affect the resistance performance of the ship. It is significant for a scientific research vessel to study the mechanism of bubble sweep-down and propose a bubble suppression scheme considering the ship's resistance characteristics. Detached-Eddy Simulation (DES) Shear Stress Transfer (SST) k-ω model combined with the Discrete Element Method (DEM) is used to simulate the process of microbubbles sweeping down to the ship's bottom under the influence of the flow at the bow of the scientific research vessel, and the deflection effect of the fairing on the bubbles, to carry out a mechanical analysis of the bubble sweep-down phenomenon. The results show that the bubble movement is closely related to the characteristics of the bow flow, the bubble's trajectory was related to the vessel speed and the bubble starting position, and the bubble at the bottom of the bow was easier to sweep down to the bottom of the ship. The fairing was a defoaming attachment with excellent resistance performance, which can physically block the air bubbles and strengthen the bottom vortex structure to change the bubbles' trajectory. Exploring the bubble sweep-down mechanism and the suppression scheme can better serve the design of the hull shape and defoaming attachment of scientific research ships.

Trajectory of a spherical bubble rising in a fully developed laminar flow

The rising trajectory of bubbles subjected to shear-induced lift force in a vertical channel is of great interest for designing and optimising a variety of electrochemical equipment. In this work, the rising trajectory of a spherical microbubble with a diameter of 0.3-1.0mm in a vertical channel with a fully developed laminar flow is investigated experimentally. Subsequently, the lift and drag forces on the bubble are assessed. Due to the limitation of the existing correlations for lift force coefficient, a new correlation is developed, which can be used to estimate the trajectory of a bubble in a vertical channel. The effect of flow velocity gradient and bubble diameter on the bubble motion is discussed to control bubble rising trajectory. It is concluded that the lift coefficient for a spherical microbubble in vertical channels with a fully developed laminar flow is slightly higher than the reported shear lift coefficient for larger bubbles rising in a high viscous liquid.

A single bubble rising in the vicinity of a vertical wall: A numerical study based on volume of fluid method

Single bubble rising in the close vicinity of a vertical wall is one of the focuses in the field of gas-liquid two-phase flow. In this work, three-dimensional direct numerical simulation based on volume of fluid (VOF) and adaptive mesh refinement method is performed to investigate the bubble rising and wake structures near a vertical wall. The bubble migration trajectory, deformation, velocity and wake structures were analyzed under various Galileo numbers and initial wall distances. Bubbles with a Galileo number of 8.8 significantly migrate away from the wall, which is driven by the repulsive force as a result of the suppression of the vortex diffusion on the bubble surface. As the Galileo number increases, the asymmetry of the shedding vortex tends to play a vital role on the repulsive force, which is exacerbated by the presence of the vertical wall. A periodic shedding of the vortex appears behind the bubble when the Galileo number reaches at 95. Meanwhile, the bubble trajectory changes from rectilinear to spiral along with a periodic oscillation. Due to the interaction between the vertical wall and the bubble wake, a transition from a rectilinear rising path to a spiral rising path is more likely to occur compared to that without a vertical wall. The presence of a vertical wall contributes to bubble-rising oscillations. A decrease of the bubble terminal velocity due to viscous effect near the vertical wall is observed for bubbles at low Galileo numbers. However, the viscous effect is less pronounced as the Galileo number increases.

Study of the bubble motion in a centrifugal rotor based on visualization in a rotating frame of reference

Single bubbles flowing in a centrifugal rotor were investigated through numerical and experimental methods, where the rotating speed, flow coefficient, and bubble diameter were varied to analyze their influence on the bubbles trajectories and drag coefficient. Results show that bubble trajectory is influenced by the flow rate and rotating speed, while drag coefficient curves are influenced by the bubble diameter and rotating speed. Finally, a comparison of the estimated drag coefficient against several drag coefficient correlations is shown. It was noted a good agreement with a model based on the Reynolds, Weber, and Eötvos numbers. Additionally, it was found a dependence between the drag coefficient and a dimensionless number, which considers the rotational acceleration of the rotor.

Characteristics of flow fields in the gas–liquid mini‐bubble columns with particle image velocimetry measurements

AbstractAs a new type of gas–liquid microreactors, the gas–liquid mini‐bubble column has potential applications. However, few studies on the flow fields in the mini‐bubble column can be found at present. In this work, particle image velocimetry (PIV) was used to visually study the velocity fields, vorticity fields and bubble dynamics in the gas–liquid mini‐bubble columns with column inner diameters of 1–3 mm and mini‐bubble diameters ranged from 0.7 to 1.3 mm. It is found that with the increase of superficial liquid velocity, bubbles rose from almost straight line to Z‐shaped or S‐shaped trajectory, and the bubble trajectory changed from one‐dimension to three‐dimension; when the bubble velocity changed, the bubble size and gas holdup decreased; bubble terminal velocity was controlled by bubble buoyancy and flow resistance, and increased slightly with bubble coalescence. These findings may provide basic reference for the design and scale‐up of such a mini‐bubble column reactor.

Transport in turbulent, recurrent flows: Time-extrapolation and statistical symmetrization

We investigate bubble dynamics in a submerged double-jet as an example of weakly coupled, Lagrangian transport. The setup resembles continuous casting of steel at Reynolds numbers Re=136000 to Re=272000. Using short time series of flow fields obtained from large-eddy simulations (LES) with a discrete bubble model, we calculate a database of transport patterns and time-extrapolate them to long durations in the framework of recurrence CFD (rCFD). A dedicated averaging procedure along bubble trajectories allows us to study their transport with large steps at little numerical costs and monitor their spatial distribution. Besides time-extrapolation for fixed Re with a single database, we demonstrate how several time series can be combined to (i) enforce symmetries and (ii) approximately model conditions in between. We compare bubble volume fraction fields and total hold-up obtained from LES and from rCFD and find very good to satisfying agreement with speed up factors of more than 500.

The motion mechanism and characteristic of bubble in a pseudo-2D tapered fluidized bed

The different carbon nanotube (CNT) particles ( @ A and @ V) were bed materials in the pseudo-2D tapered fluidized bed (TFB) with/without a distributor. A detailed investigation of the motion mechanism of bubbles was carried out. The high-speed photography and image analysis techniques were used to study bubble characteristic and mixing behavior in the tapered angle of TFB without a distributor. The fractal analysis method was used to analyze the degree of particles movement. Results showed that an S-shaped motion trajectory of bubbles was captured in the bed of @ V particles. The population of observational bubbles in the bed of @ V particles was more than that of @ A particles, and the bubble size was smaller in the bed of @ V particles than that of @ A particles. The motion mechanism of bubbles had been shown to be related to bed materials and initial bed height in terms of analysis and comparison of bubble diameter, bubble aspect ratio and bubble shape factor. Importantly, compared to the TFB with a distributor, the TFB without a distributor had been proved to be beneficial to the CNT fluidization according to the study of bubble characteristic and the degree of the particle movement. Additionally, it was found that the mixing behavior of @ V particles was better than @ A particles in the tapered angle of TFB without a distributor.

On the rupture of bubble foam in subcooled flow boiling of seawater in a vertical annulus

This paper investigates the nature of foam rupture and its effects on heat transfer during subcooled flow boiling of 3.5 wt% artificial seawater in a vertical annulus. Flow visualizations near the outlet of the heated section using a high-speed camera disclose the rupture of foam flow and the following development of slug bubble for the inlet temperatures of 55°C, 65°C, and 75°C and boiling numbers ranging from 3 × 10−4 to 11 × 10−4. The slug flow, otherwise, is absent for the case with inlet temperature of 85°C in this study and 90°C, as reported in our previous study. The nature of such foam rupture and evolution of slug flow is studied through the image processing and measurements in Image J. Geometric characteristics including the Feret diameters, equivalent diameters, angles, and aspect ratios are measured and discussed. Motion characteristics including the bubble growth rates, velocities and trajectories of slug bubbles are also measured and analyzed. It reveals that the slug bubble grows approximately linearly. However, the bubble growth rate demonstrates large variations and crucial effect on heat transfer. The trajectories of the slug bubbles reveal zigzag motions with large radial displacements. A theory is proposed to explain the ruptures of bubble foam. Such coexistence of foam flow and slug flow enhances significantly heat transfer in subcooled flow boiling, leading to a large wall temperature drop up to 10°C, compared with 1°C in those cases with stable foam flow in artificial seawater and slug flow in de-ionized water.