Bubble Migration Research Articles

Numerical investigation of electrohydrodynamically enhanced flow boiling inside minichannels: The seeding model

In the present study, three-dimensional numerical simulations are performed to examine the effect of applied direct current electric fields on the subcooled flow boiling heat transfer in a vertical minichannel. The volume of fluid model is used to capture the liquid–vapor interfaces, and the conservation laws, together with the electrohydrodynamic (EHD) equations, are solved using the finite volume method. Two mass transfer models of Lee and Fourier are evaluated, and a new seeding algorithm is developed based on the physics of the bubble formation and departure on the heated walls, integrated with the Fourier model. The early transition from slug to churn/annular flow regimes, due to EHD forces, is observed in the numerical solutions with both models, in agreement with the available experiment. Nevertheless, the Lee model tuned for the electric-free condition fails to predict the EHD-induced heat transfer enhancement, while the Fourier model with the new seeding algorithm captures this phenomenon with reasonable accuracy. Based on the present numerical results, this effect can be attributed to the migration of small bubbles toward the walls under the effect of electric fields, forming recirculating flow regions near the walls, augmenting the flow mixing, and mitigating hot spots.

Experimental study on the lateral migration of a bubble contaminated by surfactant in a linear shear flow

Adding a small amount of surfactant to a gas–liquid two-phase flow can markedly change the dynamic behavior of its bubbles. In this study, the lateral motion of a single bubble (deq = 1.99–3.33 mm, Reb = 200–420) contaminated by surfactant and rising in a linear shear flow is experimentally studied. Sodium dodecyl sulfate (SDS) is chosen as the surfactant with concentrations ranging from 10 to 50 ppm. A curved screen is used to generate a stable linear shear flow, and particle image velocimetry is used to measure the quality of the flow field. Bubble motion parameters, including trajectory, aspect ratio, instantaneous velocity, and terminal velocity, are captured using the shadow method with charge-coupled device cameras. The lift coefficient C L is obtained by a quasi-steady-state analysis. The results show that the presence of surfactant inhibits the lateral migration of bubbles rising in a shear flow and that increasing the SDS concentration and bubble equivalent diameter strengthens this inhibition effect. That is, the C L and the net lateral migration distance decreased with SDS concentration and bubble equivalent diameter. In addition, the variation trends of the quasi-steady drag coefficient, bubble terminal velocity, and bubble oscillation frequency with bubble equivalent diameter and SDS concentration also were analyzed.

A comprehensive atomistic investigation on the cascade induced helium bubble motion in bcc iron for neutron irradiated RAFM steels

Cascade induced by the primary knock-on atom could significantly influence the evolution of helium bubbles in RAFM steels. In this work, the helium bubble migration and helium bubbles coalescence under cascade overlapping, i.e., when the helium bubble was overlapped by the thermal spike of one cascade, in bcc Fe was investigated by molecular dynamics simulation. The influence of He-to-vacancy (He/V) ratio as well as cascade energy, temperature, bubble size and bubbles distance were investigated based on the statistical results of 4700 cases. A method which could quantitatively estimate the overlapping extent between the cascade and helium bubbles was put forward to confirm that the cascade did overlap with the helium bubble in all cases. It was found that under cascade overlapping, the helium bubble tends to migrate toward the thermal-vacancy-rich center. The mechanisms of migration change from vacancy diffusion to He atoms migration with the increase of the He/V ratio, and the corresponding migration distance could decrease one order of magnitude. The cascade induced coalescence of two close helium bubbles is associated with the synergy effect of migration, dissociation and joint strain field of the two bubbles. Especially, it was found that the joint strain field of two close helium bubbles could significantly promote the bubbles’ coalescence by fostering the two bubbles’ directional migration towards each other under cascade overlapping. It was also found that the stable He/V ratio of helium bubble under cascade overlapping is around 0.5.

A unified theory for bubble dynamics

In this work, we established a novel theory for the dynamics of oscillating bubbles such as cavitation bubbles, underwater explosion bubbles, and air bubbles. For the first time, we proposed bubble dynamics equations that can simultaneously take into consideration the effects of boundaries, bubble interaction, ambient flow field, gravity, bubble migration, fluid compressibility, viscosity, and surface tension while maintaining a unified and elegant mathematical form. The present theory unifies different classical bubble equations such as the Rayleigh–Plesset equation, the Gilmore equation, and the Keller–Miksis equation. Furthermore, we validated the theory with experimental data of bubbles with a variety in scales, sources, boundaries, and ambient conditions and showed the advantages of our theory over the classical theoretical models, followed by a discussion on the applicability of the present theory based on a comparison to simulation results with different numerical methods. Finally, as a demonstration of the potential of our theory, we modeled the complex multi-cycle bubble interaction with wide ranges of energy and phase differences and gained new physical insight into inter-bubble energy transfer and coupling of bubble-induced pressure waves.

Lattice damage and helium bubbles formation in KTaO3 crystals induced by helium ion implantation

Potassium tantalite (KTaO[Formula: see text] has shown an excellent performance in optoelectronic applications, proving its advantages with respect to fabricating single crystal thin films by ion implantation. This work introduces the damage formation of KTaO3 under 200 keV He ion implantation at room temperature and He bubbles accumulation. Ion implantation-induced lattice damage before and after annealing was quantitatively analyzed by using the Rutherford backscattering spectrometry in channeling technique. The crystals phase analysis of the samples under different fluences was studied by using the X-ray diffraction technique. For 200 keV He ions, the accumulation and migration of He bubbles were induced by the thermal annealing effect under high fluences of He[Formula: see text]. The He bubbles appear obviously coarsening and embrittlement with thermal annealing. The blistering phenomenon caused by He ion implantation is the physical basis for the smart-cut technique, which allows the preparation of high-quality single-crystal films.

Numerical study of dynamics of cavitation bubble collapse near oscillating walls

This paper presents a numerical study of the dynamics of an initially spherical bubble collapse near an oscillating rigid wall with a large amplitude; the wall oscillating amplitude is greater than 1% of the initial maximum bubble radius. Numerical simulations were conducted using a compressible two-phase flow model and the volume of fluid (VOF) interphase-sharpening technique on a general curvilinear moving grid. The numerical results for bubbles in the free field and near a wall were computed and compared with published experimental data. To study the effects of the oscillating wall on bubble collapse, a sinusoidal function was used for wall oscillation. The initial bubble conditions were set as a Rayleigh bubble located above the rigid wall at a dimensionless bubble-boundary distance with initial phases of 0° and 180°. During bubble collapse, the interface deformation, jetting behavior, bubble collapse time, and bubble migration were determined. Violent collapse of the bubble, jetting behavior, and shock propagation from the significant effects of the oscillating wall were observed in simulation cases with different wall motions. The effects of the non-dimensional amplitude scale and non-dimensional period timescale were considered with the initial phases in the problem. The trend lines of typical characteristics and critical points of bubble collapse were determined.

Lattice Boltzmann study of bubble dynamic behaviors and heat transfer performance during flow boiling in a serpentine microchannel

• Mesoscale study of microchannel flow boiling in U bend was conducted. • Detailed bubble dynamics and heat transfer characteristics were presented. • Effect of buoyancy on flow boiling process depends on flow orientation. • The onset of nucleate boiling is delayed as the Re number increases. Understanding the flow boiling process in a serpentine microchannel with U-bends is very important to its design and application in practice. In this study, a hybrid thermal multiphase model consisting of the pseudopotential multiphase lattice Boltzmann model and the finite difference method is employed to investigate the flow boiling heat transfer in a serpentine microchannel. Effects of curvature ratio, flow orientation, heat flux and Reynolds ( Re ) number on the bubble dynamic behaviors and heat transfer performance during flow boiling process are comprehensively evaluated. Bubble behaviors including bubble nucleation, growth, coalescence, departure, recontact, and migration are well captured, and typical flow patterns of bubbly flow and intermittent slug flow are identified. The simulation results show that increasing curvature ratio does not affect the heat transfer performance much, but generates elongated bubbles at the U-bend. Depending on the flow orientation, the buoyancy induced by gravity acceleration has both favorable and unfavorable effects on the bubble dynamic behaviors and local heat transfer characteristics in the serpentine microchannel. In addition, the vapor volume fraction is calculated under different Re numbers and heat fluxes. The lower vapor volume fraction exhibits lower wall superheat and better heat transfer performance.

Different modes of bubble migration near a vertical wall in pure water

Different modes of bubble migration near a vertical wall in pure water

Bubble migration characteristics in power transformer oil under coupled stresses of forced vibration and electrical field

AbstractThe stresses existing in power transformers lead to the generation of bubbles. They are the primary source that worsens and leads to insulation failure. Due to the complexity of bubble dynamics, it is challenging to figure out bubble mechanisms. However, creation, migration, and accumulation mechanisms require further explanation. A set of experiments has been conducted to provide a scientific overview of bubbles. The results revealed that buoyancy is dominant in low electric fields, while high electric fields affect bubbles' rise. Bubbles stretch and oscillate while moving. When the electric field is not strong, the AC field reduces the duration and volume of bubble detachment. Vibration stress on the other side is leading the bubble motion including detachment frequency and migration speed. Under the combination of AC and vibration stresses, the bubble distortion degree aggravates and bubble gathering formation is easy. This research can provide a deeper acumen of the bubble's actions beneath a power transformer under a combination of vibration and electric fields. It provides essential technique parameters to deal with while investigating bubble's problems and related engineering research areas.

Effect of different pre-existing dislocation densities on the microstructure evolution of W-0.5ZrC alloy during in-situ He+ irradiation

The evolution of dislocation loops and helium bubbles in W-0.5ZrC alloy with different pre-existing dislocation densities was investigated by in-situ transmission electron microscopy during 30 keV He+ irradiation at 800 °C. The nucleation, migration, aggregation, and growth of dislocation loops and helium bubbles occurred with the whole irradiation process. The variation trends of average size and volume number density of dislocation loops and helium bubbles with the increase of irradiation dose in the regions with different pre-existing dislocation densities were obtained. It was found that the growth of dislocation loops was inhibited in the region with a low pre-existing dislocation density (about 5.2 × 1013 /m2), and the nucleation of dislocation loops was inhibited in the region with the dispersed medium density (about 1.6 × 1014 /m2, dislocations were crossed but dispersed), while both the nucleation and growth of dislocation loops were inhibited in the region with the compact medium density (about 1.5 × 1014 /m2, dislocations were crossed and compact). During the irradiation process, the pre-existing dislocations would interact with the irradiation-induced dislocation loops and helium bubbles, but their influence weakened with the increase of irradiation dose, and finally resulted in the disappearance or deformation of pre-existing dislocations. The irradiation hardening caused by dislocation loops and helium bubbles was increased with the increase of irradiation dose, and the corresponding mechanism was analyzed.

Effect of liquid viscosity on the gas–liquid two phase countercurrent flow in the wellbore of bullheading killing

Studying the gas–liquid two-phase countercurrent flow under high liquid viscosities is key in designing the well-killing parameters of the bullheading method; however, currently, in-depth research on this issue is limited. Herein, using a self-designed vertical visualization wellbore gas–liquid two-phase flow experimental system with a tube diameter of 0.1 m and liquid viscosity in the range of 1–50 mPa s, a simulation experiment of bullheading killing was conducted. The effects of liquid viscosity on the bubble morphology, bubble migration, and gas–liquid countercurrent flow pattern were analyzed, and the mechanism of back pressing of gas during the well-killing process of the bullheading method in a large-scale tube was revealed. Moreover, the influencing factors and variation in the critical displacement of bullheading were analyzed. The results revealed that the stability of Taylor bubbles increased and the number of small bubbles decreased with the increase in liquid viscosity, resulting the gradual shrinkage or disappearance of the churn flow and bubbly flow, expansion of the region of slug flow, and downward movement of the transition boundary between bullet-cup flow and slug flow. On approaching the critical liquid displacement during the process of simulated bullheading, Taylor bubbles rapidly dissociated into small bubbles and turned around during the rising process, forming a gas–liquid downward flow, following which the gas was successfully pressed back. In addition, our analysis revealed that the press-back effect is not always positively correlated with the viscosity of the kill fluid. The critical displacement of the well-killing fluid first decreased and then increased with an increase in the liquid viscosity. Finally, based on the experimental results, an improved calculation method for the critical displacement of bullheading considering the influence of liquid viscosity was established, which could provide important theoretical guidance for the parameter design of bull heading.

The Generation and Migration of Bubbles in Oil-Pressboard Insulation Needle-Plate System

Bubbles in transformer oil can easily lead to partial discharge (PD), which can deteriorate the transformer oil and even breakdown the transformer insulation. To clarify the migration process and the characteristics of bubbles generated in an oil-immersed power transformer exposed to an extremely uneven electric field, we experimentally monitor these phenomena under an extremely nonuniform ac electric field and numerically simulate the migration distance and the migration speed of bubbles with different initial positions and sizes. The results show that the streamer discharge channel formed by a PD in oil is gasified into a bubble channel. After it collides with the surface of the pressboard, its morphology is transformed into approximately spherical bubbles due to the surface tension of the gas–liquid interface. After bubbles are generated in the oil, they move away from areas with a strong electric field due to the electric-field force and gradually approach the oil surface due to the buoyancy force. The experimental results are consistent with the simulation results, which verify the rationality of the simulation model.

The behaviors of bubble migration and pressure build-up during a dynamic shut-in procedure in deep-water drilling

A dynamic shut-in procedure is commonly adopted after a kick incident in order to build up the wellbore pressure, obtain reservoir information, and thereby handle the gas kick. In deep-water scenarios, the hydrate growth behaviors have a significant effect on gas migration and interphase mass transfer, which has not been quantitatively analyzed during the well shut-in process. In this study, a comprehensive mechanistic model of wellbore dynamics is developed considering gas migration and phase transitions. The simulation results show that the wellbore pressure field can be built up in different trends during different well shut-in periods, governed by gas seepage from the reservoir and gas migration along the wellbore, respectively. Masking the migration of free gas, the phase transition phenomena have a significant influence on the wellbore dynamics and bottomhole pressure. This work adds further insights into quantitatively characterizing the hydrate growth behaviors and interphase mass transfer rules of gas bubbles during a dynamic well shut-in procedure.

Bubble behavior under a novel metallurgy process coupling an annular gas curtain with swirling flow at tundish upper nozzle

A novel metallurgy process coupling an annular gas curtain and swirling flow technology at a tundish upper nozzle (TUN) was developed in this paper. The migration and distribution behaviors of bubbles were studied by a 1:2 scale water model, and the effects of the process and structural parameters on the distribution of gas flow and bubble size in the mold were further investigated. The results indicated that most argon bubbles float up in the tundish, forming a complete annular gas curtain around the stopper rod, and a small portion of small bubbles enter the nozzle and mold, which can realize the effect of normal argon blowing at the TUN. The liquid steel passing through the swirl slots can produce a certain swirling flow in the nozzle. Under the action of the centrifugal force of the swirling flow, lightweight bubbles will converge to the centre of the nozzle, which increases the collision and coalescence probability of bubbles. As the flow rate of argon blowing, casting speed, and number and height of swirl slots increase, the argon volume and average bubble size in the mold gradually increase.

Microstructure and mechanical property evolution of He-implanted nanochannel W film under post-annealing

Tungsten (W) has been selected as the most promising Plasma Facing materials (PMFs) due to its excellent properties. One of the most challenged problems for W is the He retention. Nowaday, a nanochannel structure film was presented to improve the He irradiation resistance of W. In this work, the microstructure and mechanical property evolution of He-implanted nanochannel W film under post-annealing from 573 to 1273 K were studied. A significant growth of He bubbles and the formation of large size holes at grain boundaries (GBs) were observed after post-annealing at 1273 K. In addition, a formation model of pores structure on surface caused by high energy He implantation was proposed. Moreover, a mitigated lattice swelling was observed at 1273 K due to the He-V clusters and He bubbles migration and coarsening. The shape of the He bubbles at 1273 K was also investigated in detail. The nanochannel film has a better resistance to the degradation of Young's modulus under the He implantation than compact film, due to the loose nanochannel. Hardness increment induced by the implantation of He was analyzed. The hardening effect of the He-implanted film annealing at 1073 K, resulting from the transition of He-V clusters or He bubbles distributions from dislocations to distributed vacancies, should be notice. Finally, the holes at GBs led to the decrease of both Young's modulus and hardness.

On the theory of void nucleation in irradiated crystals

The theory of void nucleation in irradiated materials is critically analysed and further developed. It is shown that the existing theory overestimates the rate of empty void nucleation in ion-irradiated steels by several orders of magnitude and therefore requires revision for an adequate interpretation of ion bombardment tests. To extend the nucleation model to the conditions of reactor neutron irradiation, it is modified to take into account the effect of void stabilisation by transmutation gas (He) in steels or by fission gases (Xe, Kr) in fuels. It is shown that the presence of gas atoms in a solid solution strongly reduces the free energy of void formation, but does not affect the number of void nucleation sites, which is determined by the concentration of vacancies in the irradiated material. The model developed for nucleation of small gas filled voids (bubbles) is further modified to adequately consider their further growth, stabilisation (due to athermal resolution of gas atoms) and generation of a new population of larger bubbles. The obtained analytical expression for the nucleation rate is applied to a simplified analysis of experimental data on neutron irradiation of stainless steel in PWRs and FRs. For a more accurate analysis of these tests, it is recommended to implement the new model in a fuel performance code, additionally taking into consideration the migration and coalescence of small bubbles.

Shock-induced collapse and migration of nanoscale He bubble in single crystal Al

As a major product of nuclear radiation damage, high-pressure He bubble is a potentially important factor affecting the constitutive response and the damage dynamics of the matrix material. This Letter reports the shock-induced collapse and migration process of He bubble in Al based on atomistic simulations. The He bubble will be compressed to permanent deformation while experiences a finite collapse in comparison with the complete collapse of the voided matrix. Under strong shock, the He bubble can be broken down by the nano-jet of the metal, but it returns to a reduced sphere in the molten metal after long-time evolution, driven by the He-Al interface energy. The Carnhan-Starling hard-sphere model is used to describe the thermodynamics of shocked He bubbles, by establishing the relationship between the diameter of He atom and temperature. In particular, the shock-induced migration of He bubble is revealed, which can be divided into shock acceleration and the following inertial motion. Then, an equation of migration distance related to shock intensity is proposed.

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.

Jetting and migration of a laser-induced cavitation bubble in a rectangular channel

The jetting behaviour and migratory characteristics of a laser-induced cavitation bubble in a rectangular channel are investigated both experimentally and numerically, for various combinations of the geometric and physical parameters of the system. High-speed photography is used to visualize the temporal development of the bubble shape, the formation of liquid jets during bubble collapse, and the bubble displacement in contact with the sidewalls of the channel during two oscillation cycles of the bubble. The bubble profiles, pressure contours and velocity vectors ambient to the bubble are obtained through numerical simulation results by using an Eulerian finite element method with a compressible liquid impact model. The jetting behaviour of the bubble varies between single jet formation and the formation of three liquid jets directed towards each wall of the channel. The numerical calculations indicate that the liquid jets directed towards the sidewalls of the channel reach maximum velocities of 100 m s−1while the peak velocity of the liquid jet directed towards the channel endwall is about 55 m s−1. A small bubble generated close to a sidewall of the channel develops only a single inclined jet during collapse. Such jets can reach velocities of up to 110 m s−1. A bubble displacement in contact with the sidewalls of the channels of 350 μm was observed during the first two oscillation cycles for a bubble with a maximum diameter slightly smaller than the height of the channel. The results of our investigations are compared to previous results obtained in similar configurations.

Capture capability of different intrinsic structures for helium bubbles in micro-nano composite 304L steels

• Micro-nano composite 304L stainless steel (MN304-La) is implanted with He + and annealed at 900 °C. • Nano-precipitates, grain boundaries, and dislocation are three intrinsic structures coexisting in MN304-La. • Three intrinsic structures have competing processes for capturing helium bubbles. • Nano-precipitates exhibit the strongest strength in capturing helium bubbles compared to grain boundaries and dislocations. The formation and migration of helium bubbles lead to performance degradation of structural materials in nuclear reactors. Intrinsic structures in nuclear materials can significantly influence the behavior and distribution of these helium bubbles under irradiation. Micro-nano composite 304L stainless steel (MN304-La) possesses three main kinds of intrinsic structures: grain boundaries (GBs), dislocations, and La-rich nano-precipitates (NPs). In this study, the helium bubbles on different intrinsic structures in MN304-La after He + ions implantation were characterized and analyzed. We compared the capabilities of different intrinsic structures in capturing helium bubbles and confirmed that NPs exhibit the strongest strength in capturing helium bubbles among the three kinds of intrinsic structures. We found that the competition of intrinsic structures for capturing helium bubbles depends on both the capture strength of the intrinsic structures and the capture distance to the helium bubbles.