Motion Of Gas Bubbles Research Articles

Motion characteristics of bubbles behind a sudden channel expansion

Characteristics of movement of gas bubbles behind a sudden channel expansion were studied experimentally. The data on the distribution of the gas phase and speed of movement of bubbles were obtained. It is shown that at a distance from the sudden channel expansion the bubbles slow down, which relates to the slowing down of liquid due to an increase in the flow cross section. After the reattachment point, the bubbles move along a more curved path than upstream. It is shown that bubble clusters can form in the flow recovery zone. The velocity distribution in the channel was studied by means of laser Doppler anemometry (LDA).

Microporosity formation and dendrite growth during solidification of aluminum alloys: Modeling and experiment

A coupled lattice Boltzmann-cellular automaton-finite difference (LB-CA-FD) model is proposed for the simulations of hydrogen porosity formation during dendritic solidification of binary hypoeutectic aluminum alloys. The present model involves the effect of hydrogen and solute partitioning at the solid/liquid interface, and the transport of hydrogen and solute concentrations as gas bubbles and dendrites grow in two and three dimensions. The dendrite growth and solute transport are simulated using a CA-FDM approach. The nucleation, growth, and movement of gas bubbles, as well as the transport of hydrogen, are calculated using the multi-phase LB model. After model validation by the tests of Laplace’s law and contact angle simulations on smooth and rough solid surfaces, the proposed model is applied to simulate the temporal evolution of hydrogen porosities and their interaction with dendrites during solidification of an Al-4 wt% Cu alloy. The simulated morphologies of gas pores and dendrites compare reasonably well with the experimental micrograph reported in the literature. The simulation results show that the gas bubbles nucleate at the roots of secondary dendrite arms preferentially. The competitive growth between the bubbles is visualized and found to be coherently controlled by bubble size, hydrogen supersaturation, and local hydrogen concentration in liquid. The simulations of microporosity formation together with columnar dendrite growth during directional solidification of an Al-4 wt% Cu alloy are carried out in two and three dimensions. It is found that the bubbles could move in the interdendritic liquid channel in the two-dimensional case. In the three-dimensional simulation, however, the bubbles are probably pinned by the secondary arms and thereby remain stationary. The three-dimensional simulation results are identical with the in situ experimental observation.

Mechanism and morphology control of underwater femtosecond laser microgrooving of silicon carbide ceramics.

Silicon carbide (SiC) ceramics have been widely used for microelectronics, aerospace, and other industrial fields due to their excellent chemical stability and thermal tolerance. However, hard machinability and low machining precision of SiC ceramics are the key limitations for their further applications. To address this issue, a novel method of underwater femtosecond laser machining was introduced in this study to obtain high precision and smooth surface of the microgrooves of SiC ceramics. The removal profiles were characterized in terms of width, depth, and surface morphology, which exhibited high dependence on the femtosecond laser processing parameters. The instability during the underwater processing affected by laser-induced gas bubbles and material deposition, however, limits the high surface accuracy of microgrooves and processing efficiency. The process condition transformation from a bubble-disturbed circumstance to a disturbance-free model was carefully investigated through a high speed camera for the femtosecond laser processing of SiC ceramics in water. The experiment results indicated that degree of disturbed effect was heavily dependent on size, distribution, and motion of laser-induced gas bubble. Furthermore, some typical evolution mechanisms of gas bubble and their influence on the removal profiles of microgrooves were discussed in detail. Bubble evolution has been proven to be mainly responsible for the behavior of laser propagation (focus model, total reflection, etc.), which notably affects microstructural characteristic of the microgrooves.

Экспериментальная оценка рациональных параметров предпускового ускорения для обеспечения запуска маршевых жидкостных ракетных двигателей в условиях невесомости

The paper presents the results of experimental studies of liquid fuel deposition processes in liquid rocket propulsion system tanks under the conditions of free, i.e. unperturbed, orbital and suborbital flight under the influence of small prestart G-load created by auxiliary engines before liquid sustainer engine launch. Having conducted the dimension analysis, we formed the structure of dimensionless groups, which determine the dependence of the time required for fuel deposition to the tank intake, on the g level, the degree of tank filling and the physical properties of the propellant. Experimental studies were carried out on a zero gravity test bench, using the principle of implementing below-G conditions at the free fall of the equipment under test, and on a laboratory aircraft at the "Kepler's parabola" flight. The results of experimental studies have shown that the effect of viscosity on the deposition rate is practically absent. At the same time, in experiments it was found that, starting with Bond numbers exceeding 40 ... 50, the dimensionless separation time does not depend on Bond number, the value of the latter being determined by the minimum radius of free gas inclusions, deformed during when ascending during the separation process. Findings of research show that with a considerable duration of the prestart G load pulse, the movement of gas bubbles in the liquid under the action of a G load pulse becomes quasistationary.

Numerical Simulation of Gas and Liquid Two-Phase Flow in the RH Process

A mathematical model has been established to simulate the gas–liquid two-phase flow in the RH process. The effects of interphase forces and the bubble-induced turbulence on the fluid flow and motion of gas bubbles were investigated. The interphase forces include the drag force, the virtual mass force, the lift force, and the pressure gradient force, respectively. The prediction results of the liquid velocity and the recirculation rate were compared with the experimental data measured with a Particle Image Velocimetry technique. The results indicated that the drag force and the virtual mass force dominated the gas plume shape in the up-leg snorkel, and significantly influenced the liquid velocity and the distribution of the gas phase. The lift force mainly affected the spreading of gas bubbles in the radial direction of the snorkel. However, the pressure gradient force could be neglected because it had no effect on the liquid velocity and the gas volume fraction. The increase of the bubble-induced turbulence decreased the liquid velocity, and by adjusting the lift force, it widened the shape of the individual gas plumes. To accurately simulate the gas–liquid flow behavior in the RH degasser, these appropriate parameters, such as the drag coefficient, the virtual mass coefficient, the lift coefficient, and the bubble-induced turbulence coefficient, were determined by comparing the predicted results and the measured data.

On Image Processing Algorithm for Registration of Three-Dimensional Gas Bubble Movement Using a High Speed Camera

The paper deals with an imaging computer tomography method based on simple image processing techniques for two phase flow analysis. Moreover, it has been presented the algorithm of 3D bubble trajectory reconstruction using a single high speed camera and the system of mirrors. In the experiment a glass tank filled with distilled water was used. The nozzle through which the bubbles were generated was placed in the center of the tank bottom. Through the use of basic image processing and analysis techniques such as noise reduction, smoothing, edge detection and few algorithms like close contour filling, tracking single bubble etc. proposed in paper it became possible to draw out 3D trajectories for gas bubble paths in liquid. In the paper the measurement error of imaging computer tomography method has been estimated. The maximum measurement error recorded for this method was within the limits ±0,65 [mm] for a certain set of parameters like: resolution, mirror angle and deviation error of z axis from the 90o vertical line. Trajectories of subsequently departing bubbles were visualized in the form of figures.

On Image Processing Algorithm for Registration of Three-Dimensional Gas Bubble Movement Using a High Speed Camera

The paper deals with an imaging computer tomography method based on simple image processing techniques for two phase flow analysis. Moreover, it has been presented the algorithm of 3D bubble trajectory reconstruction using a single high speed camera and the system of mirrors. In the experiment a glass tank filled with distilled water was used. The nozzle through which the bubbles were generated was placed in the center of the tank bottom. Through the use of basic image processing and analysis techniques such as noise reduction, smoothing, edge detection and few algorithms like close contour filling, tracking single bubble etc. proposed in paper it became possible to draw out 3D trajectories for gas bubble paths in liquid. In the paper the measurement error of imaging computer tomography method has been estimated. The maximum measurement error recorded for this method was within the limits ±0,65 [mm] for a certain set of parameters like: resolution, mirror angle and deviation error of z axis from the 90o vertical line. Trajectories of subsequently departing bubbles were visualized in the form of figures.

Experimental and numerical simulation of gas bubbles motion in liquid metal

Motion and heat transfer of bubbles with liquid metal have been investigated in the present paper. Theoretical studies have been provided with help of modern numerical approaches and the HYDRA-IBRAE/LM code to simulate two-phase flows. Two-fluid model has been used in the presented code to calculate thermal hydraulic processes into two-phase liquid metal coolant. To close two-fluid model, the system of constitutive relation has been used. The brief description some of the constitutive relations has been presented in the current paper. For example, some features of the wall friction coefficient calculation have been discussed. Gas injection experiments and experimental investigations of the bubbles motion and heat transfer have been provided on a liquid metal test facility. Obtained results can help to receive new information about two-phase liquid metal flows. On the base of this information a validation of numerical codes, which are already have an industrial application, can be carried out.

Characteristics of opening in the ice cover formed by the gas vents

«Proparina» (russ) is a small hole in the ice cover formed by steaming of the ice by the gas vents. Some characteristics of this phenomenon were studied by the example of formation of one proparina found in March 2015 in the ice cover of the shallow eutrophic Lake Shakshinskoye (Trans-Baikal Region). The interest in this object is due to the fact that a proparina, unlike a polynya (small water opening in ice), is formed after the establishment of the ice cover and it can appear in those parts of a reservoir where there is no clearly expressed inflow or outflow of water. Although proparinas do often occur on some water bodies, e.g. Lake Baikal, a detailed description of their structure and process of formation is not available. Research on features of the proparina in the ice of the Lake Shakshinskoye and adjacent areas of this reservoir was carried out on March 25 and 28 in 2015. Melting at the lower and upper ice cover boundaries started at that time, and it was found that the proparina under investigation was formed in the center of a dome-shaped area where the ice thickness decreased compared to the adjoining parts within a distance of 200 meters. Gradient of the lower surface in the dome was on average 5 centimeters per 100 meters at a distance from the center. We found a narrow channel in the ice through which gas came into the proparina in the form of separate portions. The maximum recorded volume of gas that came into the open proparina reached 10 l/min. The channel is supposed to be formed at the end of winter period due to the release of gas during the melting of the lower layers of the ice cover and the subsequent movement of gas bubbles into the center of the dome. To study the ice cover structure, we measured thermo-microwave self-radiation of the “ice-water” system in the centimeter range. Such measurements allow detecting changes in ice thickness with an accuracy of 1 cm. It is assumed that the accumulation of gases under the ice causes the instability of the water column due to warming by the heat fl w from the bottom layers and initiates the circulation and, thus, formation of proparina.

The shape and motion of gas bubbles in a liquid flowing through a thin annulus

We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.

A technique for measuring ensemble-averaged, three-component liquid velocity fields in two-phase, gas–liquid, intermittent pipe flows

Gas–liquid intermittent flows can be found in many engineering applications, nevertheless a detailed knowledge of this flow pattern is still not fully available. In the present work, an experimental study was conducted with the objective of developing a measurement procedure capable of providing ensemble-averaged three-component velocity fields in the liquid phase of a gas–liquid, intermittent, horizontal flow in a pipe. To this end, a high-frequency stereoscopic particle image velocimetry system (SPIV) was employed, combined with the laser induced fluorescence (LIF) technique to separate the light scattered by the liquid–gas interfaces from that emitted by the fluorescent tracer particles. A set of photogates was used to trigger the SPIV system, allowing for the measurement of velocity fields in the liquid plug, downstream of the elongated bubble, and in the liquid film, upstream of the elongated bubble nose position. The triggered measurements allowed the determination of ensemble-averaged three-component velocity fields at different positions in relation to the bubble nose, obtained from the replication of a sufficiently large number of bubble passage events. Contours of the liquid flow streamwise vorticity component in cross-stream planes upstream and downstream of the bubble nose tip were also obtained from the SPIV measurements. The photogate system was also employed to measure the bubble velocity. This information was used to transform time-based into space-based velocity field data. This allowed the construction of a three-dimensional representation of the ensemble-averaged structure of the gas bubble nose and the associated vortical structures induced in the liquid flow. The three-component velocity information obtained revealed the influence of the gas bubble motion on the liquid flow in the plug and liquid film regions.

Experimental Simulation of Hydrodynamics and Heat Transfer in Bubble and Slug Flow Regimes in a Heavy Liquid Metal

For the confirmation of the claimed design properties of a reactor plant with a heavy liquid-metal coolant, computational and theoretical studies should be performed in order to justify its safety. As one of the basic scenarios of an accident, the leakage of water into the liquid metal is considered in the case of steam generator tube decompression. The most important in the analysis of such a kind of accidents are questions concerning the motion and heat exchange of steam bubbles in the steam generator and the probability of blocking the flow area owing to freezing coolant, since the temperature of boiling feedwater in the steam generator can become lower than the melting point of lead. In this paper, we present main approaches and relationships used for the simulation of the motion of gas bubbles and heat transfer between bubbles and liquid metal flow. A brief description of the HYDRA-IBRAE/LM computational code that can be used to analyze emergency situations in a liquid metal-cooled reactor facility is also presented. It should be noted that the existing experimental data on the motion and heat transfer of gas bubbles in a heavy liquid metal are insufficient. For this reason, in order to verify the HYDRA-IBRAE/LM code models, experiments have been performed on the cooling of liquid lead by argon and on the motion of gas bubbles in the Rose’s alloy. In particular, a change in the temperature of the coolant over time has been studied, and the void fraction of gas at different flow rates of gas has been measured. A detailed description of the experiments and a comparison of the results of the calculations with the experimental data are presented. The analysis of uncertainties made it possible to reveal the main factors that exert the greatest effect on the results of calculations. The numerical analysis has shown that the models incorporated into the HYDRA-IBRAE/LM code allow one to describe to a sufficient degree of confidence the process of cooling liquid lead melt when argon bubbles pass through it, simulating the flow of water into the liquid metal in the course of steam generator tube rupture.

Investigation on the Fluid Flow and Decarburization Process in the RH Process

In the current study, mathematical models were developed to predict the fluid flow and motion of gas bubbles in the RH degasser. The multiphase model volume of fluid was employed to simulate the free surface in the vacuum chamber, while injected argon bubbles were treated as discrete-phase particles and tracked using the discrete-phase model. The expansion of argon bubbles was considered. The variation of the liquid level in the vacuum chamber and the effect of the number of nozzles on the recirculation rate were discussed in detail. The optimal number of nozzles was 6. Furthermore, the decarburization process was simulated by considering three reaction sites: the free surface, inner sites of the liquid steel in the vacuum chamber, and the bubble surface. The carbon contents predicted by the decarburization model were well validated by the measured results in the literature. The effect of the gas flow rate on the decarburization process was also investigated. With the increasing gas flow rate, the carbon content gradually decreased, and then reached the steady state. The optimal gas flow rate was 2400 L/min, which was explained by the comparison of the decarburization rate constants. Meanwhile, the decarburization reaction was in a quick decarburization stage for the first 7 minutes, and for the next 13 minutes, the decarburization rate constant rapidly decreased.

Surface Phenomena in the Smelting Bath of an Oxygen Converter

The oxygen-converter production of steel is determined by processes in the converter’s reaction zone, which consists of primary and secondary regions. The primary region is the crater formed by the collision of a supersonic gas jet with the molten-metal surface. It is filled with metal droplets (diameter 0.1–2 mm). The surrounding secondary region consists of melt with an enormous quantity of gas bubbles (diameter 0.2–4 mm). The total surface area of the droplets and bubbles is four orders of magnitude greater than the surface of the quiescent melt. That indicates the important role of processes at phase boundaries in steel production. The structure of the reaction zone and the corresponding temperature distribution are studied by hot simulation, when the molten metal is blown by oxygen in a transparent quartz crucible. The transparent walls permit photographic and video recording of the processes in the crucible. Besides the temperature distribution, the hydrodynamics of the bath may be studied directly in the injection zone. The most unexpected result of hot simulation is the motion of the bubbles in the secondary region. They move normal to the crater surface. In other words, their motion is almost horizontal, rather than vertical, as in cold simulation in water. This may be attributed to nonuniformity of the melt’s surface tension, resulting in motion of the bubbles toward higher temperatures. In liquid with a temperature gradient, the surface tension will be different ahead of and behind the bubbles. The forces pushing the bubbles from behind are greater than the forces at the front. Accordingly, they move toward the region of lower surface tension. The nonuniformity of the surface tension is due to the temperature gradient (up to 1200°C within the secondary region) and the change in concentration of the melt components, especially oxygen. The surface tension of the ferrocarbon melt changes in a complex manner with increase in temperature. The surface tension rises on heating to 1550°C, but begins to decrease beyond 1550–1600°C. With decrease in carbon content in the melt, the maximum value of the surface tension increases. The motion of gas bubbles and other phases toward lower surface tension begins at the 1550°C isotherm, which is therefore the external boundary of the secondary region, separating it from the remainder of the bath. Within this boundary, the resultant vector of the surface forces pushes the gas bubbles and slag particles, together with the molten metal, horizontally toward the crater, at increasing speed. This determines the hydrodynamics of the smelting bath and the associated redistribution of oxygen over different parts of the bath and hence the refining process as a whole.

Peculiarities of gas bubble movement in limited volume of viscous liquid

The results of an experimental study of the motion of a gas bubble in a limited volume of a viscous liquid are presented The influence of the velocity of the fluid on the location of the gas bubble in its volume is shown A method for calculating the process is proposed and a block diagram for calculating its parameters is developed In the calculations the speed of the rotational motion of the viscous fluid the volume of the gas space and the physical properties of the liquid are taken into account The gas space which is in the static position above the liquid level when rotating creates a bubble of a spherical shape that moves along the volume of the liquid to make it move in the vessel Such mixing can cause intensification of various technological processes

Effect of Slotted Anode on Gas Bubble Behaviors in Aluminum Reduction Cell

In the aluminum reduction cells, gas bubbles are generated at the bottom of the anode which eventually reduces the effective current contact area and the system efficiency. To encourage the removal of gas bubbles, slotted anode has been proposed and increasingly adopted by some industrial aluminum reduction cells. Nonetheless, the exact gas bubble removal mechanisms are yet to be fully understood. A three-dimensional (3D) transient, multiphase flow mathematical model coupled with magnetohydrodynamics has been developed to investigate the effect of slotted anode on the gas bubble movement. The Eulerian volume of fluid approach is applied to track the electrolyte (bath)–molten aluminum (metal) interface. Meanwhile, the Lagrangian discrete particle model is employed to handle the dynamics of gas bubbles with considerations of the buoyancy force, drag force, virtual mass force, and pressure gradient force. The gas bubble coalescence process is also taken into account based on the O’Rourke’s algorithm. The two-way coupling between discrete bubbles and fluids is achieved by the inter-phase momentum exchange. Numerical predictions are validated against the anode current variation in an industrial test. Comparing the results using slotted anode with the traditional one, the time-averaged gas bubble removal rate increases from 36 to 63 pct; confirming that the slotted anode provides more escaping ways and shortens the trajectories for gas bubbles. Furthermore, the slotted anode also reduces gas bubble’s residence time and the probability of coalescence. Moreover, the bubble layer thickness in aluminum cell with slotted anode is reduced about 3.5 mm (17.4 pct), so the resistance can be cut down for the sake of energy saving and the metal surface fluctuation amplitude is significantly reduced for the stable operation due to the slighter perturbation with smaller bubbles.

Combined experimental and theoretical investigation of the gas bubble motion in an acoustic field

The objective of this paper is to apply the combined experimental and theoretical method to investigate the various behaviors of gas bubbles in an acoustic field. In the experiments, high-speed video and ultrasonic processor are used to capture the transient evolution of gas bubble patterns, as well as velocity profiles. In the theoretical analysis, the theories of primary and secondary Bjerknes forces and buoyancy force are introduced to accurately demonstrate the variations of bubble volume and motion. Results are presented for gas bubbles with the radius of 1.4mm under an acoustic field with a frequency of 18kHz, for three cases, namely single bubble rising in a quiescent liquid, acoustic single bubble oscillation and two bubbles coalescence conditions. The results show that the fragments around the single gas bubble presents the periodical behaviors, namely, splitting, attraction, and secondary splitting motion. The centroid of the single gas bubble almost oscillates without motion upwards or downwards, because of the equilibrium of the primary Bjerknes force caused by acoustic waves and the effect of the buoyancy force. For the two coalescing bubbles, the resultant of buoyancy, primary and secondary Bjerknes forces acting on two bubbles are same in magnitude, but in opposite direction, which indicates that two gas bubbles attract each other and and coalesce into one.

Note on in situ (scanning) transmission electron microscopy study of liquid samples

Liquid cell (scanning) transmission electron microscopy has been developed rapidly, using amorphous SiNx membranes as electron transparent windows. The current interpretations of electron beam effects are mainly based on radiolytic processes. In this note, additional effects of the electric field due to electron-beam irradiation are discussed. The electric field can be produced by the charge accumulation due to the emission of secondary and Auger electrons. Besides various beam-induced phenomena, such as nanoparticle precipitation and gas bubble formation and motion, two other effects need to be considered; one is the change of Gibbs free energy of nucleation and the other is the violation of Brownian motion due to ion drifting driven by the electric field.

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (εmf) and the minimum fluidization velocity (Umf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017

Some approaches to numerical modelling of a phenomenon observed during steam generator tube rupture in the reactor with liquid metal coolant

The presented paper contains a description of approaches to simulate processes, observed during leakage in the steam generator of the reactor with liquid metal coolant. These approaches have been implemented in thermal hydraulic code HYDRA-IBRAE/LM. To calculate motion of gas bubbles in liquid metal flow and heat transfer of gas bubbles with metal, different relations are used in HYDRA-IBRAE/LM code. The code contains models of chemical interaction between water and sodium for modelling of tube rupture in sodium cooled fast reactors, Modelling of the experiments has been made using HYDRA-IBRAE/LM code. The results of the modelling with determined main factors are presented in the article.