Fraction Of Bubbles Research Articles

Bubble characteristics from injected air sheet through slots in a water cross-flow

While air injection has already been widely studied for stagnant water at low air injection rates for circular injectors, limited data is available on gas in liquid cross-flows injected through slots at high air flow rates. The key purpose of this work is to observe the evolution of bubble swarm characteristics for simple geometries such that, by correlation rules, these characteristics can be projected for more complicated geometries and environment. To that effect, an experimental water loop with both controllable water and air flow rates is used to generate bubble clouds with cross-flow air injection.Measurements with an optical probe allow the void fraction, the size and velocity of bubbles to be measured. The findings show that the general size of bubbles increases with an increase in the air flow rate for a constant water flow rate. Furthermore, bubbles pick up speed while moving away from the injection slot. Bigger bubbles travel faster than small ones ; they also tend to rise to the top of bubble plumes while small bubbles stay in the lower portion. Finally, the maps of void fraction indicate that its local values drop while heading away from the injection slot.

A new transient model of multi-scale bubble migration and evolution during gas-liquid flow in pipelines

This paper investigated the bubble migration and evolution during gas-liquid flow in oil-pipelines. Multi-scale bubbles migrated in the pipe while interacting with each other, causing coalescence and disintegration. The bubbles collided and coalesced, forming a new air pocket in the downward inclined pipe that blocked the transportation of both the gas and liquid phases. Consequently, the buoyance weakened the migration ability of the bubbles, which is known as re-coalescence. To investigate, a new transient model applied with the population balance model (PBM), and Eulerian-Lagrangian coupling scheme is proposed. The complex flow field was modeled using the Eulerian scheme, while the migration of the bubbles was designed in the Lagrangian mesh. Furthermore, the PBM was employed to describe the multi-scale bubble interaction and migration. Smooth particle hydrodynamics were applied to connect the bubble-dimension and pipe-dimension descriptions. Therefore, the bubble fraction could be described to predict the re-coalescence of air pockets. The required unknown model parameters were extracted from the published experimental data, while the simulation results corresponded well with the measured results. During the simulation, the effect of the liquid Froude number and pipe inclination on the size distribution of the multi-scale bubbles were investigated. The results indicated that an increase in the pipe inclination and liquid Froude number improved the bubble migration capacity, leading to a broader distribution of bubbles of all sizes. • A new transient modelfor bubble transportation and evolution in gas-liquid flow in the oil pipeline is proposed. • The migration abilities coupled with the bubble coalescence and breakup of different sizes of bubbles in the inclined pipe are analyzed. • The prediction of the transportation blockage based on bubble tracking and smooth particle hydrodynamics is introduced. • A new insight into the pipeline blockage from the bubble-dimension to the pipe-dimension is introduced. • Effects of pipe inclination, as well as liquid Froude number to the bubble size distribution in oil pipeline are analyzed.

Cavitation analysis of a Delft twisted hydrofoil using multi-process cavitation model

Cavitation is essentially formed of a combination of multiple processes. Most of the conventional cavitation models consider these in a single model, and the conservation equation for void fraction of cavitation bubbles is solved. To understand the relationship between cavitation and turbomachinery performance, not only void fraction, but also states of cavitation bubbles such as number and radius may become useful data. The multi-process cavitation model proposed by Tsuda and Watanabe is one of the solutions. This model has been implemented into commercial CFD software scFLOW. To validate the implemented cavitation model, cavitation around a Delft twisted hydrofoil was analyzed, and the result was compared with past studies. From a parametric study of the bubble growth coefficient in this cavitation model, it was found that 0.2 for expansion and 0.05 – 0.1 for shrinkage are appropriate. Cavitation shedding frequency and cavity volume agreed well with the past studies by using those coefficients. Since the multi-process cavitation model provides detailed cavitation bubble states, a new index is proposed to detect bubbly regions and its distribution compares well with experimental data. This implies that the multi-process cavitation model will help us to predict cavitation noise and erosion risk as well as to understand the mechanism of the cavitation phenomenon.

CFD study of the characteristics of a single elongated gas bubble in liquid in a moderately inclined pipe

In recent years, Computational Fluid Dynamics (CFD) modelling methods have been applied to study the behavior of a single elongated bubble in stagnant and flowing liquid. To date, only very few studies have been performed for slightly upwardly inclined pipes. This work presents mostly 2D numerical simulations based on the Volume of Fluid approach, dealing with the characteristics of a single elongated bubble injected into a liquid in a slightly upwardly inclined pipe. CFD-based results were compared with experimental results. In general, except the numerical bubble length, drift velocity, bubble fraction and bubble shape, agreed fairly with the experimental outcomes.

Prospects for the development of the concept of a safe nuclear reactor BREST of maximum limiting power for nuclear energetics of the middle of the XXI century

In Russia (JSC NIKIET) a project of a fast reactor BREST-1200 is being developed. The article analyzes the safety of the BREST reactor with maximum limiting power (BREST-2000, -2400). In BREST reactors with an electric power of 1200 MW and more, the void reactivity effect of can be positive. The most dangerous scenario is associated with the entrainment of bubbles into the central part of the core (with a volume fraction of bubbles of 50 … 60%). In this case, the magnitude of the void reactivity effect is several times higher than the effective fraction of delayed neutrons β. For the BREST-2400 reactor, the maximum void reactivity effect is about 7 β (for BREST-2000, about 6 β). When other ATWS are implemented against the background of erosion and corrosion of structural steel, the total introduced reactivity can also exceed β, which will lead to an accident. To ensure safety, it is necessary to adjust the design solutions. When used as a coolant, lead extracted from thorium ores (with a concentration of 208Pb isotope from 75 to 98%) and tungsten coatings of fuel claddings, the void reactivity effect is negative even in the BREST-2400 reactor, and when switching to a new cermet fuel (0.8 MN - 0.2 U) it is easy to ensure the safety of the reactor.

Drag coefficient and volume fraction of bubbles in a supercritical water fluidized bed

The supercritical water fluidized bed (SCWFB) is a recently introduced reactor for biomass gasification that does not release pollutants. Four groups of Geldart B-type quartz sands with different particle sizes were fluidized at a system pressure of 20–27MPa and system temperature of 410–570°C. A series of experiments were performed for determining the drag coefficient and volume fraction of bubbles. The effects of the particles’ size, superficial velocity, system pressure, and temperature on the drag coefficient and volume fraction are discussed. In addition, a correlation between experimental and computed values is demonstrated for both the drag coefficient and volume fraction in SCWFBs. The relative error of the correlation is within ±30%. The results of this study provide significant guidance for the scaling-up design of SCWFBs and for the development of supercritical water gasification technology.

Impact of fission gas bubbles on thermal conductivity of UO[formula omitted] fuels with high thermal conductivity additives

High thermal conductivity additives are being considered to create composite nuclear fuels with a higher effective thermal conductivity (ETC) to reduce the peak fuel temperatures during reactor operation. However, the benefits of the additive may be reduced, or possibly eliminated, when placed into a reactor environment. In this work, we investigate the impact of fission gas bubbles on the ETC of composite fuels. We create 2D simulations representing a small portion of a composite UO2 fuel containing a single additive particle and calculate the ETC with various volume fractions of fission gas bubbles. Our results show that fission gas bubbles, depending on the volume fraction and contact angle at the additive interface, can completely remove any benefit of the additive and even result in a lower thermal conductivity than in UO2 without the additive. We also expose a trend that relates the fraction of additive that is screened by bubbles to the ETC. We then propose a model which predicts the ETC of a composite fuel with fission gas bubbles to better estimate the benefits of a novel fuel design.

Particle Image Velocimetry study on the formation of gas bubbles and electrolyte velocity due to electrochemical reactions for different distances between the electrodes of Flooded Lead_ Acid batteries

During the charging processes in Flooded Lead_ Acid batteries (FLA), the production of gas bubbles occurs and it effects on the FLA performance. In this experimental investigation, the effect of distance between electrodes at different charge-discharge rates on the electrolyte flow velocity and bubbles behavior is investigated based on the Particle Image Velocimetry method. The reduction in the distance decreases the FLA capacity linearly at the same processes. It leads to an increase in the void fraction of bubbles and a decline in the diameter and rising velocity of the bubbles. The maximum diameter of the bubbles during the charging processes is very small compared to the distances between the electrodes. The effect of electrolyte velocity compared to the effect of the average rising velocity of bubbles on the FLA performance is negligible. The results show that the increase in the void fraction of the bubbles and the formation of bubble layers in the vicinity of the electrodes is the most critical factor at the increase of the ohmic resistance.

Modelling and simulation of H2-H2O bubbly flow through a stack of three cells in a pre-pilot filter press electrocoagulation reactor

Computational fluid dynamics simulations were carried out to describe the hydrodynamic characteristics of a two-phase bubbly flow in a filter-press flow reactor stack of three cells, which is typically used in electrocoagulation (EC). The hydrogen evolution reaction (HER) took place at the cathode; dissolution of aluminium occurred at the anode. The fundamental transport equations of momentum and electrical potential were simultaneously solved to simulate the H2-H2O flow. Continuous (H2O) and dispersed phase (H2) velocity fields were modelled via the Euler-Eulerian approach, using the biphasic Reynolds Averaged Navier-Stokes (RANS) equations and the standard k − ε turbulence model. The influence of volumetric flow rate (1.7 ≤ Q ≤ 15 cm3 s−1) and applied current density (–28 ≤ j ≤ –5 mA cm−2) was systematically addressed to calculate the fraction of dispersed phase and current distribution along the electrodes. The evolved H2 bubbles were transported away from the electrode by the liquid flow. The dispersion of H2 through the electrode gap showed a modest bubble curtain profile due to the liquid flow rate. A homogeneous current distribution along the electrode length was experienced due to the geometrical design of the electrochemical cell and the low degree of H2 dispersion. The velocity profiles of the H2-H2O mixture were different in each cell due to the change of flow direction. H2 bubbles increased the velocity of the liquid phase but the gas fraction of such bubbles resulted in a higher pressure drop. Good agreement between theoretical and experimental residence time distribution curves was achieved; the experimental aluminium dose released by the anode agreed well with the simulations.

Experimental investigation on the effect of surface characterization of electrodes on the gas bubble dynamics in electrolyte flow and performance of FLA batteries by using PIV

Generation of oxygen and hydrogen bubbles at vicinity of the electrodes FLA battery at SOC 60%. In the flooded lead_acid batteries (FLAB), gas bubbles are initially formed on the surface of the electrodes, which are produced by electrochemical reactions, and then released into the electrolyte. In the present investigation, the effect of surface characterization of electrodes of FLAB on the gas bubble dynamic parameters in the electrolyte flow at different charging/discharging rates (C-rates) are studied utilizing particle image velocimetry (PIV) method. The results show that the capacity of FLAB have a linear behavior due to changes in each of the two parameters of the surface characterization of electrodes and the C-rate. At all State of charges (SOCs) of FLAB cells in different tests, increasing average roughness ( Ra ) and average wavelength of the roughness ( λa ) in the electrode surfaces, results in an increase in average bubble diameter and bubble rising velocity. Nevertheless, a sharp decrease in the void fraction of bubbles within the electrolyte was observed due to the increment in λa and Ra . Also, the effect of the rising movement of gas bubbles within the electrolyte on the average electrolyte velocity pattern in the gap between the electrodes by changing the surface characterization of electrodes are investigated in detail.

Effect of capillary number on morphological characterizations of trapped gas bubbles: Study by using micro-tomography

A series of capillary trapping experiments were conducted in unconsolidated porous media using high-resolution micro-tomography. Morphological characterizations of trapped gas bubbles, including residual gas saturation, bubble size distribution and interfacial area, were determined from reconstructed three-dimensional images. Different flooding flow rates, corresponding to capillary number ranges from 1.0 × 10−7 to 3.8 × 10−4, were performed to investigate the impact of flow rate on these characterizations during imbibition processes. At low capillary number, where capillary forces are dominant, the residual gas saturation is independent of capillary number and the bubble size distribution shows a universal power-law behavior predicted from percolation theory. When the capillary number increases above a critical value, the residual saturation decreases sharply, and the bubble size distribution no longer exhibits power-law behavior. As capillary number increases, the volume fraction of single-pore bubbles increases from about 16% to more than 80%. Furthermore, we explored the interfacial area of trapped bubbles, including gas–solid and gas–water (meniscus) interfacial areas, which directly affect the mass transfer process inside porous media. The specific interfacial area exhibits a linear relation with residual saturation, and the effect of particle size can be normalized by multiplying the median particle diameter. Moreover, we observed that the meniscus/total interfacial area ratio increases from 0.61 to 0.78 with an increase in capillary number, because the smaller singlet has a higher interfacial contact with the wetting phase.

Mechanism and performance of a hydrofoil bubble generator utilized for bubbly drag reduction ships

Following the success of bubbly drag reduction for marine vessels by hydrofoil bubble generators (Kumagai et al., 2015), we conducted two kinds of experiments to specify and improve its working principle. Firstly, PIV measurement was carried out to elucidate the flow structure around a hydrofoil running beneath a free surface. Froude number dependence of the anti-stall characteristics and cancellation of vertical pressure gradient above the hydrofoil were confirmed, which entrain the atmospheric air into water flow. Secondly, we mounted the hydrofoil to a model ship that runs in a 100-m-towing facility and found three different two-phase flow patterns in realizing bubble generation. Volume flow rate of generated bubbles increased with the ship speed to the power of greater than two, thus wall-occupation fraction of bubbles also increases with the ship speed. Furthermore, hydrofoil drag during bubble generation was measured and used to estimate net power-saving, having proved advantage of the hydrofoil bubble generator for long ships at high speed conditions.

Validation of interfacial area concentration model for simulating bubbly and cap/slug flow behaviors in large diameter pipes using LSTF, ATLAS, and horizontal pipe experiment data

This study discusses Interfacial Area Concentration (IAC) models developed for bubbly and cap/slug flow regimes in a large diameter pipe incorporating recently developed small bubble fraction correlations in various flow conditions. The main focus of this study was the robust simulation of the behaviors of bubbly and cap/slug flows in large diameter pipes observed in Large Scale Test Facility (LSTF), Advanced Thermal-hydraulic test Loop for Accident Simulation (ATLAS), and horizontal flow test facility. Since the IAC was one of the key parameters to achieve this, an IAC model for large diameter pipes was first derived based on the existing Hibiki-Ishii IAC model for bubbly flow, and the Shen-Hibiki IAC model for cap bubbly-to-churn flow. The volume fraction of group-1 bubbles, i.e., small spherical bubbles, in the IAC model, which have been developed based on the assumption that the group-1 bubbles exponentially decay in the slug flow regime, was refined. As a result, a more physical model of the group-1 bubble number density that remains constant in the cap/slug flow regime was imposed for large diameter pipes. The validity of the IAC model for large diameter pipes in predicting the interfacial drag force between gas and liquid phases in large pipes of multiple test facilities was then examined. The validation of the proposed IAC model was conducted through simulations of 1) the loss of Residual Heat Removal (RHR) during the mid-loop operation of the LSTF, 2) the 6-inch Small Break Loss of Coolant Accident (SBLOCA) of the ATLAS, and 3) the two-phase flow behaviors under a wide range of fluid conditions in the horizontal pipe test facility. Improved results were obtained with the proposed model as it indicated that there was good agreement between the calculated results and the experimental data throughout the whole transient.

Analyzing the microstructure of a fresh sorbet with X-ray micro-computed tomography: Sampling, acquisition, and image processing

Abstract X-ray micro-computed tomography and image processing techniques were used to analyze fresh frozen sorbets at the outlet of a batch freezer. Sorbets made from water and sucrose were visualized and their microstructure was quantified with a resolution of 9 μm. Sodium iodide was confirmed to enhance the contrast between the unfrozen water and ice in sorbets. A thermostated box was employed to keep the samples at frozen state and constant temperature (close to −6 °C) during imaging. A reproducible quantification of size distributions and volume fractions of ice crystals and air bubbles were obtained. Data concerning ice crystals were in agreement with cryo-SEM imaging. Ice crystals represented approximately 50%wt of the product and their mean size was about 60 μm whereas air bubbles represented about 6% of the volume. Finally, X-ray microtomography equipped with a thermostated box was found to be a particularly relevant technique for the analysis of the microstructure of frozen desserts.

Experiment of Engine Lubricating Oil Bubble Fraction in the Driving Cycles

Cavitation phenomenon always occurs in gerotor pumps. Bubble fraction of oil has remarkable influence in bearing lubricating characteristic. Oil mass flow rate and density is measured in a gerotor oil pump performance test, bubble fraction is analyzed based on measured oil density. Experimental results show that bubble volume fraction linearly decrease with the increasing of outlet pressure at constant pump speed. With the increasing of pump speed, influence of outlet pressure on bubble mass fraction decreases gradually. In the driving cycles, bubble mass fraction also increases with the increase of pump outlet pressure, and has a closely positive correlation with oil flow and pump speed. When the temperature of lubricating oil rises from 25°C to 100°C, the average volume fraction decreases about 72.6%, and the average mass fraction decreases about 84.3%. The changing rate of oil bubble fraction apparently lags behind the pump speed, it means that oil bubble fraction in acceleration stage is much higher than deceleration stage.

Pool boiling heat transfer and bubble behavior on the treelike networks with wedge-shaped channels

This paper designs the treelike networks with the wedge-shaped channel from the idea of bionics. Pool boiling heat transfer experiments of FC-72 under different liquid subcoolings were conducted on the networks with different branch levels. The bubble phenomenon was recorded to reveal the boiling mechanism. The bubble pumpless movement towards the diverging direction in the wedge-shaped channel under the action of capillary force was observed. Under the saturated condition, the surfaces are easier to be completely covered by bubbles and the effect of treelike networks is hindered, which results in similar heat transfer performance. With the increase of liquid subcoolings, the bubble fraction decreases and the effect of the branch level comes into effect. On the surface with high branch level, the bubbles are transported immediately at small radius by the narrow wedge-shaped channel. The moving bubbles disturb the fluid and coalesce with other bubbles, which increases the heat transfer coefficient. There are also more liquid supply pathways between small bubbles, which increases the critical heat flux. The importance of this research is to provide a solution to the difficulty of bubbles vertical departure caused by the lack of buoyancy under microgravity.

Helium-induced damage behavior in high temperature nickel-based alloys with different chemical composition

The Inconel 617 and Alloy 800H have recently acquired significant attention owing to their potential applications in molten salt reactors (MSRs). Considering that helium induced damage is the main irradiation degradation pathway for the nickel-based alloys used for the MSRs, the helium ion irradiation experiments of the Inconel 617 and Alloy 800H have been performed to evaluate their potential applications in MSRs. The Hastelloy N alloy is selected as a reference alloy. The extent of microstructural and mechanical changes of the investigated alloys resulting from helium ion irradiation at 700 °C was characterized by transmission electron microscopy (TEM) and nanoindentation. The nanoindentation results showed that both the Inconel 617 and Alloy 800H possess inferior irradiation hardening resistance when compared to the Hastelloy N alloy reference alloy. Moreover, the TEM graphs and additional analyses indicated that the content of Ni, Mo, and Cr in the investigated alloys will affect the characteristics of helium bubbles and dislocation loops. The Alloy 800H demonstrates a greater potential to function as the structural material for high-temperature MSRs than the Inconel 617, which is due to its good mechanical properties and smaller volume fraction of helium bubbles.

Monitoring crustal CO<sub>2</sub> flow: methods and their applications to the mofettes in West Bohemia

Abstract. Monitoring of CO2 degassing in seismoactive areas allows the study of correlations of gas release and seismic activity. Reliable continuous monitoring of the gas flow rate in rough field conditions requires robust methods capable of measuring gas flow at different types of gas outlets such as wet mofettes, mineral springs, and boreholes. In this paper we focus on the methods and results of the long-term monitoring of CO2 degassing in the West Bohemia/Vogtland region in central Europe, which is typified by the occurrence of earthquake swarms and discharge of carbon dioxide of magmatic origin. Besides direct flow measurement using flowmeters, we introduce a novel indirect technique based on quantifying the gas bubble contents in a water column, which is capable of functioning in severe environmental conditions. The method calculates the mean bubble fraction in a water–gas mixture from the pressure difference along a fixed depth interval in a water column. Laboratory tests indicate the nonlinear dependence of the bubble fraction on the flow rate, which is confirmed by empirical models found in the chemical and nuclear engineering literature. Application of the method in a pilot borehole shows a high correlation between the bubble fraction and measured gas flow rate. This was specifically the case for two coseismic anomalies in 2008 and 2014, when the flow rate rose during a seismic swarm to a multitude of the preseismic level for several months and was followed by a long-term flow rate decline. However, three more seismic swarms occurring in the same fault zone were not associated with any significant CO2 flow anomaly. We surmise that this could be related to the slightly farther distance of the hypocenters of these swarms compared to the two ones which caused the coseismic CO2 flow rise. Further long-term CO2-flow monitoring is required to verify the mutual influence of CO2 degassing and seismic activity in the area.

Modeling intra-granular fission gas bubble evolution and coarsening in uranium dioxide during in-pile transients

The description of intra-granular fission gas behavior during irradiation is a fundamental part of models used for the calculation of fission gas release and gaseous swelling in nuclear fuel performance codes. The relevant phenomena include diffusion of gas atoms towards the grain boundaries coupled to the evolution of intra-granular bubbles. While intra-granular bubbles during normal operating conditions are limited to sizes of a few nanometers, experimental evidence exists for the appearance of a second population of bubbles during transients, characterized by coarsening to sizes of tens to hundreds of nanometers and that can significantly contribute to gaseous fuel swelling. In this work, we present a model of intra-granular fission gas behavior in uranium dioxide fuel that includes both nanometric fission gas bubble evolution and bubble coarsening during transients. While retaining a physical basis, the developed model is relatively simple and is intended for application in engineering fuel performance codes. We assess the model through comparisons to a substantial number of experimental data from SEM observations of intra-granular bubbles in power ramp tested uranium dioxide samples. The results demonstrate that the model reproduces the coarsening of a fraction of the intra-granular bubbles and correspondingly, predicts gaseous swelling during power ramps with a significantly higher accuracy than is allowed by traditional models limited to the evolution of nanometric intra-granular bubbles.

Movement and mixing behavior of a single biomass particle during combustion in a hot fluidized bed combustor

The characteristic motion of fuel particles in a fluidized bed (FB) reactor has an important impact on combustion behaviors. Therefore, monitoring the characteristic motion of fuel particles is of great significance for understanding the process of fuel conversion in fluidized bed. In this work, the characteristic motion of a single fuel particle (lignite and rice hull) in the different locations (including the dense phase, bed surface, and splash zone) was investigated in a two-dimensional visualized fluidized bed combustor. The movement behaviors of the fuel particle in the combustion process were recorded using a color video camera. The results showed that the particle was likely to be found in the dense phase during drying and devolatilization processes. As the combustion continued, the particle density decreased and the char was likely to be found floating on the bed surface. It was observed that the different fuel particles exhibited significantly different characteristic motion. To be specific, the lignite particle preferred to be observed in the dense phase, while the biomass particle tended to be found floating on the bed surface. Additionally, increasing the oxygen concentration significantly improved the probability of the particle to be found floating on the bed surface, while increasing the fluidization velocity exerted the opposite effect. At a high fluidization velocity, the probabilities of finding a particle in the dense phase and splash zone were increased as a result of the high bed voidage and bubble fraction. Replacing N2 by CO2 did not affect the probability that a particle would be found in a different location.