Single Bubble Research Articles

Effects of grain boundaries on the evolution of radiation-induced bubbles in polycrystalline tungsten: A phase-field simulation

Tungsten (W) is widely recognized as a promising material for plasma-facing components in nuclear fusion reactors, and the evolution of bubbles in polycrystalline tungsten significantly impacts its properties. This study developed a phase-field model considering elastic inhomogeneity to investigate the evolution of radiation-induced bubbles. The Gibbs free energy of helium bubbles was derived using the Van der Waals equation. An explicit nucleation algorithm accounting for the position of grain boundaries (GBs) was coupled to the model. Furthermore, the model also considered the inequality between the internal pressure of the bubble and the surface tension. The growth kinetics of a single bubble with elastic heterogeneity were examined, and based on these findings, the evolution of bubbles under irradiation was simulated, yielding results consistent with experimental observations. Furthermore, the influence of GBs on radiation-induced bubbles was investigated using the developed model. The simulation results demonstrated that both GBs, as preferential nucleation regions and as sinks for vacancy absorption, had an impact on bubble nucleation and growth. The superior radiation resistance observed in nanocrystalline tungsten was dominantly attributed to the absorption of vacancies by GBs.

Experimental study on insertion loss of air bubble layer in freestream flow

Air injection in water has several engineering advantages, such as an air lubrication system, cavitation control, and noise reduction. The present study investigates the effect of air injection rate and stream velocity on bubble layer volume fraction, thickness, and noise mitigation. The air bubble volume fraction is controlled through the injection rate of 2 to 25 SLPM at different stream velocities. The Froude number is in the range of 30.30 to 70.7. The image processing technique quantifies the air bubble projected void fraction and thickness. The air is injected through a multi-hole pipe placed on a modified NACA 0010 hydrofoil with a 100 mm chord length. The acoustic measurement system consists of a sound transmitter that projects sound waves at the frequency range of 7 to 50.5 [kHz], and on the opposite side of the air bubble layer, a hydrophone is placed to receive the transmitter sound wave. The results showed that the air bubble projected void fraction increased as the air injection and Froude number increased. Higher stream velocity promotes air bubble breaking and the formation of diverse air bubble patterns such as mono-dispersion bubbles and clustering bubbles. As the Froude number increases, the size of the bubble decreases. At various air injection rates and stream velocities, the thickness of the air bubble layer is found to be nearly constant. The bubble layer effectively reduces the noise in all frequency ranges of the present study. The sound pressure level (SPL) inhibition rate increases as the air bubble projected void increases. The insertion loss increases with stream velocity as higher flow velocity leads air bubbles to break and form a cluster of air bubbles, which is found to be more effective in reducing noise than a single bubble. Regardless of injection rate, the maximum insertion loss is 44 [dB] in a frequency of 20 [kHz] and Froude number 70.7.

Computational modelling and investigation of nucleate boiling with periodic exponential heat flux-based power-transients

The present work proposes a numerical methodology to perform single bubble nucleate boiling simulation subjected to power transients with exponential heating. Cyclic variation in transient heat flux is utilized which enables—proper characterization of the power transient parameters, parametric investigation of power transients on single bubble dynamics, and comparison between steady and transient heating condition. For constant time averaged heat flux in power cycle, it was observed that heating parameters are not having any influence on bubble departure diameter and bubble growth period whereas the bubble departure frequency is enhanced until vertical coalescence is observed. Parametric investigations based on time period of periodic heat flux and time averaged heat flux reveal the existence of a range in excursion time period which cause increase in bubble departure frequency, with applicability for controlling bubble dynamics with power transients and improving efficiency of thermal management devices using nucleate boiling.

A unified heat transfer model for gas-liquid two-phase mixing process in a rectangular channel based on steady status identification

Gas-liquid two-phase flow in rectangular channels involves thermal energy, chemical industry, and other fields. However, it is difficult to identify steady flow status and unify heat transfer model owing to the changeable mixing process and the complex industrial environment. In this work, the time series decomposition technique (i.e., variational mode decomposition) and neural network algorithm (i.e., long short-term memory) are employed to identify the steady flow pattern of the gas-liquid mixing process in vertical rectangular channels. The heat transfer model of a single bubble in a rectangular channel is established based on the geometrical characteristics of the bubble. Results demonstrate that a 30-dimensional feature vector is extracted from the conductivity time series for flow pattern recognition. The feature parameters of each intrinsic mode function are period, minimum, maximum, standard deviation, skewness, and kurtosis. Average accuracy of the new model is 94.58% with the highest at 95.83% and the proposed new model improves the recognition accuracy of steady flow pattern by 2.54–6.93% compared with other models adopted in this work. The precision of the new hybrid model is the highest while the number of decomposition layers for the conductivity time series is 5 and the bubble flow exhibits the largest heat transfer area under the same conditions. The proposed hybrid model can be used to enhance the identification accuracy of bubble flow in rectangular channels, contributing to curtailing unneeded flow patterns in industrial processes. The heat transfer model of a single bubble can provide a unified and convenient method for calculating industrial heat transfer. These findings provide a useful guideline for engineering design and energy-saving in the fields of thermal energy and chemical industry.

Laser Plasma‐Accelerated Ultra‐Intense Electron Beam for Efficiently Exciting Nuclear Isomers

AbstractUtilizing a laser plasma wakefield to accelerate ultrahigh charge and current electron beams is critical for many pioneering applications, for example, to efficiently produce nuclear isomers with short lifetimes that may be widely used. However, because of the beam loading effect, the electron charge in a single plasma bubble is limited to hundreds of picocoulombs. Herein, via a tightly focused intense laser pulse self‐guiding in dense gas, a twenty‐nanocoulomb, tens‐of‐MeV, hundred‐kiloampere collimated electron beam is produced from a chain of plasma bubbles in the saturated injection regime. This intense electron beam is utilized to excite indium nuclear isomers with an ultrahigh peak rate of particles·s‐1 via photonuclear reaction. This efficient isomer production method can be widely used for pumping isotopes with excited state lifetimes on the picosecond scale, which is beneficial for deeply understanding nuclear transition mechanisms or stimulating γ‐ray lasers.

Competitive methane bubble growth in aquatic muds

Methane (CH4) bubbles residing in shallow aquatic muds pose a significant threat to the environment. Impeded by the muddy sediment opacity and insufficient resolution for their characterization, past studies overlooked bubble interactions as they grow. Here, the competitive fracture-driven growth of CH4 bubble pair with different initial sizes in the aquatic muds is simulated, using a mechanical/reaction-transport numerical model. Mechanical and solute transport interactions were found to dominate at the different stages of the bubble growth, both retarding the smaller competitive bubble growth. The stress from large competitive bubble affects the inner pressure and diffusive CH4 flux to the smaller bubble, causing its slower initial growth (at t < 40 s). The large competitive bubble also diverts CH4 from the smaller one later on, thus inhibiting its growth even more. Bubble stress interactions may produce more laterally oriented smaller bubble and significant deformations of the larger one, both amplified under the decreasing distance between the growing bubble pair. Competitive bubble growth may shape a bubble size distribution pattern observed in the lab experiments and in situ, which may promote muddy sediment CH4 gas retention and produce gas domes. The latter acts as pockmark precursors whose formation induces a violent gas release to the water column and potentially to the atmosphere. Our study presents a basis for a proper upscaling to various effective gassy muddy sediment characteristics and gas retention evolution transient models, while preserving the single bubble growth descriptors. It contributes to evaluation and even reduction of a long-persisting uncertainty related to CH4 flux from the shallow aquatic sediments.

Effect of Low–Frequency Vibrations on the Hydrodynamic Properties of Single Bubbles at Different Gravity Levels

Effect of Low–Frequency Vibrations on the Hydrodynamic Properties of Single Bubbles at Different Gravity Levels

A lattice Boltzmann investigation of liquid viscosity effects on the evolution of a cavitation bubble attached to chemically patterned walls

The thermal lattice Boltzmann model is applied to explore liquid viscosity effects on a single cavitation bubble attached to chemically patterned walls. A conversion method based on the surface tension and the non-ideal equation of state parameters is proposed. According to the force analysis, it is found that the local pressure difference and the unbalanced Young's force are two main controlling factors for contactpoint dynamics. The dynamic contact angle is larger than the equilibrium contact angle throughout the evolution process for a hydrophilic wall, which results in a hysteresis effect in the bubble growth process due to the unbalanced Young's force and accelerates the contact point retraction velocity in the collapse stage. For hydrophobic walls, the unbalanced Young's force accelerates the contact radius expanding, resulting in a larger maximum contact radius than for a bubble attached to a hydrophilic wall. The hysteresis effects caused by the unbalanced Young's force slow down the contact points retraction in the early collapse stage and accelerate the retraction later because of dramatic interface deformation. The bubble is punctured over a larger volume with a hydrophilic wall than with a hydrophilic wall, resulting in a smaller collapse intensity. An exponential relationship between the micro-jet volume and the cosine function of the equilibrium contact angle at the collapse point is found. Furthermore, the jet volume before bubble collapse decreases, and the collapse time delays with the increase in viscosity.

Визуализация эволюции одиночного пузыря при кипении недогретой жидкости: опыт автоматизации обработки результатов скоростной видеосъемки

Subcooled liquid boiling makes it possible to remove extremely high heat flux densities (tens, and under certain conditions even hundreds of MW/m2). This method is used for cooling rocket nozzles, in metallurgy, in thermonuclear power and special installations (cooling of beam targets), pulsed magnetohydrodynamic (MHD) installations. However, the calculations of this process are based on purely empirical correlations. The lack of experimental data on the characteristics of bubbles (size evolution, life time, formation frequency) hinders the development of mathematical models and calculation methods. The paper presents a program developed in the Python 3.10 environment for automated processing of high-speed video recordings of the evolution of a single bubble during boiling of a subcooled liquid. Comparison of frame processing, carried out «manually» and with the help of the developed program, showed a fairly high agreement between the results. The developed method makes it possible to significantly reduce the time spent on processing the experimental material and to include a significantly larger array of experimental data in the analysis.

On the flow of a single bubble rising in temperature-sensitive gelled propellants

On the flow of a single bubble rising in temperature-sensitive gelled propellants

Bubble dynamic behaviors in the anode porous transport layer of proton exchange membrane electrolyzers using a microfluidic reactor

Gas bubbles in proton exchange membrane (PEM) electrolyzers are the main source of mass transport loss in the porous transport layer at high current densities. However, the complexity and opacity of the porous transport layer make it challenging. In this study, a microfluidic reactor with an electrochemical reaction boundary is designed to observe the bubble behaviors in the porous transport layer. The detailed bubble growth and detachment mechanisms are explored based on the visualization results. Results show that the gas phase evolution undergoes the unsteady growth stage and the steady stage. Bubble behaviors including free movement, coalescence, breakthrough and detachment are observed for a single bubble in the porous transport layer. The gas phase saturation in the porous transport layer as well as the applied voltage increases with increasing the current density. Moreover, as the flow rate increases, the time for the first bubble detachment and the number of bubble detachment gradually decrease, and the average detachment radius of the bubble gradually increases. A lower applied voltage is needed under a higher flow rate due to low mass transfer resistance. The proposed method can help us understand the dynamic behavior of bubbles in the porous transport layer of PEM electrolyzers.

Quantitative estimation of phospholipid molecules desorbed from a microbubble surface under ultrasound irradiation

Microbubbles have potential applications as drug and gene carriers, and drug release can be triggered by externally applied ultrasound irradiation while inside blood vessels. Desorption of molecules forming the microbubble shell can be observed under ultrasound irradiation of a single isolated microbubble, and the volume of desorbed molecules can be quantitatively estimated from the contact angle between the bubble and a glass plate. Microbubbles composed of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) shell and a poorly-soluble gas are created. When the microbubbles are exposed to a pulsed ultrasound, the contact angles increase dramatically; the percentage of DMPC molecules desorbed from the bubble surface reaches 70%. Vibration of a single bubble in the radial direction is measured using a laser Doppler vibrometer. The relationship between the vibrational characteristics and the amount of molecular desorption reveals that a larger vibrational amplitude of the bubble around the resonance size induces a larger amount of molecular desorption. These results support the possibility of controlling molecular desorption with pulsed ultrasound.

Surface activity characterization of synthesized polyvinyl alcohol-based nanogel at air-water interface under highly dynamic conditions

The ability to predict the behavior of nanogels at the air-water interface is critical for developing intelligent drug delivery systems. In this investigation, polyvinyl alcohol (PVA)-based nanogels were synthesized via the self-assembly crosslinking method employing fructose. The nanogels were characterized by means of FE-SEM, FTIR, and DLS analyses. The FE-SEM images confirmed the spherical morphology of the nanogels with an average diameter in the range of 18–30 nm. The polydispersity index of the nanogels, as assessed by the DLS test, was approximately 0.4. Subsequently, the impact of pure PVA and its produced nanogels on the hydrodynamics of a single air bubble ascending in aqueous solutions was explored, revealing that the presence of these molecules at the air-water interface results in a decrease in the local velocity profile (LVP) due to their surface activities. Finally, the results of the pendant drop measurements demonstrate that nanogels are more surface active than PVA.

Investigation of surfactant effect on ozone bubble motion and mass transfer characteristics

The low solubility and low mass transfer efficiency of ozone (O3) in water are important reasons for limiting the application of ozone oxidation technology and advanced ozone oxidation technology. Low-dose surfactants were introduced to affect the behavior of ozone bubbles, influencing the ozone mass transfer efficiency. The motion of ozone bubbles in three different types of surfactant solutions (polyoxyethylene lauryl ether (Brij35), sodium dodecylsulfate (SDS), and cetyltrimethylammonium bromide (CTAB)) was observed. The decrease in surface tension led to a decrease in bubble size, a tendency to spherical shape, an increase in gas content and residence time, a decrease in lateral displacement of a single bubble, and a more stable trajectory. The effect of surfactants on ozone mass transfer was further investigated. The addition of surfactant was found to increase the gas-liquid interface area but decrease the liquid phase mass transfer coefficient, and the extent of the two opposite effects varied for different surfactants. The analysis combined with the removal of pollutant p-Nitrophenol (PNP) showed that the addition of the nonionic surfactant Brij35 and the anionic surfactant SDS inhibited ozone mass transfer, while low concentrations of the cationic surfactant CTAB enhanced ozone mass transfer. When the CTAB concentration was 3 mg L−1, the ozone volume mass transfer coefficient reached a maximum of 0.2902 min−1.

Using a Lagrangian-Lagrangian approach for studying flow behavior inside a bubble column

Bubble columns are widely encountered in several industries, especially in the field of nuclear safety. The Eulerian-Eulerian and the Eulerian-Lagrangian methods are commonly used to investigate bubble columns. Eulerian approaches require additional tasks such as strict volume conservation at the interface and a predefined well-structured grid. In contrast, the Lagrangian approach can be easily implemented. Hence, we introduce a fully Lagrangian approach for the simulation of bubble columns using the discrete bubble model (DBM) and moving particle semi-implicit (MPS) methods. Additionally, we propose a rigorous method to estimate the volume fraction accurately, and verified it through experimental data and analytical results. The MPS method was compared with the experimental data of Dambreak. The DBM was verified by analyzing the terminal velocity of a single bubble for each bubble size. It agreed with the analytical results for each of the four drag correlations. Additionally, the improved method for calculating the volume fraction showed agreement with the Ergun equation for the pressure drop in a packed bed. The implemented MPS-DBM was used to simulate the bubble column, and the results were compared with the experimental results. We demonstrated that the MPS-DBM was in quantitative agreement with the experimental data.

Solutal Marangoni effect determines bubble dynamics during electrocatalytic hydrogen evolution.

Understanding and manipulating gas bubble evolution during electrochemical water splitting is a crucial strategy for optimizing the electrode/electrolyte/gas bubble interface. Here gas bubble dynamics are investigated during the hydrogen evolution reaction on a well-defined platinum microelectrode by varying the electrolyte composition. We find that the microbubble coalescence efficiency follows the Hofmeister series of anions in the electrolyte. This dependency yields very different types of H2 gas bubble evolution in different electrolytes, ranging from periodic detachment of a single H2 gas bubble in sulfuric acid to aperiodic detachment of small H2 gas bubbles in perchloric acid. Our results indicate that the solutal Marangoni convection, induced by the anion concentration gradient developing during the reaction, plays a critical role at practical current density conditions. The resulting Marangoni force on the H2 gas bubble and the bubble departure diameter therefore depend on how surface tension varies with concentration for different electrolytes. This insight provides new avenues for controlling bubble dynamics during electrochemical gas bubble formation.

Controlled Bubble Formation From a Microelectrode Single Bubble Generator

Abstract In this work, a new micro-electrode bubble generator is presented that employs a micro-electrode installed inside a small nozzle enabling the production of bubbles with controllable size and frequency. This bubble generator can be employed as a simple and potentially cheap method for the generation of single bubbles in a liquid, as long as it enables ion exchange, as an alternative to more complicated methods such as timely injection of a gas through a nozzle, which requires sophisticated nozzle design, manufacturing, and monitoring of the injected gas flow rate. A systematic investigation was conducted to assess the effect of the bubble generator dimensions, applied voltage, and electrolyte flow conditions on the size and frequency of the generated bubbles. It was shown that when the micro-electrode is thinly concealed within the nozzle, this bubble generator can successfully produce bubbles covering a wide range of diameters from 0.4 to 1.4 mm with a size distribution standard deviation of about 25%. The mechanism of single and continuous bubbles formation from the proposed bubble generator is also discussed. While this paper introduces this new micro-electrode bubble generator, further work is required to optimize it, enabling more accurate control over bubble size and frequency.

Towards green flotation: Investigating the effect of rhamnolipid biosurfactant on single bubble adhesion dynamics

In this study we performed an experimental analysis of the influence of rhamnolipid (RL) biosurfactant produced by Pseudomonas aeruginosa on the dynamics of bubble adhesion onto model surfaces of various hydrophobicity. The attachment time of a single bubble to a solid surface with contact angles of 6, 34, 49 and 80° was evaluated at different concentrations of RL in the range of 0 to 500 mg·dm−3, both at pH 5 (non-ionic form) and pH 10 (anionic form). The dependence of the three-phase contact (TPC) formation time on biosurfactant concentration was determined by monitoring the bubble-solid surface interactions using a high-speed camera. It was found that as the concentration of RL rises, the formation time of the TPC extends as a result of an increase in the film drainage time. A significant effect of RL on the process of TPC expansion (lower rate) and the size of the contact area between the bubble and the surface (smaller surface area) was also noted. This study provides a better understanding of the effect of one of the most popular biosurfactants on the fundamental act of the flotation process: attachment of the particle to the bubble.

NH3/O2 premixed combustion in a single bubble of fluidized bed

As a carbon - free, economical and safe energy carrier, ammonia is favoured and has been used in many different applications for combustion. However, there are few reports of NH3 being burned in a fluidized bed. This investigation of the premixed combustion of NH3 and O2 in a single bubble inside a fluidized bed was done both experimentally and with CFD-DEM simulations incorporating a skeletal mechanism of 98 elementary reactions. The results show that premixed combustion of NH3 and O2 occurred in a growing bubble. Quenching became evident at a bubble's edge. Combustion became unstable at low equivalence ratios. In addition, the equivalence ratio significantly affected the ignition delay, flame's propagation velocity, as well as the concentrations of gaseous species in the bubble and emulsion phases. Increasing bed temperature reduced the ignition delay. Nitrogen, as the background fluidising gas, exerted a negative impact on combustion, because of N2 diffusing from the emulsion to the bubble phase. The combustion of NH3/air in the bubble phase of a fluidized bed was difficult to happen in the absence of a catalyst or co-combustion with an inflammable fuel.

A method for consistent cavitation bubble generation at different voltages

A study of the dynamics of a single cavitation bubble is fundamental for understanding a wide range of applications in science and engineering. Underwater electrical discharge is a widely used method for generating cavitation bubbles to study their inception, subsequent dynamics, and collapse. In this work, an existing underwater low-voltage discharge circuit for generating cavitation bubbles is improved further to get a wider range of maximum bubble radius. In this novel electric circuit design, the operating voltage can be varied (up to 420 V in steps of 60 V) by connecting a network of capacitors in different series-parallel combinations with the help of relay-based control. Therefore, this device can generate oscillating cavitation bubbles up to a maximum radius of 14 mm by adjusting the available discharge energy. A voltage sensor circuit is included in this design to measure the drop in voltage during the sparking event, and a correlation between the delivered energy and the potential energy of the bubble is established. The dependence of bubble radius on circuit resistance, electrode resistance, and electrode material is studied for the entire voltage range. A suitably rated semiconductor field effect transistor is used as a switch that enables the generation of bubbles of a consistent maximum radius and ensures the repeatability of the experiment. A high-speed imaging system is used to estimate the bubble radius and nucleation period, which are compared with the existing theoretical models based on empty cavity collapse. Results show that delaying the oxidation of electrodes with a protective layer influences the collapse phase and the average pressure inside the spark-generated bubble.