Bubble Population Research Articles

Interface capturing simulations of bubble population effects in PWR subchannels

As the computational power of high-performance computing (HPC) facilities grows, so too does the feasibility of using first principle based simulation to study turbulent two-phase flows within complex pressurized water reactor (PWR) geometries. Direct numerical simulation (DNS), integrated with an interface capturing method, allows for the collection of high-fidelity numerical data using advanced analysis techniques. The presented research employs the massively parallel, finite-element based, unstructured mesh code, PHASTA, to simulate a set of two-phase bubbly flows through PWR subchannel geometries including auxiliary structures (spacer grids and mixing vanes). The main objective of the presented work is to analyze bubble dynamics and turbulence interactions at varying bubble concentrations to support the development of advanced two-phase flow closure models. Turbulent two-phase flows in PWR subchannels were simulated at hydraulic Reynolds numbers of 81,000 with bubble concentrations of 3%–15% by gas volume fraction (768–3928 resolved bubbles, respectively) and compared against a 1% void fraction case (262 bubbles) that had been previously simulated. The finite element mesh utilized for the study at higher bubble concentrations was composed of 1.55 billion elements, compared to the previous study which employed 1.11 billion elements, ensuring all turbulence scales and individual bubbles within the flow are fully resolved. For each case, the resolved initial bubble size was 0.65 mm in diameter (resolved with 25 grid points across the diameter). The simulations were analyzed to find flow features such as the mean velocity profile, bubble relative velocity and the effect of the bubbles on the turbulent conditions.

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.

Inversion method of bubble size distribution based on acoustic nonlinear coefficient measurement**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674074 and 61701133).

Measurements of bubble size distribution require the understanding of the acoustic characteristics of the medium. The bubbles show highly nonlinear properties under finite amplitude acoustic excitation, so the acoustic fields from bubble population are easily observed at the second harmonics as well as at the fundamental frequency, which shows that the nonlinear coefficient increases obviously. The inversion method of bubble size distribution based on nonlinear acoustic effects can peel off the influence of complex environment and obtain the size distribution coefficient information of bubbles more accurately. The previous nonlinear inversion methods of bubble size distribution are mostly based on the nonlinear scattering cross-section characteristics of bubbles. However, the stability of inversion is not high enough. In this paper, we introduce a new acoustic inversion method for bubble size distribution, which is based on the nonlinear coefficients of bubble medium. Compared with other inversion methods based on linear or nonlinear scattering cross section, the inversion method based on nonlinear coefficients of bubble medium proposed in this paper shows good robustness in both simulation and experiment.

Cavitation clusters in lipid systems - Ring-up, bubble population, and bifurcated streamer lifetime.

The processing of oils is vital to their ultimate use within the food industry. Control over the physical properties of such materials could be achieved through the application of high-intensity ultrasound (HIU). However, the exact mechanism, centred upon acoustic cavitation, is currently unclear. To investigate the cavitation environment in oils further, the ring-up of a HIU source in an oil media is studied in the presence and absence of a pre-existing bubble population. High-speed imaging and acoustic measurements within the system is demonstrated to be extremely useful in characterising the dynamics present under non steady-state conditions. The behaviour of the clusters generated in the first 1000ms under these conditions is shown to be significantly different depending on the bubble population. A bifurcated streamer (BiS), originating from a unique bi-cluster event, is only observable in the presence of a bubble population during the ring-up process to higher cluster orders. In addition, the lifetime of this BiS event is highly temperature dependent and is shown to be a good marker for the viscosity of the oil employed.

The effect of size range on ultrasound-induced translations in microbubble populations

Microbubble translations driven by ultrasound-induced radiation forces can be beneficial for applications in ultrasound molecular imaging and drug delivery. Here, the effect of size range in microbubble populations on their translations is investigated experimentally and theoretically. The displacements within five distinct size-isolated microbubble populations are driven by a standard ultrasound-imaging probe at frequencies ranging from 3 to 7 MHz, and measured using the multi-gate spectral Doppler approach. Peak microbubble displacements, reaching up to 10 μm per pulse, are found to describe transient phenomena from the resonant proportion of each bubble population. The overall trend of the statistical behavior of the bubble displacements, quantified by the total number of identified displacements, reveals significant differences between the bubble populations as a function of the transmission frequency. A good agreement is found between the experiments and theory that includes a model parameter fit, which is further supported by separate measurements of individual microbubbles to characterize the viscoelasticity of their stabilizing lipid shell. These findings may help to tune the microbubble size distribution and ultrasound transmission parameters to optimize the radiation-force translations. They also demonstrate a simple technique to characterize the microbubble shell viscosity, the fitted model parameter, from freely floating microbubble populations using a standard ultrasound-imaging probe.

Multiphase numerical modeling of a pilot-scale bubble column with a fixed poly-dispersity approach

A three-dimensional numerical study of air/water bubbly flow in a cylindrical large-scale bubble column is performed using Euler-Euler approach. The main objective is to investigate the influence of different boundary conditions such as bubble size distribution (BSD), polydisperse effects (mono and bi-dispersed approach), lift force modelling and flow rate distribution at sparger using a computationally affordable approach. In the bi-dispersed approach the population of bubbles are divided into two groups of small and large bubbles and a mean diameter is considered for each group. The division is based on the critical bubble diameter, for which the lift coefficient changes its sign from positive to negative. For air/water system Tomiyama lift coefficient model is widely used and the critical bubble diameter is equal to 5.8 mm. An alternative critical bubble diameter and lift force coefficient correlation is used in this study and compared with the well-known Tomiyama model. The numerical predictions are compared against the experimental data and the effect of different conditions is assessed on basis of comparison of axial gas fraction (local holdup) and global holdup. Better predictions are obtained by taking into account polydisperse flow with new critical bubble diameter and new lift coefficient model. Also, it was found that mass flow-rate distribution at the sparger does not affect numerical results for global and local holdup, however a different flow pattern is observed near the sparger region.

Characterization by APT and TEM of Xe nano-bubbles in CeO2

Only few experimental techniques are able to characterize the space and size distributions of gas-filled nano-cavities embedded in ceramic matrices. The aim of the current paper is to point out the potentiality of atom probe tomography for this objective. Nano-clusters enriched in Xe are evidenced in 3-D reconstructed volumes in Xe-implanted CeO2. Their size and density are comparable to bubble populations observed by TEM.

A Gaussian moment method and its augmentation via LSTM recurrent neural networks for the statistics of cavitating bubble populations

Phase-averaged dilute bubbly flow models require high-order statistical moments of the bubble population. The method of classes, which directly evolve bins of bubbles in the probability space, are accurate but computationally expensive. Moment-based methods based upon a Gaussian closure present an opportunity to accelerate this approach, particularly when the bubble size distributions are broad (polydisperse). For linear bubble dynamics a Gaussian closure is exact, but for bubbles undergoing large and nonlinear oscillations, it results in a large error from misrepresented higher-order moments. Long short-term memory recurrent neural networks, trained on Monte Carlo truth data, are proposed to improve these model predictions. The networks are used to correct the low-order moment evolution equations and improve prediction of higher-order moments based upon the low-order ones. Results show that the networks can reduce model errors to less than 1% of their unaugmented values.

Influence of frequency sweep on sonochemiluminescence and sonoluminescence.

Bubbles generated by acoustic cavitation may be efficient in light production by direct emission (sonoluminescence) or indirect emission (sonochemiluminescence) depending on operating parameters such as acoustic pressure and surface tension. These conditions are quite difficult to reach at very high frequencies, even by concentrating the acoustic power at a given location via focusing the acoustic field thanks to the transducer shape (High Intensity Focused Ultrasound). The current work aims at probing the cavitation bubble behaviour under short frequency sweeps by monitoring sonochemiluminescence and sonoluminescence activities. When the frequency was swept in reverse (negative sweep), an enhancement in the SCL, relative to the SCL observed under a single frequency irradiation, was observed. Conversely, a positive frequency sweep resulted in the quenching of SCL intensity. The degree of SCL enhancement and quenching was also dependent on the rate at which the frequency was being swept and on the change in the size of cavitation bubbles. The size of cavitation bubbles varied with varying starting sweep frequency (3.4, 3.6 and 4.2MHz), affecting both SCL and sonoluminescence (SL) emissions. The addition of a surfactant (sodium dodecyl sulphate) affected the observed results, possibly due to its influence on coalescence between cavitation bubbles. The results suggest that the enhancement and quenching are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep and the influence of the sweep rate on growth and coalescence of bubbles, which affected the population of the active bubbles.

Expansion-mediated breakup of bubbles and droplets in microfluidics

Different breakup regimes of bubbles and droplets caused by a sudden channel expansion in a microfluidic device are investigated. Without modifying the geometry and by only tuning several dimensionless parameters related to the fluid flow, a microchannel expansion region can produce mono-, bi-, or tri-disperse bubble or droplet populations.

An integrated microfluidic chip for generation and transfer of reactive species using gas plasma

Reactive species produced by atmospheric-pressure plasma (APP) are useful in many applications including disinfection, pretreatment, catalysis, detection and chemical synthesis. Most highly reactive species produced by plasma, such as ·OH, 1O2 and {text{O}}_{2}^{ cdot - } , are short-lived; therefore, in situ generation is essential to transfer plasma products to the liquid phase efficiently. A novel microfluidic device that generates a dielectric barrier discharge (DBD) plasma at the gas–liquid interface and disperses the reactive species generated using microbubbles of ca. 200 µm in diameter has been developed and tested. As the bubble size affects the mass transfer performance of the device, the effect of operating parameters and plasma discharge on generated bubbles size has been studied. The mass transfer performance of the device was evaluated by transferring the reactive species generated to an aqueous solution containing dye and measuring percentage degradation of the dye. Monodisperse microbubbles (polydispersity index between 2 and 7%) were generated under all examined conditions, but for gas flow rate exceeding a critical value, a secondary break-up event occurred after bubble formation leading to multiple monodisperse bubble populations. The generated microbubble size increased by up to ~ 8% when the device was operated with the gas plasma in the dispersed phase compared to the case without the plasma due to thermal expansion of the feed gas. At the optimal operating conditions, initial dye concentration was reduced by ~ 60% in a single pass with a residence time of 5–10 s. This microfluidic chip has the potential to play a significant role in lab-on-a-chip devices where highly reactive species are essential for the process.

Impact of bubble size on the integral characteristics of bubble plumes in quiescent and unstratified water

We present an experimental study to investigate the impact of bubble size on behavior of bubble plumes from the integral point of view. Two distinct types of bubble plume were generated under the same volumetric gas flow rate: many small bubbles using an air-stone diffuser and a few large bubbles using a single-orifice diffuser. Using Particle Image Velocimetry, we measured the velocity field in the plumes, and derived the integral characteristics for comparison, including the evolution of plume width, centerline velocity, volume flux, momentum flux, momentum amplification factor, and the entrainment coefficient. The measurement data show that the evolution of all the parameters in the plumes are influenced strongly by the different bubble sizes and population. The plumes with many small bubbles behave closer to the single-phase coherent plumes, showing a linear growth of plume width and a decreasing centerline velocity. These characteristics are modified in the plumes with fewer but larger bubbles. With the same total initial buoyancy flux from the source, fewer but larger bubbles give rise to weaker mean flow characteristics, i.e., smaller plume velocity and weaker fluxes of volume and momentum. This modification is attributed to the different mechanisms in fundamental transport of energy and momentum between mean and fluctuating components. In the single-orifice cases, the plumes are influenced by stronger but less frequent bubble wakes, compared to those more coherent bubble plumes in the air-stone cases. Higher momentum amplification factors were found in bubble plumes with fewer but larger bubbles, indicating a stronger ratio of turbulence to mean in these plumes. Despite the modification, all bubble plumes support a typical shear entrainment process within 40 cm height-of-rise. Hence, the integral model and the universal scaling using the plume length scale D and the bubble slip velocity Ws were found to describe the integral behavior of bubble plumes generally well in the range of our experimental parameters, regardless of the different bubble sizes and population.

Effects of wind-generated bubbles layer on sound propagation underneath rough sea surface in shallow water

The reflection coefficient of the flat sea surface in the ideal condition to the incident sound wave is –1. The perfect reflection effect does not introduce reflection loss. However, the sea surface is usually rough due to the wind. The wind-generated rough sea surface has not only reflection effect, but also scattering effect on the sound wave. At the same time, the wind-generated bubbles layer also has significant effect on the sound propagation. On the one hand, the bubbles layer can change the sound speed profile and result in the refraction of the incident sound wave. On the other hand, the bubbles layer has scattering effect and absorption effect on the incident sound wave and leads to the sound wave to attenuate. In fact, the rough sea surface and the bubbles layer are two main factors affecting the sound propagation in the windy weather at sea. Many researchers have paid much attention to the effect of the wind-generated rough sea surface on the sound propagation, but few of them have considered the effect of the wind-generated bubbles layer on the sound propagation. Based on the Ramsurf sound propagation model under the rough sea surface, the effects of wind-generated bubbles layer underneath rough sea surface on reflection loss and sound propagation with different wind speeds are analyzed. Based on the Hall-Novarini bubbles population model, the sound speed profile in the bubbles layer is modified and the attenuation coefficient due to scattering and absorption of the bubbles layer is calculated. The simulation results shows that when the wind speed is 10 m/s, the effect of the bubbles layer is significant on reflection loss with the frequencies higher than 2 kHz. In the given underwater acoustic environment, for a frequency of 3 kHz, when the source depth and the receiver depth are both 7 m, the enhancement of the transmission loss due to the bubbles layer is 2.6 dB for a wind speed of 13 m/s, and the enhancement is 8.1 dB for a wind speed of 16 m/s. And when the source depth and the receiver depth are both 18 m, the enhancement of the transmission loss due to the bubbles layer is 2.5 dB for a wind speed of 13 m/s and the enhancement is 4 dB for a wind speed of 16 m/s.

Gravity‐Induced Bubble Ripening in Porous Media and Its Impact on Capillary Trapping Stability

AbstractCapillary or residual trapping is considered one of the safest geologic CO2 storage mechanisms due to its hydrodynamic stability. We present a first study of the impact of gravity on Ostwald ripening in porous media and show it may render capillary trapping thermodynamically unstable. Gravity induces a vertical chemical potential gradient that leads to the upwards diffusion of CO2. Thus, bubbles at shallower depths grow at the expense of bubbles in deeper strata, leading to the formation of an overriding gas cap. We first develop a pore‐scale model for two bubbles trapped within adjacent pores and then upscale it to obtain a one‐dimensional continuum model. We use the latter to predict the macroscopic evolution of a trapped bubble population. Factors controlling the ripening process are isolated to assist in selecting CO2 storage sites. Gravity‐induced ripening may also play a role in geologic fluid emplacement and migration over millions of years.

Cavity formation and hardness change in He implanted EUROFER97 and EU-ODS EUROFER

Abstract RAFM steels EUROFER97 and EU-ODS EUROFER samples have been implanted with He ions at 40 keV (ion penetration depth ~300 nm) at a dose of ~ 1 × 1015 ion/cm2 and different temperatures up to 550 °C. Post-irradiation examination of the samples has been performed using nanoindentation along with conventional and scanning transmission electron microscopy (CTEM/STEM in Annular Dark Field mode). The specimens were indented up to 500 nm by CSM method (Continuous Stiffness Measurement), in order to assess the changes in nanoindentation hardness values due to the irradiation. After indentation tests, lamella was extracted from each implanted sample which included at least one nanoindentation cross-section. The changes in hardness were correlated with any microstructure modification detected by TEM. A clear trend can be observed; the hardness values increased with irradiation temperature, as well as the He bubble nucleation and population density. A remarkable change in bubble distribution in EUROFER97 was found at the two highest irradiation temperatures (450 °C and 550 °C). In contrast, an increase in the cavities size was observed in EU-ODS EUROFER, but no new nucleation was observed for the same temperatures. These observations suggest that for these particular conditions, the cavity growth is enhanced, rather than new nucleation, which depends strongly on the material microstructure.

Response of Superheated Emulsion Detectors to low energy alpha irradiation

Superheated Emulsion Detectors (SED) are traditionally employed in the detection of neutrons. In this work the focus is on the detection of alpha particles for an eventual alpha spectrometer using C2ClF5 as the target liquid. The alpha-droplet interaction is examined via computational studies, and a geometric model developed to describe the anticipated detector response. Experiments with alpha-emitting uranium- and samarium-doped SEDs at temperatures of 5-12 ºC confirm that the event rate is related to the size of the droplets, and are in model agreement for temperatures below 8 ºC; above this temperature, the acoustic sensitivity is reduced by signal attenuation as a result of the increasing bubble population, for which the addition of an attenuation coefficient restores the agreement with experiment. The results suggest an SED-based alpha spectrometer.

A Novel Supercritical CO2 Foam System Stabilized With a Mixture of Zwitterionic Surfactant and Silica Nanoparticles for Enhanced Oil Recovery.

In order to improve the CO2 foam stability at high temperature and salinity, hydrophilic silica nanoparticles (NPs) were added into a dilute zwitterionic surfactant solution to stabilize supercritical CO2 (SC-CO2) foam. In the present paper, the foaming capacity and stability of SC-CO2 foam were investigated as a function of NP concentration at elevated temperatures and pressures. It was observed that the drainage rate of SC-CO2 foam was initially fast and then became slower with NPs adsorption at the gas-liquid interface. The improved foam stability at high temperature was attributed to the enhanced disjoining pressure with addition of NPs. Furthermore, an obvious increase in the foam stability was noticed with the increasing salinity due to the screening of NP charges at the interface. The rheological characteristics including apparent viscosity and surface elasticity, resistance factor, and microstructures of SC-CO2 foam were also analyzed at high temperature and pressure. With addition of 0.7% NPs, SC-CO2 foam was stabilized with apparent viscosity increased up to 80 mPa·s and resistance factor up to 200. Based on the stochastic bubble population (SBP) model, the resistance factor of SC-CO2 foam was simulated by considering the foam generation rate and maximum bubble density. The microstructural characteristics of SC-CO2 foam were detected by optical microscopy. It was found that the effluent bubble size ranged between 20 and 30 μm and the coalescence rate of SC-CO2 foam became slow with the increasing NP concentration. Oscillation measurements revealed that the NPs enhanced surface elasticity between CO2 and foam agents for resisting external disturbances, thus resulting in enhanced film stability and excellent rheological properties.

X‐ray imaging of a high‐temperature furnace applied to glass melting

AbstractThe dynamics of soda‐lime‐silica glass grain melting is investigated experimentally using a nonintrusive technique. A cylindrical alumina crucible is filled with glass cullet and placed into a furnace illuminated by an X‐ray source. This glass granular bed is gradually heated up to 1100°C, leading to its melting and the generation of a size‐distributed population of bubbles rising in the molten glass. An image processing algorithm of X‐ray images of the cullet bed during melting allows the characterization of bubbles size distribution in the crucible as well as their velocity. The introduction of tin dioxide μ‐particles in the glass matrix before melting enhances the texture of the images and makes possible the determination of the bubble‐induced molten glass velocity field by an optical flow technique. The bubble size distribution can be fitted by a log‐normal law, suggesting that it is closely related to the initial size distribution in the cullet bed. The liquid motion induced by the bubbles in Stokes' regime is strongly affected by the flow confinement and the determination of bubble rising velocity along its trajectory unveils the existence of local tiny temperature fluctuations in the crucible. Overall, the measuring techniques developed in this work seem to be very promising for the improvement of models and optimization of industrial glass furnaces.

Bubble population balance modelling for stationary and rotating columns under zero-gravity environment: Numerical study

Two axisymmetric dimensional Eulerian/Eulerian simulation of two-phase (gas/liquid) transient flow was performed for both stationary and rotating columns under zero gravity. Multiphase flow simulation is modelled using flow algorithm based on the finite-volume method to simulate bubble population balance modelling. Two coalescence kernels, one for laminar flow and the other for laminar shear flow, are experimented under zero gravity using special user defined functions. Effects of rotation speed are investigated at angular velocities of 0, 0.25, 0.5, 0.75 and 1 rad/s, and effects of air inlet velocity are conducted at 1, 2, 3 and 4 cm/s. The numerical results show that the centrifugal force is pulling the bubbles towards the axis of rotation and shifts them away from the wall. Also, rotation could accelerate the moving bubbles throughout the column under zero-gravity conditions. Increasing the inlet air velocity accelerates the coalescence rate with a decrease in the inlet bubble diameter. Under zero gravity, the bubbles are formed with limited size distribution and may not be represented by an equivalent phase with Sauter mean diameter. Coalescence and break-up phenomena must be considered under normal gravity and coalescence phenomenon only under zero gravity.

Influence of resonance-driven bubble clouds on fine-scale behaviour of common carp (Cyprinus carpio)

Acoustic-bubble barriers show potential as a management tool for fish passage. Although greater efforts are now being made to better understand the fundamentals that underpin reactions of fish to such barriers, the phenomenon of bubble coalescence has not yet been considered. When introducing air underwater smaller bubbles coalesce with successor bubbles at the orifice so that without vibration the bubble which enters the liquid is large. Orifice diameter has historically been used as a measure of bubble size, however as detachment and fragmentation occur unpredictably this method is unreliable, indicating insufficient control over the stimulus generated. This study used lower air flows (6 l min−1 m−1) and haptic feedback motors to vibrate the injection nozzles and reduce coalescence. Greater uniformity in bubble size allowed matching of the bubble population to a sound field to which the bubbles were resonant. In a proof-of-concept study, fish response to two resonant acoustic-bubble barriers was tested and compared to two non-resonant acoustic-bubble walls. The scattering and absorption coefficients for all segments of the bubble population were calculated to determine the most important bubble sizes for attenuation. Analysis of fish movement and orientation was used to examine fish behaviour in relation to the sound fields generated.