Bubble Population Research Articles

Numerical studies on effects of geometrical parameters of internals on bubble column hydrodynamics

Internals are an important part of bubble column design for applications in different industrial processes. A properly designed and configured internal can significantly improve the performance of the reactor. In this work, the Eulerian-Eulerian (E-E) Computational Fluid Dynamics (CFD) model using the discrete bubble population balance model (PBM) has been used to investigate effects of two important geometrical parameters for concentric tube bundle internal. These are tube-to-tube spacing and elevation of the tube bundle from column bottom. The simulations are conducted in 2D space by suitably accounting for vertical perforations in the concentric tube bundle. The results obtained compare well with trends of experimental simulation results from literature studies. It is determined that tube-to-tube spacing and the elevation of the bundle clearly alter mixing patterns in the column, requiring careful evaluations for a given application. The proposed simulation method based on 2D modeling can be a quick means to compare alternative configurations of internals.

Bubbles in a sand fluidized bed unit for the gasification of coffee waste biomass. A probabilistic based fluid‐dynamic description

AbstractThe present study shows the applicability of the chemical reactor engineering centre (CREC) Optiprobes engineered with a graded refractive index (GRIN) lens and fibreoptics to establish both the bubble rising velocity (BRV) and the bubble axial chord (BAC) of bubbles in a 240–955 μm sand fluidized bed, filled with different types and loadings of biomasses. It is confirmed via the application of the CREC Optiprobes that the formed bubbles display both BRV and BAC normal probabilistic distributions, leading to characteristic BRV–BAC bands of bubble behaviour, with this being true for an ample range of superficial gas velocities (0.188–0.282 m/s) and broza biomass loadings (0–30 vol.%). It is also proven that the observed BRV and BAC distributions in biomass‐loaded sand fluidized beds are of the quasi‐normal distribution type, as attested by the Shapiro–Wilk test (S–W test). Furthermore, a probabilistic prediction model (PPM) proposed in a previous contribution can be used effectively to predict, in all cases, that a close to 80% of the total bubble population falls within a proposed probabilistic band of bubble behaviour.

On the theory of void nucleation in irradiated crystals

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

Experimental investigation on bubbles generated by porous pipes

Experiments were carried out to determine the characteristics of bubbles originated in still tap water by a porous polyethylene pipe used as an air sparger. Local bubble populations were sampled at various regions of the pipe cross-section and at various air flow-rates to assess the effects of the surface local orientation and flow conditions on bubble dimensions. It was discovered that at the bottom of the pipe the size distribution of the bubbles is almost invariant with respect to air flow. On the contrary, at other locations bubble dimensions increased with the air flow-rate and the surface inclination. Statistical analyses proved that the distributions of bubble diameters and their mean values are better modeled by the Gamma distribution than the Log-Normal distribution. Local populations were merged to get the corresponding global populations. It was discovered that the bubbles from the upper half of the pipe cross-section have a higher weight in determining the properties of global populations. Global bubble populations from the whole cross-section of the pipe were directly measured at low air flow-rates. Their mean diameters were used to validate the merging procedure. Two Koide-like correlations were successfully implemented to model as a function of the Froude and Weber numbers the dimensionless mean diameter and the dimensionless Sauter mean diameter of the global bubble populations. The Froude and Weber numbers were computed based on the flow of air through the porous wall, which was successfully modeled with the Forchheimer equation for compressible fluids.

The Effect of Seawater Salinity and Seawater Temperature on Sea Salt Aerosol Production

AbstractTo improve our understanding of the impact of sea salt aerosols (SSA) on the Earth's climate, it is critical to understand the physical mechanisms which determine the size‐resolved SSA production flux. Of the factors affecting SSA emissions, seawater salinity has perhaps received the least attention in the literature and previous studies have produced conflicting results. Here, we present a series of laboratory experiments designed to investigate the role of salinity on aerosol production from artificial seawater using a continuous plunging jet. During these experiments, the aerosol and surface bubble size distributions were monitored while the salinity was decreased from 35 to 0 g kg−1. Three distinct salinity regimes were identified: (a) A high salinity regime, 10–35 g kg−1, where lower salinity resulted in only minor reductions in particle number flux but notable reductions in particle volume flux; (b) an intermediate salinity regime, 5–10 g kg−1, with a local maximum in particle number flux; (c) a low salinity regime, <5 g kg−1, characterized by a rapid decrease in particle number flux at lower salinities and dominated by small particles and larger bubbles. We discuss the implications of our results through comparison of the size‐resolved aerosol flux and the surface bubble population at different salinities. Finally, by varying the seawater temperature at three specific salinities we have also developed a simple parameterization of the particle production flux as a function of seawater temperature and salinity. The range of seawater salinity and temperature studied is representative of the global oceans and lower salinity water bodies such as the Baltic Sea.

Modeling acoustic cavitation with inhomogeneous polydisperse bubble population on a large scale

A model for acoustic cavitation flows able to depict large geometries and time scales is proposed. It is based on the Euler–Lagrange approach incorporating a novel Helmholtz solver with a non-linear acoustic attenuation model. The method is able to depict a polydisperse bubble population, which may vary locally. The model is verified and analyzed in a setup with a large sonotrode. Influences of the initial void fraction and the population type are studied. The results show that the velocity is strongly influenced by these parameters. Furthermore, the largest bubbles determine the highest pressure amplitude reached in the domain, which corresponds to the Blake threshold of these bubbles. Additionally, a validation is performed with a small sonotrode. The model reproduces most of the experimentally observed phenomena. In the experiments, neighboring bubbles are found which move in different directions depending on their size. The numerical results show that the responsible mechanism here is the reversal of the primary Bjerknes force at a certain pressure amplitude.

Computational study on the three phase displacement characteristics of foam fluids in porous media

Foam technology has shown great potentials not only on enhancing oil recovery (EOR) but also on the Carbon Capture Storage Utilization (CCUS) applications. In this paper, the foam three phase displacement characteristics in the porous media is investigated based on the mechanistic Stochastic Bubble Population Balance (SBPB) model. With taking the foam generation rate Kg and maximum bubble density nmax as the key influential parameters, the dynamic distributions of the water, oil and gas phase saturations as well as the bubble densities along the foam flow direction are successfully obtained. Firstly, the overall computation frame is validated based on good agreements between the numerical and the reported experimental results on the foam displacement efficiency on water and oil phases. Then, the parameter studies are thoroughly performed concerning the two key factors of Kg and nmax in the SBPB model. Numerical results show the parameter of Kg dominates the foam flow behavoir in the inlet region of the porous media therefore doesn't show significant effect on the final oil and gas phase recovery rates. On the other hand, it is found larger nmax could lead to higher oil and water recovery ratio, which indicates to increase the nmax values of the foam fluid could act as the favorable means in oil recovery enhancement practices.

Study on optical scattering detection method of bubbly wake of targets in water

A method for the detection of the bubbly wake of targets in water is presented, which is based on the features of optical frequency domain of the scattering optical signal. A simulation on the optical frequency domain characteristics of the bubbly wake scattering signal is performed by statistical test method. The influence of the bubble population and the velocity distribution on the optical frequency domain characteristics is analyzed. On this basis, a test platform for the characteristics of optical frequency domain of the bubbly wake scattering signal is designed and established. The characteristics of the bubbly wake scattering signal are studied experimentally. Experimental results agree with the theoretical analysis. The results show that the amplitude and the cut-off frequency of the shift spectra will increase with the increasing bubble population and range of velocity distribution. It indicates the effectiveness of the detection method.

The motion mechanism and characteristic of bubble in a pseudo-2D tapered fluidized bed

The different carbon nanotube (CNT) particles ( @ A and @ V) were bed materials in the pseudo-2D tapered fluidized bed (TFB) with/without a distributor. A detailed investigation of the motion mechanism of bubbles was carried out. The high-speed photography and image analysis techniques were used to study bubble characteristic and mixing behavior in the tapered angle of TFB without a distributor. The fractal analysis method was used to analyze the degree of particles movement. Results showed that an S-shaped motion trajectory of bubbles was captured in the bed of @ V particles. The population of observational bubbles in the bed of @ V particles was more than that of @ A particles, and the bubble size was smaller in the bed of @ V particles than that of @ A particles. The motion mechanism of bubbles had been shown to be related to bed materials and initial bed height in terms of analysis and comparison of bubble diameter, bubble aspect ratio and bubble shape factor. Importantly, compared to the TFB with a distributor, the TFB without a distributor had been proved to be beneficial to the CNT fluidization according to the study of bubble characteristic and the degree of the particle movement. Additionally, it was found that the mixing behavior of @ V particles was better than @ A particles in the tapered angle of TFB without a distributor.

Modelling the growth and evolution of statistically significant populations of intergranular fission gas bubbles by IPM

The time-evolution of an intergranular bubble population is considered using the Included Phase Model (IPM). The model considers the generation and coupled transport of vacancies and gas atoms driven by interfacial energy, elasticity, and internal energy in a framework that admits complex interface morphology. The model predicts overpressure, bubble growth, coarsening, and coalescence leading to the formation of interconnected high-aspect-ratio bubbles on the grain face. The computational efficiency of this model is leveraged to simulate sets of 250 bubbles over time, which compare well with the CAGR-UOX-SWELL SEM measurements and the established SIFGRS model. Analysis of the simulation results highlights the importance of the bubble density as an indicator of bubble structure formed by coalescence and suggests that the two-point autocorrelation function captures the formation of an interconnected network which could improve release thresholds.

Ocean bubbles under high wind conditions – Part 2: Bubble size distributions and implications for models of bubble dynamics

Abstract. Bubbles formed by breaking waves in the open ocean influence many surface processes but are poorly understood. We report here on detailed bubble size distributions measured during the High Wind Speed Gas Exchange Study (HiWinGS) in the North Atlantic, during four separate storms with hourly averaged wind speeds from 10–27 m s−1. The measurements focus on the deeper plumes formed by advection downwards (at 2 m depth and below), rather than the initial surface distributions. Our results suggest that bubbles reaching a depth of 2 m have already evolved to form a heterogeneous but statistically stable population in the top 1–2 m of the ocean. These shallow bubble populations are carried downwards by coherent near-surface circulations; bubble evolution at greater depths is consistent with control by local gas saturation, surfactant coatings and pressure. We find that at 2 m the maximum bubble radius observed has a very weak wind speed dependence and is too small to be explained by simple buoyancy arguments. For void fractions greater than 10−6, bubble size distributions at 2 m can be fitted by a two-slope power law (with slopes of −0.3 for bubbles of radius <80 µm and −4.4 for larger sizes). If normalised by void fraction, these distributions collapse to a very narrow range, implying that the bubble population is relatively stable and the void fraction is determined by bubbles spreading out in space rather than changing their size over time. In regions with these relatively high void fractions we see no evidence for slow bubble dissolution. When void fractions are below 10−6, the peak volume of the bubble size distribution is more variable and can change systematically across a plume at lower wind speeds, tracking the void fraction. Relatively large bubbles (80 µm in radius) are observed to persist for several hours in some cases, following periods of very high wind. Our results suggest that local gas supersaturation around the bubble plume may have a strong influence on bubble lifetime, but significantly, the gas in the bubbles contained in the deep plumes cannot be responsible for this supersaturation. We propose that the supersaturation is predominately controlled by the dissolution of bubbles in the top metre of the ocean, and that this bulk water is then drawn downwards, surrounding the deep bubble plume and influencing its lifetime. In this scenario, oxygen uptake is associated with deep bubble plumes but is not driven directly by them. We suggest that as bubbles move to depths greater than 2 m, sudden collapse may be more significant as a bubble termination mechanism than slow dissolution, especially in regions of high void fraction. Finally, we present a proposal for the processes and timescales which form and control these deeper bubble plumes.

Identifying Unique Acoustic Signatures from Chemically-Crosslinked Microbubble Clusters Using Deep Learning.

Ultrasound contrast agents (UCA) are gas encapsulated microspheres that oscillate volumetrically when exposed to an ultrasound field producing a backscattered signal which can be used for improved ultrasound imaging and drug delivery. UCA's are being used widely for contrast-enhanced ultrasound imaging, but there is a need for improved UCAs to develop faster and more accurate contrast agent detection algorithms. Recently, we introduced a new class of lipid based UCAs called Chemically Cross-linked Microbubble Clusters (CCMCs). CCMCs are formed by the physical tethering of individual lipid microbubbles into a larger aggregate cluster. The advantages of these novel CCMCs are their ability to fuse together when exposed to low intensity pulsed ultrasound (US), potentially generating unique acoustic signatures that can enable better contrast agent detection. In this study, our main objective is to demonstrate that the acoustic response of CCMCs is unique and distinct when compared to individual UCAs using deep learning algorithms. Acoustic characterization of CCMCs and individual bubbles was performed using a broadband hydrophone or a clinical transducer attached to a Verasonics Vantage 256. A simple artificial neural network (ANN) was trained and used to classify raw 1D RF ultrasound data as either from CCMC or non-tethered individual bubble populations of UCAs. The ANN was able to classify CCMCs at an accuracy of 93.8% for data collected from broadband hydrophone and 90% for data collected using Verasonics with a clinical transducer. The results obtained suggest the acoustic response of CCMCs is unique and has the potential to be used in developing a novel contrast agent detection technique.

Current and Potential Distribution in Two-Phase (Gas Evolving) Electrochemical Reactors by the Finite Volume Method

A solver was developed and implemented in the OpenFOAM framework in order to predict the current distribution in gas evolving electrochemical reactors. The solver takes into account both liquid and gas flows under turbulent conditions and assumes a bubble population balance with a range of bubble sizes. The possibility of coalescence and breakup of bubbles is also considered. The comparison between experimental results of gas fraction and current distribution, from this research and from previous ones, demonstrates that the solver is able to predict the behavior of this kind of electrochemical systems. The model presented is supplied as a free source code.

Pore-network modeling of Ostwald ripening in porous media: How do trapped bubbles equilibrate?

Ostwald ripening of trapped bubbles in porous media is relevant to several subsurface (e.g., CO2 storage) and manufacturing (e.g., fuel cells) applications. A fundamental question common to both is: how does an initially heterogeneous distribution of bubble sizes, PDFo, evolves towards a stable equilibrium distribution, PDFe? And what is PDFe? We develop a pore-network model (PNM) that simulates the temporal evolution of a partially miscible population of bubbles from PDFo to PDFe. The PNM is validated against published micromodel experiments. We subsequently propose a theory that can predict PDFe directly. The predictions are compared against PNM simulations and found to be in close agreement. Using the PNM, we also show that bubble equilibration in heterogeneous porous media is a lot slower than homogeneous media. Limitations and potential extensions of both the PNM and the theory are discussed.

Pore-scale bubble population dynamics of CO2-foam at reservoir pressure

The flow of CO2 foam for mobility control in porous media is dictated by the foam texture, or bubble density, which is commonly expressed as the number of bubbles per unit of flowing gas. In most high-pressure laboratory studies of foam in porous media, the local foam texture cannot be determined due to opaque flow systems. Here, we unlock real-time foam texture dynamics at high pressure (100 bar) by utilizing a realistic pore network with an extended field of view. We identified snap-off as the dominant foam generation mechanism, with additional fining of foam texture caused by backward foam propagation. Foam coalescence during continuous CO2 injection resulted in large gas channels parallel to the general flow direction that reduced the overall foam apparent viscosity. A large fraction of the CO2 foam remained trapped (Xt > 0.97) and stationary in pores to divert CO2 flow and increase sweep efficiency. The gas mobility was calculated from the fraction of trapped bubbles at the pore-scale, and the apparent foam viscosity agreed with similar injection test performed at core-scale. Hence, improved understanding of CO2 foam texture evolution (nf) can strengthen the validation of numerical foam models for upscaling of flow phenomena, instrumental in the development of field scale implementation of CO2 foam for in carbon utilization and storage applications.

Multiwavelength emission from leptonic processes in ageing galaxy bubbles

ABSTRACT The evolutionary behaviour and multiwavelength emission properties of bubbles around galaxies, such as the Fermi bubbles of the Milky Way, is unsettled. We perform 3D magneto-hydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3-Myr intense explosive outburst from the nucleus of Milky-Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multiwavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to γ-rays, and construct emission maps in four energy bands to show the the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV γ-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after ∼1 Myr. In contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that γ-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with, e.g. the upcoming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.

Effect of the complex air–sea interface on a hybrid atmosphere-underwater optical wireless communications system

Wireless optical communication (WOC) through the atmosphere–ocean system is fundamentally complex because of the diversity, concentration, types and vertical thickness of the constituents and poses a major challenge from the optical characteristics of bubbles at the air–sea interface and water constituents (phytoplankton, suspended sediments, and dissolved organic matter), and stratification (medium inhomogeneity). The present study aims to analyse the effects of atmospheric aerosols (their intensity, diversity, and vertical thickness), waves/bubbles at the air–sea interface, seawater constituents (phytoplankton, suspended sediments, and dissolved organic matter), and medium inhomogeneity along the path between a laser transmitter in the airborne platform and a receiver in the underwater platform using Monte Carlo technique. For the first time, our analysis showed that for a homogeneous atmosphere–ocean channel (7 km in the atmosphere column and 42 m in the ocean medium), the estimated normalized received power reduced to 17 dB and the time delay spread increased to 1.01 ns (considering a receiver of 7” and an aperture FOV of 180°) when compared to those values for a stratified atmosphere–ocean channel. The bubbles (both clean and coated) relative to the particulates in open oceanic water under high wind speeds have a significant impact on the spatio-temporal spread of the laser beam. There is also a well pronounced effect of coated bubbles on the received signal power in open ocean water. In contrast, the particulates have a higher impact on the received signal power than the clean/coated bubbles in turbid coastal water. These results suggest that the normalized received power when calculated for the cases of dense bubble populations under high wind speeds (at a wind speed 18 ms−1) reduced to 8–9 dB and increased to 0.0025 scintillation index. The coated bubbles at the air–water interface reduced the normalized received power by 1.41 dB compared to the clean bubbles. Further, our analysis indicated that the expansion of beam width and the reduction of beam divergence angle can be utilized to mitigate the performance degradation (power loss) caused by bubbles at the air–sea interface. These results and analyses will be helpful for a system designer to accurately define/decide the system parameters and build a functional communication system for the inhomogeneous atmosphere–ocean system.

Part II - The bubble population: an analytic view into mutual forces and allied energy exchange

Part II - The bubble population: an analytic view into mutual forces and allied energy exchange

Development of an optical flow through detector for bubbles, crystals and particles in oils.

The characterisation of bubbles or particles in an oil poses some unique challenges. In contrast to water solutions, the use of electrochemical detection approaches is more difficult in an oil. However, optical sensing systems have considerable potential in this area. Here we use a flow through channel approach and monitor the light propagation through this structure in an optical transmission sensor arrangement (OTS). This simple approach is demonstrated to be useful at detecting bubbles produced in the oil as a result of cavitation induced by high intensity ultrasound (HIU). The optical technique is shown to have an analytical basis. Bubble detection from an operating HIU source is shown to depend on position of the sensor with respect to the source. Critically, the bubble population can be followed for extended time periods after the ultrasonic source has been terminated. The detection of crystals is also demonstrated. Hence, this technique is ideal for the study of the effects of HIU on oils as they crystallise over extended time periods.

Clean hydrogen production by ultrasound (sonochemistry): The effect of noble gases

Power ultrasonic (>100 ​kHz) splits water into free radicals and hydrogen. As a result, water sonochemistry is considered as an alternative clean and fossil-fuel-free hydrogen production technique. In this research work, the impact of rare gases (Xe, Ar, and He) on the sonochemical production of hydrogen as well as the population of active bubbles has been investigated computationally for various sonicated frequencies (213–515 ​kHz) and intensities (1–2 ​W/cm2). It has been found that both the H2 yielding and the bubble population size for H2 yielding are in the order Xe ​> ​Ar ​> ​He, whatever the imposed sonolytic parameters (i.e., frequency and power). These findings were principally ascribed to the thermal conductivity of the saturating gases, which is in the reverse order (He ​> ​Ar ​> ​Xe). Besides, the difference between Ar and Xe is condensed in comparison with the He gas. For wave frequencies larger than 213 ​kHz, however, all saturating gases (Xe, Ar, and He) behave identically, with the influence of thermal conductivity of these gases on the optimal radius muted. At 213 ​kHz, however, this impact is plainly visible (Ropt (Ar and Xe)>Ropt (He)). As per the results obtained, helium's inefficiency as a saturating gas for hydrogen production is verified, but xenon's maximal efficacy is reached when water is saturated with it. These results support the fewer experimental data reported in this emerging branch of sonochemistry, while the discussed results in the present paper (i.e., noble gases effect on sono-hydrogen production) are treated for the first time. Consequently, our work is considered as a guideline for increasing the efficacy of hydrogen production in a sonochemical reactor.