Scale Bubble Research Articles

A Unified Multiscale Framework for Simulating Electrochemical Gas Evolving Reactions

The presence of gas bubbles in electrochemical systems, such as water-splitting, significantly increases overpotential and diminishes energy efficiency. High-fidelity simulation holds significant promise to gain mechanistic understanding of bubble dynamics and guide high-performance electrolytic cell design. However, the extreme length scales involved into electrochemical gas evolving systems, from sub-nanometer dictated by the electrical double layer (EDL) to a few centimeters featured by the electrolytic cell, have posed a huge challenge to enable sufficient numerical accuracy and superior computational efficiency. As a result, state-of-the-art numerical approaches either can only simulate a nanoscale computational domain or neglect the electrochemical kinetics within the EDL, impeding an in-depth understanding of how bubbles could intervene with the electrochemical process.In this work, we demonstrate a full-field simulation approach of electrochemical gas evolving reactions that can capture the electrochemical kinetics of the EDL in a centimeter-scale electrolytical cell with the presence of micro-to-millimeter scale bubbles. To resolve all characteristic length scales, the EDL is geometrically decoupled from the electrolytical cell but physically coupled with the bulk electrolyte and gas bubble through a quasi-1D treatment. As a result, the Nernst-Planck-Poisson-Boltzmann model can be rigorously solved in both the EDL and the rest of the electrolytical cell with highly affordable computational cost. Taking hydrogen evolution reaction as an example, we validated our approach by comparing with a variety of existing numerical and experimental results. For the first time, the impact of bubbles on the increase of overpotential observed in experiments can be quantitatively confirmed through numerical simulation. More notably, compared to state-of-the-art high-fidelity simulations, our approach exhibits a remarkable computational efficiency, which reduced the computational time by a factor of 100,000. This work provides a viable solution to simulate electrochemical gas evolving reactions with highly desirable numerical accuracy and unprecedented computational efficiency, which can serve as an effective tool to understand bubble dynamics and guide the design of next-generation electrolytical cells.

Morphological Evidence for the eROSITA Bubbles Being Giant and Distant Structures

There are two contradictory views of the eROSITA bubbles: either a 104 pc scale pair of giant bubbles blown by the Galactic center (GC), or a 102 pc scale local structure coincidentally located in the direction of GC. A key element of this controversy is the distance to the bubbles. Based on the 3D dust distribution in the Galactic plane, we found three isolated, distant (500–800 pc) clouds at intermediate Galactic latitudes. Their projected morphologies perfectly match the X-ray shadows on the defining features of the north eROSITA bubble, i.e., the North Polar Spur (NPS) and the Lotus Petal Cloud (LPC), indicating that both the NPS and LPC are distant, with a distance lower limit of nearly 1 kpc. In the X-ray-dark region between the NPS and LPC, we found a few polarized radio arcs and attributed them to the bubble’s shock front. These arcs match up perfectly with the outer border of the NPS and LPC and provide a way to define the bubble’s border. The border defined in this way can be well described by the line-of-sight tangent of a 3D skewed cup model rooted in the GC. We conclude that, instead of being two independent, distant features, the NPS and LPC compose a single, giant bubble, which therefore is most plausibly a 10 kpc scale bubble rooted at the GC.

Study on the Dynamic Characteristics of Single Cavitation Bubble Motion near the Wall Based on the Keller–Miksis Model

The dynamic model of cavitation bubbles serves as the foundation for the study of all cavitation phenomena. Solving the cavitation bubble dynamics equation can better elucidate the physical principles of bubble dynamics, assisting with the design of hydraulic machinery and fluid control. This paper employs a fourth-order explicit Runge–Kutta numerical method to solve the translational Keller–Miksis model for cavitation bubbles. It analyzes the collapse time, velocity, as well as the motion and force characteristics of bubbles under different wall distances γ values. The results indicate that as the distance between the cavitation bubble and the wall decreases, the cavitation bubble collapse time increases, the displacement of the center of mass and the amplitude of translational velocity of the cavitation bubble increase, and the minimum radius of the cavitation bubble gradually decreases linearly. During the stage when the cavitation bubble collapses to its minimum radius, the Bjerknes force and resistance experienced by the bubble also increase as the distance to the wall decreases. Especially in the cases where γ = 1.3 and 1.5, during the rebound stage of the bubble, the Bjerknes force and resistance increase, causing the bubble to move away from the wall. Meanwhile, this study proposes a radiation pressure coefficient to characterize the radial vibration behavior of cavitation bubbles when analyzing the radiation sound pressure. It is found that the wall distance has a relatively minor effect on the radiation pressure coefficient, providing an important basis for future research on the effects of different scale bubbles and multiple bubbles. The overall idea of this paper is to numerically solve the bubble dynamics equation, explore the characteristics of bubble dynamics, and elucidate the specific manifestations of physical quantities that affect bubble motion. This provides theoretical references for further engineering applications and flow analysis.

Hybrid AI modeling techniques for pilot scale bubble column aeration: A comparative study

With increased accessibility of process data from the production lines in chemical and biochemical production plants, the use of data-based modeling methods is gaining interest. In this work, three different data-based modeling approaches are applied for modeling aeration in a pilot scale bubble column. In all three modeling approaches the same serial hybrid-model structure is used, combining a species conservation balance based on first-principles with different data-based models for the overall volumetric mass transfer coefficient (KLa). Simple empirical correlations with parameters fit to process data provide transparent models but lack the accuracy of Artificial Neural Networks (ANNs). ANNs provide models with high accuracy within the operation regimes used for training, however, the models are prone to overfitting, and their black-box nature results in models that are difficult to interpret. As an alternative, a symbolic regression-inspired technique is used for discovering symbolic equations, resulting in interpretable models with accuracy that is comparable to that of the ANN.

Development of extrusion blown films of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) blends for flexible packaging

AbstractPoly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) is a biodegradable polymer with significant potential for use in food packaging. However, its limited melt strength poses a challenge when employing film‐blowing techniques to produce flexible packaging. To overcome this obstacle, we developed blends consisting of 70 wt% PHBV and 30 wt% poly(butylene‐co‐succinate‐co‐adipate) (PBSA). Organic peroxides such as dicumyl peroxide and 2,5‐dimethyl‐2,5‐di‐(tert‐butylperoxy)hexane, were utilized as reactive compatibilizers to enhance the interfacial adhesion between the polymers. Additionally, acetyl tributyl citrate (ATBC) was employed as a plasticizer to improve processability and ductility. The inclusion of organic peroxides resulted in the formation of long‐branched structures, as confirmed by the van‐Gurp‐Palmen plot. The melt flow index decreased from 30 to 9.8 g/10 min without ATBC and 15.5 g/10 min with ATBC. Successful production of blown PHBV/PBSA films was achieved on a pilot scale (bubble height 180 cm). These films exhibited heat‐sealing capability and increased impact strength (7.7 kJ/m2). Moreover, the films maintained a maximum elongation at break of 4% during a 3‐month storage experiment with frozen food. Food safety was assessed through overall migration experiments, and the non‐plasticized films received approval. In conclusion, the compatibilized PHBV/PBSA blends demonstrate great potential as materials for manufacturing film‐blown flexible packaging.

Multi-Scale Simulations of Bubble Transport in an Alkaline Water Electrolyzer

The existence of bubbles can lead to the loss of performance by decreasing the electrochemically active surface area and disrupting the transport of hydroxide ions. These effects caused by the bubbles have yet to be sufficiently clarified. A comprehensive understanding of the mechanisms of dynamics inside an electrolyzer from the molecular scale up to the whole electrolyzer scale is needed. Furthermore, establishing relations between these dynamics at a wide range of scales is important. In this study, numerical simulations of bubble transport coupling fluid dynamics, mass transport and electrochemical reactions were conducted on micro and macro scales, respectively. The micro scale bubble simulation (Fig. 1(a)) incorporates the concentration field of the chemical species into the numerical model and deals with how bubbles grow on and detach from an electrode. In this model, the chemical species generated by the electrochemical reactions dissolve into the liquid phase, and their transfer into the gas phase leads to bubble growth. Since the spatial size distribution of the bubbles can be one of the factors that affect electrolysis performance, in the macro scale bubble simulation (Fig. 1(b)), we study how different sizes of bubbles are transported and distributed inside the electrolyzer. To simulate this phenomenon, a four-phase flow analysis is performed in which small, medium, and large diameters are selected as representatives of bubble diameters, and each is treated as a different phase.[Acknowledgements]This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Fig. 1 Simulation systems of bubble transport at (a) micro scale and (b) macro scale. Figure 1

Improving gas dispersion and decreasing bubble size in saline water with oscillatory air supply

The present paper describes gas dispersion in an air-sparged laboratory scale bubble column containing aqueous solutions of single electrolytes (NaCl, CaCl2, MgCl2, MgSO4, and AlCl3) and mixed electrolytes (NaCl-CaCl2 and NaCl-CaCl2-MgCl2-AlCl3) at different ionic strengths and air flow rates, with oscillatory air supply and conventional steady air supply, respectively. The gas dispersion was investigated by a light-scattering technique (turbidity measurement) and bubble size measurement. The oscillatory air supply was generated from steady air flow using a fast-switching solenoid valve and was characterized by measuring the pressure of the output air of the solenoid valve as a function of time. It was found that for each electrolyte solution tested, oscillatory air supply at a certain frequency and amplitude could give better gas dispersion than steady air supply, and the degree of improvement of gas dispersion varied with the electrolyte type and concentration and air flow rate. At each air supply mode tested, either the intensity of scattered light or the Sauter mean diameter of bubbles would scale with the ionic strength, following a decreasing trend. A cooperative effect of electrolyte addition and oscillatory air supply on gas dispersion was generally found. The combined effect of electrolyte addition and oscillatory air supply on bubble size was significant, being up to almost a 2/3 decrease in the Sauter mean diameter. The present work highlights the importance of flow condition for controlling bubble generation and coalescence in saline water and has significant implications for seawater flotation of minerals.

Process engineering strategy for large scale outdoor cultivation of Tetradesmus obliquus CT02 coupled with pH guided CO2 feeding

A novel CO2 tolerant microalga Tetradesmus obliquus CT02, was previously evaluated to be a suitable bio refinery platform for synthesis of bioactive molecules, biodiesel, and biofertilizer. In the present study, a process engineering strategy was developed targeting improved growth performance of the strain at large scale under fluctuating outdoor environmental conditions. The strategy relies on maintaining pH of the culture at its optimal value via cascade control with CO2 feeding. The strategy was developed at laboratory scale bubble column photobioreactor under diurnal variation of simulated sunlight intensity and was further validated through growth performance of the strain under outdoor conditions in a 100L airlift bioreactor. Under laboratory condition, 53.3% and 85.16% improvement in biomass concentration (1.87gL-1) and productivity (114.8mgL-1 day-1) was achieved as compared to the uncontrolled pH, respectively. The strategy demonstrated a significant improvement in biomass concentration and productivity by 225.7% and 121.6% respectively, compared to the pH uncontrolled batch, even under outdoor fluctuating environmental condition.

Stability of ex situ biological methanation of H2/CO2 with a mixed microbial culture in a pilot scale bubble column reactor

Biological methanation is a promising technology for gas and carbon valorisation. Therefore, process stability is required to allow its scale up and development. A pilot scale bubble column reactor was used for ex situ biological methanation with Mixed Microbial Culture (MMC). A 16S rRNA high throughput sequencing analysis revealed the MMC reached a stable composition with 50–60% Methanobacterium in closed liquid mode, a robust genus adapted to large scale constraints. Class MBA03 was identified as an indicator of process stability. Methanogenic genera moved toward 50% of Methanothermobacter when intensifying the process, and proteolytic activity was identified while 94% of H2/CO2 was converted into methane at 4NL.L−1.d−1. This study gives clarifications on the origin of volatile fatty acids (VFA) apparitions. Acetate and propionate accumulated when methanogenic activity weakened due to nutritive deficiency, and when PH2 reached 0.7 bar. The MMC withstood a storage period of 34d at room temperature indicating its suitability for industrial constraints.

On Inter-bubble distances and bubble clustering in bubbly Flows: An experimental study

• Positional relationships of bubbles were studied experimentally in the dense swarms. • Dimensionless inter-bubble distances were extracted from X-ray measurement data. • The distance of nearest neighbor bubbles is close to that of FCC structure. • Vertical clustering is observed for Eo ≥ 3.82. • Clustering orientation was found to be independent of the gas holdup in this study. Swarm effects in bubble columns result from the interaction of the bubbles at higher gas holdup. The flow around individual bubbles in a swarm influences the movement of the following bubbles and this has a feedback on the flow regime on a larger scale. That is, hydrodynamics that can no longer be described with models for single rising bubbles. The experimental investigation of the multiscale hydrodynamics in three-dimensional bubble columns was so far limited by the available measurement techniques. In this study, ultrafast X-ray tomography (UFXCT) was applied on bubbly flows in a lab scale bubble column to determine inter-bubble distances and clustering characteristics in the vicinity of individual bubbles for different gas holdups and bubble diameters. Applying the pair correlation function reveals a pronounced vertical clustering for large bubbles with Eo ≥ 3.82, whereas no clustering has been observed for bubbles of lower Eo . It was found that clustering orientation mainly depends on deformability of the bubbles but not on gas holdup.

Spatio-temporal 1D gas–liquid model for biological methanation in lab scale and industrial bubble column

Numerical simulation of biological methanation in bubble column reactors is challenging due to strong coupling between hydrodynamics, mass transfer and bioreaction. A satisfactory prediction of the mass transfer coefficient and the gas solubility is crucial to couple hydrodynamics and biokinetics. A comprehensive 1D multispecies multiple-way coupled gas–liquid model is developed. The hydrodynamics of the model is validated using experimental gas loading data at very low gas holdup. The local mass transfer is validated using literature data on CO2 mass transfer in the large-scale. The local gas holdup and interfacial CO2 mass transfer flux are correctly described thanks to a two-way coupling between hydrodynamics and mass transfer, including changes in bubble diameter and pressure effects. When a mixture of H2, CO2 and CH4 is used, highly non-linear mass transfer profiles due to differences in solubilities are observed. An original flux-based metabolic model is proposed to simulate an industrial biological methanation process. This allows smooth transition between biologically and physically controlled regimes as the culture reaches a steady-state. The comprehensive model enlightens that the biological methanation performances are governed by the H2 mass transfer limitation balanced by growth in the transient state and biological maintenance in the steady-state.

Stability of Ex Situ Biological Methanation of H2/Co2 with a Mixed Microbial Culture in a Pilot Scale Bubble Column Reactor

Stability of Ex Situ Biological Methanation of H2/Co2 with a Mixed Microbial Culture in a Pilot Scale Bubble Column Reactor

Numerical study of effects of stand-off distance and gravity on large scale bubbles near a breach

When a ship is subjected to a near-field underwater explosion, the strong shock waves may cause local damage or a breach in the ship’s structure. The large-scale bubbles that subsequently form will expand and collapse according to the effects of the nearby breach. In the present paper, a bubble numerical setup for the region near a breach is established based on the Eulerian finite element method. This numerical setup is first verified through bubble experiments in a vacuum water tank that produces a strong buoyancy effect. The mechanism of coupling between the large-scale bubble and the breach is then studied under different standoff distances and gravitational effects. It can be concluded that the standoff distance determines the inrush pattern, with the various patterns including inrush with a bursting bubble, inrush with a gourd-shaped bubble, and inrush with a floating bubble. The volume and initial velocity of the inrush water varies with the standoff distance. Finally, the influence of gravity on the inrush process is analyzed. The initial stage of inrush is almost independent of gravity, although this soon gives way to a stage in which gravity has an obvious influence. As the bubbles float up and drive the surrounding water, the volume of inrush water suddenly increases under the effect of gravity. This paper aims to provide references for ship protection under UNDEX load.

Ozonation of diclofenac in a laboratory scale bubble column: Intermediates, mechanism, and mass transfer study

Diclofenac (DCF), a non-steroidal analgesic drug, has been recurrently detected in surface and groundwater during the past two decades. Traces of DCF pass through the conventional wastewater treatment facilities without any alteration due to its resistance and high stability. This study aims the complete degradation and partial mineralization of DCF by an ozonation process. The findings of the study suggest that increasing pH of the system and ozone supply rate, and generation of hydroxyl radicals in situ enhance the degradation process. In contrast, the increasing initial concentration of DCF slows down the process. The pseudo-first-order reaction rate constants were in the range of 0.0742 – –0.0979 min−1. A model has been developed to predict the behavior of the rate constant with various operating variables. High volumetric mass transfer coefficients (i.e., 1.150–2.717 × 103 s−1) was observed during the mass transfer study of ozone in water. A probable mechanism for ozonation is suggested along with the structures of major intermediates formed during the oxidation. Distinct orange pigments appeared during the reaction, which confirmed the production of DCF-2, 5-iminoquinone. Di-chloro aniline, 5-hydroxy DCF. The carboxylic acids were mainly detected as the metabolites. The treatment cost was estimated to be USD 0.80 for treating 1 m3 of the solution containing 50 mg dm−3 DCF. In addition, the effect of other water matrices on DCF degradation was also investigated and reported.

Environment-friendly surface cleaning using micro-nano bubbles

Cleaning a surface using a solution containing a large number of micro to nano scale bubbles has significant advantage regarding environmental protection. This review first briefly introduces the cleaning mechanism of micro-nano bubbles (MNBs), including physical and chemical effects. Then the applications of MNBs in cleaning of metal parts, precision parts, cultural relics or food are introduced. After that, coupled cleaning method of ultrasound and bubbles is introduced. Finally, the characterization methods for the cleaning effect are introduced, which mainly focuses on the changes of physico-chemical properties (mass or cleaning area, infiltration, colony number and light scattering intensity) of the cleaned parts or that (like conductivity) of the solvent. It is believed that MNBs technology will be applied in a broader range of surface cleaning applications.

An investigation of the gravity effects on pool boiling heat transfer via high-fidelity simulations

Nucleate pool boiling simulations are carried out to study the effects of gravity on the dynamics of bubble formation, bubble departure, and the mutual bubble interactions. We focus on identifying the dominant trends associated with the flow and thermal behavior at different gravity levels using fixed values of wall superheat and liquid subcooling. This is accomplished by measuring the contributions to heat flux associated with turbulent flow resulting from the dynamics of bubble interaction. The time averaged fluctuations of velocity and temperature indicate that the wall heat flux conditioned by the wetted surface area - the liquid heat flux - provides a useful indication of high heat flux hot spots. Furthermore, despite substantial differences in the dynamics of large scale bubbles at different gravity levels, the average heat flux in the liquid region is found to be dominated by the sliding and merger type events associated with small scale bubbles over a wide range of gravity levels. This significance of near-wall, small scale bubble dynamics is reflected in the transition of vorticity structures from hairpin type vortices in the near-wall region to vortex ring patterns away from the wall. While the maximum value of average turbulent fluctuations occurs away from the wall for earth normal gravity, the near-wall peak is only slightly lower and is similar in magnitude to that in the case of low gravity. These observations reveal the significance of small scale dynamics in affecting the heat transfer which was previously thought to be dominated by large scale bubbles, particularly at low gravity levels.

Macroscopic, layered onion shell like magnetic domain structure generated in YIG films using ultrashort, megagauss magnetic pulses

Study of the formation and evolution of large scale, ordered structures is an enduring theme in science. Generation, evolution and control of large sized magnetic domains are challenging tasks, given the complex nature of competing interactions in a magnetic system. Here, we demonstrate large scale non-coplanar ordering of spins, driven by picosecond, megagauss magnetic pulses derived from a high intensity, femtosecond laser. Our studies on a specially designed yttrium iron garnet (YIG) dielectric/metal film sandwich target, show the creation of complex, large, concentric, elliptical shaped magnetic domains which resemble the layered shell structure of an onion. The largest shell has a major axis over hundreds of micrometers, in stark contrast to sub micrometer scale polygonal, striped or bubble shaped magnetic domains in magnetic materials, or large dumbbell shaped domains produced in magnetic films irradiated with accelerator based relativistic electron beams. Micromagnetic simulations show that the giant magnetic field pulses create ultrafast terahertz (THz) spin waves and a snapshot of these fast-propagating spin waves is stored as the layered onion shell shaped domains in the YIG film. Typically, information transport via spin waves in magnonic devices occurs in the gigahertz regime, where devices are susceptible to thermal disturbances at room temperature. Our intense laser light pulse—YIG sandwich target combination, paves the way for room temperature table-top THz spin wave devices, operating just above or in the range of the thermal noise floor. This dissipation-less device offers ultrafast control of spin information over distances of few hundreds of microns.

High cell density culture of recombinant E. coli in the miniaturized bubble columns.

Miniaturized bubble columns (MBCs) can provide mass transfer characteristics similar to stirred tank bioreactors. In this study, a new application was developed for MBCs to investigate the effect of feeding strategy and medium type on the fed-batch culture of recombinant E. coli. The results showed that the exponential feeding strategy and defined M9 medium were more suitable to achieve the high cell density culture (HCDC). The maximum obtained cell concentration in exponential feeding strategy in the defined medium without induction, was at OD600 of 169, while glucose concentration was maintained under 2g/L. To the best of our knowledge, this cell concentration cannot be achieved in lab or pilot scale bubble columns. At the end of the process, adverse effect of the metabolic burden due to induction and mass transfer limitations decreased the obtained final cell concentration to OD600 of 116. Finally, a comparison of the results for fed-batch culture in the stirred tank bioreactor with those of the MBCs showed that their lower cell concentrations were due to the hydrodynamics limitations of MBCs. Yet, it was found that the MBCs are efficient tools in development of feeding strategies and evaluation of medium components for HCDC of recombinant E. coli.

Numerical simulation of continuum scale electrochemical hydrogen bubble evolution

One of the important aspects in improving the efficiency of electrochemical processes, such as water electrolysis, is the efficient removal of bubbles which evolve from the electrodes. Numerical modelling based on Computational Fluid Dynamics (CFD) can describe the process, provide insights into its complexity, elucidate the underlying mechanisms of how bubbles evolve and their effect as well as aid in developing strategies to reduce the impact of the bubble.In this paper, a Volume of Fluid (VOF) based simulation framework to study the evolution of hydrogen bubbles in the order of few hundred micrometers, refered to as continuum scale bubbles, is proposed. The framework accounts for the multiphase nature of the process, electrochemical reactions, dissolved gas transport, charge transport, interfacial mass transfer and associated bubble growth. The proposed solver is verified, for two-dimensional cases, by comparison to analytical solution of bubble growth in supersaturated solutions, stationary bubble, rising bubbles and qualitative analysis based on experimental observations of the variations in current based on static simulations. The proposed solver is used to simulate the evolution of a single bubble under various wetting conditions of the electrode as well as the coalescence driven evolution of two bubbles. The results show that as the bubbles detach, its surface oscillates and the shape of the rising bubble is determined by the balance between drag force and surface tension. These surface oscillations, which causes the bubble to get flattened and elongated, results in temporal variation of the electrical current. The reduction of current due to bubble growth is visible only when these surface oscillations have reduced. The simulations also show the current as a function of the position of the bubble in the interelectrode gap. The framework also predicts the increase in current as a result of bubbles leaving the surface which is larger when the process is coalescence driven. The simulations indicate that bubble coalescence is the underlying mechanism for continuum scale bubble detachment.

Advanced analysis of bubble columns: Comparison of Euler/Lagrange simulations and experiments under CO2 chemisorption conditions

The chemical absorption reaction of CO2 in a circular bubble column reactor was simulated with an Euler/Lagrange CFD method and results were compared with data from a laboratory scale bubble column. The experimental data comprises gas holdup and bubble size distributions obtained with ultrafast X-ray tomography and hydroxide ion conversion obtained with a wire-mesh sensor. Large Eddy Simulation (LES) is used for calculating the fluid flow and modelling the turbulence in the continuous phase, considering the effect of sub-grid-scale (SGS) turbulence on bubble motion, and also SGS turbulence modification by bubbles. Lagrangian bubble tracking is conducted considering all relevant forces, bubble oscillation via stochastic generation of eccentricity and motion angle and consequently non-steady mass transfer via a dynamic Sherwood number. The comparison of experimental and simulation results shows very good agreement for the given data.