Bubble Merging Research Articles

Boiling Heat Transfer in Copper Foam Bilayers in Positive and Inverse Gradients of Pore Density

Gradient metal foam is an advanced heat transfer material that decreases resistance to bubble escape and enhances the transfer of boiling heat. In this study, boiling heat transfer and bubble behavior were studied in an experimental set-up with copper foam bilayers configurated either in positive or inverse gradients, utilizing deionized water as working fluid. Positive gradient refers to arranging metal foam layers with high pore density at the bottom, above the heat source, and low pore density on the top. Results show that the heat transfer is higher for gradient metal foam surfaces, of 6.14×105 W/m2, versus a uniform metal foam surface, of 3.94×105 W/m2. For the positive gradient configuration, boiling heat transfer performance first increased with the pore density, then decreased when the pore density was higher than 60 pores per inch (PPI). By contrast, for the inverse gradient, the heat transfer performance was nearly constant with increasing pore density. At the low pore density, the inverse gradient performed better than the positive gradient during the whole boiling process. At high pore density, the positive gradient structure performed better in heat transfer at the early boiling stage. Three main types of bubble escape were observed: For the positive gradient bilayer, the bubbles moved up or down without lateral interference. In contrast, for inverse gradient, the bubbles mostly escape from the sides, which is easy to induce bubble merging. The inverse gradient surface generates larger bubbles, while the positive gradient surface produces a higher frequency of bubble detachment. Accordingly, two liquid replenishment models are proposed: for the positive gradient, external liquid replenishes from the side into the copper foam, while for inverse gradient, the liquid is mainly replenished from the top.

PHANGS–JWST First Results: A Statistical View on Bubble Evolution in NGC 628

The first JWST observations of nearby galaxies have unveiled a rich population of bubbles that trace the stellar-feedback mechanisms responsible for their creation. Studying these bubbles therefore allows us to chart the interaction between stellar feedback and the interstellar medium, and the larger galactic flows needed to regulate star formation processes globally. We present the first catalog of bubbles in NGC 628, visually identified using Mid-Infrared Instrument F770W Physics at High Angular resolution in Nearby GalaxieS (PHANGS)–JWST observations, and use them to statistically evaluate bubble characteristics. We classify 1694 structures as bubbles with radii between 6 and 552 pc. Of these, 31% contain at least one smaller bubble at their edge, indicating that previous generations of star formation have a local impact on where new stars form. On large scales, most bubbles lie near a spiral arm, and their radii increase downstream compared to upstream. Furthermore, bubbles are elongated in a similar direction to the spiral-arm ridgeline. These azimuthal trends demonstrate that star formation is intimately connected to the spiral-arm passage. Finally, the bubble size distribution follows a power law of index p = −2.2 ± 0.1, which is slightly shallower than the theoretical value by 1–3.5σ that did not include bubble mergers. The fraction of bubbles identified within the shells of larger bubbles suggests that bubble merging is a common process. Our analysis therefore allows us to quantify the number of star-forming regions that are influenced by an earlier generation, and the role feedback processes have in setting the global star formation rate. With the full PHANGS–JWST sample, we can do this for more galaxies.

Mesoscopic study on effect of electric field and heater characteristics on saturated pool boiling heat transfer

The phase change lattice Boltzmann (LB) model combined with the electric field model is employed to investigate the heat transfer performance of saturated pool boiling. Particular attention is paid to the influence of heater surface wettability and heater length on bubble behaviors, including generation, merging, and fracture during boiling in a uniform electric field. Moreover, the effects of the bubble behavior on heat transfer performance are also investigated. The study results indicate that the enhancement of boiling heat transfer by the electric field is dependent on both the heater length and the wettability. In the case of a hydrophilic surface, when the heater length <inline-formula><tex-math id="M5">\begin{document}$L_H^*\leqslant 6.25$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M5.png"/></alternatives></inline-formula>, the bubble interaction force generated on the heater surface during boiling is weak due to the small size of the heater. Thus the effect of a uniform electric field on the bubble dynamic behaviors is mainly manifested by reducing the bubble size. As a result, the whole boiling phase is suppressed in this case. In the case of <inline-formula><tex-math id="M6">\begin{document}$6.25 < L_H^*\leqslant9.375$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M6.png"/></alternatives></inline-formula>, the uniform electric field enhances the critical heat flux (CHF), and the enhancement degree increases with electric field strength increasing. This can be attributed to the longer heater providing sufficient space for bubble generation, resulting in increased bubble nucleation sites and stronger interaction forces between bubbles. On the other hand, the distance between adjacent bubbles increases with the heater length increasing,thus further contributing to the improved CHF percentage. When <inline-formula><tex-math id="M7">\begin{document}$L_H^*>9.375$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M7.png"/></alternatives></inline-formula>, the rewetting resistance increases with heater length increasing. So the vapor generated in the boiling process is prone to be closely adhered to the heating surface under the action of electric field force, forming a thin layer of vapor on the heater surface. The vapor not only increases the heat transfer thermal resistance between the solid and the fluid but also creates no vortex near the bubble. This is not conducive to the movement of the bubble to the middle of the heater, thereby slowing down the heat mass exchange between the hot fluid on the heating surface and the colder fluid on both sides. As a result, the improved percentage of CHF decreases gradually with the increase in the heater length. In the case of hydrophobic surfaces, the increased percentage of CHF initially increases with heater length increasing and then decreases. However, comparing with the hydrophilic surface, the increase of the heater source length corresponds to the beginning of the decrease of critical heat flux.

Molecular dynamics simulation of bubble growth under surface of tungsten under helium irradiation

Understanding the fuzz growth mechanism on tungsten (W) surface under the helium (He) particles irradiation is an important guide for structural design of the first wall materials. In this work, molecular dynamics simulation was used to explore the process of He bubble formation, growth and rupture under irradiation to understand the initial stage of fuzz nanostructure formation. It was found that He clusters had the most movable ability at the surface temperature around 800∼1800 K and thus big bubbles were favorable to be aggregated. We simulated the growth, rupture and merging of He bubbles to reproduce the process of protrusion's formation on the surface which induced the formation of fuzz nanostructure. It was found that the shallow bubble burst led to the pinhole in surface layer. The dislocation slip in the growth process of deep bubbles was the main factor for the configurations of protrusion on surface. It mainly pushes the W above He bubble along the <111> directions, while the different surfaces have different relation with the <111> directions. Thus, the different morphology of protrusion induced by the bubble appeared on different surfaces as observed in experiments.

Enhanced pool boiling heat transfer of multilayer copper-nanoparticle-packed beds by copper mesh coating

Pool boiling heat transfer has drawn a wide interest in dissipating high density heat flux produced by electronic products etc. In this work, the advantages of the copper mesh and the multilayer nanoparticle-packed beds in improving pool-boiling heat transfer was combined to give a good performance. The copper-nanoparticle-packed beds were firstly prepared, and then the effects of the layer thickness and porosity in the bed on the pool boiling heat transfer were explored. After obtaining the optimal thickness and porosity of the multilayer beds, the copper mesh was further coated on the multilayer beds to further enhance the pool-boiling heat transfer. Results turn out that: The increased thickness of the beds has a negative effect on enhancing heat transfer coefficient (HTC) because of the increased bubble-escape resistance, however, this effect will be set-off by the positive effect of the increased wetted area when the thickness was increase to be larger than 3 mm in this work. The onset of nucleate boiling of the optimal 4-layer beds without coating is about 1.7 °C which is much lower than most reported values, and the HTC is about 9 × 104 W/(m2·k) which is 9 times higher than that of the smooth surface. With the optimal bed further coated with copper mesh, the HTC can be further increased to 9.8 × 104 W/(m2·k). This enhancement arrives from the fact that the copper mesh prevents the continuous merging of bubbles, thus reducing the bubble detachment diameter by 30% and increasing detachment frequency of bubbles by 0.5 times.

Controllable and Directional Transportation of Bubbles on Asymmetric Hexagonal Cage Substrate in Aqueous Environment.

Controllable and directional bubble transport is usually the critical step in applications involving bubbles. However, current bubble transport strategies either are limited in controllability and transport distance or require the assistance of a specific external field. Here, we propose a strategy for bubble transport in an asymmetric hexagonal cage (ASHC), which works smoothly even under antibuoyancy conditions. The transport efficiency of bubbles can be greatly improved by adjusting the structural parameters of the cage. The control of the bubble depends only on the change of the bubble's volume, so there is no strict restriction on the driving force, which can be pressure, photothermal, electrothermal, and even acoustic-thermal forces. Moreover, we demonstrate that long-distance transport and controllable merging of bubbles can be easily achieved by cascading multistage ASHC structures. This investigation offers a simple, low-cost, extensible, and versatile construction for bubble transport for fundamental research and practical applications.

Investigation on visualization and heat transfer performance study of the mini-channel flow boiling

In recent years, mini-channel water flow boiling has been proven to have favorable cooling capability, especially for electronic component heat dissipation. Nevertheless, the fundamental principles related to flow boiling in mini-channel water lack clear comprehension, and the relevant experimental data on heat transfer performance and CHF visualization need to be supplemented. The behavior of vapor bubbles was explored in detail through visualization experiments to elucidate the mechanism of flow boiling and heat transfer in mini-channels. Meanwhile, the effects of mass flux, heat flux and tilt angle on the flow boiling of water and the CHF characteristics were revealed clearly though a series of crossover experiments. The results showed that in mini-channel water flow boiling, mass flux and heat flux will have effects on vapor bubble behavior, especially the merging of vapor bubbles, and then modify the heat transfer coefficient, and the mass flux in the optimal range exists. The tilt angle has less effect on the heat transfer coefficient, but affects the rate of vapor bubble growth. After local dryness the heat transfer mechanism transitions directly to film boiling instead of transition boiling, the boiling curve grows linearly with the wall superheat, and the CHF increases with increased mass flux. The research content of this paper helps to fill the gap in the experimental study of flow boiling in mini-channel and provides guidance for related practical applications.

Study on the recovery characteristics of disc-type superconducting fault current limiter coils from the perspective of boiling heat transfer and bubble behaviors

The disc-type superconducting fault current limiter (SFCL) encapsulated by yttrium barium copper oxide (YBCO) superconducting material has broad application prospects in the current limiting field. However, the difficulty to recover quickly and safely after quench remains the bottleneck of the disc-type SFCL. Existing work on the SFCL mainly focused on the resistance recovery while the bubble behavior and distribution, which are important factors affecting the temperature distribution of the superconducting tape, are rarely noticed. In this paper, a visualization platform for the superconducting tape impulse experiment is built, and the dynamic behavior and distribution characteristics of bubble clusters are analyzed by image processing technology. Two arrangements (vertical and horizontal) disc-type coils are comparatively studied. Firstly, the huge differences in the recovery performance of the superconducting tape at different positions are observed and compared. In the vertical coil, the fastest one can recover in 1.414 s and the slowest one takes 2.426 s to recover to the superconducting state. The maximum temperature difference of superconducting tapes at different positions in the coil is up to 10.8 K. In the horizontal coil, the recovery times of the two samples are 1.497 s and 1.595 s, respectively, indicating that the recovery is significant uniform. The whole process of bubble generation, merging and detachment in the transient boiling process is comprehensively and qualitatively analyzed with the help of the high-speed camera. Secondly, it is observed a phenomenon of bubble retention is common in the vertical coil while the horizontal coil is not, which is not conducive to the safe operation of the system. Therefore, the bubble recovery is as important as the resistance and the temperature as a recovery reference. In summary, the horizontal coil is superior to the vertical coil in terms of safe recovery and uniform cooling based on the experimental results in this paper, the coupling analysis of bubble dissipation behavior and thermodynamic characteristics of superconducting tape during quench and recovery is of theoretical significance to the future research work on accelerated recovery and the SFCL structural design.

Boiling Heat Transfer on Cylindrical Surface: An Experimental Study

In this paper, pool boiling heat transfer data from experiments are presented to investigate bubble formation patterns and film growth on a large sized cylinder dipped in a pool. A wide range of heat flux has been applied over the cylinder to observe different regimes in boiling heat transfer. Nucleation, bubble merging during the transition, and sliding are captured through careful experimental efforts. Bubble sliding on the cylinder after inclining it with respect to horizontal has also been captured during the experiment which prompts the formation of large-sized bubbles on the surface. It has been established that the heat transfer coefficient varies nearly 2 times based on the azimuthal location of a large cylinder, bottom location of the cylinder showing the highest. Studies from inclined cylinder showed azimuthally bottom-most point of higher end has faster bubble release and highest heat transfer coefficient (∼2 times higher than top-most point of lower end). Spiral-like bubble motion has been noticed after detachment from nucleation sites over the inclined surface. Effect of cylinder surface inclination on boiling pattern has been also presented along with observations of bubble formation.

Collapsing behavior of a spark-induced cavitation bubble near the air bubble attached to the tube nozzle

Air entrainment has been proved to be an effective method to weaken the cavitation damage. The objective of this paper is to apply high-speed photography and schlieren optical method to investigate the collapse behavior of cavitation bubble near the air bubble. The corresponding data such as bubble collapse time, collapse position and the velocity of collapse jet are analyzed. Results are presented for a cavitation bubble generated directly above an air bubble under a wide range of dimensionless standoff distances. The results show that (1) four distinctive patterns are identified by the collapse direction of the cavitation bubble and the interaction between the cavitation bubble and the air bubble. (2) The dynamic features of the cavitation bubble at collapse stage vary evidently at different patterns, including the collapse direction, high-speed jet and shock waves. (3) Compared with the cases that the cavitation bubble does not merge with an air bubble, the velocity of collapse jet and the intensity of shock waves are extremely smaller when the cavitation bubble merging with the air bubble. Investigating the influence of air bubble on the cavitation bubble has great significance to understand the mechanism of air entrainment to reduce cavitation damage.

Effect of T-shaped micro-fins on pool boiling heat transfer performance of surfaces

The saturated pool boiling heat transfer performance on the surface of T-shaped micro-fins was investigated with deionized water as working medium. Ten different T-shaped micro-fins surfaces were fabricated by machining method and the pool boiling experiments were carried out. By comparing the influence of varios parameters on the boiling heat transfer performance, the results showed that reducing the width of upper fin, increasing the gap or height of upper fin can improve the boiling heat transfer coefficient. With the increase of the width of the lower fins, the boiling heat transfer performance showed a different trend, that is, it increased at first and then decreased. The maximum boiling heat transfer coefficient of the surface S1 to the smooth surface was 184.7%. The visualization results showed that the merging of bubbles on the outside of the channel surface could strengthen the vertical and horizontal replenishment of the liquid to dry spot in the channel, thereby delaying the occurrence of critical heat flux and effectively reducing the wall temperature. The simplified semi-analytical model to determine the total heat flux of the micro channeal was proposed, which provided a heat flux prediction with an error of ± 40%.

A fractional-step lattice Boltzmann method for multiphase flows with complex interfacial behavior and large density contrast

In the present study, a robust fractional-step lattice Boltzmann (FSLB) method is proposed to simulate the mass transfer phenomenon in incompressible multiphase flows with complex interfacial behavior and large density contrast. The previous simplified lattice Boltzmann method recovers the continuity equation in first-order accuracy and reconstructs the corrector step by directly applying the complex central difference scheme on the macroscopic variables. However, the present FSLB method employs the Chapman-Enskog expansion analysis to reconstruct the convection and diffusion terms of the macroscopic governing equations, and uses the equilibrium and non-equilibrium distribution functions to establish the predictor-corrector step. The intermediate variables are predicted by the equilibrium distribution functions without the consideration of the source terms, and then the physical variables are corrected by the non-equilibrium distribution functions and the source terms without the evolution of the distribution functions. The reconstructed governing equations in both the predictor and corrector steps can be recovered into fully second-order accuracy through the C-E expansion analysis and the Taylor series expansion analysis. The present FSLB method inherits the excellent performance of kinetic theory from the conventional LB method and the good numerical stability from the matured fractional-step method, which is validated by several benchmark problems, such as Laplace law, bubble merging, interfacial deformation under a magnetic field, Rayleigh-Taylor instability at Reynolds number of 3000, Kelvin-Helmholtz instability at Reynolds number of 5000, and bubble rising at density ratio of 1000. A good agreement between the present numerical results with the published numerical data verifies the capability and reliability of the present FSLB method to handle the multiphase problems with complex interfacial behavior and large density contrast.

CFD Comparison of multiphase models in the pool boiling state

Abstract With the development of simulation technology and programs, it became necessary to study the models that control equations’ solutions and influence the results. The models having control over solving equations of multiple phases and materials are investigated. They include (Volume of Fluid (VOF), mixture, Eulerian) controlling the governing equations. The study was conducted depending on the boiling point of the water. The activation of these three models is carried out to find out which one is better for solving the issue of boiling compared to previous numerical and empirical research with the study of the surface tension coefficient that affects the behavior of phases in a contaminated manner. The best model explored in the case of boiling is VOF for the merging of steam bubbles, the velocity of flows 0.257 m/s for both water and steam, and the phase transition. The effectiveness of the VOF model is mirrored by higher efficiency and accuracy of the solution with velocity 0.257 m/s and volume fraction 0.9997. The activation of the surface tension factor 0.072 property simulates the real conditions surrounding the materials used in boiling, but it significantly increases the turbulence and distribution of gas bubbles.

Multiple-relaxation-time lattice Boltzmann method for three dimensional free-surface flows with multi-bubble model

Flow with bubbles is a commonly occurring phenomenon in nature and industrial applications. This study provides a multi-bubble model for the free surface lattice Boltzmann method (FSLBM) which can be used to simulate fluid flow with bubbles as well as their splitting and merging processes. FSLBM neglects the flow of gas and considers only the gas pressure, and thus simplifies the gas-liquid two-phase flow to a single-phase free surface. The compression of the bubble will influence its pressure and lead to movement, splitting, or merging processes within the bubble flow. The multi-bubble model considers the evolution of bubble's pressure by enhancing the model for free surface which simulates the behavior of gas-liquid interface inside the liquid. The Smagorinsky subgrid-scale model (SGS) and the more advanced multiple-relaxation-time (MRT) collision model is used for high Reynolds number. In this study, a bubble algorithm to handle the bubble splitting and merging without incurring significant computational costs is provided. The verification and analysis of this model are given by a comparison between both analytical solution and an experiment. Therefore, the multi-bubble as an extension of FSLBM can be used to solve a wider range of two-phase flow problems more efficiently, such as bubble formation, metal foaming process, and development in chemical reactors.

Two-dimensional lattice Boltzmann method to study the influence of nucleation distance on heat flux during bubble coalescence

The lattice Boltzmann method studied the merging of double bubbles under different spacings in pool boiling. This paper analyzes the influence of bubble nucleation distance on heat transfer performance. According to the difference of nucleation distance within the scope of this article, the impact of bubble coalescence on bubble departure frequency can be divided into three regions: promotion, inhibitive, and promotion. The dynamic behavior of the bubble is linked with the heat flux, the heat flux evolution characteristics of different bubble separation distances are analyzed. The optimal nucleation distances were found to exist to achieve the maximum thermal transfer performance. The optimal distance is correlated with environmental factors such as superheat, heater length, and gravity. The main reason affecting the optimal separation distance is the bubble diameter. The larger the bubble leaves the diameter, the larger the optimal separation distance.

(Invited) Investigating Preferential Pathways for Oxygen Removal through Porous Transport Layers of Polymer Electrolyte Water Electrolyzer Using Operando X-Ray CT

Polymer electrolyte water electrolyzer (PEWE) is one of the most appealing options to produce hydrogen and oxygen from renewable resources[1]. Understanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the cell performance of PEWE. In order to probe this more fully, operando x-ray computed tomography (CT)[2], machine learning, and LBM simulation[3] were performed on a model electrolyzer at different water flowrates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. X-ray CT was employed to create 3D images (Figure 1A) and visualize oxygen content in the model electrolyzer (Figure 1C and 1D). Machine learning was used to quantify the oxygen content in the electrolyzer geometry, as well as any pathways or patterns the oxygen took as it exited the electrolyzer through the PTL. Computational fluid dynamics (CFD) was used to investigate the characteristics of oxygen transport in the PTL under different operating conditions.Our work has demonstrated new findings in oxygen transport behavior in the PTL. We report a direct observation of oxygen taking preferential pathways through the PTL regardless of the water flowrate or current density (1-4 A/cm2). The spatially periodic oxygen front has been observed in this study for the first time. Oxygen distribution in the PTL had a periodic behavior with period of 400 μm[4]. CFD model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer. A conceptual schematic showing transport of oxygen in the PTL is shown in Figure 1B. Once the transport pathway through the thickness of the PTL is established, oxygen bubbles will preferentially follow the pathway and no new pathways will be introduced within the current densities range studied here. These findings should be useful in improving mass transport and enhancing our understanding of processes in PEWE.

Advanced boiling heat transfer model for a horizontal tube with numerical analysis of bubble behaviours

The boiling heat transfer model for a horizontal tube is developed herein, with numerical investigations to realistically simulate bubble nucleation site distributions and bubble behaviours. Accordingly, a new heat partitioning model was proposed to reflect the bubble sliding phenomena more realistically than the extant heat partitioning models for horizontal tubes. The bubble trajectories, merging, lift-off, and distribution of nucleation sites were numerically analysed based on the force balance model, bubble tracking method, and Latin-hypercube sampling. The proposed approach can overcome the limitations of the arithmetic heat partitioning model, which does not reflect bubble mergers and the random nature of the nucleation site distribution. Distinctive bubble behaviours on a horizontal tube can be reproduced by the present model, including bubble sliding, departure, lift-off, and merger. The numerical model presented here considers bubble sliding based on a mechanistic force balance model and bubble merger based on the bubble tracking method. The Monte Carlo method was applied to determine the spatial distribution of the nucleation sites, and statistical information on the wall heat transfer and contribution of each heat transfer mechanism were obtained via extensive calculations. The method developed herein was applied to evaluate the wall heat transfer in horizontal heaters. Further, the proposed numerical boiling heat transfer model was validated against an extant experimental database of horizontal heat exchanger tubes, and the model was observed to predict the wall heat flux with satisfactory accuracy.

A Lyman-α protocluster at redshift 6.9

Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6 (refs. 1–6). Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium7,8. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-α galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported9,10. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of 3.7 × 1015 solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process. The protocluster LAGER-z7OD1 is discovered at a redshift of 6.93, identified by an overdensity of 6 times the average galaxy density and 16 spectroscopically confirmed members. It shows an elongated shape, indicative of a past merger, and its intergalactic medium is almost fully ionized.

A high order spectral difference-based phase field lattice Boltzmann method for incompressible two-phase flows

In this paper, a high order spectral difference-based phase field lattice Boltzmann method (SD-PFLBM) is proposed for simulating incompressible two-phase flows. The spectral difference method (SDM) is used to discretize the convection term and the gradient term of the discrete Boltzmann equation for obtaining the flow field. Moreover, the SDM is also adopted to discretize the convection term and the high order partial derivative term of the Cahn–Hilliard equation for interface tracking. The proposed method can overcome the drawback of the standard LBM such as tie-up between the time step and the mesh spacing. Meanwhile, the present method still holds the locality of the standard LBM because each cell only needs its own information to complete the discretization. Numerical validations of the proposed method are implemented by simulating rigid-body rotation of Zalesak’s disk, layered Poiseuille flows, bubble deformation in shear flow, Rayleigh–Taylor instability, and bubble merging. More satisfactory interface shapes and flow properties can be achieved as compared with the published data in the literature. In addition, the convergence studies are also given, which prove that the current SD-PFLBM can achieve high order accuracy by increasing the order of cell local polynomials.

Observation of Preferential Pathways for Oxygen Removal through Porous Transport Layers of Polymer Electrolyte Water Electrolyzers.

SummaryUnderstanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the polymer electrolyte water electrolyzer (PEWE) performance. Operando X-ray computed tomography and machine learning were performed on a model electrolyzer at different water flow rates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. We report a direct observation of oxygen taking preferential pathways through the PTL, regardless of the water flow rate or current density (1-4 A/cm2). Oxygen distribution in the PTL had a periodic behavior with period of 400 μm. A computational fluid dynamics model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer.