Bubble Velocity Research Articles

Numerical investigation on the bubble rising dynamics in ratchet channels filled with viscoelastic liquids

In this paper, we investigated the dynamics of a bubble rising inside ratchet channels filled with viscoelastic liquids by means of volume-of-fluid-based direct numerical simulations. The exponential Phan–Thien–Tanner constitutive model was used to describe the rheological behaviors of the nonlinear viscoelastic fluid. The effects of fluid elasticity [characterized by the relaxation time (λ)] and ratchet angle (θ) are mainly discussed in respect of bubble dynamics (e.g., rising velocity, flow field, and stress field, etc.). Our results found that the bubble rise velocity increases with fluid elasticity, and the average bubble velocity can be reduced up to 20% at low elasticity in ratchet channels. In addition, the periodic arrangement of the ratchet influences the distribution of the stress field, the vorticity component, and also the deformation of the entangled polymers in the flow. It was observed that the distributions of the stress field and the trace of the conformation tensor change significantly in a dense ratchet channel compared to a sparse one. Interestingly, the bubble velocity gradually increases after the bubble emerges from the convergent section, whereas it decreases on approaching the convergent section. The dynamical bubbles can be manipulated by the surrounding fluid viscoelasticity and ratchet channels, which will be useful in oil extraction and chemical process involving complex non-Newtonian fluids.

Prediction of bubble shape deformation in aqueous solutions of short-chain alcohols

The behaviour of bubbles in aqueous solutions of short-chain alcohols depends significantly on the alcohol concentration. At low concentrations, alcohols act as surfactants and slow the rising velocity of the bubble, whereas at higher concentrations, primarily due to the internal arrangement at the molecular level, the desorption of molecules from the bubble surface is facilitated and the mobility of the bubble surface is increased. These factors significantly affect both bubble velocity and deformation. On the basis of experimental data (aqueous solutions of ethanol and propanol in the entire concentration range), two relations were proposed for the estimation of bubble deformation depending on the dimensionless Weber number. The model was successfully verified on an independent data set of the velocity and deformation of bubbles that rise in a stationary liquid.

Temporally consistent reconstruction of 3D clothed human surface with warp field

Implicit functions are widely used in 3D human surface reconstruction due to their advantage to represent details. However, human reconstruction based on implicit functions struggles to maintain the integrity (unbroken body structure) and accuracy (no non-human parts) of human models. To address these issues, we propose a method, called TCR, for temporally consistent reconstruction of 3D clothed human surface with warp field. The fact that the general shape of a person does not change largely over time inspires us to exploit the temporally consistent shape information from previous frames to refine the human model of current frame. Therefore, we construct a canonical space and then store the shape information by updating the canonical model. To align the observed space with the canonical space, a warp field is firstly estimated for the forward and inverse warping of the human model. A probabilistic fusion strategy is then used to update the canonical model. In addition, the reconstructed result is further refined through the orthogonality constraints between the surface and its normal, which fully exploits the detailed information of estimated normal maps. Experiments on the Adobe and MonoPerfCap datasets show that TCR achieves the state-of-the-art performance. Furthermore, TCR is more robust and can maintain the integrity and accuracy of the reconstructed human body even with extreme poses and partial occlusions.

Comparative CFD-DEM study of flow regimes in spout-fluid beds

Spout-fluid beds are unique systems that require thorough study prior to their industrial application. In this study, the hydrodynamics of spout-fluid beds were investigated using 3D computational fluid dynamics coupled with discrete element method (CFD-DEM). Three flow regimes, including jet-in-fluidized bed, spouting-with-aeration, and intermediate/spout-fluidization were studied, and the particle mixing was quantified in these regimes using the Lacey mixing index. The results showed that both axial and lateral mixing rates are better in jet-in-fluidized bed and the spouting-with-aeration flow regimes, with the axial mixing being superior to the lateral in all flow regimes. Examining the diffusivity coefficient revealed that mixing in the jet-in-fluidized bed flow regime is better due to the formation and eruption of bubbles in the annulus. Additionally, the granular temperature was analyzed in all flow regimes, and higher particle velocity fluctuations were observed in the spouting-with-aeration and the jet-in-fluidized bed flow regimes due to the higher spout gas velocity and formation of bubbles in the annulus. This study provides valuable insights into the hydrodynamics of spout-fluid beds in different flow regimes, which can aid in the design and optimization of spout-fluid bed reactors for various industrial applications.

Investigating the Mechanism of Intravascular Bubble Formation in Designed Arrays of Vascularized Systems on a Chip.

Vascular gas embolism is a rare medical condition, resulting from the existence of air or gas in the venous or arterial system. Gas embolism is associated with a wide range of circulatory, cardiovascular, and neurological complications that can lead to sudden and unexplained death. Despite the recent increase in related studies, gas embolism remains under-reported with a poor understanding of its genesis and pathophysiology. In this work, intravascular bubble formation is investigated in an array of biomimetic microscale systems, where the endogenous generation of gas bubbles is induced by variations in the surrounding pressure. Microfluidic devices, based on polydimethylsiloxane, are designed and fabricated as vascularized systems on a chip with one main channel at two different diameters (30 µm, and 40 µm), surrounded by a pressure chamber (200 µm) on each side, at a separation of 50 µm. Two blood-equivalent solutions, at 20% and 46% hematocrit concentrations were prepared from a glycerin and xanthan gum mixture to mimic the physicochemical characteristics of the blood. As the volume of injected air increased, the events related to gas embolism were occurring at shorter timespans with more significant characteristics, i.e., length and number of bubbles. Additionally, correlations were established between the input parameters, i.e., the vascular diameter and equivalent hematocrit concentration, and the output parameters, i.e., the bubble size, velocity, frequency, and nucleation sites.Clinical Relevance- The reported results constitute a reproducible observation and quantification of intravascular bubble formation induced by global pressure variations, where the emergence of bubbles exhibits different patterns depending on biological characteristics related to gender and medical history.

Large Eddy Simulation of Bubble Column Bubbly Flow Considering Subgrid‐Scale Turbulent Diffusion Effects and Bubble Oscillation

AbstractThrough Euler/Euler large eddy simulation (LES) modeling, it is demonstrated that turbulent dispersion of bubbles can effectively indicate the impact of turbulent eddies on the bubble dynamics, i.e., the bubble oscillation behavior. This finding builds on previous work using the Euler/Lagrange LES modeling approach and leads to a significant improvement in predicting bubble lateral dispersion. Spatially filtered terms were proposed for the subgrid‐scale (SGS) turbulent dispersion and added mass stress force models, with a modification made to the SGS eddy viscosity to reflect bubble turbulent dispersion and oscillations. The proposed model substantially improves the prediction of bubble volume fraction distribution, bubble and liquid phase velocity profiles, the turbulent kinetic energy spectrum, and mass transfer.

Bubble dynamics properties of B-particles in a quasi-2D gas-solid fluidized bed: Computational particle fluid dynamics numerical simulation and post-processed by digital image analysis technique

Bubble dynamics properties play a crucial and significant role in the design and optimization of gas-solid fluidized beds. In this study, the bubble dynamics properties of four B-particles were investigated in a quasi-two-dimensional (quasi-2D) fluidized bed, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble radial position, and bubble velocity. The studies were performed by computational particle fluid dynamics (CPFD) numerical simulation and post-processed with digital image analysis (DIA) technique, at superficial gas velocities ranging from 2umf to 7umf. The simulated results shown that the CPFD simulation combining with DIA technique post-processing could be used as a reliable method for simulating bubble dynamics properties in quasi-2D gas-solid fluidized beds. However, it seemed not desirable for the simulation of bubble motion near the air distributor at higher superficial gas velocity from the simulated average bubble density distribution. The superficial gas velocity significantly affected the bubble equivalent diameter and evolution, while it had little influence on bubble size distribution and bubble aspect ratio distribution for the same particles. Both time-averaged bubble hold-up and bed expansion ratio increased with the increase of superficial gas velocity. Two core-annular flow structures could be found in the fluidized bed for all cases. The average bubble rising velocity increased with the increasing bubble equivalent diameter. For bubble lateral movement, the smaller bubbles might be more susceptible, and superficial gas velocity had a little influence on the absolute lateral velocity of bubbles. The simulated results presented a valuable and novel approach for studying bubble dynamics properties. The comprehensive understanding of bubble dynamics behaviors in quasi-2D gas-solid fluidized beds would provide support in the design, operation, and optimization of gas-solid fluidized bed reactors.

Migration velocities of intergranular He gas bubbles under thermal gradients in Fe by phase-field modeling

We compute the migration velocity of intergranular He bubbles in Fe under thermal gradients using phase-field (PF) modeling. Grain boundaries (GBs) with energies in the range of 0.88−1.6J/m2 are considered. To develop a suitable PF model, the influences of the PF parameterization strategy, thermal conductivity in grain boundaries, and GB diffusion coefficients on intergranular He bubble geometry are addressed. Good agreement between our simulations and a theoretical model are found for the contact angle at the junction of the bubble and GB. Under the application of a thermal gradient, we find that intergranular He bubbles migrate slower than He bubbles in an Fe matrix. We show that this is a result of the smaller temperature gradient at the bubble/GB junction when the bubble is at the GB, than when the bubble is in the matrix. A linear relationship between bubble migration velocity and temperature gradient is observed for intergranular He bubbles for all GB energies studied in this work, consistent with predictions from a theoretical model. Intergranular bubble migration velocity is also found to depend linearly on the inverse of bubble size. However, we find the shape of the bubble influences intergranular bubble migration velocity. An analytical model for bubble migration velocity that considers the shape of an intergranular bubble is proposed and shown to capture the computed PF data for He bubbles.

Simulation Study on Dynamic Characteristics of Bubbles in Transformer Oil under Extremely Nonuniform Field

Bubbles in transformer oil seriously threaten the deterioration of internal insulation performance during its movement, leading to severe transformer failure. A gas-liquid two-phase flow simulation model is established based on the phase field method to study the effect of bubbles on the insulation performance of transformer oil. The dynamic characteristics of bubbles in transformer oil under extremely non-uniform electric fields are studied. The results show that the electric field force has a pushing effect on the bubble. As the voltage increases, the pushing effect is more significant, and the displacement distance of the bubble in the horizontal direction is larger. The velocity curve of a single bubble in the vertical direction shows an oscillating attenuation trend. The pushing effect of electric field force on the bubble causes the bubble velocity near the needle electrode to decrease, but the stretching effect increases the average bubble velocity. When the electric field strength is further improved due to the enhanced push-off impact of the electric field force on the bubbles, the bubbles will gather under the needle electrode to form cluster bubbles, further reducing the insulation performance of the transformer oil. This needs to be paid attention to in the design of the insulation structure to avoid the accumulation of bubbles caused by too high an electric field intensity at a particular place.

CFD-VOF-DPM numerical simulation of enhanced boiling heat transfer characteristics of microencapsulated phase change material slurry

Two-dimensional (2D) computational fluid dynamics (CFD) simulations of water-based microencapsulated phase change material suspension (MPCMS) on the flow boiling in a gas–liquid–solid flow system was performed with volume of fluid (VOF) method and discrete particle model (DPM). The Lagrangian particles were linked to the Eulerian phases through the interchange terms such as the drag force in the respective momentum equations. Influences of particle properties including mass fraction and core phase transition temperature, fluid properties including liquid surface tension force and viscosity, detachment time and rise velocity of gas bubbles and particle entrainment in the gas–liquid–solid flow under ambient conditions were numerically investigated. The effects of particles on bubble nucleation, growth and rupture during boiling were studied by visualization. The results show that MPCM can enhance the boiling heat transfer ability of the base liquid. The maximum heat transfer enhancement rate of MPCMS (28 °C) is 4.8%, the maximum heat transfer enhancement rate of MPCMS (90 °C) is 5.1%, the maximum heat transfer enhancement rate of MPCMS (110 °C) can reach 6.7%. MPCM can promote the formation of new bubbles and the rupture of large bubbles, reduce the departure diameter of bubbles, and enhance the boiling heat transfer capacity of the base liquid. The MPCM with core phase change temperature higher than the boiling temperature of base fluid has the best enhancement effect. Through the combination of numerical simulation methods such as VOF and DPM, the complex phase transition heat transfer process of gas-liquid-solid particle coupling of latent thermal functional thermal fluid can be accurately simulated. The work lays a foundation for further explorations on the gas–liquid–solid flows and possible industry applications.

Pixel-Wise Warping for Deep Image Stitching

Existing image stitching approaches based on global or local homography estimation are not free from the parallax problem and suffer from undesired artifacts. In this paper, instead of relying on the homography-based warp, we propose a novel deep image stitching framework exploiting the pixel-wise warp field to handle the large-parallax problem. The proposed deep image stitching framework consists of a Pixel-wise Warping Module (PWM) and a Stitched Image Generating Module (SIGMo). For PWM, we obtain pixel-wise warp in a similar manner as estimating an optical flow (OF). In the stitching scenario, the input images usually include non-overlap (NOV) regions of which warp cannot be directly estimated, unlike the overlap (OV) regions. To help the PWM predict a reasonable warp on the NOV region, we impose two geometrical constraints: an epipolar loss and a line-preservation loss. With the obtained warp field, we relocate the pixels of the target image using forward warping. Finally, the SIGMo is trained by the proposed multi-branch training framework to generate a stitched image from a reference image and a warped target image. For training and evaluating the proposed framework, we build and publish a novel dataset including image pairs with corresponding pixel-wise ground truth warp and stitched result images. We show that the results of the proposed framework are quantitatively and qualitatively superior to those of the conventional methods.

Bubbles morphology, frequency and rising velocity in yield stress fluid under downward gas injection

Mixing in a reactor can be achieved mechanically or by the injection of a gas. This can be done by injecting a gas towards the bottom of the tank. The induced flow recirculation and then the stirring quality depends on the bubbles morphology, rising velocity and frequency detachment. This study focus on the case where the stirred fluid exhibits a yield stress. It is in line with previous studies on the gas cavity at the injection point [1], the induced flow recirculation [2] and the numerical simulation [3]. Here we quantified the bubbles diameters, aspect ratios and velocities as a function of the air flow rate, rheological parameters and tank size. Scaling laws and correlations based on the characteristic dimensionless numbers governing the flow have been proposed and validated experimentally. It was found that bubble frequencies and consequently bubble volumes and velocities are mainly governed by the balance between inertial and buoyancy forces. However, viscous and yield stress effects play an important role along buoyancy and inertial in defining the bubble aspect ratios. Weakly inertial flows are characterized by elongated bubbles while high inertial flows produce almost spherical bubbles. These experimental results are of practical interest for the design of reactor or digester where yield stress fluids have to be stirred.

One-dimensional kinetic model with heat transfer and axial dispersion of molten-metal bubble column reactors for hydrogen production via methane pyrolysis

A one-dimensional (1D) molten-metal-based bubble-column reactor (MMBCR) model coupled with heat transfer and axial dispersion was developed for non-oxidative CH4 pyrolysis to produce low-carbon H2. The MMBCR model included mass, momentum, and energy balances, accounting for catalytic and non-catalytic reaction kinetics, and hydrodynamic parameters such as the gas holdup (αG), gas velocity (uG), bubble size (db), and specific interfacial surface area of bubbles (as). The 1D MMBCR model was compared with other 1D reactor models and bench-scale experimental data for CH4 conversion (XM). The MMBCR model agreed well with the experimental data. When heat transfer and axial dispersion were considered for an industrial-scale MMBCR to produce 10,000 Nm3/h, the reactor length increased to 2.4 times higher than that estimated by the model without axial dispersion to meet 80% XM. The MMBCR model has the potential to design industrial-scale MMBCRs and optimize operating conditions.

Experimental measurements on the microbubble characteristics and dissolved oxygen (DO) in water using single and twin-Venturi type microbubble generators

The aim of this paper is to develop low cost and low power consumption microbubble generator to improve the mass transfer of gas to liquid. Microbubble aeration used in aquaculture to improve dissolve oxygen level, water recycling through dissolved air flotation (DAF), oil refinement and reduction of skin friction on ship Hull. A single Venturi and 3 twin-Venturis microbubble generators are manufactured and tested in this study. Experimental data on microbubbles diameters distribution and dissolved oxygen are presented. The main conclusions are: i) the mean microbubbles diameters produced by twin-Venturi are smaller by 10 µm than single Venturi generator; ii) the dissolved oxygen concentration was higher for twin-Venturi microbubble generator; iii) the microbubbles size distribution from twin-Venturis have a narrower distribution which reduces the probability of bubbles coalesce as they have very close values of velocities; iv) the bubbles diameters and velocities increase with the height in the water tank.

Escape of an air bubble from a droplet under power ultrasound

Escape of air bubbles from a droplet driven by ultrasound is of great relevance to many industrial processes, featuring the passage of air bubbles through a moving interface between two immiscible liquids. In this study, we experimentally investigated the interactions between an air bubble and a moving liquid–liquid interface, including the identification of different interaction outcomes, bubble velocity change, and the role of bubble-to-droplet size ratio. Five bubble-interface interaction outcomes were observed, namely, upward and downward penetration, semi-penetration, inward bouncing and interface locking. A theoretical model was developed for predicting the interaction outcome between a bubble and a moving interface. The results revealed that the driving pressure of power ultrasound that yields an acoustic radiation force acted as the main mechanism driving the bubble to penetrate the fluid interface. On the contrary, the interfacial tension strengthened bubble-droplet attachment and suppressed the bubble’s passage. The effect of the confined liquid size was found to be marginal. The study presents the knowledge that could be used to inform the design and operation of relevant technologies where ultrasound is applied to control bubble dynamics in the confined liquid.

Effects of pressure and particle size on bubble behaviors in a pseudo 2D pressured fluidized bed with Geldart A/B, B and D particles

Bubble behaviors of three types of particles from Geldart A/B, B and D groups were comprehensively investigated in a pseudo 2D pressurized fluidized bed via digital image analysis. Based on the analysis of a large quantity of experimental data, the effects of pressure and particle size on the spatial distribution of bubbles, bubble diameter, rise velocity, bubble fraction and flow regimes were obtained. Results show that bubbles are smaller and uniformly dispersed throughout the bed of Geldart A/B particles, while they become densely populated and grow into much larger sizes in the bed of Geldart B particles. A single peak of large bubbles in the center and emptying of bubble near the walls is formed in the bed of Geldart D particles due to the preferable rising path of bubbles towards the center and the significant coalescence of bubbles. Bubble diameter decreases with increasing pressure, and the effect of pressure is more pronounced on Geldart D particles. Bubble rise velocity increases with the increasing particle size and decreases as pressure rises. Large bubbles/slugs hardly appear in the bed of Geldart A/B particles, while they are clearly visible in the bed of Geldart B and D particles. The flow regime transition from slugging to fast fluidization shifts to the lower gas velocity for Geldart D particles compared with Geldart B particles.

Effect of zero-mean-shear turbulence on rise velocity of in-chain bubbles from marine natural seeps

This paper reports on a laboratory experiment that investigates the impact of ocean turbulence on the rise velocity of bubbles released from natural seeps. To simulate the turbulence conditions of ocean bottoms, we used an oscillating grid-stirred turbulence (OGT) tank to generate nearly homogeneous and isotropic turbulence (HIT) with a range of turbulence dissipation rates. By isolating the effect of zero-mean-shear turbulence on the bubble rise from the effect of cross flows, we found that the presence of turbulence reduced the bubble rise velocity by up to 19% across six different gas flow rates. Our data show a linear relationship between the normalized bubble rise velocity and the log-value of the normalized turbulence dissipation rate, with two different slopes. Our analysis suggests that turbulence increased the horizontal motion of the bubbles, causing them to scatter laterally, thereby reducing their rise velocity. Lastly, we propose a semi-empirical equation that can be used to calculate the rise velocity of in-chain bubbles in turbulent waters. These findings have important implications for our understanding of the role of turbulence in the transport and fate of seep bubbles in ocean waters.

Effect of bubble volume on the sweeping velocity of air bubbles in horizontal pipelines in water supply system

Trapped air bubbles in pipelines significantly affect the safety and efficiency of water supply systems, potentially inducing water hammer and leading to pipeline explosions and interruptions. This study investigates the sweeping velocity of air bubbles in horizontal pipelines by analyzing the forces acting on a single air bubble and deriving a theoretical expression for calculating the sweeping velocity. Physical model experiments and numerical simulations are conducted to examine the variations in sweeping velocity with respect to bubble volume and dimensionless bubble volume. Results indicate that the sweeping velocity increases with bubble volume, and the diameter of the pipeline significantly affects the movement of air bubbles. These findings contribute to a better understanding of air bubble behavior in horizontal pipelines and can inform strategies for mitigating risks associated with trapped air bubbles, ultimately improving the safety and reliability of water supply systems.

Effect of long-wavelength perturbations in nonlinear evolution of the ablative Rayleigh–Taylor mixing

We investigate herein how long-wavelength perturbations affect the nonlinear evolution of the multimode ablative Rayleigh–Taylor instability (ARTI). A single-mode ARTI with an initial small amplitude is first investigated to validate the reliability of the proposed simulation code. The results show that both linear growth rates and asymptotic bubble velocities obtained from simulations are in reasonable agreement with theoretical results. Initial perturbations with different long-wavelength perturbations are compared to investigate the contribution of the long-wavelength perturbations to the nonlinear evolution of the ARTI mixing. Beyond the nonlinear saturation limit [S. W. Haan, Phys. Rev. A 39, 5812 (1989)], the long-wavelength perturbation promotes the ARTI mixing and facilitates the development of the large-scale structure on the ablation surface. In the self-similar analysis, the simulation results indicate that the self-similar growth parameters decrease with increasing initial longest-wavelength modes.

Study of Fluidization Behavior Transition from Geldart B to A Induced by High Temperature Using Electrical Capacitance Tomography

The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (Umb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (Umf) and Umb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.