Bubble Velocity Research Articles

Magneto-Hydrodynamic effects on Diesel/CNG bubbly flow enhanced by Fe3O4 Nanoparticles: Experimental and mathematical assessments

In the present study, the magneto hydrodynamic (MHD) was applied to the motion analysis of compressed natural gas (CNG) bubbles in laminar diesel flow after enhancing the liquid fuel with iron oxide nanoparticles (Fe3O4) in a horizontal pipe using experimental and mathematical approaches. Two distinct techniques were adopted for the experimental measurements: High speed video camera system (HSVC) and particle image velocimetry (PIV). The result of the PIV investigation indicates that increasing the intensity of the magnetic field changed the axial velocity profile of the diesel/nanoparticle mixture from the parabolic to a relatively flat shape. Whilst to illustrate the effect of magnetic field on bubble velocity, the axial and vertical components of the bubble velocity were computed by keeping track of the geometric center of the bubble from frame to frame recorded by HSVC. Moreover, the fundamental equation of motion of a single CNG bubble based upon Newton’s law under a forced magnetic field were derived and presented. The origination processes of the velocity dispersion of the liquid and gas stages of the bubbly flow were equally discussed. The governing partial-differential equations were numerically determined by means of a 4th order Runge-Kutta method. The outcomes from the model solution indicate that at magnetic field intensity (B = 0), the comparison between mathematical and experimental results in literature for the average axial and perpendicular components of bubble velocity resulted in mean percentage differences of 10.4 % and 15.6 % respectively.

Thin-shell effects on nonlinear bubble evolution in the ablative Rayleigh–Taylor instability

The influence of thin-shell effects on the nonlinear evolution of two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) is studied in the parameter range of inertial confinement fusion implosions. A new phase of unsaturated nonlinear bubble evolution caused by thin-shell effects is found. This is different from the traditional opinion that the bubble velocity becomes saturated after the ARTI evolution enters a highly nonlinear regime. A modified bubble velocity formula is proposed, based on the Betti–Sanz model [Betti and Sanz, Phys. Rev. Lett. 97, 205002 (2006)], considering the thin-shell effects. It is shown that the bubble velocity becomes saturated in the thick-target case after the ARTI evolution enters a highly nonlinear regime. In this case, the Betti–Sanz bubble dynamics model can predict the evolution of bubble velocity. However, when the thin-shell effects become significant in the case of kD0 < 100, where D0 is the initial thickness of the target and k is the perturbation wavenumber, the difference of the average acceleration between the bubble vertex and the spike tip can be much more significant than that of the thick-target case. In this situation, the nonlinear evolution of the ARTI bubbles will accelerate without saturation until the target breakup, which cannot be depicted by the Betti–Sanz model while the improved theory formula is applicative. The Betti–Sanz model and the improved theory formula are independent of the initial perturbation amplitude.

Oxygen transfer of microbubble clouds in aqueous solutions – Application to wastewater

This study aims at improving the knowledge on the effects of gas injection, bubbles sizes and contaminants on oxygen transfer in microbubble clouds. First the effects of gas injection on oxygen transfer are studied and linked to several parameters that change together with changes in flow rate, namely bubble sizes and rise velocities. Oxygen transfer is then studied in the presence of contaminants that are shown to affect bubble size distribution, modify bubble dynamics and interfacial mass transfer. Oxygen transfer efficiencies are also measured in wastewater and compared with those obtained in aqueous solutions. The agreement between contaminated water in the lab (Triton X100) and wastewater experiments is emphasised as this offers the possibility to develop fundamental understanding relevant to wastewater under laboratory conditions. The role of the surfactants on the volumetric oxygen transfer coefficient is further analysed in terms of specific interfacial area and transfer coefficients, respectively. Interestingly, this shows that the increase in oxygen transfer efficiency as the concentration in Pentanol increases is due to the increase in interfacial area while the transfer coefficients decrease.

On the velocity jump discontinuity for single bubbles rising in a viscoelastic fluid

<h2>Abstract</h2> It is well known that bubbles rising in viscoelastic liquids may exhibit a jump discontinuity of the rise velocity as a critical bubble volume is exceeded. This phenomenon has been extensively investigated in the literature, both by means of experiments and numerical simulations. The occurrence of the velocity jump seems to be associated with a change of the bubble shape under formation of a pointed tip at the rear and to the appearance of a so-called negative wake, with the liquid velocity behind the bubble pointing in the opposite direction to that in viscous Newtonian fluids. We revisit this topic, building mainly on our recent numerical work. Besides detailed information on the employed methods and techniques, this talk reports on findings from a local conformation tensor analysis, which is able to explain the velocity discontinuity in the terminal bubble rise velocity by a self-amplifying mechanism.

Heat transfer, dynamic characteristics and disturbance to water of bubbles generated on vertical surfaces featuring extreme wettability patterns

Surfaces with mixed wettability patterns are one of the potential ways to improve energy conversion efficiency of industrial equipment. It can significantly change the boiling heat transfer proformance of the surface by affecting the bubble dynamics, as well as the flow field properties. Accordingly, this work simultaneously investigates the thermodynamic, bubble dynamics, and water velocity vector field of the bubbles generated on the designed surfaces, through a combination of thermal parameter measurements, high-speed photography, and particle image velocimetry (PIV). The surfaces are designed in six types, and coated by alternating superhydrophilic materials (contact angle of 9.8°) and superhydrophobic materials (contact angle of 154.8°) at intervals with widths of 5 mm, 10 mm, and 15 mm, respectively. The results showed that the different wettability of the pattern at the bubble generation leads to different bubble formation modes, which in turn affects the heat transfer and the initial dynamics of the bubble. The different width of the pattern affects the bubble floatation and water motion. After comparison, it is found that the bubbles have the best heat transfer proformance when they are generated on a superhydrophilic pattern with a width of 15 mm. However, regardless of pattern wettability and width, the bubble frequency, size, and velocity, as well as the water vorticity and velocity, increase with decreasing supercooling, and first increase and then decrease with increasing heat flux. Based on the experimental results, the correlations of bubble velocity and water velocity are modified by introducing the influence factor of wettability patterns. It is tested that it is not only adaptable to the data of this experiment, but can also be generalized to the data of other works.

Innovative water-based dynamic liquid bubble membrane generation device for gas/vapour separation

Gas and vapour separation by membrane technology has become a promising method because of its high energy efficiency and low capital investment. Here, we demonstrate a proof of concept of the design of a new type of liquid membrane called the “dynamic liquid bubble membrane” (DLBM) connected by plateau borders. The DLBM is composed of a network of bubbles formed by passing a gaseous mixture through water containing a surfactant. The bubble network serves as a membrane to selectively separate the gases/vapours of mixtures according to the different solubilities of the gases in the bubble liquid. To the best of our knowledge, this type of liquid foam has not been described previously. The effects of key experimental factors, such as the surfactant concentration in the water of the membrane solution and the bubble velocity (controlled by the slope of the device) of the DLBM, on the CO2/CH4 separation performance were investigated. The retentate gas (CH4) remained over 99.9 % for the high CH4 concentration injection mixture (>66.7 % CH4). Then, O2/CO2 was used to simulate the fluid of the alveolar lining during the full in-breath process. We found that carbon dioxide transferred outside the bubble film, while oxygen was retained inside the bubble. This finding supports a further understanding of the CO2 transport mechanism of alveoli. We further demonstrated that the thickness of the bubble film was closely correlated to mass transfer resistance theory, while the gas selectivity was completely unaffected. This innovative DLBM generation device is a highly attractive and promising prototype for designing a green and highly efficient gas/vapour separation membrane technology.

Dynamics of Rising Bubbles and Their Impact with Viscoelastic Fluid Interfaces.

Bubble dynamics plays a significant role in a wide range of industrial fields, such as food, pharmacy and chemical engineering. The physicochemical properties of complex fluids can greatly affect the speed with which bubbles rise, and the lifetime of bubbles, which in turn can affect the efficiency of food and drug manufacturing and also sewage purification. Therefore, it is of great scientific and practical significance to study the influence mechanism of nanoparticles and surfactants on bubble rising and impact in a complex fluid interface. This paper selects a mixed dispersion liquid of nanoparticles (SiO2) and a surfactant (SDS) as the objects of the study, observes in real-time the entire processes of bubbles rising, impact at the gas-liquid interface, and rupture, and analyzes the dynamic mechanism of bubble impact in a complex fluid interface. By analyzing the morphological changes of the rising bubbles, the rising velocity and the lifetime of the bubbles, it is found that the surfactant molecules are distributed in the ultrapure water liquid pool and the liquid film surrounding the bubbles. Such distribution can reduce the viscoelasticity between bubbles and the liquid surface, and lower the surface tension of the liquid, which can reduce the rising velocity of bubbles, delay the drainage process of bubbles on a liquid surface, and enhance the lifetime of bubbles. If the liquid surface is covered with nanoparticles, a reticulate structure will be formed on the bubble liquid film, which can inhibit bubble discharge and prolong bubble lifetime. In addition, the influence of such a reticulate structure on liquid surface tension is limited and its function is far smaller than a surfactant.

Numerical Investigation on the Flow Instability of Dispersed Bubbly Flow in a Horizontal Contraction Section

Dispersed bubbly flow is important to understand when working in a wide variety of hydrodynamic engineering areas; the main objective of this work is to numerically study bubble-induced instability. Surface tension and bubble-induced turbulence effects are considered with the momentum and k-ω transport equations. Steady dispersed bubbly flow is generated at the inlet surface using time-step and user-defined functions. In order to track the interface between the liquid and gas phases, the volume of fraction method is used. Several calculation conditions are considered in order to determine the effects of bubble diameter, bubble distribution, bubble velocity and bubble density on flow instability and void fraction. The void fraction of the domain is set to no more than 0.5% under different bubbly (micro/small) flow conditions; and the order of magnitude of the Reynolds number is 106. Results from the simulation indicate that velocity fluctuation induced by bubble swarm increases with increasing bubble diameter. Bubble density and bubble distribution seem to have a complex influence on flow instability. Bubble-induced turbulence results indicate that small bubbles produce a significant disturbance near the boundary region of bubble swarm; this indicates that induced bubble swarm has a potential capability of enhancing heat and mass transfer in the velocity boundary layer. Results from this study are useful for two-phase flow, bubble floatation and other hydrodynamic engineering applications.

Bubble velocimetry using the conventional and CNN-based optical flow algorithms

In the present study, we introduce new bubble velocimetry methods based on the optical flow, which were validated (compared) with the conventional particle tracking velocimetry (PTV) for various gas–liquid two-phase flows. For the optical flow algorithms, the convolutional neural network (CNN)-based models as well as the original schemes like the Lucas-Kanade and Farnebäck methods are considered. In particular, the CNN-based method was re-trained (fine-tuned) using the synthetic bubble images produced by varying the density, diameter, and velocity distribution. While all models accurately measured the unsteady velocities of a single bubble rising with a lateral oscillation, the pre-trained CNN-based method showed the discrepancy in the averaged velocities in both directions for the dilute bubble plume. In terms of the fluctuating velocity components, the fine-tuned CNN-based model produced the closest results to that from PTV, while the conventional optical flow methods under- or over-estimated them owing to the intensity assumption. When the void fraction increases much higher (e.g., over 10%) in the bubble plume, the PTV failed to evaluate the bubble velocities because of the overlapped bubble images and significant bubble deformation, which is clearly overcome by the optical flow bubble velocimetry. This is quite encouraging in experimentally investigating the gas–liquid two-phase flows of a high void fraction. Furthermore, the fine-tuned CNN-based model captures the individual motion of overlapped bubbles most faithfully while saving the computing time, compared to the Farnebäck method.

Discrete Boltzmann modeling of Rayleigh-Taylor instability: Effects of interfacial tension, viscosity, and heat conductivity.

The two-dimensional Rayleigh-Taylor instability (RTI) in compressible flow with intermolecular interactions is probed via the discrete Boltzmann method. The effects of interfacial tension, viscosity, and heat conduction are investigated. It is found that the influences of interfacial tension on the perturbation amplitude, bubble velocity, and two kinds of entropy production rates all show differences at different stages of RTI evolution. It inhibits the RTI evolution at the bubble acceleration stage, while at the asymptotic velocity stage, it first promotes and then inhibits the RTI evolution. Viscosity and heat conduction inhibit the RTI evolution. Viscosity shows a suppressive effect on the entropy generation rate related to heat flow at the early stage but a first promotive and then suppressive effect on the entropy generation rate related to heat flow at a later stage. Heat conduction shows a promotive effect on the entropy generation rate related to heat flow at an early stage. Still, it offers a first promotive and then suppressive effect on the entropy generation rate related to heat flow at a later stage. By introducing the morphological boundary length, we find that the stage of exponential growth of the interface length with time corresponds to the bubble acceleration stage. The first maximum point of the interface length change rate and the first maximum point of the change rate of the entropy generation rate related to viscous stress can be used as a criterion for RTI to enter the asymptotic velocity stage.

Statistical Analysis Of Bubble Parameters From A Model Bubble Column With And Without Countercurrent Flow

Bubble columns are widely used in numerous industrial processes, because of their advantages in operation, design and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially in continuous mode with counter-current liquid flow. For the design and optimization of such reactors precise numerical simulations and modelling is needed. These simulations and models have to be validated with experimental data. For this reason experiments were carried out in a laboratory-scale bubble column with shadow imaging technique with and without countercurrent liquid flow. In the experiments two types of gases –relatively badly soluble air and good soluble CO2 – are applied and the bubbles were generated with three different capillary diameters. With changing gas and liquid flow rates overall 108 different flow conditions were investigated. The gathered data has been statistically evaluated over the whole 1 m measurement area and bubble parameters have been obtained such as bubble diameter, velocity, aspect ratio, bubble motion direction and inclination. The analysis of these parameters shows, that the countercurrent liquid flow has a noticeable influence on the bubble parameters, especially on the bubble velocity and motion direction. In the case of CO 2 bubbles remarkable bubble shrinkage has been observed at counter-current liquid flow due to the enhanced mass transfer. The comprehensive and extensive bubble data obtained in this study will now be used as a source for the validation of numerical simulations and models.

Hydrodynamics and Scaling Laws of Gas–Liquid Taylor Flow in Viscous Liquids in a Microchannel

The hydrodynamics of gas–liquid Taylor flow in microchannels is vital to the microreactor’s application, but its characteristics in viscous liquids are rarely reported. Accordingly, this study concentrates on the characteristics of bubble behavior in viscous liquids (45.6–240.5 mPa·s). The results show that the viscosity effect could dominate the bubble morphology. Importantly, considering the liquid film distribution, a novel parameter as the equivalent liquid film thickness is proposed for the square microchannel and it is strongly related to the capillary number and gas–liquid flow rate ratio. Besides, the bubble velocity first decreases to a minimum value and then becomes faster with the flow rate of the liquid, and the ideality of the multiphase system decreases with the increase of liquid viscosity and pressure drop. Finally, the viscosity term is introduced to the pressure drop equation based on the Lockhart–Martinelli type. This work enriches the understanding of the viscosity effect on gas–liquid microflows.

Effects of gas concentration on hydrodynamics of gas absorption in a microchannel

AbstractIn this article, the effects of gas concentration on hydrodynamics of gas–liquid two‐phase flow with mass transfer during gas absorption in a microchannel are investigated, by using 2‐amino‐2‐methyl‐1‐propanol (AMP) solution to absorb mixtures of CO2 and N2 with various volume fractions. The concentration of CO2 not only affects the driving force of gas–liquid mass transfer, but also affects the pressure drop of gas–liquid two‐phase flow. The average linear velocity of the liquid phase is estimated by introducing the void fraction, which accurately characterizes the difference in the bubble velocity versus the liquid velocity. On this basis, the pressure drop model of gas–liquid two‐phase flow with mass transfer in a microchannel is established. Through the pressure drop model, the influence mechanism of CO2 concentration on the pressure drop during gas absorption in a microchannel is revealed.

Weakly nonlinear incompressible Kelvin–Helmholtz instability in plane geometry

A weakly nonlinear (WN) theoretical model for the two-dimensional incompressible Kelvin–Helmholtz instability (KHI) is proposed. Its solution form is the complete expansion in real space. The transition from linear to nonlinear growth is analytically studied via third-order solutions of plane KHI initiated by a single-mode surface perturbation. The difference between the WN growth of the Rayleigh–Taylor instability (RTI) in plane geometry and the WN growth of the KHI in plane geometry is discussed. It is found that there are resonance solutions in the higher harmonics of KHI but not in RTI. The vertex of spikes and bubbles is deflected because of the shearing effect in KHI compared with that in RTI. The spike velocity increases with the Atwood number before a particular time and then inversely after that time. There is no such reversal in RTI. However, the bubble velocity with the Atwood number of KHI has the same evolution rule as that of RTI. In addition to the influence of initial perturbation on the nonlinear saturation amplitude, the influence of Atwood number on the nonlinear saturation amplitude of the fundamental mode is obtained. Finally, compared with the numerical simulations, at the same initial conditions, the analytical solutions up to the third harmonics are well consistent with the simulated datum in the linear and weakly nonlinear stages for a widely varied Atwood number.

First principles determination of bubble wall velocity

The terminal wall velocity of a first-order phase transition bubble can be calculated from a set of fluid equations describing the scalar fields and the plasma's state. We rederive these equations from the energy-momentum tensor conservation and the Boltzmann equation, without linearizing in the background temperature and fluid velocity. The resulting equations have a finite solution for any wall velocity. We propose a spectral method to integrate the Boltzmann equation, which is simple, efficient and accurate. As an example, we apply this new methodology to the singlet scalar extension of the standard model. We find that all solutions are naturally categorized as deflagrations ($v_w\sim c_s$) or ultrarelativistic detonations ($\gamma_w\gtrsim10$). Furthermore, the contributions from out-of-equilibrium effects are, most of the time, subdominant. Finally, we use these results to propose several approximation schemes with increasing levels of complexity and accuracy. They can be used to considerably simplify the methodology while correctly describing the qualitative behavior of the bubble wall.

Self-generated magnetic field in ablative Rayleigh–Taylor instability

Self-generated magnetic fields in single-mode ablative Rayleigh–Taylor instability (ARTI) relevant to the acceleration phase of inertial confinement fusion (ICF) implosions are studied via two dimensional simulations. In ARTI, ∼100 T magnetic fields can be generated via the Biermann battery source without considering the Nernst effect. The Nernst effect significantly compresses the magnetic field against the electron temperature gradient and amplifies the peak value by more than three times. A scaling law for the magnetic flux is obtained, and it well predicts the evolution of the magnetic field from linear to deeply nonlinear phases of ARTI. The self-generated magnetic field reduces the ablation near the spike and reduces the width of bubbles by magnetizing the electron heat flows, which results in higher magnitude vorticity inside the bubble and enhances the nonlinear ARTI bubble penetration velocity for short-wavelength modes. The bubble velocity boosting due to self-generated magnetic field indicates the larger impact of the short-wavelength ARTI modes on ICF implosion performance than previously expected.

ProgressiveMotionSeg: Mutually Reinforced Framework for Event-Based Motion Segmentation

Dynamic Vision Sensor (DVS) can asynchronously output the events reflecting apparent motion of objects with microsecond resolution, and shows great application potential in monitoring and other fields. However, the output event stream of existing DVS inevitably contains background activity noise (BA noise) due to dark current and junction leakage current, which will affect the temporal correlation of objects, resulting in deteriorated motion estimation performance. Particularly, the existing filter-based denoising methods cannot be directly applied to suppress the noise in event stream, since there is no spatial correlation. To address this issue, this paper presents a novel progressive framework, in which a Motion Estimation (ME) module and an Event Denoising (ED) module are jointly optimized in a mutually reinforced manner. Specifically, based on the maximum sharpness criterion, ME module divides the input event into several segments by adaptive clustering in a motion compensating warp field, and captures the temporal correlation of event stream according to the clustered motion parameters. Taking temporal correlation as guidance, ED module calculates the confidence that each event belongs to real activity events, and transmits it to ME module to update energy function of motion segmentation for noise suppression. The two steps are iteratively updated until stable motion segmentation results are obtained. Extensive experimental results on both synthetic and real datasets demonstrate the superiority of our proposed approaches against the State-Of-The-Art (SOTA) methods.

Eulerian simulations of bubbling and jetting regimes in a fluidized bed

The mode of gas-injection is known to influence the local bubbling and jetting behavior in gas–solid fluidized beds. The resultant bubbling behavior influences the mixing and distribution of the gas and solid phases, which in turn can influence heat and mass transfer, and reaction performance in large-scale gas–solid fluidized beds. In the present work, we simulated unary gas–solid flow of particles differing in density, fluidized using uniform and two-jet distributors at different UG. The predictions are validated using the measured local gas-phase area fraction fluctuations, bubble size distribution, and bubble rise velocity. The effect of the models used for calculation of gas–solid drag (βgs), solids frictional pressure (Ps,f), and specularity coefficient (φ) on the bubbling characteristics under dense and dilute flow conditions are analysed. Under dense bed condition (UG = 1.1 Umf), an increase in the Ps,f and φ led to an increase in solids viscosity, which in turn led to a decrease in the bubble rise velocity and size. In the case of the two-jet distributor, an increase in βgs predicted merging of the larger jets and formation of larger bubbles. Further, to predict the different jetting regimes (isolated jets, breakage/merging of jets, and generation of larger bubbles) at different UG correctly, we show that different βgs models are required. Whereas, in the case of gas–solid flows comprised of particles of different density fluidized with the uniform distributor, a single βgs model predicted the bubbling characteristics reasonably well with measurements.

Effect of Gas Input Conditions and Ultrasound on the Dynamic Behavior of Flotation Bubbles.

Ultrasonic flotation is useful for fine low-rank coal purification; however, the efficiency of ultrasonic flotation still needs to be improved. Because the dynamic behavior of flotation bubbles has significant effects on their flotation efficiency, it was investigated under different gas input conditions with and without ultrasound using the volume of fluid method and h-speed imaging. The results indicated that the gas input method can influence the final kinetic behavior of the flotation bubbles by changing the morphology of the initial bubble. With an increase in the size and aspect ratio of the bubble, the bubble deformation and velocity increased, and the range of motion of the bubble increased and then decreased. Meanwhile, the size of the bubble increased with an increase in the thickness of the vibrating plate of the ultrasonic transducer owing to the aggregation of the bubbles under the influence of ultrasound.

Characteristics of flow fields in the gas–liquid mini‐bubble columns with particle image velocimetry measurements

AbstractAs a new type of gas–liquid microreactors, the gas–liquid mini‐bubble column has potential applications. However, few studies on the flow fields in the mini‐bubble column can be found at present. In this work, particle image velocimetry (PIV) was used to visually study the velocity fields, vorticity fields and bubble dynamics in the gas–liquid mini‐bubble columns with column inner diameters of 1–3 mm and mini‐bubble diameters ranged from 0.7 to 1.3 mm. It is found that with the increase of superficial liquid velocity, bubbles rose from almost straight line to Z‐shaped or S‐shaped trajectory, and the bubble trajectory changed from one‐dimension to three‐dimension; when the bubble velocity changed, the bubble size and gas holdup decreased; bubble terminal velocity was controlled by bubble buoyancy and flow resistance, and increased slightly with bubble coalescence. These findings may provide basic reference for the design and scale‐up of such a mini‐bubble column reactor.