Free Bubbles Research Articles

Modeling Fluidized Bed Combustion of Liquid Fuel.(Dept.M)

A mathematical model has been developed 10 predict the combustion of liquid fuel in a fluidized bed. From the fluid-dynamic point of view, the model is based on the modified two-phase theory of fluidization where the gas in the bed is assumed to have three phases. A jet-bubble phase which is fuel rich bubbles formed at fuel feeding jet, a distributor-bubble phase which is fuel free bubbles generated at distributor plate and an emulsion phase. Each phase has its composition and exchanges masses with the other two ones. The bubble is assumed to have a constant size throughout the bed. Gas mixing due to bubbles coalescence is taken into consideration based on an equivalent mass interchange coefficient between the two-bubble phases. The coefficient has been derived based on bubble size growth and probability of coalescence among jet and distributor bubbles. Two-step fuel reaction has been applied. The fuel firstly reacts to carbon monoxide and water vapor. In the second step, carbon monoxide reacts with oxygen to form carbon dioxide. Mass balance differential equations have been derived for various reacting species (CxHy, O2, CO, H2O and CO2) in the three phases. The heavy liquid fuel has been considered as a base case whereas gasoline and diesel fuel have been studied for comparison. The numerical solution of the model yields the axial concentration profiles of different gaseous species within the three phases. The in-bed fuel combustion has been estimated. The relative importance of gas mixing due to bubble coalescence has been assessed. The influences of different parameters on the combustion performance have been studied and discussed. Among the later parameters, jet velocity has a major effect on combustion performance. Experimental tests have been carried out to validate the model using a bubbling fluidized bed combustor of 300 mm inner diameter. The comparison between the model and experimental results shows a good agreement, especially, at higher jet velocity.

Reflection of sound pulses from the boundary of the bubble medium

The reflection of sound and radiation of a bubble medium under excitation by a short sound wave are studied for different values of the volume concentration of bubbles. It is shown that at the beginning of the interaction of the wave with the medium, reflection occurs as from a structureless soft boundary, which at the end of the interaction is replaced by radiation from an excited medium. The dependences of the reflection coefficient are obtained and the spectral characteristics of the radiation of the bubble medium are determined at the initial stage of the process and at large times when the radiation is steady-state. It is shown that the emission spectrum of the layer is continuous with a maximum in the frequency range of volume pulsations of free bubbles.

Acoustic propagation in a seagrass meadow over diurnal and seasonal time scales

Acoustic propagation in seagrass meadows is highly sensitive to oxygen production through photosynthesis. In addition to gas volumes encapsulated within the seagrass, free bubbles are released into the water column as oxygen diffuses through the plant tissue, affecting acoustic dispersion, absorption, and scattering. Because the oxygen production cycle is largely driven by sunlight, these effects exhibit a diurnal dependence. Previous work explored using acoustics to monitor seagrass photosynthetic activity, but this study presents new results that span both diurnal and seasonal time scales. Acoustic propagation experiments were conducted in a seagrass meadow in a shallow bay on the Texas Gulf Coast. A piezoelectric sound source transmitted frequency-modulated chirps (0.1–100 kHz) over several diurnal cycles, and the received acoustic signals were match-filtered to obtain band-limited impulse responses. Water temperature, salinity, depth, dissolved oxygen, and solar irradiance were concurrently measured with oceanographic probes. Measurements were taken both in winter and summer to examine the seasonal dependence of seagrass photosynthesis and its effect on the acoustic propagation environment. Dependence of the received acoustic signal on various environmental parameters will be discussed with the goal of using acoustics to study seagrass photosynthesis and productivity. [Work supported by ARL:UT IR&D and ONR.]

Cystoduodenal fistula: unusual complication after acute necrotizing pancreatitis with collection.

A 56-year-old man was admitted to our hospital with severe acute necrotizing gallstone pancreatitis. Computed tomography showed a 15-cm infected walled-off pancreatic necrosis (WOPN) extending to the anterior pararenal space with free air bubbles inside. After multidisciplinary evaluation and according to septic status, endoscopic ultrasound-guided drainage was planned.

A high‐order phase‐field based lattice Boltzmann model for simulating complex multiphase flows with large density ratios

SummaryBased on phase‐field theory, we develop a simple and robust single relaxation time (SRT) lattice Boltzmann (LB) model for simulating complex multiphase flows with large density ratios (up to 2000). The approach utilizes two LB equations (LBE), one is used to describe the interface behavior and the other is used to calculate the hydrodynamic properties. To improve the accuracy and stability in capturing interface, the high‐order LB model derived through the fourth‐order Chapman‐Enskog expansion analysis is applied to Cahn‐Hilliard equation. For solution of the flow field, a modified particle distribution function in the pressure‐velocity formulation is constructed to correctly consider the effect of local density variation and continuous pressure field. With such improvement, the proposed multiphase LB model is able to maintain numerical stability for the problem with very large density ratio, lower relaxation parameter and complex interface. For validation, a series of benchmark cases are carried out. Specially, the full potential of the proposed model is validated by bubble bursting at a free surface, bubble rising and droplet splashing with a density ratio of 2000. In all test cases, the obtained numerical results agree well with the reference data or the analytical solutions.

Temperature-regulated self-assembly of lipids at free bubbles interface: A green and simple method to prepare micro/nano bubbles

Micro/nanobubbles play an essential role in ultrasound-based biomedical applications. Here, a green and simple method to fabricate micro/nanobubbles was developed by the temperature-regulated self-assembly of lipids in the presence of free bubbles. The self-assembly mechanism of lipids interacting with gas-water interfaces was investigated, and the ultrasound imaging of the obtained lipid-encapsulated bubbles (LBs) was further confirmed. Above the phase transition temperature (Tm), fluid lipids transform from vesicles to micelles, and further assemble to the free bubbles interface to be a compressed monolayer, resulting in lipid shelled microbubbles. Cooling below Tm induces the lipid shell to glassy state and stables the LBs. Moreover, increasing the 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG2K) content in lipids formulation can further manipulate the shell curvature and reduce the LBs size into nanobubbles. LBs with diameters of 1.68 ± 0.11 µm, 704 ± 7 nm and 208 ± 6 nm were successfully prepared. The in vitro and in vivo ultrasound imaging results showed that all of the LBs had excellent echogenicity. The nanosized LBs revealed elongated imaging duration time and greater microvascular details for the liver tissue. Avoiding the organic solvent and complicated multiple preparation process, this method has great potential in construction of various multifunctional micro/nanobubbles with size control for theranostic applications.

Conceptual features for improving the flow part of the multiphase stages of ESP submersible plants for small and medium feeds for extracting stratal liquid with a high free gas content

AnnotationThis article analyzes the operation numerical simulation for serial and multiphase stages of various manufacturers on water and gas-liquid mixtures (GLM). Are identified the local gas separation areas in serial stages, which can lead to the formation of gas plugs and disruption of the pump supply.These factors include: the creation of a finely dispersed flow structure at the entrance of the stage, the preservation along the flow with small free gas bubbles diameters, the sustenance of the value for the allowed free gas content by circulation means of fluid part in the impeller, a defined relationship between the pressure gradient and the GLM flow rate. A mathematical model is selected, hydrodynamic calculations of structural options, optimization are carried out, a conceptual option is selected. Bench tests were carried out, confirming the high parameters of the product when working on GLM.

Axially-distributed bubble-bubble interaction under a coustic excitation in pipeline

Acoustically-excited bubble dynamics is the foundation of pipeline bubble detection based on acoustic technology. Due to the existence of multiple bubbles in pipeline flow, the Bjerknes forces among arbitrary bubbles under acoustic excitation may enforce bubble-bubble interaction and then change the features of bubble dynamics. Based on traditional free bubble’s Rayleigh-Plesset (R-P) model, this paper tries to establish bubble-bubble interaction model in consideration of the second Bjerknes force and bubble distribution in the pipeline axial direction. Meanwhile, the influence of finite wave speed in compressible fluid is considered. The proposed model is numerically calculated by the fourth-order Runge-Kutta method. Firstly, the differences in bubble feature between the free bubble’s R-P model and bubble-bubble interaction model are compared under excitation with different frequencies and amplitudes. Results show that the differences in bubble dynamics are minor when the bubble’s distance is large enough. When the bubble’s distance is fixed, the differences are significant on condition that the frequency of acoustic excitation is nearly the resonant frequency of bubbles. Secondly, through establishing compressible model and incompressible fluid model, we compare the differences between the two models. Numerical calculations show that the second Bjerknes force under the compressible assumption acts as an external force and forces the bubble to vibrate. On the other hand, the second Bjerknes force under the incompressible assumption changes the dynamics of bubble-bubble interaction as well as the resonant features. Finally, we study the effect of bubble-bubble distance and bubble’s axial position on bubble vibration characteristics. The bubble-bubble distance affects the second Bjerknes force and may lead the bubbles to vibrate nonlinearly. The bubble’s axial position changes the phase of external acoustic force and leads to the difference in initial vibration feature. When this difference is coupled with the second Bjerknes force, the bubble-bubble interaction may be changed even into nonlinear vibration, leading the bubble’s oscillation spectrum to differ from linear vibrations significantly. These results demonstrate that the resonant state of a small bubble may be converted into nonlinear vibration state if the second Bjerknes force is present. On the other hand, the resonant state of a large bubble can keep linear vibration when the second Bjerknes force is not obvious.

Predicting droplet velocity in a Hele-Shaw cell

We study the motion of low viscous non-wetting droplet in a Hele-Shaw cell, while it is pushed by an external phase of imposed flow rate, at low capillary numbers. In this regime, the droplet's mobility, defined as the ratio between the droplet velocity and the external phase mean velocity, evolves non linearly with the capillary number, a signature of the different dissipation mechanisms at play. Experiments are performed with surfactant free air bubbles in fluorinated oil, and with surfactant laden fluorinated oil droplets in water. We propose a model based on a power balance which takes into account the dissipation in the thin wetting film trapped between the bubble (or the drop) and the channel wall. The full topography of this thin film is obtained theoretically for the bubble case. By contrast, the presence of surfactants in the drop case induces uncontrolled boundary conditions at the interface, thus imposing to use the experimental topography measured in the previous paper [Reichert et al., J. Fluid. Mech., 850 p.708 (2018)]. Remarkably, the model reproduces the experimental velocities and shows that the velocity can be strongly affected by a stagnant cap effect at the rear of the drop, even if localized in less than a few percents of the total film area.

Mechanism of partial discharges in free helium bubbles in transformer oil

Partial discharges (PD) in floating bubbles and in microspheres made from thin glass walls in transformer oil were studied experimentally and theoretically. In a test cell, the PD occurrence was a very rare event. Sometimes PD in bubbles occurred after exposure for more than 10 hours at a voltage that corresponds to a value three times the voltage in accordance with Paschen's law. In the case of a glass microsphere, PD takes place at a voltage according to Paschen's law after several hours but afterwards PD occurs more frequently. The reason for this is in the starting electrons which initiate the first avalanche. It is shown with 100 keV X-rays that PD occurs in all bubbles and its inception voltage corresponds to Paschen's law. Theoretical analysis shows that, at the conditions of PD occurrence in helium bubbles, breakdown fits the streamer mechanism. Registrations of electrical and optical signals of PD were performed to estimate apparent and true charges and the number of radiated photons. As a result of the evaluation, the number of electrons is approximately equal to the number of photons.

Registration and simulation of partial discharges in free bubbles at AC voltage

Partial discharges in free bubbles in transformer oil are investigated both experimentally and theoretically. Optical, electric and optoelectronic registration are performed to obtain a picture of partial discharge. The time duration of electrical and optoelectronic signals is practically the same, 50 ns. Much longer time is needed for bubble deformation and its breaking, approximately several milliseconds. Electric properties obtained in experiments are confirmed by the simulation of the electrical field, “true” and “apparent” charges of partial discharge. Hydrodynamic behavior in experiments corresponds well to the simulation with the lattice Boltzmann method.

Experimental Study on Influences of Surface Materials on Cavitation Flow Around Hydrofoils

In order to resist on the cavitation erosion, many researchers try to change the solidity and tenacity of the coatings, but ignore the influence of surface characteristics of materials on cavitation flow and the interaction with each other. In this paper, high speed visualization system is used to observe the cavitation flow patterns in different stage. After comparing the characteristics of cavitation flow around hydrofoils made of aluminum (Foil A), stainless steel (Foil B) and the hydrofoil painted with epoxy coating (Foil C), the study shows that material has a significant effect on the cavitation flow. Firstly, when the incipient cavitation occurs, cavitation number of Foil A is highest among three hydrofoils, generating horseshoe vortex randomly. For Foil B and Foil C, it shows in the form of free bubbles. When the sheet cavitation occurs, Foil A has the highest cavitation number and shortest period, which is contrary to Foil C. And cavity consists of lots of small finger-like cavities. For Foil B and Foil C, it both constitutes with many bubbles. Compared with the high-density and small-scale cavities over surface of Foil C, the cavity of Foil B has larger scale and less density, which causes a minimal scope of influence of the re-entrant jet and strong randomness. When the cloud cavitation occurs, Foil C has the lowest cavitation number and shortest period. Secondly, compared with aluminum, both of stainless steel and epoxy coating restrains the occurrence and development of cavitation, and stainless steel and epoxy coating performs better than aluminum. For inception and sheet cavitation, stainless steel performs better than epoxy coating and aluminum. For cloud cavitation, epoxy coating performs better than stainless steel and aluminum. The objective of this paper is applied experimental method to investigate the effect of surface materials on cavitation around Clark-Y hydrofoils.

Acoustic characteristics and micro-distribution prediction during hydrate dissociation in sediments from the South China Sea

Different micro-distribution patterns in gas hydrates may produce different acoustic effects on the deposition medium. The influence of hydrate distributions on the reservoir and the acoustic response of the hydrate during the dissociation process are very important. In this paper, ultrasonic velocity and hydrate saturation were measured by ultrasonic measurement and time domain reflection (TDR) techniques, respectively. The relationship between the wave velocities and saturation of the hydrate along with X-ray computed tomography (X-CT) scanning results were used to analyze hydrate distribution patterns and their acoustic response characteristics. The results show that the rate of decrease in hydrate saturation in the dissociation stage remains basically unchanged, and the hydrate dissociation rate remains constant. During the process of hydrate dissociation, the hydrate first disrupts the stable sedimentary skeleton structure, and then breaks down at the contacts with sediment particles. Then, the hydrate gradually decomposes into the pore space away from the particle contacts and the hydrate is mainly present in suspended mode. Finally, hydrate decomposes entirely and free bubbles are in contact with the hydrate. In the early stage of hydrate dissociation, compressional and shear wave velocities decreased rapidly by 453 m/s and 307 m/s, respectively. In the later stage of hydrate dissociation, compressional and shear wave velocities decreased by 271 m/s and 119 m/s, respectively, with smaller decreases in wave velocities than in the early hydrate dissociation stage. The results show that the measured changes in P-wave velocity are consistent with results obtained from a site in the Shenhu area. The micro-distribution modes during the hydrate dissociation process were inferred. At the initial stage of dissociation, the hydrate decomposed at the contacts with sediment particles, whereas the hydrate was mainly suspended in the pores in the later stage.

Turbulent flow investigation of the vacuum boiler at boiling

Viscosity coefficient defining methods taking into account the turbulent component under the boiler various operating conditions at the subatmospheric pressureare considered in the paper. Currently, addressing this problem is of particular relevance for the vacuum boilers heat-exchange problems solution. The viscosity coefficient dependences in a wide range of the specific heat fluxes at the free convection and bubble boiling are obtained. The study results were obtained by means of the software complex ANSYS CFX and RPI model.

Free fall motion of a floating body: Bubble formation and its effect

Abstract The hydrodynamic problem of a floating body moving downwards into water with prescribed velocity or in free fall motion is solved based on the incompressible velocity potential theory through the boundary element method in the time domain. In particular, the focus is on the whole process of flow detachment, the formation of an open cavity, closure of the cavity or formation of entrapped air bubble, collision of the inner jet with the body surface and jet impact with free surface. The whole problem is divided into several stages and methodology is introduced to resolve the numerical challenge at each stage. In particular the dual coordinate system method is adopted to resolve the local sharp spatial and temporal variation in the impact zone, and the domain decomposition method with Riemann second sheet is adopted to resolve domain overlapping. An auxiliary function method is used to decouple the nonlinear mutual dependence of fluid loading, body motion and bubble deformation. Simulations are undertaken for a floating body moving into water at constant velocity or constant acceleration, and in free motion, respectively. Detailed results for pressure, free surface profile, bubble deformation and body motion are provided, and their physical implications are discussed.

Behavior of bubbles moving along horizontal flat plates with different surface wettability

This paper presents experimental findings on the behavior of bubbles moving along upper flat plates with different surface wettability in a horizontal channel. The size and velocity of the bubbles are measured by image processing to evaluate the wettability dependence in the range of channel Reynolds number, Re (2000 ≤ Re ≤ 5000) and bubble Weber number, We (1 < We < 10). When the bubbles move along a hydrophilic flat plate, their wall-sliding velocity increases with the bubble diameter, and no bubbles adhere to the plate even at low Re. By contrast, their behavior depends strongly on Re when a hydrophobic plate is used. At Re = 5000, an intrinsic cycle of coalescence and breakup of bubbles takes place. In this cycle, large wall-creeping bubbles become stretched by shear, and then each tends to break up into a small bubble that adheres to the plate surface and a medium-sized bubble that is detached from the plate. Because the small, adhered bubbles grow to large creeping bubbles through coalescence with bubbles supplied upstream, this intrinsic cycle repeats downstream. As a consequence, the hydrophobic plate produces two groups of bubbles within the turbulent boundary layer of the plate: one consisting of medium-sized free bubbles and one consisting of small wall-adhering bubbles to large wall-creeping bubbles.

Investigation on the Properties of Aqueous Foams Stabilized by Cetyltrimethylammonium Bromide in Terms of Free Drainage and Bubble Size

AbstractAlthough foams stabilized by surfactants have been the subject of massive investigations and great achievements were made over the past few years, many questions concerning their properties are still not well understood. The aim of this study is to examine the effect of the cetyltrimethylammonium bromide (CTAB) concentration on the foam properties and illustrate the interaction between liquid drainage and bubble size evolution. Experiments were carried out at varying CTAB concentrations ranging from 0.1 to 5.0 times the critical micelle concentration (CMC), where the variation of the liquid content of foam and bubble size was simultaneously determined using a commercially available FoamScan device. The results showed that the foam properties (both foamability and foam stability) of CTAB were largely dependent on the surfactant concentration when concentrations c &lt; CMC but this effect did not scale linearly with concentration. When c ≥ CMC, both foamablity and foam stability were nearly concentration independent, while the latter showed a small decrease due to the formation of micelles. In addition, the correlation between free drainage and bubble size evolution indicated that the increase of bubble size could indeed enhance the foam drainage.

An experimental study of cavitation damage on tissue of Carassius auratus in a jet fish pump

The jet fish pump is a potentially valuable tool for fish transport. Though jet fish pumps have been shown to cause the least damage to fish, cavitation as a commonly encountered flow phenomenon can lead to injury or death of fish. Experiments were conducted to investigate damage to tissue of Carassius auratus caused by different cavitation stages in a jet fish pump. Mortality was assessed for 48 h after the fish passed through the device. Tissues from swim bladders, livers and gills were examined. Fish were also examined for several important enzymes that are indicators of tissue injury. The results showed that cumulative mortalities of fish in cavitation operating conditions were higher than those in non-cavitation operating conditions at each sampling time. The cumulative mortality in the stable cavitation condition reached 55% within 48 h after the experiment. Two types of swim bladder damage, crumpling and rupturing, were found in 60% of the dead fish. The damage rate of swim bladders rose with the increase of cavitation, and broken swim bladders were found only in cavitation operating conditions. In addition, liver damage was found in those dead fish experiencing intensive cavitation operating conditions, and significantly high serum ALT and AST activity verified this damage. Free bubbles, an essential symptom of the gas bubble disease, were found in the gill filament of those fish experiencing intensive cavitation operating conditions. The significantly high serum LDH activity indicated that fish experienced intense anaerobic respiration during intensive cavitation operating conditions. Judging from the significant serum Cr content, cavitation in the jet fish pump also posed a potential risk for the kidneys of fish. Moreover, a pressure damage experiment showed that rupturing of swim bladders occurred in the recompression process; environmental pressure between 40 kPa and 58 kPa was a transition area for pressure damage to swim bladders in vitro. Swim bladders in vitro were safe when environmental pressure was above 58 kPa, yet they were broken when environmental pressure was less than 40 kPa. This study illustrates that intensive cavitation is a dangerous flow phenomenon for fish in a jet fish pump and should be avoided in practical applications.

Free drainage and bubble size for aqueous foams stabilized by sodium dodecyl benzene sulphonate

Free drainage and bubble size for aqueous foams stabilized by sodium dodecyl benzene sulphonate

A dynamic model of fluidized-bed flotation

Abstract The concept of fluidized-bed flotation (FBF) (e.g. HydroFloatTM) was recently introduced to improve the recovery of coarse particles (425 to 1180 µm). In spite of encouraging results from the first industrial applications dating back more than a decade, FBF still remains an emerging technology in the global mining industry. This paper presents a dynamic model of a FBF cell based on first principles describing volume conservation balances and macro hydrodynamic conditions. It was specifically developed to address process design, monitoring, and control problems. The model relies on the drift-flux theory to predict the flow of three populations (free particles, attached particles and bubbles) throughout the proposed vertical mixer-in-series framework. A calibration of empirical parameters on literature data demonstrates the ability to reproduce the effect of operating parameters (airflow rate, fluidization water flow rate and bed height) on steady-state recovery as long as the equipment operates at low turbulence.