Dominant Bubble Research Articles

Multi-scale characteristics investigation of sheet/cloud cavitation

Sheet and cloud cavitation involve the formation of large-scale cavities and microbubbles across a wide range of length scales. In this study, a two-way coupling Eulerian-Lagrangian algorithm was utilized to simulate the cavitating flow around a NACA66 hydrofoil at an incidence of 8 degrees and a cavitation number of 1.2. The large eddy simulation (LES) and volume of fraction (VOF) methods are used to capture the large scale vapor structures in Eulerian frame. Meanwhile, the discrete bubble model (DBM) and simplified Rayleigh-Plesset equation are implied to solve the dynamic of Lagrangian microbubbles smaller than local grid scale. The results indicate that large-scale cavities are periodically shed downstream, influenced alternatively by the re-entrant jet and shock wave mechanisms, resulting in periodic variations of both the number and Sauter mean diameter of microscale bubbles. During the quasi-periodic evolution process of sheet/cloud cavitation, the microbubbles scatter around the large scale cavities where have strong turbulence intensity and high vorticity, and the probability density function of discrete bubble diameter similarly conforms to Gamma distribution with the dominant bubble size between 30 and 70μm. During the growth of an attached cavity, the number density of bubbles follows a power-law scaling with a value of -5/3 on radius. For the other stages, the bubble size spectrum of smaller microbubbles exhibits a -2/3 power-law scaling when the diameter of the microbubble is less than 200 μm. For larger bubbles with a diameter greater than 300 μm, the bubble density is proportional to the bubble radius to the power of -6.

On the flow of a bubble pair rising in temperature-sensitive gel propellants

The gel propellant is a novel type of propellant created by introducing gelling agents to the liquid propellant. Similar to other types of propellants, the occurrence of air bubbles in the gels is common and unavoidable during the manufacturing and casting process. The presence of bubbles can adversely impact the stable combustion of the engine and may even pose safety hazards when they enter the combustion chamber. In order to gain a better understanding of the bubble interaction within the gel propellant, we have conducted a study on the motion of a pair of bubbles rising in a quiescent water-gel. In this study, the volume of fluid (VOF) method is adopted for modeling the multiphase feature of the flow. The shear and temperature dependent viscosity of the gel has been modeled using the temperature-coupled Carreau–Yasuda model. The effects of various factors, including the initial distance between the bubble pair, bubble diameters, and the temperature, on the motion characteristics of the bubble pair are studied. It is found that an increase in the initial distance leads to a reduction in the bubble interaction; for side-by-side bubble pairs, the bubbles exhibit slower ascent rates and reduced lateral displacement; for inline bubble pairs, the coalescence of the bubble pair becomes slower. Increasing the diameter of the dominant bubble accelerates the coalescence of the bubble pair and the ascent. Elevating the temperature results in a decrease in viscosity, thereby accelerating the coalescence and ascent of inline bubble pairs and producing viscosity reduction around the bubble pair.

Experiments to understand bubble base growth mechanism(s) on hydrophobic surfaces under the influence of bulk flow inertia during nucleate boiling regime

Wetting characteristics of substrate surface is one of the parameters that strongly influence the phenomena of boiling heat transfer. In this direction, importance of hydrophobic surfaces has been emphasized in the literature due to their potential to offer high boiling heat transfer rates at low heat fluxes. For such surfaces, it has been shown that microlayer ceases to exist with contact line evaporation being the dominant bubble base growth mechanism during nucleate pool boiling regime. The present work investigates the plausible mechanism(s) of bubble growth on hydrophobic surfaces under nucleate flow boiling conditions. Possibility of formation of microlayer even on low wettability surfaces under the influence of bulk flow inertia has been examined. Thin-film interferometry, in conjunction with high-speed videography, has been configured to record the microlayer dynamics and the bubble growth process in tandem. For a direct comparison, experiments under identical flow and bulk subcooled conditions have also been performed on a hydrophilic substrate, which is known to promote microlayer formation during the bubble growth process. Observations made through thin-film interferograms confirmed the formation of microlayer on hydrophobic surfaces. Experiments further revealed that the rate of growth of dry patch is higher on hydrophobic surfaces than that in the case of hydrophilic substrates. Vapor bubbles have been observed to be pinned on the hydrophobic surface whereas the phenomenon of bubble lift-off has been distinctly observed on hydrophilic substrate surface.

Long-lasting stagnant equatorial plasma bubble event and the related scintillation over the Brazilian region

Diverse studies about equatorial plasma bubble structures and their relation with ionospheric scintillation have been performed in the last decades. Among many findings, the investigations usually reported dominant plasma bubble eastward velocity with a magnitude of few tens of m/s and larger amplitude scintillation for transionospheric signals aligned with these depleted structures. However, an uncommon long-lasting event with negligible average zonal drift prevailing for hundreds of minutes was registered over the Brazilian region, allowing a case study of the scintillation pattern under this particular condition. Data from ionosondes, all-sky imager (6300 Å filter), geostationary satellites and Global Navigation Satellite System were used here, covering the eastern portion of the Brazilian ionosphere. The results show that the scintillation was less intense than in other nights around the event, suggesting that larger plasma density gradients and sequential bubble structures seem to cause more severe scintillation scenario than the alignment condition alone, even though the last also contributes to worsen the scintillation effects. Regarding the stagnant bubble pattern, the results suggest prevailing E region Hall conductivity and equatorward wind as potential agents.

Pore-scale mechanisms and simulations for gas–water two-phase transport processes in natural gas reservoirs

During the development of natural gas reservoirs, predicting the bubble size distribution and transport velocity in the study of gas-water two-phase flow through porous media is extremely important in natural gas recovery. Considering the different forces that act on small and elongated bubbles, a double-bubble phase population balance model was introduced to establish an improved bubble flow model. Accordingly, a two-dimensional visualized model was designed to analyze the bubble diameter distribution and average velocity of bubbles. Dominant bubble coalescence, breakup, and coexistence flows were simulated to evaluate the uniqueness of the diameter distribution of bubbles, the change in the median bubble size at different dimensionless distances, and the average velocity distribution. The peak distribution of the bubble diameter was different in the three flow conditions. Owing to the difference in the bubble breakup and coalescence rates, the peak values of bubbles varied with the increase in the dimensionless distance. Higher bubble breakup or coalescence rates were accompanied by a rapid change in the median bubble size. Furthermore, the average velocity of the transition from small bubbles to elongated bubbles decreased significantly. Thus, the proposed pore-scale gas-water two-phase model can effectively predict the occurrence and transport processes in natural gas reservoirs.

Late-time turbulent mixing induced by multimode Richtmyer–Meshkov instability in cylindrical geometry

Turbulent mixing induced by Richtmyer–Meshkov instability (RMI) in convergent geometry widely exists in natural phenomena and in engineering applications. In the present work, high-resolution numerical simulations of RMI at a complete cylindrical interface, with the imploding shock wave initially passing from the heavy fluid to the light fluid, are presented. Two different initial perturbations are applied. The mixing zone finally reaches a convergence ratio Cr ≈ 1.6 in both cases. Compared to classical RM instability, the more complex wave system, as well as the geometrical effect induced by the radial movement of mixing fluid, modifies the evolution of the mixing zone. The growth rate of the mixing width is analyzed in terms of the stretching or compression effect and species-penetration effect. In a cylindrical geometry, the stretching or compression effect is mainly induced by the wave system and the nonplanar geometric environment. The late-time turbulent mixing width induced by the penetration effect scales as (t−t0)θ, as with the evolution of planar RMI. For both cases, the mass-fraction profiles are collapsed at the late time if the radial coordinate is first shifted with the spike-front position and then scaled by the mixing width. By analyzing the distribution of the bubble (spike) contour, the dominant bubble (spike) diameter [D¯b(s)] is obtained. The ratios [βb(s)] between the dominant bubble (spike) diameter and the bubble (spike) amplitude [Wb(s)] are calculated, and a stable ratio of spike βs is observed during the late stage. Meanwhile, the ratio of the bubble βb is greater than 1 at late time.

Research on cavitation gain induced by nanosecond pulsed laser based on microparticles

The cavitation gain refers to the phenomenon of increasing the number and intensity of cavitation by lowering the threshold of cavitation. Microparticles play a direct and critical role in the cavitation process. Laser cavitation gain based on microparticles has been found. To study the mechanism of cavitation gain, the finite-difference time-domain simulation model is established to analyze the field enhancement effect around the microparticle under laser irradiation. A laser-induced cavitation experiment platform is built, and the pulsation process of cavitation is recorded by a high-speed camera. Combined with FLUENT bubble collapse vector model, the impact of the cavitation water jet on the wall is analyzed. The result shows that microparticles reduce the cavitation threshold of liquid. Owing to the field enhancement effect at the bottom of microparticles, the number of primary cavitation bubbles in the mixed solution is greater than that in distilled water under the same experimental conditions, and the dominant cavitation bubble has a larger radius and a longer pulsation period. The bubble collapse near the wall produces an obvious water jet and wall-leaning effect, and the pressure on the wall in the microparticles mixture is higher than that in distilled water. Cavitation gain of microparticles in the laser-induced cavitation processing is helpful in revealing the mechanism of cavitation erosion and solving the problem of cavitation damage. The behavior of cavitation gain can help us rationally utilize water jet force produced by laser-induced cavitation in microparticle mixtures, which has a profound influence on the innovative development of material surface micromachining.

Experimental study on bubble and droplet entrainment in vertical churn and annular flows and their relationship

Bubble and droplet entrainment plays an important role in the mass transfer between the liquid film and the gas core in churn and annular flow. In this work, bubble and droplet entrainments in vertical churn and annular flows are systematically studied in an air-water two-phase vertical flow experiment using a high-speed camera. The results show that there are three kinds of bubble entrainment and five kinds of droplet entrainment in churn and annular flows. The entrainment can be categorized as primary entrainment and secondary entrainment. The primary entrainment includes one kind of bubble entrainment, which is a large wave roll and overturn, and three kinds of droplet entrainment, which are shearing-off of waves containing bubbles, bag break-up of waves, and ligament break-up of waves. The secondary entrainment includes two kinds of bubble entrainment, which are synchronized bubble entrainment along with droplet entrainment and droplets from the gas core impacting the liquid film, and two kinds of droplet entrainment, which are droplets impacting the liquid film and the burst of bubbles. With the flow pattern transition from churn to annular flow, the dominant bubble entrainment changes from large wave roll and overturn to the synchronized bubble entrainment along with droplet entrainment accompanied by a decreasing intensity. The dominant droplet entrainment changes from shearing-off of waves containing bubbles to ligament break-up of waves with the intensity first decreasing and then increasing. There is a close relationship between bubble entrainment and droplet entrainment. The entrained bubbles reduce the strength of large-scale waves and facilitate the shearing-off of waves. A small number of droplets are entrained due to the burst of residual bubbles in the liquid film. Droplets that are generated by bag or ligament break-up of waves can directly impact the nearby liquid film, leading to synchronous bubble entrainment. A small number of bubbles can be entrapped by droplets from the gas core impacting the liquid film. The primary entrainment intensity is significantly greater than that of the secondary entrainment. With the transition from churn to annular flow, the change in the primary entrainment intensity determines the variety of the secondary entrainment intensity.

Dynamic evolution of Rayleigh-Taylor bubbles from sinusoidal, W-shaped, and random perturbations.

Implicit large eddy simulations of two-dimensional Rayleigh-Taylor instability at different density ratios (i.e., Atwood number A=0.05, 0.5, and 0.9) are conducted to investigate the late-time dynamics of bubbles. To produce a flow field full of bounded, semibounded, and chaotic bubbles, three problems with distinct perturbations are simulated: (I) periodic sinusoidal perturbation, (II) isolated W-shaped perturbation, and (III) random short-wave perturbations. The evolution of height h, velocity v, and diameter D of the (dominant) bubble with time t are formulated and analyzed. In problem I, during the quasisteady stage, the simulations confirm Goncharov's prediction of the terminal speed v_{∞}=Frsqrt[Agλ/(1+A)], where Fr=1/sqrt[3π]. Moreover, the diameter D at this stage is found to be proportional to the initial perturbation wavelength λ as D≈λ. This differed from Daly's simulation result of D=λ(1+A)/2. In problem II, a W-shaped perturbation is designed to produce a bubble environment similar to that of chaotic bubbles in problem III. We obtain a similar terminal speed relationship as above, but Fr is replaced by Fr_{w}≈0.63. In problem III, the simulations show that h grows quadratically with the bubble acceleration constant α≡h/(Agt^{2})≈0.05, and D expands self-similarly with a steady aspect ratio β≡D/h≈(1+A)/2, which differs from existing theories. Therefore, following the mechanism of self-similar growth, we derive a relationship of β=4α(1+A)/Fr_{w}^{2} to relate the evolution of chaotic bubbles in problem III to that of semibounded bubbles in problem II. The validity of this relationship highlights the fact that the dynamics of chaotic bubbles in problem III are similar to the semibounded isolated bubbles in problem II, but not to that of bounded periodic bubbles in problem I.

Evolution of a small hydrothermal eruption episode through a mud pool of varying depth and rheology, White Island, NZ

White Island volcano, New Zealand was a host to multiple hydrothermal eruptive episodes within a mud-sulphur pool in 2013. Although hydrothermal activity is common at White Island, past events have largely gone undescribed in favour of the larger phreatomagmatic and magmatic eruptions. Here, we detail the first and longest hydrothermal episode of 2013, lasting from 15 January to 7 February using video and photo analysis from tour operators and staff responsible for monitoring the volcano. Differences in the dominant bubble burst style across this episode led to the classification of four distinct eruption regimes: (1) multiple irregular bursts on the pool surface, (2) larger distinct symmetric hemispheres with starbursts and/or followed by mud heaves, (3) no initial pool surface deformation but a vertical steam jet followed by a sometimes large directed mud heave and (4) no lake and continuous pulsating dry ash and block venting. The progression through these regimes is associated with a lowering lake level and a concomitantly increasing viscosity of the pool, which initially comprises a low viscosity muddy water, and partially evaporates to yield a shallow layer of high viscosity mud that ends with the complete drying up of the mud pool. Formation of primary mud hemispheres or gas jets is followed by heaves or secondary upheaval events. The heights of these heaves are used as a measure of explosivity. Heights increase from ∼8 m during regime 1 on 15 January to ∼102 m during regime 3 on 28 January. Venting of dry mud during regime 4 developed on 29 January before a regression back to regime 1 took place on 7 February as the pool re-established. Through observations of the shapes of ejected mud clots, we propose that the increasing explosivity of higher number regimes is primarily due to increasing slug bubble lengths teamed with increasing mud pool viscosity. We attribute a lesser control to the decreasing depth of the pool during its progressive desiccation, which may in turn influence the bubble burst depth. Occasionally, visible yellowing of the steam/gas plume led us to suggest that elemental sulphur may also be present in the conduit and may also play a role in regulating bubble release dynamics. Although, evidence for magmatic/phreatomagmatic eruptions was present during eruptions later in 2013, we found no evidence for juvenile magma in the January-February eruption episode described here. However, we concur with other investigators that magma was probably intruded to shallow levels and may have driven heat and gas flux. Our explanation for the correlation of pool depth, mud viscosity and eruption regime is based on a conceptual model in which a pool is perched above a two phase hydrothermal system and is sensitive to changes in the heat and gas flux from shallow magma. The variable release of gas and thermal perturbations in the course of the January-February eruptive episode impacted the pool level, the water to sediment ratio in the pool, and thus its viscosity, and in turn modulated the eruption regime. The varying degree of explosivity throughout this episode calls for a new consideration of pool properties in assessing eruption hazards at this frequently visited volcano. We additionally emphasise that ballistic hazards from small eruptions exist coupled with a range of seismic signals and that the hazard was greatest during infrasound tremor.

Monitoring of particle motions in gas-solid fluidized beds by electrostatic sensors

Gas-solid fluidized beds are widely applied in numerous industrial processes. Particle motions significantly affect the performance of fluidized bed reactors and the characterization of particle movements is therefore important for fluidization quality monitoring and scale-up of reactors. Electrostatic charge signals in the fluidized bed contain much dynamic information on particle motions, which are poorly understood and explored. In this work, correlation velocities of Geldart B and D particles were measured, analyzed and compared by induced electrostatic sensors combined with cross-correlation method in the fluidized bed. The results indicated that the average correlation velocity of particle clouds increased and the normalized probability density distributions of correlation velocities broadened when the superficial gas velocity increased in the dense-phase region. Both upward and downward correlation velocities could be acquired in the dynamic bed level region. Under the same excess gas velocity, the average correlation velocity of Geldart D particles was significantly smaller than that of Geldart B particles, which was caused by the smaller bubble sizes caused by the dominant bubble split over coalescence and less volume of gas forming bubbles for Geldart D particles. The experimental results verified the reliability and repeatability of particle correlation velocity measurement by induced electrostatic sensors in the gas-solid fluidized bed, which provides definite potential in monitoring of particle motions.

Influences of wake-effects on bubble dynamics by utilizing micro-pin-finned surfaces under microgravity

The influences of wake on bubble dynamics under various heat fluxes have been studied in microgravity. Nucleate pool boiling experiments of FC-72 on silicon chips fabricated with micro-pin-fins were conducted in 10−3 gravitational acceleration for 3.6s. Experimental results indicated that wake field had little influence on bubble dynamics at low heat flux, q=12.53W/cm2, but its effects on bubble dynamics became very great at moderate heat flux, q=27.89W/cm2, and high heat flux, q=39.54W/cm2. More importantly, wake-effects appeared even more significant in microgravity. In the wake field, the horizontal flow of the liquid on micro-pin-finned silicon chips promoted the collision, coalescence and movement of bubbles, and the vertical flow of liquid phase exerted an upward force on bubbles, which can effectively shorten the growth cycle and decrease the departure radius of bubbles. Furthermore, fresh liquid could easily be inhaled into the micro-pin-finned structure owning to the negative pressure in wake region, which can supply sufficient liquid for the growth of bubble, avoiding film boiling. The interaction between the micro-pin-finned structure and the wake effect promoted the process of bubble coalescence and departure effectively, so the process of heat transfer was significantly improved on micro-pin-finned surface. In addition, the flow field and bubble behavior in wake region were also briefly analyzed based on some reasonable simplifications and hypotheses. The theoretical analyses showed that the durations of wake-effects lasted longer than the time periods of the bubble in next generation, and the bubble diameters were also smaller than the thickness of wake region. Therefore, the dominant bubble of next generation was influenced by the wake field during its ebullition cycle, which is consistent with the experimental phenomenon.

Assessment of polydisperse drag models for the size segregation in a bubbling fluidized bed using discrete particle method

Polydisperse gas-particle flow is often encountered in industry and many polydisperse drag models have been developed in literature. In this work, discrete particle method was employed to assess polydisperse drag models for the segregation and mixing of binary gas-particle flow in a bubbling fluidized bed. The degree of particle segregation and the characteristic bubble frequency using different polydisperse drag models were analyzed. It was shown that the results predicted by the model of Rong et al. (2014) are in a best agreement with experimental data with 5.3% errors on average, and two dominant bubble frequencies were found by analyzing the fluctuations of average particle height.

Sonofragmentation: Effect of Ultrasound Frequency and Power on Particle Breakage

This paper investigates, for the first time, the breaking mechanism of particles exposed to implosions of stable and transient cavitation bubbles via Kapur function analysis. The effect of ultrasonic frequencies of 30–1140 kHz and powers of 4–200 W on particle breakage of paracetamol crystals was studied. The dominant cavitation bubble type was defined via sonoluminescence measurements. The breakage rate of seed crystals with a median size of 75 μm was found to be independent of the applied power when ultrasonically generated stable cavitation bubbles were generated. Furthermore, a particle size threshold of ca. 35 μm was observed. The particle size could not be reduced below this size regardless of the applied power or frequency. For transient bubbles, in contrast, higher powers lead to considerably smaller particles, with no threshold size within the investigated power range. The Kapur function analysis suggests that stable bubbles are more efficient than transient bubbles to break coarse particles with...

A deep-sea, high-speed, stereoscopic imaging system for in situ measurement of natural seep bubble and droplet characteristics

Development, testing, and application of a deep-sea, high-speed, stereoscopic imaging system are presented. The new system is designed for field-ready deployment, focusing on measurement of the characteristics of natural seep bubbles and droplets with high-speed and high-resolution image capture. The stereo view configuration allows precise evaluation of the physical scale of the moving particles in image pairs. Two laboratory validation experiments (a continuous bubble chain and an airstone bubble plume) were carried out to test the calibration procedure, performance of image processing and bubble matching algorithms, three-dimensional viewing, and estimation of bubble size distribution and volumetric flow rate. The results showed that the stereo view was able to improve the individual bubble size measurement over the single-camera view by up to 90% in the two validation cases, with the single-camera being biased toward overestimation of the flow rate. We also present the first application of this imaging system in a study of natural gas seeps in the Gulf of Mexico. The high-speed images reveal the rigidity of the transparent bubble interface, indicating the presence of clathrate hydrate skins on the natural gas bubbles near the source (lowest measurement 1.3m above the vent). We estimated the dominant bubble size at the seep site Sleeping Dragon in Mississippi Canyon block 118 to be in the range of 2–4mm and the volumetric flow rate to be 0.2–0.3L/min during our measurements from 17 to 21 July 2014.

Exploration of the 1891 Foerstner submarine vent site (Pantelleria, Italy): insights into the formation of basaltic balloons

On October 17, 1891, a submarine eruption started at Foerstner volcano located within the Pantelleria Rift of the Strait of Sicily (Italy). Activity occurred for a period of 1 week from an eruptive vent located 4 km northwest of the island of Pantelleria at a water depth of 250 m. The eruption produced lava balloons that discharged gas at the surface and eventually sank to the seafloor. Remotely operated vehicle (ROV) video footage and high-resolution multi-beam mapping of the Foerstner vent site were used to create a geologic map of the AD 1891 deposits and conduct the first detailed study of the source area associated with this unusual type of submarine volcanism. The main Foerstner vent consists of two overlapping circular mounds with a total volume of 6.3 × 105 m3 and relief of 60 m. It is dominantly constructed of clastic scoriaceous deposits with some interbedded pillow lavas. Petrographic and geochemical analyses of Foerstner samples by X-ray fluorescence and inductively coupled plasma mass spectrometry reveal that the majority of the deposits are vesicular, hypocrystalline basanite scoria that display porphyritic, hyaloophitic, and vitrophyric textures. An intact lava balloon recovered from the seafloor consists of a large interior gas cavity surrounded by a thin lava shell comprising two distinct layers: a thin, oxidized, quenched crust surrounding the exterior of the balloon and a dark gray, tachylite layer lying beneath it. Ostwald ripening is proposed to be the dominant bubble growth mechanism of four representative Foerstner scoria samples as inferred by vesicle size distributions. Characterization of the diversity of deposit facies observed at Foerstner in conjunction with quantitative rock texture analysis indicates that submarine Strombolian-like activity is the most likely mechanism for the formation of lava balloons. The deposit facies observed at the main Foerstner vent are very similar to those produced by other known submarine Strombolian eruptions (short pillow flow lobes, large scoriaceous clasts, spatter-like vent facies). Balloons were likely formed from the rapid cooling of extremely vesicular magma fragments as a result of a gas-rich frothy magma source. The exterior of these fragments hyperquenched forming a vesicular glassy shell that acted as an insulating layer preventing magmatic gas in its interior from escaping and thus allowing flotation as densities reached less than 1,000 kg/m3. We believe that lava balloons are a common eruptive product, as the conditions required to generate these products are likely to be present in a variety of submarine volcanic environments. Additionally, the facies relationships observed at Foerstner may be used as a paleoenvironmental indicator for modern and ancient basaltic shallow submarine eruptions because of the relatively narrow depth range over which they likely occur (200–400 m).

An acoustic technique for measurement of bubble solids mass loading: (b) Monitoring of Jameson cell flotation performance by passive acoustic emissions

A passive acoustic emission (AE) monitoring system has been developed and tested in Australian coal washeries. The system detects naturally occurring stress waves in the pulp and froth zones of a flotation unit, utilising volumetric (hydrophone) and surface (broadband accelerometer) sensing technologies. The analyser has been proof-of-concept trialled on a coal washery Jameson cell. AE signals associated with both hydrophones mounted near and accelerometers mounted on a downcomer are shown to be sensitive to cell process conditions including aeration, feed solids and reagent addition rates. Accelerometers mounted on a Jameson cell downcomer are shown to be particularly sensitive to aeration rate with AE signals containing features that can be interpreted in terms of Minnaert bubble resonance theory. The frequencies of peaks in the Fourier power spectra are in this case used to estimate dominant bubble size and solids loading inside the downcomer. Passive AE monitoring is a promising approach for flotation cell performance on-line monitoring.

Effect of coalescence and breakup on bubble size distributions in a two-dimensional packed bed

Bubble size distributions in a two-dimensional packed bed are investigated as a function of axial direction by using image processing techniques with a large number of bubble samples. Two inlet conditions, controlled sized bubbles and uncontrolled sized bubbles, are conducted to study the characteristic behaviors of bubbles. With the uncontrolled sized bubbles, the average bubble size corresponding to the two-dimensional bed is found, and is not affected by decrease or increase of flow rates. With the controlled sized bubbles, dominant bubble breakup and coalescence flows are separately simulated to investigate bubble breakup and coalescence rates. Unique behavior of bubble size distributions for dominant bubble breakup and coalescence has been seen, and changes in bubble size distributions along axial direction are studied with median bubble size. Near the inlet the median changes rapidly due to the dominant bubble mechanism of either coalescence or breakup, and far away from the inlet the median reaches asymptotic value due to the balance of bubble breakup and coalescence. For both dominant breakup and coalescence flows, the asymptotic values are close to the average bubble size. Therefore, the average bubble size, resulting from the balance of bubble coalescence and breakup at far downstream, is irrespective of inlet flow conditions.

Bubble mechanisms and characteristics at pore scale in a packed-bed reactor

An image processing technique was used to study dominant bubble mechanisms in a two-dimensional packed-bed at pore level under the bubbly flow regime. Bubble breakup and coalescence were identified as dominant mechanisms using a large number of image samples. Two types of coalescence mechanisms were identified that occur due to compression and deceleration associated with the bubbles and three breakup mechanisms were identified that are result of liquid shear force, bubble acceleration, and bubble impact. Data on various two-phase parameters, such as local void fraction, bubble velocity, size, number, and shape were obtained from the images. Results indicated that when a flow regime changed from bubbly to either trickling or pulsing flow, the number of average sized bubbles significantly decreased and the shape of the majority of the bubbles was no longer spherical. Although a mean bubble velocity of all sized bubbles was uniform for given gas and liquid superficial velocities, individual bubble velocities were quite different depending on the bubble location in the pore. The present bubble size distributions were compared with previous studies and the results on bubble size are in general agreement.

Identification of the State of a Magneto-Fluidized Bed by Pressure Fluctuations

Local instantaneous pressure signals obtained through a magneto-fluidized bed have been analyzed using both classical and advanced signal analysis methods, which can deliver the necessary information about the presence of the bubbling and turbulent flow pattern. The conventional signal processing tool such as autocorrelation and cross correlation were used as preliminary tools to analyze the data. Evaluation of the dominant bubble frequency was completed using the autocorrelation function and power spectral density function. Mutual information function was used to identify the periodicity and the predictability of the local instantaneous pressure signals. Since it does not demand any particular functional relationships between the data points, it is a better method (compared to autocorrelation function) for measuring the predictability of nonlinear systems.