Bubble Expansion Research Articles

Investigation on the Alloy Mixing and Inclusion Removement through Using a New Slot-Porous Matched Tuyeres

A mathematical model was developed to investigate the refining process through using slot-porous matched dual tuyeres. In this model, the bubble expansion is considered through the ideal gas state equation. The density variation of bubbles is calculated through the mass conservation equation. The coalescence and breakup of bubbles are tracked using the discrete particle model (DPM). The transient flow is computed through the k-ε model with three continuous phases (air–slag–steel). Comparisons were made between traditional porous–porous and the new slot–porous tuyeres, mainly focusing on the mixing behavior and inclusion removement ratio. The results show that, for traditional porous–porous matched tuyeres (P-P mode), the inclusion removement ratio is 34.14%; by comparison, after employing the slot–porous matched tuyeres (S-P mode), the inclusion removements ratio rises to 36.34%. The mixing time is also shortened by 18.74% by using the S-P mode. The reason for this phenomenon is because the slot tuyere produces a strong asymmetry that drives more liquid at the bottom of the ladle. The new tuyere match takes advantage of porous and slot tuyeres, and the mixing behavior and inclusion removements are improved. The result is beneficial for future refining.

Modeling and study of microlayer effects on flow boiling in a mini-channel

The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel.

Experimental and numerical investigation of gas-side mass transfer in Taylor flow

Taylor flows are of great interest for industrial application where efficient gas absorption is needed. Literature however scarcely relates to the gas-side mass transfer coefficient kG in Taylor flows. In this work, the authors selected and developed an experimental method, based on bubble expansion record, to estimate kG value in a millimetric channel of circular section, fed with nitrogen gas and pure ethyl acetate (EA). The experiments provided an order of magnitude of a few mm.s−1 at least for kG, for a two-phase velocity UTP ∼ 0.1 m.s−1.CFD simulations completed the experimental investigation and further estimated the order of magnitude, with kG∼10-2 m.s−1 when 0.09<UTP<0.18 m.s−1. The transport of EA inside the bubble suggests a significant contribution of diffusion in regard with the convective effects. kG is found to be sensitive to UTP, and to the temperature via the gas diffusivity and also via the bubble expansion leading to an increase in bubble velocity. The dependence of the gas-side Sherwood number to the Péclet number is similar to previously reported observations concerning a spherical bubble freely rising in quiescent liquid.This study thus provides the means to evaluate the gas-side resistance for any gas–liquid system in Taylor flows, a resistance that was usually simply dismissed in previous works. As an example, the authors demonstrated that, in Taylor flows, for a gas–liquid system of industrial interest (diluted CO2 in 30 wt.% N-methyldiethanolamine aqueous solution), gas-side mass transfer resistance is lower than the liquid-side one by at least one order of magnitude.

How to control the uniform micro bubbles generation on underwater superaerophobic surface?

Bubbles are an important medium to increase the efficiency of gas/liquid mass transfer. Smaller and more homogeneous bubbles can improve environmental treatment efficiency, microfluidic stability, and medical imaging capabilities. At present, it is still difficult to produce uniform micro bubbles and stably apply them into complex environment. Herein, a strategy of aerating micro bubbles by designing four different structures via laser ablation is proposed. The underwater superaerophobic polyimide (SPI) with micro volcano-like channels has been verified to produce bubbles with a volume of nanoliter. It is attributed to the combination of the micro-nano rough structures covering the surface and the micro-volcanic structure limiting the expansion of bubbles. The SPI with micro volcano-like structure, as a porous aeration film, can make various gases pass, which can be applied to water purification and gas detection. It is also found that the designed SPI with micro volcano-like structure can resist the damage of complex application environments, such as extreme water environments and ultraviolet radiation. As indicated in all the results, the SPI is applicable and flexible. The femtosecond laser can realize high precision and good controllability of the superaerophobic surface, which is difficult for other traditional methods to fabricate micropores. This distinctive membrane provides a new inspiration to design advanced materials for applications in gas transport and collection, wastewater treatment, and chemical reactions. It is also helpful to promote the basic research and applications of micro and nano bubbles generation.

Effect of magnetic field on expansion of ferrofluid-encapsulated microbubble

Magnetic microbubbles (MMBs) have great potential applications in drug delivery and target therapy because they can be controlled by magnetic fields. In this paper, dynamic equations are derived by Lagrangian formalism and the behavior of MMBs subject to a combination field of magnetic and ultrasound field in an incompressible infinite fluid is analyzed numerically. The results show that the magnetic field can promote bubble expansion and hinder its translational motion, and both the enhancement and obstruction effects will weaken with the decrease of bubble size. The initial translational velocity has almost no effect on bubbles motion. Besides, the maximum expansion radius of MMBs increases with the ferrofluid shell thickness, while that of the common MBs is just the opposite. In addition, the periodic change of Levich viscous drag caused by the rebound leads to the step-like translational motion. Finally, the ferrofluid-shell model can be replaced by the model of non-magnetic microbubbles in magnetic liquid at high driving frequency.

Polarization and phase control of electron injection and acceleration in the plasma by a self-steepening laser pulse

We describe an interplay between two injection mechanism of background electrons into an evolving plasma bubble behind an intense laser pulse: one due to the overall bubble expansion, and another due to its periodic undulation. The two mechanisms occur simultaneously when an intense laser pulse propagating inside a plasma forms a shock-like steepened front. Periodic undulations of the plasma bubble along the laser propagation path can either inhibit or conspire with electron injection due to bubble expansion. We show that carrier-envelope-phase (CEP) controlled plasma bubble undulation induced by the self-steepening laser pulse produces a unique electron injector—expanding phase-controlled undulating bubble (EPUB). The longitudinal structure of the electron bunch injected by the EPUB can be controlled by laser polarization and power, resulting in high-charge (multiple nano-Coulombs) high-current (tens of kilo-amperes) electron beams with ultra-short (femtosecond-scale) temporal structure. Generation of high-energy betatron radiation with polarization- and CEP-controlled energy spectrum and angular distribution is analyzed as a promising application of EPUB-produced beams.

Microbubble Delivery Platform for Ultrasound-Mediated Therapy in Brain Cancers.

The blood-brain barrier (BBB) is one of the most selective endothelial barriers that protect the brain and maintains homeostasis in neural microenvironments. This barrier restricts the passage of molecules into the brain, except for gaseous or extremely small hydrophobic molecules. Thus, the BBB hinders the delivery of drugs with large molecular weights for the treatment of brain cancers. Various methods have been used to deliver drugs to the brain by circumventing the BBB; however, they have limitations such as drug diversity and low delivery efficiency. To overcome this challenge, microbubbles (MBs)-based drug delivery systems have garnered a lot of interest in recent years. MBs are widely used as contrast agents and are recently being researched as a vehicle for delivering drugs, proteins, and gene complexes. The MBs are 1-10 μm in size and consist of a gas core and an organic shell, which cause physical changes, such as bubble expansion, contraction, vibration, and collapse, in response to ultrasound. The physical changes in the MBs and the resulting energy lead to biological changes in the BBB and cause the drug to penetrate it, thus enhancing the therapeutic effect. Particularly, this review describes a state-of-the-art strategy for fabricating MB-based delivery platforms and their use with ultrasound in brain cancer therapy.

Responses to Drought Stress in Poplar: What Do We Know and What Can We Learn?

Poplar (Populus spp.) is a high-value crop for wood and biomass production and a model organism for tree physiology and genomics. The early release, in 2006, of the complete genome sequence of P. trichocarpa was followed by a wealth of studies that significantly enriched our knowledge of complex pathways inherent to woody plants, such as lignin biosynthesis and secondary cell wall deposition. Recently, in the attempt to cope with the challenges posed by ongoing climate change, fundamental studies and breeding programs with poplar have gradually shifted their focus to address the responses to abiotic stresses, particularly drought. Taking advantage from a set of modern genomic and phenotyping tools, these studies are now shedding light on important processes, including embolism formation (the entry and expansion of air bubbles in the xylem) and repair, the impact of drought stress on biomass yield and quality, and the long-term effects of drought events. In this review, we summarize the status of the research on the molecular bases of the responses to drought in poplar. We highlight how this knowledge can be exploited to select more tolerant genotypes and how it can be translated to other tree species to improve our understanding of forest dynamics under rapidly changing environmental conditions.

Instability of bubble expansion at zero temperature

In the context of false vacuum decay at zero temperature, it is well known that bubbles expand with uniform proper acceleration. We show that this uniformly accelerating expansion suffers from an instability related to the bubble size. This can be observed in Minkowski spacetime as a tachyonic mode in the spectrum of fluctuations for the energy functional in the reference frame in which the uniformly accelerating bubble wall appears static. In such a frame, arbitrary small perturbations cause an amplifying departure from the static wall solution. This implies that the nucleated bubble is not a critical point of the energy functional in the rest frame of nucleation but becomes one in the accelerating frame. The aforementioned instability for vacuum bubbles can be related to the well-known instability for the nucleated critical static bubbles during finite-temperature phase transitions in the rest frame of the plasma. It is therefore proposed that zero-temperature vacuum decays as seen from accelerating frames have a dual description in terms of finite-temperature phase transitions.

Resonance behaviors of encapsulated microbubbles oscillating nonlinearly with ultrasonic excitation

The resonance behaviors of a few lipid-coated microbubbles acoustically activated in viscoelastic media were comprehensively examined via radius response analysis. The size polydispersity and random spatial distribution of the interacting microbubbles, the rheological properties of the lipid shell and the viscoelasticity of the surrounding medium were considered simultaneously. The obtained radius response curves present a successive occurrence of linear resonances, nonlinear harmonic and sub-harmonic resonances with the acoustic pressure increasing. The microbubble resonance is radius-, pressure- and frequency-dependent. Specifically, the maximum bubble expansion ratio at the main resonance peak increases but the resonant radius decreases as the ultrasound pressure increases, while both of them decrease with the ultrasound frequency increasing. Moreover, compared to an isolated microbubble case, it is found that large microbubbles in close proximity prominently suppress the resonant oscillations while slightly increase the resonant radii for both harmonic and subharmonic resonances, even leading to the disappearance of the subharmonic resonance with the influences increasing to a certain degree. In addition, the results also suggest that both the encapsulating shell and surrounding medium can substantially dampen the harmonic and subharmonic resonances while increase the resonant radii, which seem to be affected by the medium viscoelasticity to a greater degree rather than the shell properties. This work offers valuable insights into the resonance behaviors of microbubbles oscillating in viscoelastic biological media, greatly contributing to further optimizing their biomedical applications.

Numerical Simulations of Wettability on Anodic Bubble Expansion in Aluminum Electrolysis Cell Using Lattice Boltzmann Method

Numerical Simulations of Wettability on Anodic Bubble Expansion in Aluminum Electrolysis Cell Using Lattice Boltzmann Method

Mechanism of enhanced phase-change process on structured surface: Evolution of solid-liquid-gas interface

Bubble behaviors in nucleate boiling have been continuously concerned, traditional experiment believes that micro/nano structures can improve the bubble behaviors by increasing the capillary force to enhance the heat efficiency of nucleate boiling. However, due to the scale limitation of the macro experiment, the evolution of the solid-liquid-gas interface (three-phase contact line) during the bubble formation and expansion on the structured surface is still not clear. Therefore, a three-phase contact line evolution model with a groove structure was constructed in this work. Results showed that the heat transfer performance of the structured surface was better than the smooth surface even though the heating area was the same and the structure was adiabatic. The three-phase contact line appeared on the structured surface but did not appear on the smooth surface when the superheat was low, which was due to the structure hindered the heat convection inner the liquid and enhanced the heat accumulation in a smaller region, then enhanced the evaporation. It was also found that structures were conducive to the liquid transport from the unheated area to the heating area, which hindered the expansion of the three-phase contact line and improved the evaporation of the microlayer that surrounded the three-phase contact line. These phenomena and mechanisms will contribute to explaining why some micro/nano structures can improve the expansion bubbles and reduce the departure radius, and can provide a more effective theoretical basis for designing the surface with the nanostructure.

Interferometric Fiber Optic Probe for Measurements of Cavitation Bubble Expansion Velocity and Bubble Oscillation Time.

Cavitation bubbles are used in medicine as a mechanism to generate shock waves. The study of cavitation bubble dynamics plays a crucial role in understanding and utilizing such phenomena for practical applications and purposes. Since the lifetime of cavitation bubbles is in the range of hundreds of microseconds and the radii are in the millimeter range, the observation of bubble dynamics requires complicated and expensive equipment. High-speed cameras or other optical techniques require transparent containers or at least a transparent optical window to access the region. Fiber optic probe tips are commonly used to monitor water pressure, density, and temperature, but no study has used a fiber tip sensor in an interferometric setup to measure cavitation bubble dynamics. We present how a fiber tip sensor system, originally intended as a hydrophone, can be used to track the expansion and contraction of cavitation bubbles. The measurement is based on interference between light reflected from the fiber tip surface and light reflected from the cavitation bubble itself. We used a continuous-wave laser to generate cavitation bubbles and a high-speed camera to validate our measurements. The shock wave resulting from the collapse of a bubble can also be measured with a delay in the order of 1 µs since the probe tip can be placed less than 1 mm away from the origin of the cavitation bubble. By combining the information on the bubble expansion velocity and the time of bubble collapse, the lifetime of a bubble can be estimated. The bubble expansion velocity is measured with a spatial resolution of 488 nm, half the wavelength of the measuring laser. Our results demonstrate an alternative method for monitoring bubble dynamics without the need for expensive equipment. The method is flexible and can be adapted to different environmental conditions, opening up new perspectives in many application areas.

Flow‐To‐Fracture Transition of Linear Maxwell‐Type Versus Yield Strength Fluids by Air Injection—Implications for Magma Fracturing

AbstractTo illuminate brittle and ductile fracturing of magma, we investigated bubble expansion and fracturing in two contrasting fluids: a Maxwell‐type viscoelastic fluid and a Bingham‐type yield‐strength fluid. Measurements of the complex shear modulus, G′ + iG″ (i is the imaginary unit), under small‐strain oscillation showed that both fluids are elastic (G′ &gt; G″) with similar rigidity. Viscous behavior (G′ &lt; G″) appeared at lower frequency in the Maxwell fluid but at larger strain in the Bingham fluid. When we injected air into the Maxwell fluid, bubbles expanded viscously at low flux, fractured in a brittle manner at high flux, and behaved transitionally at intermediate flux. In contrast, we observed no fracturing in the Bingham fluid. This demonstrates that the G′ &gt; G″ condition is insufficient to infer that brittle fracturing can occur. Brittle fracturing of the Maxwell fluid occurred not at a critical strain rate but under decreasing strain rate and increasing stress.

Growth of elongated vapor bubbles during flow boiling heat transfer in wavy microchannels

Flow boiling within microchannels is an efficient heat removal mechanism by utilizing the latent heat of the fluid. Numerical simulations can resolve the small temporal and spatial scales of the vapor bubbles in the flow boiling process, thus accessing many details of the flow which is unattainable experimentally. Most of the previous studies of flow boiling heat transfer used straight microchannels, even though curved microchannels have potential to further enhance the heat transfer performance. In this study, the flow boiling heat transfer in wavy microchannels is studied via numerical simulation by focusing on the growth of the vapor bubbles. Straight and wavy microchannels are considered for comparison and analysis. The numerical simulations are conducted using the finite volume method to solve the flow and the heat transfer, and the volume of fluid (VOF) method to capture the interface dynamics. The effects of key dimensionless numbers on bubble growth rate and wall heat transfer are quantified, including the Weber number, the Capillary number, and the Jakob number. The results indicate that the wavy structure of the microchannel has a strong effect on the bubble growth and the wall heat transfer in microchannels. As the waviness of the microchannel increases, the bubble grows faster because of the bubble deformation by the wavy channel, the bubble moves faster due to the bubble expansion, and the heat transfer is enhanced by the perturbation of the bubble to the flow. With the large velocity perpendicular to the channel flow direction induced by the waviness, the local Nusselt number of the wavy channel could be high up to 2.6 times higher than the straight channel.

Hydrodynamic backreaction force of cosmological bubble expansion

As a promising probe for the new physics beyond the standard model of particle physics in the early Universe, the predictions for the stochastic gravitational wave background from a cosmological first-order phase transition heavily rely on the bubble wall velocity determined by the bubble expansion dynamics. The bubble expansion dynamics is governed by the competition between the driving force from the effective potential difference and the backreaction force from a sum of the thermal force and friction force induced by the temperature jumping and out-of-equilibrium effects across the bubble wall, respectively. In this paper, we propose a hydrodynamic evaluation on this total backreaction force for a non-runaway steady-state bubble expansion, which, after evaluated at the wall interface, exactly reproduces the pressure difference $\Delta_\mathrm{wall}[(\bar{\gamma}^2-1)w]$ obtained previously from the junction condition of the total energy-momentum tensor at the wall interface, where $w$ is the enthalpy and $\bar{\gamma}\equiv(1-\bar{v}^2)^{-1/2}$ is the Lorentz factor of the wall-frame fluid velocity $\bar{v}$.

Simulation of dynamic process of double bubble coupled acoustic cavitation

In order to simulate the acoustic cavitation process of double-bubble coupling, based on the fluid dynamics governing equation and fluid volume fraction model, this paper proposes a three-dimensional finite element simulation model of double-bubble coupled ultrasonic cavitation in Fluent software, and numerically simulates the dynamic process of double-bubble coupled acoustic cavitation in fluid driven by ultrasonic wave. The nonlinear dynamic characteristics of coupled acoustic cavitation with double bubbles are studied by evaluating the variation of the acoustic field around the cavitation bubble. The results indicate that under the ultrasonic drive, the spherical bubbles gradually widen first to the maximal radius, then shrink swiftly, and finally collapse. There is an interaction force between the paired double bubbles, which inhibits the expansion of cavitation bubbles and increases the bubble contraction time. The ability of the cavitation bubble to converse the energy is strengthened in the contraction stage. Compared with the acoustic cavitation of the single bubble, the pressure inside the bubble is expanded when the coupled double bubble collapses. The analysis results in this paper will provide the reference for the dynamic process simulation of the ultrasonic cavitation bubbles.

The Current Situation and Differences of the Real Estate Bubble between China and the United States

With the continuous expansion of real estate bubbles in China and the United States, the two countries' governments have introduced various policies according to their respective national conditions to prevent the bursting of the real estate bubbles. This article describes how the real estate bubbles in China and the United States formed, the status quo, and the differences between the real estate bubbles in China and the United States. Because the economic systems and management methods of China and the United States are very different, studying the differences in real estate bubbles between China and the United States has excellent enlightenment and significance for China.

Impact of a suspension drop onto a hot substrate: diminution of splash and prevention of film boiling.

In the present study, the effect of graphite lubricant additives on the dynamics of a single drop impact onto a heated surface has been investigated in the nucleate boiling and thermal atomization regimes. In the nucleate boiling regime the drop impact is accompanied by the nucleation and expansion of multiple vapor bubbles. The drop residence time at the substrate is determined by the time of its mass loss due to splash and evaporation. At higher temperatures, above the Leidenfrost point, impact may lead to drop rebound. In this experimental and theoretical study the effect of additives on the outcome of drop impact, in particular, the addition of solid graphite particles, is investigated. The residence time of the drop has been measured for various initial drop temperatures and suspension concentrations. The addition of the particles leads to some increase of the residence time, while its dependence on the substrate temperature follows the scaling relation obtained in the theory. Moreover, the presence of the particles in the drop leads to suppression of splash and a significant increase of the drop rebound temperature, which is often associated with the Leidenfrost point. These effects are caused by the properties of the deposited layer, and pinning of the contact line of the entire drop and of each vapor bubble, preventing bubble coalescence and drop rebound. The phenomena are also explained by a significant increase of the liquid viscosity caused by the evaporation of the bulk liquid at high wall temperatures.

EXPERIMENTAL STUDY OF THE INTERACTION BETWEEN A BUBBLE AND A POROUS PLATE

This paper aims to experimentally investigate the interaction between a bubble and a plate with multiple holes. The bubble is generated under the plate by underwater electric discharge method, and the top surface of the plate is flush with the water-air interface. In order to reveal the bubble dynamics and the droplets formation caused by bubble oscillation, the interaction process is recorded by high-speed photography. Similar to the interaction between bubble and plate with a single hole, there exist two jets, the primary slow jet and the secondary fast jet, caused by bubble expansion and collapse, respectively. The former starts to be pumped out as liquid columns and breaks up into fine droplets over time, while the latter erupts directly as droplets with more instability. As the distance between bubble and porous plate decreases, the speed of droplets increases and the velocity difference in droplets through different holes becomes greater. Besides, with the longer oscillation time the bubble is less likely to detach from the plate, and its maximum volume will be larger than expected. The effects of plate thickness and hole diameter on bubble dynamics and droplets formation are also investigated. Increasing the plate thickness and decreasing the hole diameter make the porous plate have a similar effect as the rigid wall. In this case, the bubble tends to migrate to the plate and inside the bubble, a jet toward the solid boundary will be generated. The amount of liquid being squeezed out through holes is reduced and the liquid columns are more likely to break up into sprays. In contrast, decreasing the plate thickness and increasing the hole diameter enhance the effect of free liquid on the bubble dynamics. Also, the efficiency of the oscillating bubble to pump liquids has increased.