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Flow-induced noise is closely related to the flow characteristics through an adiabatic capillary tube and a transition pipe, most existing methods for suppressing flow-induced noise are passive. An aerating technique is proposed based on the pressure feedback in the transition pipe to actively suppress flowinduced noise. The 3-D simulations of flashing are presented by performing the Schnerr-Sauer cavitation model coupling with the mixture model. For the turbulence model, the large eddy simulation approach is used. With the installation of aerating module, the pressure fluctuation in the transition pipe is weakened significantly, and the phenomenon of bubble collapse is suppressed. Numerical results illustrate that the transient pressure of the monitoring points downstream of the capillary outlet oscillates seriously due to the bubble bursting. The shedding process of the bubble group is observed according to the vaporliquid interface in the transition pipe. In addition, the oscillations of monitored transient pressure are suppressed with the application of aerating module. Then the noise source can be partially reduced actively in essence. This paper is devoted to understanding the two-phase flow characteristics of refrigerants in a transition pipe and presents a practical method to suppress noise near the capillary outlet. [ABSTRACT FROM AUTHOR]

It has been reported that the addition of microbubbles in a water flow enhances a cleaning efficiency of oil adhering to a channel wall. However, the enhancement mechanism of the cleaning effect has not been sufficiently understood yet. Experiments on the removal effects of water flows in a horizontal channel with and without microbubbles were therefore conducted to investigate the removal process by microbubbles. Silicone oil adhering to the bottom wall of the channel was used as dirt. The local thickness of the oil was measured during cleaning from micrographs of fluorescence intensity. The amount of oil removed by the water flow increased with increasing the Reynolds number. Adding microbubbles to the water flow increased the amount of removed oil. Microbubbles attached to the oil interface under turbulent conditions, and the bubbles grew by mass transfer through the gas-liquid interface and bubble coalescence. The detachment of bubbles grown on the oil interface led to oil peel-off, resulting in an improvement of cleaning effect. [Display omitted] • The thickness of adhesion oil was measured using laser-induced fluorescence. • The presence of microbubbles was confirmed to enhance oil removal. • The removal rate was modeled as a superposition of shear and bubble effect terms. • Microbubbles adhering to oil can be grown by mass transfer and induce oil peeling. [ABSTRACT FROM AUTHOR]

For potential use in wastewater management and health control, this study investigates the disinfection effectiveness of bulk ultrafine bubbles (UFBs) with different bubble number densities and solution pH. Initially, neutral UFB solutions with different bubble concentrations were mixed with E. coli suspension for 120 min, but these solutions did not achieve sterilization. The bubble number density did not affect the disinfection ability of the neutral solution. Next, the pH of the UFB solutions was fixed at 5, 7, and 9. When mixed with E. coli suspension, the acidic UFB solutions reduced the colony counts by 12% after 30 min of cultivation and by 66% after 60 min of cultivation. The colony counts increased slightly in neutral and significantly in alkaline UFB solutions. The acidic UFB solutions had lower zeta potentials and smaller number densities after cultivation, implying that the number density reduced through bubble coalescence rather than increased by bubble collapse. Additionally, the UFBs exhibited insignificant fluorescence intensity, suggesting that the colony counts increased by generated ∙OH radicals. This study revealed that the effect of UFB on E. coli significantly depends on the solution pH. Further, an acidified solvent achieves a bactericidal effect, whereas a neutral or alkaline solvent enhances the growth effect. This result is important when using actual wastewater. [ABSTRACT FROM AUTHOR]

Visualization experiments and numerical simulations of two-phase flow are conducted to study the pressure fluctuation characteristics of oil-gas flow in horizontal pipes. The two-fluid model based on the Eulerian–Eulerian method is adopted, and the bubble number density equation (BNDE) is introduced to the simulation to predict the bubble size and distribution within the pipe. The bubble size and pressure variations in the pipe obtained from the simulations agree well with the recorded values from the experiments. The fast Fourier transform (FFT) algorithm is used to analyze the characteristics of pressure fluctuations, and the results show that the sudden pressure increase in the pipe sections is related to gas injection. The bubble number density increases with liquid flow rate (Ql), which causes the oil-gas flow to be more turbulent while increasing the amplitude of high-frequency fluctuations. The maximum amplitude for the dominant frequency is observed near the pump inlet for low liquid flow rates. At high liquid flow rates, more liquid vortices are found near the gas orifice, and there is a maximum amplitude for the dominant frequency in this section. Due to the high swirling strength at larger inlet gas volume fraction (IGVF), there is an obvious increase in the amplitude of low-frequency fluctuations, while the amplitude of high-frequency fluctuations is nearly the same under all IGVF. [ABSTRACT FROM AUTHOR]

For potential use in wastewater management and health control, this study investigates the disinfection effectiveness of bulk ultrafine bubbles (UFBs) with different bubble number densities and solution pH. Initially, neutral UFB solutions with different bubble concentrations were mixed with E. coli suspension for 120 min, but these solutions did not achieve sterilization. The bubble number density did not affect the disinfection ability of the neutral solution. Next, the pH of the UFB solutions was fixed at 5, 7, and 9. When mixed with E. coli suspension, the acidic UFB solutions reduced the colony counts by 12% after 30 min of cultivation and by 66% after 60 min of cultivation. The colony counts increased slightly in neutral and significantly in alkaline UFB solutions. The acidic UFB solutions had lower zeta potentials and smaller number densities after cultivation, implying that the number density reduced through bubble coalescence rather than increased by bubble collapse. Additionally, the UFBs exhibited insignificant fluorescence intensity, suggesting that the colony counts increased by generated ∙OH radicals. This study revealed that the effect of UFB on E. coli significantly depends on the solution pH. Further, an acidified solvent achieves a bactericidal effect, whereas a neutral or alkaline solvent enhances the growth effect. This result is important when using actual wastewater.

The eruption of the Tambora volcano in 1815 was initiated by two precursory Plinian falls and formed two generations of pyroclastic density current (PDC) deposits. In this study, we found slight changes in phenocrysts (modal mineralogy, content, and size), bulk-rock and feldspar microlite chemical compositions, and bubble and microlite number densities through the stratigraphic position. Plinian fall units are characterized by a lower phenocryst abundance (avg. of 5.1%), smaller phenocryst size (avg. of 0.06 mm2), and higher silica content (bulk-rock, 58–58.5 wt.%). The PDC deposits are characterized by a relatively higher crystal abundance (avg. of 12.1%), larger phenocryst sizes (avg. of 0.13 mm2), and lower silica content (bulk-rock, 56.7–57.9 wt.%). Therefore, the deposit stratigraphy and analyses suggest that phenocryst stratification in the magma chamber was established prior to the 1815 eruption and was thus responsible for yielding a slight contrast in bulk compositions. Feldspar microlite moves toward slightly more albitic compositions from Plinian falls to the PDCs, suggesting a slight decrease in the initial melt temperature from the upper to the lower magma chamber portion. Because the Plinian eruptions extracted the hottest magma, the degree of supercooling became low, and consequently yielded microlite-poor juveniles. By contrast, the PDCs experienced a larger degree of supercooling because the temperature was relatively low, thus yielding microlite-rich juveniles. Moreover, such temperature stratification coupled with the evidence of homogeneous melt composition (58.5–58.9 wt.% SiO2) and the minor evidence of crystal mush (at most 27%) might suggest that the Tambora case is still in the early stage of magmatic evolution under cooling from the surrounding rocks. [ABSTRACT FROM AUTHOR]

Mass transfer has been identified as a major bottleneck in gas fermentation and microbial conversion of carbon dioxide to chemicals. We present a pragmatic and validated Computational Fluid Dynamics (CFD) model for mass transfer in bioelectrochemical systems. Experiments were conducted to measure mixing times and mass transfer in a Duran bottle and an H‐cell. An Eulerian–Eulerian framework with a simplified model for the bubble size distribution (BSD) was developed that utilized only one additional equation for the bubble number density while including the breakup and coalescence. Validations of the CFD model for mixing times showed that the predictions were within the confidence intervals of the measurements, verifying the model's capability in simulating the hydrodynamics. Further validations were performed using constant and varying bubble diameters for the mass transfer. The results showed the benefits of a simplified BSD model, as it yielded improvements of seven and four times in accuracy when assessed against the experimental data for the Duran bottle and H‐cell, respectively. Modeling of the H‐cell predicted that a lower stirring rate improves mass transfer compared with higher stirring rates, which is of great importance when designing microbial cultivation processes. The model offers a feasible framework for advanced modeling of gas fermentation and microbial electrosynthesis. [ABSTRACT FROM AUTHOR]

Recent researches show that the pressure increment of a multiphase pump is affected by bubble size and distribution. In order to study the bubble distribution characteristics in such pumps, a novel approach describing the variable bubble size in the pump is proposed. The bubble number density equation, which has taken into account the phenomena of break-up and coalescence, is introduced into the flow simulation, and the drag coefficient is revised because of the interaction of multiple bubbles. The reliability of the approach is verified by comparison with the experiment. It was established that the bubbles move to the impeller hub due to the difference in centrifugal force between gas and liquid. Despite the high collision rate near the hub, bubble size changes little with the stirring action of the impeller. The mixture flows in a disorderly way and the bubble diameter increases due to the rotor–stator interaction. Owing to the increasing flow area in the diffuser, bubbles move to the mainstream region, and bubble size reaches its maximum owing to the flow separation near the hub. The distribution of bubbles is also analyzed under a different inlet gas volume fraction (IGVF) and inlet bubble diameter (d0). Bigger IGVF brings about a higher collision rate of bubbles, while smaller d0 makes the diffusion of bubbles easier.

Volcanism accompanies transfer of volatiles such as H2O and CO2 from the Earth interior to the atmosphere. The underlying process for transfer of the volatiles is magma degassing. As magma rises toward the Earth surface, the volatiles in silicate melts become supersaturated due to decompression of magma so that nucleation of volatile-bearing bubbles and subsequent growth of the bubble occur. In this study, bubble nucleation rate of water in rhyolite melts above 1000 K, which determines the style of the eruption of the magma, is calculated by using molecular cluster model. Calculation results indicate that the homogeneous nucleation of water in the rhyolite melts is possible when the supersaturation pressure exceeds 110 MPa and the number density of bubbles formed increases as increase in the degree of the supesaturation. Growth of bubble formed from the critical cluster was also calculated and the results were compared with observed bubble sizes. When the bubble number density is greater than 1015/m3, depletion of the concentration of volatiles hinders further growth of bubbles. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of bubbly flows at relatively high void fractions using an advanced image processing method. Bubble overlapping is a common problem in such flows and the past studies often treat the overlapping bubbles as a whole, which introduces considerable measurement uncertainties. In this study, a hybrid method combining intersection point detection and watershed segmentation is used to separate the overlapping bubbles. In order to reconstruct bubbles from separated segments, a systematic procedure is developed which can preserve more features captured in the raw image compared to the simple ellipse fitting method. The distributions of void fraction, interfacial area concentration, number density and velocity are obtained from the extracted bubble information. High-speed images of air-water bubbly flows are acquired and processed for eight test runs conducted in a 30 mm × 10 mm rectangular channel. The developed image processing scheme can effectively separate overlapping bubbles and the results compare well with the measurements by the gas flow meter and double-sensor conductivity probe. The development of flows in transverse and mainstream directions are analyzed and compared with the prediction made by the one-dimensional interfacial area transport equation (IATE) and the bubble number density transport equation. [ABSTRACT FROM AUTHOR]

This paper presents a robust image processing technique for bubbly flow measurement over a wide range of void fractions. The proposed algorithm combines geometrical, optical and topological information recorded with high speed cameras to separate and reconstruct the overlapping bubbles. The common difficulties such as overlapping, irregular bubble shape, surface deformation and large clustering in digital image processing are solved by combining different information based on a preset decision table and flow chart. Test with synthetic bubble images is performed to evaluate the reliability of the algorithm and quantify the uncertainty of the data. The result shows that the proposed algorithm can accurately measure bubbly flows with void fraction up to 18% for large bubbles. Four runs of bubbly flow images in a 30 mm × 10 mm rectangular channel are then recorded by three high speed cameras. The area-averaged void fraction of these test runs range from 2.4% to 9.1%. The axial and lateral distributions of bubble number density are obtained by the present algorithm for studying the characteristics of these flows. [ABSTRACT FROM AUTHOR]

Melt degassing by bubble nucleation and growth is a driving mechanism of magma ascent. Therefore, decompression experiments with hydrous silicate melts were used to investigate the onset and the dynamics of H2O degassing. Nominally H2O-undersaturated trachytic Campi Flegrei and phonolitic Vesuvius melts representative for the magma compositions of the Campi Flegrei volcanic system were decompressed at a super-liquidus temperature of 1050 °C from 200 MPa to final pressures (Pfinal) of 100, 75, and 60 MPa using continuous decompression rates of 0.024 and 0.17 MPa/s. Experiments started from either massive glass cylinders or glass powder to demonstrate the influence of the starting material on melt degassing. Glass powder can be used to shorten the equilibration time (teq) prior to decompression for dissolution of H2O in the melt. The decompressed samples were quenched and compared in terms of bubble number density (NV), porosity, and residual H2O content in the melt. Decompression of all glass cylinder samples led to homogeneous bubble nucleation with high NV of ~105 mm-3. The supersaturation pressures for homogeneous bubble nucleation were estimated to be <76 MPa for the trachytic and <70 MPa for the phonolitic melt. In contrast to glass cylinders, the usage of glass powder equilibrated for 24 h before decompression prevented homogeneous bubble nucleation during decompression. We suggest that trapped air in the powder pore space resulted in the formation of tiny H2O-N2 bubbles throughout the samples prior to decompression. Degassing of these glass powder samples was facilitated by diffusive growth of these pre-existing bubbles and thus did not require significant H2O supersaturation of the melt. This is evidenced by several orders of magnitude lower NV and lower residual H2O contents at correspondingly higher porosities compared to the glass cylinder samples. However, a significant extension of teq to 96 h in the glass powder experiments led to degassing results comparable to the glass cylinder samples. This effect is probably due to Ostwald ripening, coalescence, and the ascent of the pre-existing bubbles during the extended teq prior to decompression. The NV of the glass cylinder samples were used to test the applicability of the vesiculation model provided by Toramaru (2006). For the applied decompression rates, the experimental NV are up to 5 orders of magnitude higher than the values predicted by the model. This may be mainly attributed to the usage of the macroscopic surface tension and the total H2O diffusivity in the model to describe the molecular process of bubble nucleation. A significant increase in modeled NV can be achieved by application of a reduced surface tension in combination with the lower diffusivity of network formers as a limiting parameter for the formation of a bubble nucleus. This study demonstrates that the investigation of homogeneous bubble nucleation necessitates an optimized experimental protocol. We strongly recommend performing experiments with massive glass cylinders as starting material. The timescale of decompression is a limiting parameter and must be short enough to minimize the opportunity for a reduction of NV by bubble coalescence. Considering our comparably high NV, the samples of many previous experimental studies that were used to calibrate models for homogeneous bubble nucleation were probably subject to significant NV reduction. Newly derived data from optimized experiments will require improved models for homogeneous bubble nucleation during magma ascent. [ABSTRACT FROM AUTHOR]

Despite increasing recognition of silicic pumice-bearing deposits in the deep marine environment, the processes involved in explosive silicic submarine eruptions remain in question. Here we present data on bubble sizes and number densities (number of bubbles per unit of melt matrix) for deep submarine-erupted pumices from three volcanoes (Healy, Raoul SW and Havre) along the Kermadec arc (SW Pacific) to investigate the effects of a significant (>~1 km) overlying water column and the associated increased hydrostatic pressure on magma vesiculation and fragmentation. We compare these textural data with those from chemically similar, subaerially-erupted pyroclasts from nearby Raoul volcano as well as submarine-erupted ‘Tangaroan’ fragments derived by non-explosive, buoyant detachment of foaming magma from Macauley volcano, also along the Kermadec arc. Deep submarine-erupted pumices are macroscopically similar (colour, density, texture) to subaerial or shallow submarine-erupted pumices, but show contrasting microscopic bubble textures. Deep submarine-erupted pyroclasts have fewer small (< 10 μm diameter) bubbles and narrower bubble size distributions (BSDs) when compared to subaerially erupted pyroclasts from Raoul (35–55 μm vs. 20–70 μm range in volume based median bubble size, respectively). Bubble number density (BND) values are consistently lower than subaerial-erupted pyroclasts and do not display the same trends of decreasing BND with increasing vesicularity. We interpret these textural differences to result from deep submarine eruptions entering the water column at higher pressures than subaerial eruptions entering the atmosphere (~ 10 MPa vs. 0.1 MPa for a vent at 1000 mbsl). The presence of an overlying water column acts to suppress rapid acceleration of magma, as occurs in the upper conduit of subaerial eruptions, therefore suppressing coalescence, permeability development and gas loss, amounting to closed-system degassing conditions. The higher confining pressure environment of deep submarine settings hinders extensive post-fragmentation clast expansion, coalescence of bubbles, and thinning of bubble walls, causing clasts to have similar BND values regardless of their vesicularity. Although deep submarine-erupted pyroclasts are closely similar to their subaerial counterparts on the basis of bulk vesicularities and macroscopic appearance, they differ markedly in their microscopic textures, allowing them to be fingerprinted in modern and ancient pumiceous marine sediments. [ABSTRACT FROM AUTHOR]

The cavitation dynamics of cryogenic fluids are substantially impacted by the thermodynamic effects since these fluids are generally operated close to the critical point and the vapor pressure assumes strong dependence on the temperature. Developing a robust numerical methodology to model the rich physics involved in this phenomenon remains a challenging problem. This paper tries to develop an effective computational strategy to simulate cryogenic cavitation by implementing the “Schnerr–Sauer cavitation model”, coupled with the energy equation. Numerical simulations of cavitation are presented for flows over a two dimensional (2-D) hydrofoil and an axisymmetric ogive. Predicted temperature and pressure depressions within the cavity are compared with measurements by Hord et al. in NASA for liquid hydrogen and nitrogen. Specifically, the global sensibility of the cavitation solution with respect to bubble number density is investigated in detail. The Schnerr–Sauer cavitation model with corrected nuclei density provides solutions with comparable accuracy to the quasi-steady sheet cavitation in cryogenic fluids for the two geometries. [ABSTRACT FROM AUTHOR]

• An approach to simulate bubbly flows using particle-center-averaging is formed. • A physically motivated wall-contact force model is introduced. • Particle-center-averaging recovers force consistency in the Euler-Euler model. • Particle-center-averaging provides mesh independent solutions. • Experimental data for bubbly pipe flows (MTLoop) are used in validation. The standard Euler-Euler model is based on the phase-averaging method and each bubble force is a function of the local gas volume fraction. As a result, the coherent motion of each bubble as a whole is not enforced when the bubble diameter is larger than the size of a computational cell. However, the bubble force models are typically developed by tracking the bubbles' centers of mass and assuming that the forces act on these locations. In simulations, this inconsistency can lead to a nonphysical gas concentration in the center or near the wall of a channel when the bubble diameter is larger than the cell size. Besides, a mesh-independent solution may not exist in such cases. In the present contribution, a particle-center-averaging method is used to average the solution variables for the disperse phase, which allows to represent the bubble forces as forces that act on the bubbles' centers of mass. An Euler-Euler approach for bubbly flow simulation is formed by combining this averaging method with a Gaussian convolution method to represent the spatial extent of the bubbles. The remediation of the inconsistency in the standard Euler-Euler model by the particle-center-averaging method is demonstrated using a simplified two-dimensional test case. The test results illustrate that the particle-center-averaging method can recover the bubble force consistency and provide mesh independent solutions. Furthermore, a comparison of the standard Euler-Euler model and the particle-center-averaged Euler-Euler model is shown for several bubbly pipe flow cases where experimental data are available. The results show that the particle-center-averaging method can alleviate the over-prediction of the gas volume fraction peaks for wall-peaking and finely dispersed flow cases. The gas velocities simulated by both approaches are similar. [ABSTRACT FROM AUTHOR]

Micro-bubbles are in use in many industrial fields such as water treatment, purification of lake water, chemical engineering, washing processes and housing equipment by virtues of their large interfacial area concentration and long residence time in liquid. A pressurized dissolution method based on decompression of liquid with dissolved gas is one of promising methods for generating fine micro-bubbles at high number density. Since the mechanism of micro-bubble generation is not clarified yet, design and improvement of micro-bubble generators are based on trial and error. In this study, effects of liquid volume flux at a decompression nozzle and dissolved gas concentration in the upstream region of the nozzle on diameter and number density of generated micro-bubbles are examined to understand generation mechanism of micro-bubbles in a pressurized dissolution method. The diameter and the number density of micro-bubbles are measured by using phase Doppler anemometry (PDA) in the downstream region of the nozzle, and the flow patterns in the nozzle are visualized by using a high-speed camera. The experimental results show that diameter and number density of generated micro-bubbles depend on cavitation pattern at the nozzle, and that cavitation bubbles containing not only vapor but also non-condensable gas become micro-bubbles due to their shrink caused by condensation of the vapor in the downstream region of the nozzle. [ABSTRACT FROM AUTHOR]

Highlights: [•] Bubble number density and bubble size under rolling motion are studied. [•] Automatic processing algorithm dealing with bubble parameters is proposed. [•] The fluctuation amplitude is mainly affected by rolling amplitude. [•] Variation of the effective wall superheat is the main reason for the fluctuation. [ABSTRACT FROM AUTHOR]

Abstract: Explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation, even during a single eruptive event. The increasing amount of fine pyroclasts is often uncritically related to explosive magma–water interaction (i.e., hydromagmatic fragmentation). Here we report examination of two examples of major explosive eruptions from the Quaternary Vulsini Volcanic District (central Italy), in which the fine-grained nature of deposits, even in near-vent settings, indicates negligible effect of transport and implies the eruption of highly fragmented magmas. SEM morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation. We apply a recently developed Stereo-Scanning Electron Microscopy (SSEM) technique (Proussevitch et al., 2011) to determine vesicularity features (e.g., bubble size distribution and bubble number density; hereafter BSD and BND, respectively) of ash particles. SSEM analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash-rich eruptions. We conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs. [Copyright &y& Elsevier]

Abstract: Subcooled flow boiling is a commonly applied technique for achieving efficient heat transfer. Although numerous works have been done on two-phase flow phenomena, most of these works focus on land-based channels. Therefore, for identifying the heat transfer phenomena in the barge-mounted system, it is necessary to know the local flow condition in an oscillating acceleration field. In this study, forced convection subcooled water boiling experiments are conducted in narrow rectangular channels at low frequency oscillations. The bubble size, bubble velocity and bubble number density have been statistically analyzed under different heaving conditions and at different flow rates. The results of the bubble size distribution have been presented as cumulative distribution functions, which exhibit in reality a very wide spread of bubble size. The results show that an increase of the oscillation frequency causes an increase fluctuation of bubble size, bubble velocity and bubble number density. Under the same heaving condition, an increase of mass flow rate leads to a decrease fluctuation of bubble size and bubble velocity. A correlation has been sought for the fluctuation of bubble diameter due to heaving motion. The proposed model agrees well with the experimental data within the averaged relative deviation of ±19.2%. [Copyright &y& Elsevier]

Abstract: The relationship between injection pressure fluctuations and unsteady cavitation processes inside the high-pressure diesel nozzle holes have been numerically analyzed by using a two-fluid approach. In order to improve prediction of nozzle hole cavitation content under high-pressure injection conditions, a new adjustment model of bubble number density has also been developed through the analysis of cavitation bubble dynamics and internal flow characteristics of nozzle hole. Model validation results verify that this model is applicable for a wide range of diesel injection pressures. Based on simulation results, it has been found that cavitation bubbles in recirculation zone and its wake flow show totally different responses to the variations of upstream pressure, and the evolution of cavitation content shows a close association with the time derivatives of upstream pressure. [Copyright &y& Elsevier]

Decompression experiments of a crystal-free rhyolitic liquid with ≈ 6.6 wt. % H2O were carried out at a pressure range from 250 MPa to 30–75 MPa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics, especially on the bubble number density (BND, the number of bubbles produced per unit volume of liquid). A first series of experiments at 800°C and fast decompression rates (10–90 MPa/s) produced huge BNDs (≈ 2 × 1014 m−3 at 10 MPa/s ; ≈ 2 × 1015 m−3 at 90 MPa/s), comparable to those in natural silicic pumices from Plinian eruptions (1015–1016 m−3). A second series of experiments at 700°C and 1 MPa/s produced BNDs (≈ 9×1012 m−3) close to those observed at 800°C and 1 MPa/s (≈ 6 × 1012 m−3), showing that temperature has an insignificant effect on BNDs at a given decompression rate. Our study strengthens the theory that the BNDs are good markers of the decompression rate of magmas in volcanic conduits, irrespective of temperature. Huge number densities of small bubbles in natural silicic pumices from Plinian eruptions imply that a major nucleation event occurs just below the fragmentation level, at which the decompression rate of ascending magmas is a maximum (≥ 1 MPa/s). [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) . The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. “Transient” cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, “stable” cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble–bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both “stable” and “transient” cavitation bubbles which pulsate nonlinearly with the period of ultrasound. [Copyright &y& Elsevier]

Abstract: Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate. [Copyright &y& Elsevier]

Abstract: A new analytical technique for the statistical analysis of bubble populations in volcanic rocks [Proussevitch, A.A., Sahagian, D.L. and Tsentalovich, E.P., 2007-this issue. Statistical analysis of bubble and crystal size distributions: Formulations and procedures. J. Volc. Geotherm. Res.] has been applied to a collection of Colorado Plateau basalts (96 samples). A variety of mono- and polymodal distributions has been found in the samples, all of which belong to the logarithmic family of statistical functions. Most samples have bimodal log normal distributions, while the others are represented by mono- or bimodal log logistic, and Weibull distributions. We have grouped the observed distributions into 11 groups depending on distribution types, mode location, and intensity. The nature of the curves within these groups can be interpreted as evolution of vesiculation processes. We conclude that within bimodal log normal distributions, the mode of smaller bubbles is the result of a second nucleation and growth event in a lava flow after eruption. In the case of log logistic distributions the larger mode results from coalescence of bubbles. Coalescence processes are reflected in growth of a larger mode and decreasing bubble number density. Another style of population evolution leads to a monomodal Weibull (or exponential) distribution as a result of superposition of multiple log normal distributions in which the modes are comparable in size and intensity. These various population distribution styles can be interpreted with an understanding of vesiculation processes that can be gained through appropriate numerical models of coalescence and population evolution. The applicable vesiculation processes include: a) a single nucleation-growth event, b) continuous multiple nucleation-growth events, c) coalescence, and d) Ostwald ripening. [Copyright &y& Elsevier]

Abstract: This decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions. A comparison between the results of experiments and a model for bubble nucleation shows that bubble number density (BND) can be calculated as a function of decompression rate for given physical properties, such as the diffusivity of water in a silicate melt and interfacial tension, within an acceptable margin of error. Conversely, in this paper, we propose a method (hereafter referred to as BND decompression rate meter) to estimate the decompression rate by using BND data of natural pumice samples from volcanic eruptions. As a result of the application of the BND decompression rate meter to pumice and scoria from explosive eruptions, it is found that the decompression rates increase in the range from 106 to 108 (Pa/s) with the eruption column heights; these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates. The absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows. In order to explain such high rates of decompression and the correlation with discharge rate, we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit. [Copyright &y& Elsevier]

• Bubbles nucleate more efficiently on Fe-Ti oxides than on other mineral type. • Number density of oxide microlites is more relevant than volume fraction. • Bubble coalescence is enhanced by phenocrysts. Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style - effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microlite- and phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (∼900-1100 °C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on Fe-Ti oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma. [ABSTRACT FROM AUTHOR]

Volcanic eruptions are driven by the growth of gas bubbles in magma. The timing and rate of bubble growth are important because they determine whether enough gas pressure can develop to fragment the melt. Bubbles grow in response to decompression and diffusive transport of dissolved volatiles (predominantly H 2 O) that exsolve into the bubbles. Growth is resisted by the viscosity of the melt. Both melt viscosity and H 2 O diffusivity have non-linear dependence on the concentration of H 2 O dissolved in the melt, which necessitates a numerical approach to modelling bubble growth. Several bubble growth models have previously been published and applied, but none of them has been validated against continuous, in situ experimental data or provided as a user-friendly tool. Here we present a numerical bubble growth model, implemented in MATLAB, which allows for arbitrary temperature and pressure pathways, and accounts for the impact of spatial variations in dissolved H 2 O concentration on viscosity and diffusivity. We validate the model against two sets of experimental data: (1) New continuous data for gas-volume fraction as a function of time, collected using optical dilatometry of vesiculating hydrous obsidian samples which were heated from 930 °C to 1000 °C at atmospheric pressure. This dataset captures isobaric, isothermal bubble growth under strongly disequilibrium conditions. (2) Discrete data from published decompression experiments at 825 °C and pressures from 200 MPa to ~5 MPa with decompression rates from 0.1 MPa s−1 to 10 MPa s−1. These experiments represent isothermal, decompression-driven bubble growth spanning equilibrium to strongly disequilibrium conditions. The numerical model closely reproduces the experimental data across all conditions, providing validation against contrasting bubble growth scenarios. The validated model has a wide range of potential volcanological applications, including forward modelling of bubble growth phenomena, and inverse modelling to reconstruct pressure–temperature–time pathways from textures and volatile contents of eruptive products. • Flexible and general numerical model for bubble growth in magma available for download • Model validated against in-situ and decompression experiments across a range of disequilibrium conditions • Model suitable for predicting bubble growth under arbitrary pressure-temperature-time pathways • Extendable and user-configurable to include material models (solubility, diffusivity, viscosity) for different compositions [ABSTRACT FROM AUTHOR]

Bubble formation during continuous decompression from ~400 to ~70 MPa was investigated experimentally in hydrous andesitic melts at T = 1030 °C and at an oxygen fugacity (fo2) of about log(fo2/bar) = QFM+1 (QFM: quartz-fayalite-magnetite buffer). Experiments were carried out at variable decompression rates (r), ranging from 0.0005 to 0.1 MPa/s. The samples were directly quenched after decompression, allowing the investigation of the influence of r on the bubble formation. The effect of variable annealing times (tA) after decompression was also investigated for experiments performed at a decompression rate of 0.1 MPa/s. These samples were annealed for tA = 0 to 72 h at final pressure (70 MPa) to study changes in vesiculation during magma storage at shallow depths after fast ascent. Backscattered electron (BSE) images of the samples were analyzed to determine bubble number densities (BND). The BND values increase strongly with increasing r and vary from about 102.2 mm-3 at 0.0005 MPa/s to about 104.6 mm-3 at 0.1 MPa/s. After fast decompression (r ~ 0.1 MPa/s), the BND decrease significantly with tA, i.e., from ~104.6 mm-3 at tA = 0 h to ~102.9 mm-3 at tA = 72 h. A comparison of the derived BND values with recently published experimental data demonstrates the essential role of the decompression path on bubble formation. The BND are higher in experiments with multi- or single-step decompression when compared to continuous decompression. The new data show that H2O-undersaturated andesitic melts are characterized by 1 to 2 log units higher BND values than H2O-saturated rhyolitic melts after decompression with the same rate, indicating a strong influence of melt composition on bubble nucleation. This compositional effect is not predicted accurately by existing models and the interpretation of the vesicularity of dacitic to andesitic melts may lead to overestimations of magma ascent rates by about an order of magnitude. [ABSTRACT FROM AUTHOR]

A steady-state model has been built for an air-water bubble column. The bubble number density constitutive equation has been formulated through integrating the bubble transport equation. Proper kernels for bubble breakage and coalescence rate have been chosen. The momentum balance of the gas phase is included in the model which leads to a set of non-linear differential equations. The model has been successfully solved by using the Least-Squares Method (LSM) with high accuracy and fast convergence. The successive iteration has been applied to the linearised equation set. The model shows excellent agreements with experimental data. [ABSTRACT FROM AUTHOR]