Actual Bubble Size Research Articles

Gas Bubble Dynamics During Methane Hydrate Formation and its Influence on Geophysical Properties of Sediment Using High-Resolution Synchrotron Imaging and Rock Physics Modeling

Gas bubble in aquatic sediments has a significant effect on its geophysical and geomechanical properties. Recent studies have shown that methane gas and hydrate can coexist in gas hydrate–bearing sediments. Accurate calibration and understanding of the fundamental processes regarding such coexisting gas bubble dynamics is essential for geophysical characterization and hazard mitigation. We conducted high-resolution synchrotron imaging of methane hydrate formation from methane gas in water-saturated sand. While previous hydrate synchrotron imaging has focused on hydrate evolution, here we focus on the gas bubble dynamics. We used a novel semantic segmentation technique based on convolutional neural networks to observe bubble dynamics before and during hydrate formation. Our results show that bubbles change shape and size even before hydrate formation. Hydrate forms on the outer surface of the bubbles, leading to reduction in bubble size, connectivity of bubbles, and the development of nano-to micro-sized bubbles. Interestingly, methane gas bubble size does not monotonously decrease with hydrate formation; rather, we observe some bubbles being completely used up during hydrate formation, while bubbles originate from hydrates in other parts. This indicates the dynamic nature of gas and hydrate formation. We also used an effective medium model including gas bubble resonance effects to study how these bubble sizes affect the geophysical properties. Gas bubble resonance modeling for field or experimental data generally considers an average or equivalent bubble size. We use synchrotron imaging data to extract individual gas bubble volumes and equivalent spherical radii from the segmented images and implement this into the rock physics model. Our modeling results show that using actual bubble size distribution has a different effect on the geophysical properties compared to the using mean and median bubble size distributions. Our imaging and modeling studies show that the existence of these small gas bubbles of a specific size range, compared to a bigger bubble of equivalent volume, may give rise to significant uncertainties in the geophysical inversion of gas quantification.

Investigation of air bubble properties: Relevance to prevention of coronary air embolism during cardiac surgery.

Although de-airing procedures are commonly performed during cardiac surgery, use of these procedures is not necessarily based on evidence. Uncertainly remains around the size of bubbles that can be detected by echocardiography, whether embolized air or carbon dioxide can be absorbed, and the reasons for embolic events occurring despite extensive de-airing. Since air bubbles are invisible in the blood, we used simple experimental models employing water and 10% dextran solution to determine the correlation between actual bubble size and the depicted size on echocardiography, bubble size, and floatation velocity and the absorption of carbon dioxide under embolization and irrigation conditions. Bubbles depicted as larger than 1mm were overestimated by echocardiography: the actual size was larger than 0.4mm in diameter. While bubbles of 0.5mm had a floatation velocity of 2 to 3cm/s, the buoyancy of bubbles smaller than 0.3mm was negligible. Thus, bubbles that are depicted as larger than 1mm on echocardiography or that present with apparent buoyancy should be visible and need to be meticulously removed. However, echocardiography cannot distinguish bubbles of around 0.1mm in diameter from those of capillary size (<10μm). Thus, we advise continuous venting of dense bubbles until they become sparse. While carbon dioxide was rapidly absorbed when circulating, the absorption of embolized carbon dioxide was negligible. These results suggest that detected intracardiac air represents residual "air," with carbon dioxide already absorbed. Therefore, the use of conventional de-airing procedures needs reconsideration: air and buoyant bubbles should be removed from the heart before they are expelled into the aorta; this requires timely and precise assessment with transesophageal echocardiography and effective collaboration between surgeons, anesthesiologists, and perfusionists.

Automated contact time apparatus and measurement procedure for bubble-particle interaction analysis

The novel Automated Contact Time Apparatus (ACTA) presented in this paper serves as a diagnostic tool that allows the detection of changes in bubble-particle attachment probability and therefore floatability caused by alterations in the chemical environment and particle properties.The apparatus consists of six identical capillaries where bubbles with defined size are produced simultaneously in a measurement chamber. The bubbles at the needle tips are placed in contact with the submerged particle bed for specific time periods, controlled with the help of automatic actuators. The advantage of the instrument is that hundreds of bubble-particle contacts can be measured automatically within a short time period. Microscopy pictures of each measured bubble are taken while recording the movement of the bubble before, during and after contact with the solid particles. The recorded pictures can be used to determine the actual bubble size and its corresponding deviation, and to detect the attachment of particles. The attached particles are collected in a detachable chamber for subsequent characterization. Furthermore, the device is portable and can be taken to the mineral processing plants for quick evaluation of particle-bubble attachment efficiency with particles and process water sampled directly from real processes.

A new method of relating a chord length distribution to a bubble size distribution for vertical bubbly flows

Measurements of a bubbly flow using an intrusive probe give a set of random chord length values whose distribution differs considerably from the actual bubble size distribution (BSD). Considering the interaction of bubbles in groups of different sizes and shapes with the probe, a new method of relating the measured chord length distribution (CLD) to the BSD is proposed. The CLD is decomposed into simple distributions, which are considered as the CLDs of individual bubble groups and translated into BSDs using a developed analytical transformation. Entire BSD is constructed statistically from the obtained BSDs. The new method is applied to analyzing the CLDs measured by a five-sensor conductivity probe for a vertical bubbly flow in a narrow rectangular channel at different flow conditions. The results show a good agreement between the predicted BSDs and the experimental distributions measured simultaneously by a high-speed video camera. Valuable information on the statistical characteristics estimated from the CLDs and BSDs, including the chord length, bubble size and shape, and bubble population fraction, are evaluated and discussed in detail.

Multislice simulation of transmission electron microscopy imaging of helium bubbles in Fe

Formation of nanoscale helium (He) bubbles in reduced activation ferritic/martensitic steels may lead to degradation of mechanical properties of materials. Transmission electron microscopy (TEM) has commonly been used to image the Fresnel contrast of He bubbles, using an underfocus of 0.5-1µm. This paper presents our study of multislice simulation of the size correlation between imaged Fresnel rings and the actual He bubbles. It was found that for bubbles equal to or >3nm in diameter, the imaged bubble size, represented by its inner diameter of the first dark Fresnel ring (D(in)) in underfocused imaging conditions, increases with increasing electron-beam incoherency, but decreases with increasing underfocus. The electron-beam accelerating voltage, bubble size, bubble position and TEM sample thickness were found to have no significant influence on the deviation of D(in) from the actual bubble size (D(0)). However, for bubbles equal to or <2nm, D(in)/D(0) increases dramatically with increasing underfocus when it is above a threshold limit (e.g. Δf=-1µm for a 2-nm bubble). The results of this study also suggested that He bubbles can be differentiated from argon (Ar) bubbles by contrast differences.

Comparison of bubble growth obtained from pressure fluctuation measurements to optical probing and literature correlations

Pressure fluctuation measurements (PFM) along with optical probing (OP) are conducted in a 145mm ID glass column fluidized bed with Geldart B particles to evaluate and compare the measured bubble growth with commonly used literature correlations. Bubble growth from PFM is evaluated by spectral data decomposition, while OP is evaluated by a bubble linking algorithm. It is shown that bubble sizes obtained from literature correlations vary widely for the same fluidization condition due to different methods in their determination. Compared to optical probe measurements, the best agreement is found with the correlation of Horio and Nonaka [A generalized bubble diameter correlation for gas-solid fluidized beds. AIChE J. 33 (1987) 1865–1872]. This agreement is in particular good for gas velocities u0/umf>5 and bed heights below the influence of slugging. Since PFM yield a characteristic length scale only proportional to the actual bubble size, comparison with bubble sizes from OP and literature correlations is not straightforward. However, this proportionality can be assessed by a physical model of Puncochár and Drahos [Origin of pressure fluctuations in fluidized beds. Chem. Eng. Sci. 60 (2005) 1193–1197]. Their physical model yields a proportionality constant of 3 which matches well the PFM data for gas velocities u0/umf>5.

On the measurement of local gas hold-up and interfacial area in gas–liquid contactors via light sheet and image analysis

Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid contactors. Aim of this work is that of providing a numerical procedure able to reconstruct local gas hold-up and specific interfacial area from images obtained by laser sheet illumination. A correction for measured quantities accounting for laser sheet thickness is proposed and tested by means of Monte Carlo simulations. The algorithms proposed are robust and independent of any measured parameters of the actual bubble size distribution.

On the measurement of bubble size distribution in gas–liquid contactors via light sheet and image analysis

Abstract Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid mechanically agitated vessels. When trying to analyze image information the problem arises that bubble sizes are generally underestimated, due to the fact that the laser sheet used for lighting the system randomly cuts bubbles over non-diametrical planes, leading to an apparent bubble size distribution even in the ideal case of single sized bubbles. Clearly in the case of bubbles with a size distribution the experimental information obtained is affected by the superposition of effects. Aim of this work is that of providing a numerical procedure able to reconstruct actual bubble size distributions from relevant apparent size distributions obtained by laser sheet illumination and image analysis. The procedure proposed is robust and viable and can account for laser sheet thickness. The procedure is shown to provide fully satisfactory results even with quite extreme distributions. BSD resolution dependence on the numerousness of raw data processed is discussed. Use of the proposed procedure for extracting BSD from bubble chord raw data obtained by other devices, such as point probes, is straightforward.

AREA-TO-VOLUME DATA TRANSLATION IN THE MEASUREMENT OF BUBBLE SIZE DISTRIBUTIONS VIA LASER SHEET AND IMAGE ANALYSIS

A nov el experimental technique for measuring local bubble size distributions in gasliquid systems, is proposed. The technique is based on laser sheet illumination of the gas-liquid dispersion and synchronized camera, i.e. on equipment typically available within PIV set-ups. By suitably arranging experiments and subsequent image analysis, bubbles intercepted by the laser sheet are clearly identified. The size distribution of observed intercept areas can therefore be assessed. This is however different from the actual bubble size distribution needed for modelling and interfacial area estimation purposes. In this paper a simple statistical correction procedure able to reconstruct actual bubble size distributions from relevant intercept size distributions is proposed and discussed. Preliminary data obtained in stirred gas-liquid dispersions confirm the technique viability.

Oxygen transfer prediction in aeration tanks using CFD

In order to optimize aeration in the activated sludge processes, an experimentally validated numerical tool, based on computational fluid dynamics and able to predict flow and oxygen transfer characteristics in aeration tanks equipped with fine bubble diffusers and axial slow speed mixers, is proposed. For four different aeration tanks (1;1493;8191 and 29 , 313 m 3 ), this tool allows to precisely reproduce experimental results in terms of axial liquid velocities, local gas hold-ups. Predicted oxygen transfer coefficients are within ± 5% of experimental results for different operating conditions (varying pumping flow rates of the mixers and air flow rates). The actual bubble size must be known with precision in order to have a reliable estimation of the oxygen transfer coefficients.

Investigating AGN heating in a sample of nearby clusters

We analyse those objects in the Brightest 55 sample of clusters of galaxies which have a short central cooling time and a central temperature drop. Such clusters are likely to require some form of heating. Where clear radio bubbles are observed in these clusters, their energy injection is compared to the X-ray cooling rate. Of the 20 clusters requiring heating, at least 14 have clear bubbles, implying a duty cycle for the bubbling activity of at least 70 per cent. The average distance out to which the bubbles can offset the X-ray cooling, r heat , is given by r heat /r cool = 0.86 ± 0.11 where r cool is defined as the radius as which the radiative cooling time is 3 Gyr. 10 out of 16 clusters have r hcat /r cool ? 1, but there is a large range in values. The clusters which require heating but show no clear bubbles were combined with those clusters which have a radio core to form a second subsample. Using r heat = 0.86r cool we calculate the size of an average bubble expected in these clusters. In five cases (3C129.1, A2063, A2204, A3112 and A3391) the radio morphology is bi-lobed and its extent similar to the expected bubble sizes. A comparison between the actual bubble size and the maximum expected if they were to offset the X-ray cooling exactly, R max , shows a peak at R bubble ∼ 0.7R max with a tail extending to larger R bubble /R max . The offset from the expected value of R bubble ∼ R max may indicate the presence of a non-thermal component in the innermost intracluster medium of most clusters, with a pressure comparable to the thermal pressure.

Hypolimnetic aeration: field test of the empirical sizing method

A hypolimnetic aeration system was recently installed in a small (16 ha S α ) eutrophic lake and a comparison made between measured performance and predicted performance from an empirical sizing method. The design variables used to size the system were: hypolimnetic volume 451,600 m 3; maximum hypolimnetic oxygen consumption 0.2 mg l −1 d −1; aerator input rate 2 mg l −1; water velocity 0.76 m s −1 and depth of air release 12.2 m. A 3.7 kW compressor (0.57 m 3 min −1) generated a water velocity of 0.46 m s −1, a water flow of 17.7 m 3 min −1 and a theoretical hypolimnetic circulation period of 18 days. Dissolved oxygen increased by an average of 1.6 mg l −1 on each cycle through the aerator, and aerator input rates ranged from 0.6 to 2.6 mg l −1. Hypolimnetic oxygen consumption averaged 0.12 mg l −1 d −1 and ranged between 0.02 and 0.21 mg l −1 d −1. The aeration system was unable to meet the daily oxygen demand (90 kg) as the water velocity was slower than expected (0.46 m s −1). To avoid undersizing future aeration installations the following recommendations should be considered when using the empirical sizing formula: (1) estimates of oxygen consumption should be annual maximums from aerobic hypolimnia; (2) aerator input rates should be conservative (e.g. 1–4 mg l −1) and increase with depth; (3) water velocity of 0.45–0.50 m s −1 should initially be used when no information on actual bubble size or velocity is available; (4) aeration start-up should be timed to avoid periods of accumulated oxygen demands.

COMPARISON OF THE DATA OF YAMAZAKI AND MIYAUCHI WITH THE BUBBLING BED MODEL

The data of Yamazaki and Miyauchi9) for heat transfer between emulsion phase and gas bubbles in fluid-bed catalytic contactors are analyzed in terms of the Bubbling Bed Model of Kunii and Levenspiel5). It is shown that the effective bubble size needed to fit the Bubbling Bed Model to the reported transport rates is much smaller than the actual bubble size; however, more realistic assumptions regarding temperature measurements indicate that their reported transport rates are much too high. Temperature-corrected data are shown to be reasonably estimated by the Bubbling Bed Model using the measured bubble size.