Majority Of Bubbles Research Articles

Bubble breakage and aggregation characteristics in a vortex pump under bubble inflow

The behaviors of bubbles in a vortex pump play a crucial role in its performance when handling gas–liquid flows. However, not much research has been done on the distribution of the gas–liquid phases and the characteristics of bubble breakage and aggregation in vortex pumps. This lack of understanding hinders the improvement of pump performance in gas–liquid flow transport. This study aims to investigate the bubble characteristics in a vortex pump using the population balance model, focusing on the variation of bubble size and the influence of the inlet gas volume fraction (IGVF) on bubble breakage and aggregation. The results show that as the IGVF increases, the gas volume fraction in the impeller becomes larger than that in the bladeless chamber. The majority of bubbles in the impeller are concentrated near the hub, while they also remain in the circulating-flow zone of the bladeless chamber. Under low IGVF conditions, the average diameter of bubbles decreases from the pump inlet to the outlet. The bladeless chamber has a larger average bubble diameter and a higher percentage of large bubbles compared to other parts of the pump. Moreover, the bubble number density at the pump outlet increases with the IGVF, indicating the production of more bubbles, while the increase in IGVF also results in an increase in the percentage of large bubbles. The study also discusses the mechanism of bubble breakage and aggregation in vortex pumps. It suggests that the effective breakage frequency and effective aggregation frequency are responsible for bubble breakage and aggregation in the vortex pump. The gas volume fraction and turbulent dissipation rate are identified as important parameters affecting the effective breakage and aggregation frequency. These findings provide new insights into understanding the characteristics of bubble breakage and aggregation in vortex pumps.

Ozone nanobubble treatment in freshwater effectively reduced pathogenic fish bacteria and is safe for Nile tilapia (Oreochromis niloticus)

High concentrations of certain pathogenic bacteria in water usually results in outbreaks of bacterial diseases in farmed fish. Here, we explore the potential application of an emerging nanobubble technology in freshwater aquaculture, specifically aimed to reduce the concentrations of pathogenic fish bacteria in freshwater, and assess whether nanobubbles are safe for Nile tilapia (Oreochromis niloticus). An ozone nanobubble (NB-O3) treatment protocol was established, based on examination of nanobubble size, concentration, disinfection property, and impact on fish health. A 10-min treatment with NB-O3 in 50 L water generated approximately 2–3 × 107 bubbles/mL, with the majority of bubbles being less than 130 nm in diameter and an ozone level of 834 ± 22 mV oxidation-reduction potential (ORP). A single treatment with water spiked with either Streptococcus agalactiae or Aeromonas veronii effectively reduced the bacterial load by 26–48 fold or 96.11–97.92%. This same protocol was repeated three times. The result was a 22,058 to 109,978 fold reduction in bacteria or 99.93–99.99% decrease. In comparison, bacterial concentrations in the control tanks remained unchanged during the experiments. In Nile tilapia-cultured water with the presence of organic matter (e.g. mucus, feces, bacterial flora, feed, etc.), the disinfection property of NB-O3 was reduced; however, we still observe a reduction of 59.63%, 87.25%, and 99.29% after the first, second, and third consecutive treatments, respectively. To evaluate the safety of NB-O3 on fish, juvenile Nile tilapia were exposed to NB-O3 treatment for 10 min. No mortality was observed during the treatment or 48 h post treatment. Gill histology examination revealed that a single NB-O3 treatment caused no alteration in cell morphology. However, damage in the gill filaments, such as blood congestion, aggregates of basal cells at the secondary lamellae or loss of the secondary lamella was noticed in the fish receiving two or three consecutive exposures within the same day. Results of the experiments conducted in this study suggest that NB-O3 technology is promising for reducing pathogenic bacteria in aquaculture systems and may be useful at reducing the risk of bacterial disease outbreaks in farmed fish.

Bubble-loop complexes induced by helium irradiation and effect of pre-irradiation and post-irradiation of hydrogen in reduced-activation ferritic/martensitic steel

ABSTRACTIn the present work, the synergistic effect of high concentration hydrogen and helium on the dislocation loops and bubbles as well as their correlations in reduced-activation ferritic/martensitic (RAFM) steels is investigated. Such an effect was transmuted from 14 MeV neutron irradiation and has been one of the most challenging issues for RAFM steels for future fusion reactors. After low dose (0.18 dpa) high concentration (5000 appm) single-ion helium irradiation at 723 K, very large dislocation loops were observed, and the majority of bubbles were inside dislocation loops, forming bubble-loop complexes. These bubble-loop complexes defects were also present in hydrogen/helium and helium/hydrogen sequential-ion irradiated steels. Pre-irradiated hydrogen ion effectively inhibited the later growth of loops induced by helium post-irradiation, and the higher the ratio of hydrogen to helium fluence, the greater the effect of inhibition. At high fluence of hydrogen pre-irradiation, the structure of bubble-loop complexes disappeared. On the other hand, hydrogen post-irradiation promoted the growth of loops induced by helium pre-irradiation, and the higher the ratio of hydrogen to helium fluence, the greater the effect of promotion. The mechanisms for hydrogen/helium synergistic effects are discussed.

Numerical investigation of two-phase flow characteristics of subcooled boiling in IC engine cooling passages using a new 3D two-fluid model

Modern engine cooling systems use subcooled boiling as an effective heat transfer form. The significantly higher heat transfer rate achievable with boiling can keep the temperature in key thermal areas within an acceptable level. In this study, a three-dimensional two-fluid model coupled with RPI wall boiling model was adopted to simulate the local two-phase characteristics of boiling at different passages under low-pressure condition. Moreover, a modified bubble departure diameter correlation was developed and implemented in computational fluid dynamics code with user-defined function, and the correlation was verified by two sets of experimental data under different flow conditions. Results showed an improved agreement with the measured local void fractions. Finally, the verified model was applied to a practical cooling passage for a water-cooled diesel engine. The simulation results showed that present two-fluid model could get an accurate temperature field for cylinder head. The distribution of vapor phase in cooling passage was obtained, and it could be used as a reference to evaluate the reliability of local boiling. Moreover, due to the function of interphase forces, the majority of bubbles were quickly removed from near wall and the void fraction was below 0.149.

Size distribution of air bubbles entering the brain during cardiac surgery.

BackgroundThousands of air bubbles enter the cerebral circulation during cardiac surgery, but whether high numbers of bubbles explain post-operative cognitive decline is currently controversial. This study estimates the size distribution of air bubbles and volume of air entering the cerebral arteries intra-operatively based on analysis of transcranial Doppler ultrasound data.MethodsTranscranial Doppler ultrasound recordings from ten patients undergoing heart surgery were analysed for the presence of embolic signals. The backscattered intensity of each embolic signal was modelled based on ultrasound scattering theory to provide an estimate of bubble diameter. The impact of showers of bubbles on cerebral blood-flow was then investigated using patient-specific Monte-Carlo simulations to model the accumulation and clearance of bubbles within a model vasculature.ResultsAnalysis of Doppler ultrasound recordings revealed a minimum of 371 and maximum of 6476 bubbles entering the middle cerebral artery territories during surgery. This was estimated to correspond to a total volume of air ranging between 0.003 and 0.12 mL. Based on analysis of a total of 18667 embolic signals, the median diameter of bubbles entering the cerebral arteries was 33 μm (IQR: 18 to 69 μm). Although bubble diameters ranged from ~5 μm to 3.5 mm, the majority (85%) were less than 100 μm. Numerous small bubbles detected during cardiopulmonary bypass were estimated by Monte-Carlo simulation to be benign. However, during weaning from bypass, showers containing large macro-bubbles were observed, which were estimated to transiently affect up to 2.2% of arterioles.ConclusionsDetailed analysis of Doppler ultrasound data can be used to provide an estimate of bubble diameter, total volume of air, and the likely impact of embolic showers on cerebral blood flow. Although bubbles are alarmingly numerous during surgery, our simulations suggest that the majority of bubbles are too small to be harmful.

Bubble size distribution analysis and control in high frequency ultrasonic cleaning processes

In the semiconductor industry, the ongoing down-scaling of nanoelectronic elements has lead to an increasing complexity of their fabrication. Hence, the individual fabrication processes become increasingly difficult to handle. To minimize cross-contamination, intermediate surface cleaning and preparation steps are inevitable parts of the semiconductor process chain. Here, one major challenge is the removal of residual nano-particulate contamination resulting from abrasive processes such as polishing and etching. In the past, physical cleaning techniques such as megasonic cleaning have been proposed as suitable solutions. However, the soaring fragility of the smallest structures is constraining the forces of the involved physical removal mechanisms. In the case of "megasonic" cleaning –cleaning with ultrasound in the MHz-domain – the main cleaning action arises from strongly oscillating microbubbles which emerge from the periodically changing tensile strain in the cleaning liquid during sonication. These bubbles grow, oscillate and collapse due to a complex interplay of rectified diffusion, bubble coalescence, non-linear pulsation and the onset of shape instabilities. Hence, the resulting bubble size distribution does not remain static but alternates continuously. Only microbubbles in this distribution that show a high oscillatory response are responsible for the cleaning action. Therefore, the cleaning process efficiency can be improved by keeping the majority of bubbles around their resonance size. In this paper, we propose a method to control and characterize the bubble size distribution by means of "pulsed" sonication and measurements of acoustic cavitation spectra, respectively. We show that the so-obtained bubble size distributions can be related to theoretical predictions of the oscillatory responses of and the onset of shape instabilities for the respective bubbles. We also propose a mechanism to explain the enhancement of both acoustic and cleaning activity observed when turning the ultrasound periodically on and off in comparison to the case of continuous sonication.

Local bubble size distributions in agitated vessels

Local bubble size distributions (BSD) were measured from 12, 14 and 194 dm3 agitated vessels with capillary suction probe (CSP) technique. The four investigated systems were air–water, air–aqueous NaCl, air–aqueous starch and CO2–n-butanol. Several gassing rates and stirring speeds were investigated to find their effect on local BSDs. The reproducibility tests and the comparison against photography showed that Sauter mean bubble sizes and volumetric BSDs can be measured accurately from the air–water system with the capillary probe. In the CO2–n-butanol experiments the variation of BSDs with the agitation conditions was reasonable, although the majority of bubbles was likely smaller than the capillary diameter. In the air–water system, bubble size varied more in the Rushton turbine than in the flat-blade turbine (FBT) agitated vessel because of differences in flow fields. The addition of a small amount of NaCl caused a sharp decrease of bubble size in the vicinity of liquid surface. In the impeller discharge flow, bubble size decreased less. The lower coalescence rates due to higher apparent viscosity likely explain the smaller bubbles in the air–aqueous starch system than in the air–water system. In the CO2–n-butanol system, bubbles were notably smaller compared to other systems and the bubble size decreased unexpectedly in the flow from the impeller discharge to the liquid surface. The measured local BSDs are useful for the validation of population balance models that are, as connected to a computational fluid dynamic code, useful tools for the simulation of local mass transfer areas in agitated gas–liquid reactors.

High-speed simultaneous observation of plasma and keyhole behavior during high power CO2 laser welding: Effect of shielding gas on porosity formation

Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size and shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal with the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined by using two synchronized high-speed cameras and an x-ray transmission observation system. Especially, the effect of shielding gas on porosity formation was investigated in terms of plasma and keyhole behavior. In the case of He shielding gas, metallic plasma emanated from a keyhole, and the keyhole was open continuously. On the other hand, in the case of N2 shielding gas, a big gas plasma was formed above the weld bead periodically, and metallic plasma and keyhole disappeared just like in the pulsed laser welding. Such periodical interval and duty were different depending on the materials used and exerted an effect on porosity formation tendency.

On the Theory of the Bubble Chamber

An attempt is made to analyze the factors which determine the operation of the bubble chamber. It is concluded that the majority of bubbles in conventional chambers are nucleated by moderately energetic free electrons produced by the incident particles in Coulomb encounters. Nuclei are displaced too infrequently by Coulomb encounters to account for the observed densities of bubbles. The electrons deposit their kinetic energy in highly localized regions which then are the source of explosions which produce bubbles of greater than critical size in a time of the order of 10−10 or 10−11 sec. The bubbles grow subsequently by evaporation of the fluid. The temperature of the fluid should be sufficiently close to the critical temperature that the energy required to produce the bubble of critical size can be provided by an electron with a range comparable to or less than the diameter of the bubble of critical size. Otherwise the electron will be unable to localize its energy in a sufficiently small volume in any but highly improbable cases. It is also concluded that the viscosity of the liquid plays a very important role in determining the threshold energy for forming a bubble of critical size when it has a value near 1 centipoise or larger.