Sub-millimeter Bubbles Research Articles

Mechanical and physicochemical characteristics of sub-millimeter and macrobubbles: Insights from AFM analysis, zeta potential measurements and rise velocity

This study presents the change in mechanical and physicochemical behaviour of macrobubbles and sub-millimeter bubbles with respect to pH. While macrobubbles conventionally range from 100 μm to 2 mm in diameter, this study focuses on the comparatively less-explored sub-millimeter bubble range, centered around a median diameter of 500 μm. The difference in sub-millimeter bubbles (500 μm) compared to their larger counterparts (1100 μm) within macrobubble size range were evidenced through atomic force microscopy (AFM) analysis, zeta potential measurements, and rise velocity experiments. The adhesion force and energy between bubbles and the silicon nitride cantilever tip showed a decreasing trend with increasing pH, notably diminishing by about 50% beyond pH 9. Although no significant differences were statistically revealed between the two bubble sizes at lower pH, the adhesion force and energy were consistently smaller for macrobubble compared to sub-millimeter bubble at higher pH beyond pH 9. This finding suggests that the electrical double layer (EDL) interactions are influenced by the specific surface area, particularly in the alkaline region. Zeta potential measurements exhibited a cubic trend, with an isoelectric point at pH 3.5 and a minimum zeta potential at pH 11. The study also depicted a similar trend in the rise velocity of bubbles, suggesting a direct relationship between bubble stiffness and its rise velocity. Therefore, changes in pH affected the bubbles' properties, while on the other hand, certain physicochemical properties are also influenced by bubble sizes.

Parametrically excited shape distortion of a submillimeter bubble

The existence of finite amplitude shape distortion caused by parametrically excited surface instabilities for a gas bubble in water driven by a temporally periodic, spatially uniform pressure field in an axisymmetric geometry is investigated. Employing a nonlinear coupled system of equations which includes shape mode interactions to third order, the resultant spherical oscillations, translation, and shape distortion of the bubble are modelled, placing no restriction on the size of the spherical oscillations. The model accounts for viscous and thermal damping with compressibility effects. The existence of synchronous and higher order parametrically induced sustained, finite amplitude, periodic shape deformation is demonstrated. The excitement of an odd shape mode via the synchronous mechanism is shown to give rise to linear bubble self-propulsion. For larger driving amplitudes, it is shown that more than one shape mode can be parametrically excited at the same driving frequency but by different resonance mechanisms, leading to more involved shape deformation and the increased possibility of bubble self-propulsion.

Investigation into drag coefficient for sub-millimeter bubbles in gas–liquid bubbly flow: Experiments and CFD simulations

The sub-millimeter bubble technique can enhance the gas–liquid inter-phase mass transfer and show immense potential to increase production efficiency. The two-fluid model (TFM), suitable for the CFD simulations on the scale of actual reactors, is a powerful tool for exploring the application of the sub-millimeter bubble technique. Although the accuracy of TFM is largely affected by the drag models, the applicability of the drag models for gas–liquid two-phase flow of sub-millimeter bubbles is scarcely investigated. In the present work, the applicability of the single-bubble drag models was apprised with experimental data of single sub-millimeter bubbles. Then, the bubble swarm effect and its influence on the calculation of drag coefficients were investigated for the gas–liquid two-phase flow of sub-millimeter bubbles under a wide range of gas holdups (0.02–0.21). The accelerating effect was observed in the experiments of gas–liquid two-phase flows. A reasonable explanation for the accelerating effect was discussed based on the bubble size distribution obtained by applying our original image processing method. Based on the experiments, an equation for the drag modification factors was proposed to include the bubble swarm effect. The accuracy and applicability of the proposed equation were validated by applying it together with the accurate single-bubble drag model in the CFD simulation by TFM. The present work investigated the hydrodynamic characteristics of sub-millimeter bubbly flows experimentally and provided an accurate CFD method for further investigation.

Experimental investigation of the effect of air bubbles injection on the performance of a plate heat exchanger

• Air is injected in cold water to enhance the performance of plate heat exchanger . • A first PHEX performance test at various air and hot water flow rates is reported. • NTU and effectiveness are enhanced by up to 12.4 and 14.6 %, respectively. • Entropy generation rate increases up to 4.1 W/K with air bubble injection. • Witte-Shamsundar efficiency reaches a maximum of 96.9% for lowest water flow rate. Injection of sub-millimeter scale air bubbles into liquid flows has been recognized recently as an effective method for thermal performance enhancement of heat exchangers. However, no studies have been reported so far on air injection into the cold liquid stream of plate heat exchangers (PHEX). This approach is investigated experimentally in the present study for the first time in PHEXs to map the energetic and exergetic performance characteristics of a typical PHEXs in terms of the number of transfer units (NTU), effectiveness, entropy generation number, augmentation entropy generation number, rational effectiveness, and Witte-Shamsundar efficiency. Air (150–840 L/h) and hot water (280–880 L/h) streams are mixed in a T-junction mixing chamber before entering the PHEX as a two-phase flow. The results show a remarkable increase in NTU by up to 12.4% due to injected air bubbles, whereas the effectiveness increases by up to 14.6%. On the other hand, considerable increments in entropy generation rate are noticed, reaching a maximum of 4.1 W/K, but this increment was less than 3 folds for most examined cases. Overall, the Witte-Shamsundar efficiency, representing the combined energetic-exergetic performance, reached a maximum of 96.9% at air and hot water flow rates of 350 and 280 L/h, respectively.

Energy optimization of wet air oxidation reactors with sub-millimeter bubble intensification

The huge investment in equipment and energy consumption required for wet air oxidation (WAO) severely limits its industrial application due to harsh operating conditions with high temperatures and pressures. In this work, the energy consumption of phenol WAO was calculated considering gas compression, pumping, preheating and bubble generation. The relationship between energy consumption, bubble diameter and phenol conversion was constructed to evaluate the main influencing factors. It was found that sub-millimeter bubbles can effectively improve the WAO process by enhancing the oxygen mass transfer, resulting in a decrease in operating temperature and pressure. Considering the maximum conversion and minimum cost, the initial air bubble diameter for phenolic WAO is preferably between 0.2 ∼ 0.4 mm. The model was validated using experiments with a WAO reactor dominated by sub-millimeter air bubble, and good agreement was obtained between simulated results and plant data.

A deep learning-based image processing method for bubble detection, segmentation, and shape reconstruction in high gas holdup sub-millimeter bubbly flows

The sub-millimeter bubble technique can enhance the gas–liquid inter-phase mass transfer by significantly reducing the bubble size and increasing the gas–liquid interfacial area. To accurately describe the flow and mass transfer characteristics, it is necessary to characterize bubble parameters. High-speed photography followed by image processing is an effective way to characterize the gas bubbles in the multiphase flows. However, the efficient image processing method for the sub-millimeter bubbly flows with high gas holdup and high bubble overlap has not been reported yet. The present work developed a novel deep learning-based image processing method for bubble detection, segmentation, and shape reconstruction in high gas holdup sub-millimeter bubbly flows. In order to segment the highly overlapping sub-millimeter bubbles, our method was built based on Mask R-CNN, with which the pixel-level segmentation masks can be obtained, and the shape of the overlapping bubbles can be accurately described. The feature pyramid architecture was coupled with ResNet101 and Feature Pyramid Network to detect sub-millimeter bubbles with significant size differences. A shape reconstruction module was proposed to restore the real shape of overlapping bubbles and improve prediction accuracy. In order to sufficiently validate the proposed method, adequate images of sub-millimeter bubbly flows were obtained by changing the experimental media (air-tap water, air-sodium dodecyl sulphate aqueous solution, air-diesel, and air-diesel-fine catalyst particles), reactor configurations (3D beds and 2D beds), lenses, and photography (shadowgraphy and front illumination). Our method shows high accuracy under the experimental conditions and can process sub-millimeter bubble images under gas holdup up to 20%.

Numerical simulation on the terminal rise velocity and mass transfer rate of single sub-millimeter bubbles

Gas–liquid reactors containing bubbles are intensively used in the chemical industry. However, the gas–liquid mass transfer is usually the rate-limiting step. One possible way to intensify the mass transfer is the application of sub-millimeter bubbles (1–1000 μm) which efficiently increase interfacial area. Literature usually focused on bubbles larger than 1000 μm, and it still needs to be studied whether the frequently-used literature equations for terminal rise velocity and mass transfer can accurately predict the behavior of sub-millimeter bubbles. Therefore, the terminal rise velocity and mass transfer rate of single sub-millimeter bubbles were obtained by numerical simulation. It was found that the literature equations could not accurately predict the behavior of sub-millimeter bubbles over the whole size range. New equations were proposed to improve calculation accuracy, with which the terminal rise velocity and mass transfer rate of sub-millimeter bubbles can be accurately predicted.

A Systematic Analysis of the Salinity Effect on Air Bubbles Evolution: Laboratory Experiments in a Breaking Wave Analog

AbstractThe evolution of air bubbles after breaking waves plays an important role in gas and particle exchange between water bodies and the atmosphere. To improve our understanding of the impacts of salinity on this process, we systematically investigate the effect of salt concentrations ranging from 0 to 40 g/kg on the volume and size distributions of subsurface bubble plumes and surface foams in a laboratory breaking wave analog. Our experimental setup utilizes an intermittent plunging sheet to simulate breaking waves, while two synchronized digital cameras were used to monitor the temporal evolution of bubble plumes and surface foams. We first highlight the importance of plunging sheet intermittency on surface foam evolution. We then show that increasing salinity enhances the entrainment of submillimeter bubbles but has a less significant effect on larger supramillimeter bubbles. We observed that the foam area in saltier waters is consistently higher than that in freshwater throughout the foam decay phase. Furthermore, our investigation of surface bubble sizes shows that salinity has a more distinct effect on smaller (sub 2 mm) than on larger bubbles. This suggests that salinity may have a more pronounced impact on jet than on film drops ejection mechanisms. Finally, we conclude that the change in salinity within the typical oceanographic range is likely not a major factor for bubble‐mediated interactions at the water surface during breaking waves. However, even low‐salt concentrations greatly alter air entrainment characteristics in freshwater systems.

Influence of Newtonian and non-Newtonian fluid behaviour on void fraction and bubble size for a gas-liquid flow of sub-millimeter bubbles at low void fractions

This study compares the effect of Newtonian and non-Newtonian blood-mimicking test liquids on void fraction and bubble size distribution (BSD) for the case of gas–liquid bubbly flow. Along with this comparison, the novelty of the present work is extended to the examined conditions, which combine low void fractions (<10−1) with bubbles smaller than 1 mm, resembling bubbly flow in human bloodstream during Decompression Sickness. Such conditions, however, can be also found in common two-phase applications such as subcooled flow boiling in macro-channels. Experiments are performed in co-current upward bubbly flow. Void fraction and BSD are determined by a non-intrusive electrical impedance technique and by image analysis of bubbly flow images, respectively. Xanthan gum is employed (0, 150 and 1000 ppm) to impart non-Newtonian (shear-thinning) behaviour to the baseline aqueous glycerol/NaCl test liquid. The role of surface tension (by adding Triton X-100) and gas/liquid superficial velocities (Usg, Usl) on acquired data is studied as well. Results demonstrate that increase of bubble size and decrease of void fraction is primarily due to the non-Newtonian liquid character rather than the increase of dynamic viscosity caused by the addition of high Xanthan gum concentration. The addition of surfactant (Triton X-100) decreases bubble size and increases void fraction. Furthermore, bubble size and void fraction increase with Usg and decrease with Usl. Void fraction signal fluctuations show a strong dependence on bubble size. Furthermore, the performance of an empirical equation that predicts average bubble diameter from void fraction statistical quantities is tested and seems to be unaffected by the Newtonian/non-Newtonian liquid behaviour. Of particular interest are the intense peaks noticed in void fraction signals for the highest examined Xanthan gum concentration (1000 ppm) and the lowest examined liquid superficial velocity (2.89 cm/s). These peaks correspond to voluminous bubbles clusters rising among isolated bubbles, which do not appear at higher liquid velocities.

On-site cycling drag analysis with the Ring of Fire

The Ring of Fire (RoF) measurement concept, introduced by Terra et al. (Exp Fluids 58:83. https://doi.org/10.1007/s00348-017-2331-0, 2017; Experiments in Fluids 59:120, 2018), is applied to real cyclists to enable the aerodynamic drag determination during sport action. This principle is based on large-scale stereoscopic particle image velocimetry (PIV) measurements over a plane crossed by the athlete during cycling. The momentum before and after the passage of the athlete poses the basis for the control volume analysis in the athlete’s frame of reference, which returns the aerodynamic drag. This approach extrapolates aerodynamic studies towards more realistic conditions, compared to experiments performed in wind tunnels with scaled or stationary athletes. The measurement concept is termed Ring of Fire as the rider crosses a region of intense light. Two experiments are conducted, indoor and outdoor, with attention placed on the effects of the environmental conditions and the confinement of the measurement region. Stereo-PIV measurements feature a plane of approximately 2 × 2 m2, using neutrally buoyant sub-millimeter helium-filled soap bubbles (HFSB) as flow tracers. The drag measurement is obtained examining the wake produced by the athlete. It is observed that the drag value becomes independent of time after about 5 torso lengths from the passage. A statistical estimate of the drag is produced combining the results of several passages. Fluctuations of the drag value during a single passage are associated with the unsteady wake flow. Overall fluctuations among different transits are ascribed to the varying conditions of the airflow prior to the passage of the athlete. The experiments conducted outdoor exhibit significantly larger dispersion of the drag value, compared to the quieter conditions indoor. Repetition of the transit 10–30 times yields a basis for statistical convergence of the average drag value. The flow topology past the cyclist compares satisfactorily between both experiments and with wind tunnel experiments reported in literature. The current measurements clearly separate drag values from upright and time–trial athlete’s positions, indicating the suitability of this principle for aerodynamic analysis and optimization studies.Graphical abstract.

Surfactant-Controlled Photothermal Assembly of Nanoparticles and Microparticles for Rapid Concentration Measurement of Microbes

Light-induced heating on a solid-liquid interface can generate a vapor submillimeter bubble and fluid flow, which enables us to densely and rapidly assemble dispersoids into a desired position (photothermal assembly). Here, we revealed that the surface modulation of the light-induced bubble by a surfactant dominates the assembly dynamics of nanoparticles and microparticles as dispersoids, which results in highly efficient photothermal assembly under the surfactant-controlled fluid flow. This mechanism can facilitate the concentration measurement of small objects (microparticles, bacteria, viruses, etc.). Particularly, we found that the surfactant-controlled fluid flow and bubble enable high-density assembly of dispersoids and remarkable enhancement of assembly efficiency, achieving 10-20 times in comparison with the case of no surfactant. This result can extend the limit of measurable concentration by one order. Furthermore, this study revealed the influence of concentration, size, and constituent material of the dispersoids on the assembly efficiency for the improvement of measurement precision. These findings are crucial for laser-induced assembly for the rapid concentration measurement of various microbes without a cultivation process as bioanalysis, for the high-sensitivity detection of harmful particles, and for the colloidal lithography.

Using the terminal velocity for determining the size of minute gas bubbles in water

Gas dissolved in ordinary tap water is released and forms bubbles when poured into a vessel. The sub-millimeter bubbles move towards the surface with a terminal velocity of only few millimeters per second. The terminal velocity can be used for estimating the average size of the bubbles. The result is confirmed by observing the bubbles visually in a microscope.

Gas-liquid flow of sub-millimeter bubbles at low void fractions: Void fraction prediction using drift-flux model

Drift-flux model is a simple and reliable tool for void fraction prediction in two-phase flows. This work examines its performance in gas-liquid flow of small bubbles (<1 mm) at low void fractions (<10 −1) that resembles bubbly flow in human bloodstream during Decompression Sickness and can be also found in other two-phase applications such as flow boiling in macro-channels. Drift-flux model predictions are compared with experimental data measured in co-current upward bubbly flow for varying gas/liquid flow properties. Water and blood simulant are used as test liquids, while bubble size is controlled using prescribed surfactant (SDS) concentrations. Homogeneous flow model, which is a sub-case of drift-flux model, predicts adequately void fraction in water and blood simulant when the mean bubble size is lower than 200 μm. For larger bubbles in water (mean size up to 800 μm) and in blood simulant (mean size up to 350 μm), the performance of thirteen drift-flux models from literature is examined. Ten of them concern specifically bubbly flow, whereas three models are applicable to a wide range of flow conditions. To our knowledge, this is the first time that drift-flux models are tested at combined low void fractions (between ∼10−3 and ∼10−1) and sub-millimeter bubble sizes, so the determination of drift-flux parameters (distribution parameter, C0 and drift velocity, Ugm) is elaborated. On this account, a bubble size dependent correlation is proposed for Ugm determination.

Gas–liquid flow of sub-millimeter bubbles at low void fractions: Experimental study of bubble size distribution and void fraction

This work studies gas–liquid flow of small bubbles (<1 mm) at low void fractions (<10−1) that is encountered in human bloodstream during Decompression Sickness and is also relevant to two-phase applications such as flow boiling in macro-channels. Two fundamental parameters are experimentally investigated: Bubble Size Distribution (BSD) and void fraction. Experiments are conducted in co-current upward bubbly flow. Water and blood simulant are used as test liquids, while bubble size is controlled using prescribed surfactant (SDS) concentrations. BSDs are determined employing digital image analysis of bubbly flow images captured at three radial positions across the flow cross-section. Volumetric and cross-sectional area averaged void fraction is measured at three axial locations along the flow by Differential Pressure (ΔP) and Electrical Resistance Tomography (ERT), respectively. BSDs are well-fitted by the log-normal distribution. ERT and ΔP measurements are in fair agreement, with void fraction being practically equal along the flow. The influence of gas/liquid phase velocities and surfactant concentration on the measured void fraction and BSDs’ average value and width is discussed in detail. Interestingly, high SDS concentration in blood simulant results in the formation of bubble clusters, whose role on the examined parameters is investigated.

Enhancement of Focused Liquid Jets by Surface Bubbles.

We investigate the enhancement of the velocity of focused liquid jets by surface bubbles preformed on the inner surface of the container. The focused jets are created from the impact on a liquid-filled cylindrical tube at cavitation numbers of 0.37 (strong impact where cavitation is likely to occur on unprocessed surfaces) and 2.1 (weak impact where cavitation does not occur from the impact). The bubbles with a base diameter up to hundreds of micrometers were formed via the process of solvent exchange using air-equilibrated ethanol and water. Our measurements by high-speed imaging show that at both cavitation numbers, the jet velocities with preformed bubbles are significantly higher than those without preformed bubbles. Furthermore, our results show that after the process of solvent exchange, a large number of expanding bubbles are observed at cavitation number of 0.37, indicating that possibly both sub-millimeter and sub-micrometer bubbles on the surface contribute to the jet velocity enhancement. At the cavitation number of 2.1, the surface bubbles are observed to oscillate immediately after the impact. The measurements of the liquid pressure after the impact reveal that at both cavitation numbers, the negative pressure is damped by the preformed surface bubbles, contributing to the increase of the jet velocity. This work sheds light on the crucial role of surface bubbles on the impulsive motion of liquids. Our findings have significant implications for the focusing jet technology, opening the opportunities for jetting fragile samples such as biological samples.

Steam generator leakage in lead cooled fast reactors: Modeling of void transport to the core

Steam generator tube leakage and/or rupture (SGTL/R) is one of the safety issues for pool type liquid metal cooled fast reactors. During SGTL/R, water is injected from high-pressure secondary side to low-pressure primary side. The possible consequences of such an event include void transport to the core that has adverse effects on the reactor performance including heat transfer deterioration and reactivity insertion.This paper addresses the potential transport of steam bubbles to the core and subsequent void accumulation in the primary system in ELSY conceptual reactor. A CFD model of the primary coolant system for nominal operation is developed and verified. Bubble motion is simulated using Lagrangian tracking of steam bubbles in Eulerian flow field. The effects of uncertainties in the bubble size distribution and bubble drag are addressed. A probabilistic methodology to estimate the core and primary system voiding rates is proposed and demonstrated.A family of drag correlations by Tomiyama et al. (1998) provide the best agreement with the available experimental data. Primary system and core voiding analysis demonstrate that the smallest (sub-millimeter) bubbles have the highest probability to be entrained and remain in the coolant flow. It is found that leaks at the bottom region of the SG result in larger rates of void accumulation.

Void fraction, bubble size and interfacial area measurements in co-current downflow bubble column reactor with microbubble dispersion

Microbubbles dispersed in bubble column reactors have received great interest in recent years, due to their small size, stability, high gas-liquid interfacial area concentrations and longer residence times. The high gas-liquid interfacial area concentrations lead to high mass transfer rates compared to conventional bubble column reactors. In the present work, experiments have been performed in a downflow bubble column reactor with microbubbles generated and dispersed by a novel mechanism to determine the gas-liquid interfacial area concentrations by measuring the void fraction and bubble size distributions. Gamma-ray densitometry has been employed to determine the axial and radial distributions of void fraction and a high speed camera equipped with a borescope is used to measure the axial and radial variations of bubble sizes. Also, the effects of superficial gas and liquid velocities on the two-phase flow characteristics have been investigated. Further, reconstruction techniques of the radial void fraction profiles from the gamma densitometry's chordal void fraction measurements are discussed and compared for a bubble column reactor with dispersed microbubbles. Empirical correlations are also proposed to predict the Sauter mean bubble diameter. The results demonstrate that the new bubble generation technique offers high interfacial area concentrations (1000–4500 m2/m3) with sub-millimeter bubbles (500–900µm) and high overall void fractions (10–60%) in comparison with previous bubble column reactor designs.

Controllable Generation of a Submillimeter Single Bubble in Molten Metal Using a Low-Pressure Macrosized Cavity

We develop a method for generation of a single gas bubble in a pool of molten metal. The method can be useful for applications and research studies where a controllable generation of a single submillimeter bubble in opaque hot liquid is required. The method resolves difficulties with bubble detachment from the orifice, wettability issues, capillary channel and orifice surfaces degradation due to contact with corrosive hot liquid, etc. The macrosized, 5- to 50-mm3 cavity is drilled in the solid part of the pool. Flushing the cavity with gas, vacuuming it to low pressure, as well as sealing and consequent remelting cause cavity implosion due to a few orders in magnitude pressure difference between the cavity and the molten pool. We experimentally demonstrate a controllable production of single bubbles ranging from a few milliliters down to submillimeter size. The uncertainties in size and bubble release timing are estimated and compared with experimental observations for bubbles ranging within 0.16 to 4 mm in equivalent-volume sphere diameter. Our results are obtained in heavy liquid metals such as Wood’s and Lead-Bismuth eutectics at 353 K to 423 K (80 °C to 150 °C).

Heat Transfer Enhancement in vertical Mounted Tube Subjected to Uniform Heat Flux by using Electrolysis Bubble

In the present work, experimental and numerical investigations had been carried out to investigate the effect of sub-millimeter bubbles injection on heat transfer coefficient of upward flowing water in vertical mounted tube subjected to uniform heat flux. The experimental apparatus consists of a test rig designed and built to conduct the experiments. A circular tube, test section was designed and constructed from the copper and heated by an electrical heater on its outer surface. The dimensions of copper pipe was (length= 0.7m, diameter= 0.05 m, thickness= 1.5 mm). Water temperature at inlet was kept constant at (32°C). Thermocouples distributed longitudinally at different radial distances between cylinder surface and its center at seven sections, in addition to the fluid inlet and outlet were used to measure temperatures. Bubbles generation was performed in test section by using a proper ionization current that will be passed across the anode and cathode electrodes to produce hydrogen bubbles and oxygen bubbles at different intensities. The experiments were conducted using heat fluxes (13641 and 22736) W/m, water mass flow of (2, 3 and 4) lit/min, mass flow rate of hydrogen and oxygen bubbles were (0.02 , 0.025) lit/min respectively and Reynolds number (1214, 1783 and 2300) for water. The results showed that an enhancement of 25.5% was obtained in the averaged Nusselt number with using ionization bubbles compared with the case without bubbles.

Control of Submillimeter Phase Transition by Collective Photothermal Effect

Local molecular states and biological materials in small spaces ranging from the microscale to nanoscale can be modulated for medical and biological applications using the photothermal effect (PTE). However, there have been only a few reports on exploiting the collective phenomena of localized surface plasmons (LSPs) to increase the amount of light-induced heat for the control of material states and the generation of macroscopic assembled structures. Here, we clarify that microbeads covered with a vast number of Ag nanoparticles can induce a large PTE and generate a submillimeter bubble within several tens of seconds under the synergetic effect of the light-induced force (LIF) and photothermal convection enhanced by collective phenomena of LSPs. Control of the phase transition induced by such a “collective photothermal effect” enables rapid assembling of macroscopic structures consisting of nanomaterials, which would be used for detection of a small amount of proteins based on light-induced heat coagulation.