Bubble Generation Process Research Articles

Investigation of multi-inlet arrangement effect on mesoscale bubble dynamics in the gas–liquid-solid reactor via VOF-DEM

Multi-channel gas–liquid-solid systems have attracted considerable attention across chemical, energy, and biochemical industries due to their potential applications, yet the intricate interplay between different scales of interphase interactions remains complex. This study employs the volume of fluid method coupled with the discrete element method (VOF-DEM) to investigate the multi-scale flow hydrodynamics within gas–liquid-solid systems featuring various inlet arrangements. Following thorough validation of the numerical methodology, the intricate interphase interactions, bubble dynamics, and the influence of multi-channel configurations are examined. The findings elucidate that the presence of a densely packed particle bed fundamentally alters the bubble generation process, inhibiting lateral expansion while promoting vertical growth. Consequently, this phenomenon increases the likelihood of collision and coalescence between adjacent rising bubbles. Additionally, higher Reynolds and Weber numbers facilitate vortex shedding and create open unsteady wakes, thereby enhancing mixing effects within the system. Consequently, it is advisable to adjust both the inlet size and gas flow rate to achieve Reynolds numbers exceeding 3000 and Weber numbers surpassing 10. Moreover, reactor design considerations must include proper channel spacing to mitigate the formation of large merged bubbles, which may enhance local stirring but impede global mixing. Optimizing the spacing between gas inlets enhances lateral bubble extrusion by particles, fosters a meandering ascent for bubbles, and boosts both gas–liquid contact area and gas hold-up. The insights obtained from this study provide valuable contributions to understanding the multi-channel flow behaviors in gas–liquid-solid systems and offer practical guidance for the design and optimization of such reactors.

One-Step Preparation of Magnetic Lipid Bubbles: Magnetothermal Effect Induces the Simultaneous Formation of Gas Nuclei and Self-Assembly of Phospholipids.

In recent years, enveloped micro-nanobubbles have garnered significant attention in research due to their commendable stability, biocompatibility, and other notable properties. Currently, the preparation methods of enveloped micro-nanobubbles have limitations such as complicated preparation process, large bubble size, wide distribution range, low yield, etc. There exists an urgent demand to devise a simple and efficient method for the preparation of enveloped micro-nanobubbles, ensuring both high concentration and a uniform particle size distribution. Magnetic lipid bubbles (MLBs) are a multifunctional type of enveloped micro-nanobubble combining magnetic nanoparticles with lipid-coated bubbles. In this study, MLBs are prepared simply and efficiently by a magneto internal heat bubble generation process based on the interfacial self-assembly of iron oxide nanoparticles induced by the thermogenic effect in an alternating magnetic field. The mean hydrodynamic diameter of the MLBs obtained was 384.9 ± 8.5 nm, with a polydispersity index (PDI) of 0.248 ± 0.021, a zeta potential of -30.5 ± 1.0 mV, and a concentration of (7.92 ± 0.46) × 109 bubbles/mL. Electron microscopy results show that the MLBs have a regular spherical stable core-shell structure. The superparamagnetic iron oxide nanoparticles (SPIONs) and phospholipid layers adsorbed around the spherical gas nuclei of the MLBs, leading the particles to demonstrate commendable superparamagnetic and magnetic properties. In addition, the effects of process parameters on the morphology of MLBs, including phospholipid concentration, phospholipid proportiona, current intensity, magnetothermal time, and SPION concentration, were investigated and discussed to achieve controlled preparation of MLBs. In vitro imaging results reveal that the higher the concentration of MLBs loaded with iron oxide nanoparticles, the better the in vitro ultrasound (US) imaging and magnetic resonance imaging (MRI) results. This study proves that the magneto internal heat bubble generation process is a simple and efficient technique for preparing MLBs with high concentration, regular structure, and commendable properties. These findings lay a robust foundation for the mass production and application of enveloped micro-nanobubbles, particularly in biomedical fields and other related domains.

Effect of inorganic cation on dynamic characteristics of bubble generation

Understanding the effect of inorganic cations on dynamic bubble generation is an important precursor for predicting the bubble characteristics in actual coal flotation. Using the high-speed motion acquisition system, we investigated the bubble growth in different inorganic salt solutions. The bubble generation process was divided into Formation, Expansion, and Contraction stages according to the contact angle between the bubble and the capillary as 90°. Na+, Mg2+, Ca2+, and Al3+ were found to promote bubble growth during the Formation and Expansion stages while restraining the growth during the Contraction stage. Al3+ roughly presented the most obvious promotion of the bubble generation. The dynamic bubble width and length during different bubble growth stages were analyzed to discuss the bubble pressure. The internal pressure increased as the bubble grew. The pressure difference inside and outside the bubble generally followed the order Na+ > Mg2+ > Ca2+ > Al3+. The forces acting on the bubble were attempted to be discussed. The net force acting on the bubble first kept constant and then decreased with the concentrations of Na+, Mg2+, and Ca2+, while it first decreased and then increased in the presence of Al3+. Our results can provide valuable insight into the development of the technology for mineral flotation.

Generation of microbubbles at high gas concentrations via cavitation

A novel method of continuous microbubble generation is proposed in which an air–water mixture is pressurized in a chamber (up to 47 atm), leading to complete gas dissolution, before being expelled through a restriction to induce bubble nucleation. This approach differs from existing methods for small-scale bubble generation due to the high pressure gas–liquid mixture, nucleation approach, and large gas concentrations, which range from saturation ratios of 2.85 to 9.56 at normal temperature and pressure. The flow of bubbles produced by the system is characterized through optical microscopy to determine the influence of various operating parameters on the volumetric bubble size distributions (VBSDs) at a fixed flow rate. In particular, the gas concentration, compressor pressure, restriction diameter, and restriction length are varied for two classes of restrictions: capillaries with large length-to-diameter ratios, and orifices with comparatively low ratios. VBSDs are modestly polydisperse under all conditions, but with tunable distributions in the 10–70 μm diameter range. The VBSDs for each configuration collapse when non-dimensionalized by the Sauter mean diameter, which provides an opportunity to describe bubble size distributions using a predictor model for this reduced order quantity. The coefficients from the predictor model reveal a low sensitivity of the Sauter mean diameter to the gas concentration and a reduction of bubble diameters when the residence time of solution in the restriction is decreased. Analysis of the technique of bubble generation yields an energy balance to quantify the relative energy ratio of the bubble generation process, which is presently less than 0.2%. Insight from the energy balance suggests that the process could be optimized to generate smaller bubbles.

Characteristics of micro-discharge process in saline solution with pin-to-pin electrodes driven by a low-voltage high-frequency AC power supply

Low-temperature plasma ablation has been clinically used in minimally invasive surgeries. However, there is still a lack of research on its discharge process and ablation mechanism. This paper investigates the bubble generation process and micro-discharge phenomena of pin-to-pin surgical electrodes in NaCl solution driven by a high-frequency AC power supply at a level of (100–150) V. Microbubbles will occur around electrodes and merge to form a vapor layer that can completely cover the electrodes. Then, micro-discharges in the form of microspark would occur around the grounded electrode. The effects of geometrical and electrical parameters on the generation of vapor layers and micro-discharges are analyzed by the statistical results. It is found that the conductivity of the solution has an important influence on the generation probability and stability of vapor layers together with the occurrence position of micro-discharges. The simulation results of the discharge process and the experimental results match well with each other, and they demonstrate jointly that the discharge process is mainly influenced by the electrolytic effect.

Surfactant effect on mass transfer characteristics in the generation and flow stages of gas–liquid Taylor flow in a microchannel

The understanding of surfactant effect on mass transfer behaviors in microchannel devices remains insufficient even though it has been frequently added into gas–liquid systems to promote process stability. Accordingly, this work concentrates on the surfactant effect on mass transfer performances by decoupling the whole process as bubble generation and flow stage. The results show that the role of gas absorption during bubble generation process always exceeds 30 % of the total amount in the microdevice. The surfactant concentration remarkably influences the mass transfer in the bubble generation stage but slightly affects the flow stage. Importantly, the mass transfer coefficient in the generation stage is dozens of times that in the flow stage, and the inherent mechanism is revealed. Finally, the semi-empirical correlations containing the surfactant content are developed for mass transfer coefficients in two stages. This work provides some indispensable information about the selection of surfactants in microfluidic technology.

Experimental and Numerical Analysis of the Influence of Microchannel Size and Structure on Boiling Heat Transfer

Computational fluid dynamics was used and a numerical simulation analysis of boiling heat transfer in microchannels with three depths and three cross-sectional profiles was conducted. The heat transfer coefficient and bubble generation process of three microchannel structures with a width of 80 μm and a depth of 40, 60, and 80 μm were compared during the boiling process, and the factors influencing bubble generation were studied. A visual test bench was built, and test substrates of different sizes were prepared using a micro-nano laser. During the test, the behavior characteristics of the bubbles on the boiling surface and the temperature change of the heated wall were collected with a high-speed camera and a temperature sensor. It was found that the microchannel with a depth of 80 μm had the largest heat transfer coefficient and shortest bubble growth period, the rectangular channel had a larger peak heat transfer coefficient and a lower frequency of bubble occurrence, while the V-shaped channel had the shortest growth period, i.e., the highest frequency of bubble occurrence, but its heat transfer coefficient was smaller than that of the rectangular channel.

Underwater bubble escape volume measurement based on passive acoustic under noise factors: Simulation and experimental research

Passive acoustic methods are suitable for long-term seabed gas leak monitoring with high accuracy under the condition of low cost and simple experimental equipment. However, the impact of noise in complex underwater environments poses great challenges for measurements. In this paper, aiming at the quantitative detection problem, a theoretical model of the bubble generation process by underwater gas escape is established, and the mechanism of bubble acoustic signal generation is clarified. The model identification of bubble volume oscillation is realized through the complementary ensemble empirical mode decomposition (CEEMD) method. Then continuous bubbles are segmented by the normalized energy method to achieve a high-precision measurement of volume inversion. A measurement prototype is developed and tested in the lake with multiple noises caused by seawater disturbance, sea wind, and ships. The results showed that the measurement errors could be kept within 10% even under the condition of strong noise interference.

Study on the recovery characteristics of disc-type superconducting fault current limiter coils from the perspective of boiling heat transfer and bubble behaviors

The disc-type superconducting fault current limiter (SFCL) encapsulated by yttrium barium copper oxide (YBCO) superconducting material has broad application prospects in the current limiting field. However, the difficulty to recover quickly and safely after quench remains the bottleneck of the disc-type SFCL. Existing work on the SFCL mainly focused on the resistance recovery while the bubble behavior and distribution, which are important factors affecting the temperature distribution of the superconducting tape, are rarely noticed. In this paper, a visualization platform for the superconducting tape impulse experiment is built, and the dynamic behavior and distribution characteristics of bubble clusters are analyzed by image processing technology. Two arrangements (vertical and horizontal) disc-type coils are comparatively studied. Firstly, the huge differences in the recovery performance of the superconducting tape at different positions are observed and compared. In the vertical coil, the fastest one can recover in 1.414 s and the slowest one takes 2.426 s to recover to the superconducting state. The maximum temperature difference of superconducting tapes at different positions in the coil is up to 10.8 K. In the horizontal coil, the recovery times of the two samples are 1.497 s and 1.595 s, respectively, indicating that the recovery is significant uniform. The whole process of bubble generation, merging and detachment in the transient boiling process is comprehensively and qualitatively analyzed with the help of the high-speed camera. Secondly, it is observed a phenomenon of bubble retention is common in the vertical coil while the horizontal coil is not, which is not conducive to the safe operation of the system. Therefore, the bubble recovery is as important as the resistance and the temperature as a recovery reference. In summary, the horizontal coil is superior to the vertical coil in terms of safe recovery and uniform cooling based on the experimental results in this paper, the coupling analysis of bubble dissipation behavior and thermodynamic characteristics of superconducting tape during quench and recovery is of theoretical significance to the future research work on accelerated recovery and the SFCL structural design.

Bubbles help male flowers overcome obstacles during pollination in Hydrilla verticillata.

Pollination in many aquatic plants takes place on the water surface, and the male flowers or stamens often produce gas bubbles underwater; however, the generation mechanism and function of these bubbles are unknown. A common submerged plant, Hydrilla verticillata, was used as experimental material to observe the structure of male flowers, analyze the process of bubble generation, and simulate the movement process of the male flower with attached gas bubble in water. The aerenchyma inside the male plants of H. verticillata transported the gas produced by the plant's branches during photosynthesis to the male flower, and the formed gas bubbles became attached to the edge of the perianth. The gas accumulation rate in the attached bubbles increased with light intensity. Once the bubble diameter increased to approximately 3.3 mm, the male flowers with the bubble detached from the plant and floated to the water surface. The removal of the attached bubbles did not affect the male flower detached from the plant; however, the surfacing of male flowers without gas bubbles was easily prevented by the plant's branches in the water, and they could not reach the water surface to complete pollen dispersal. The gas bubbles produced by male flowers of H. verticillata came from the gas produced by branches under light. These bubbles can help ascending male flowers bypass the obstacles in water and reach the surface to complete pollination.

Using modified nano-silica to prevent bubble Ostwald ripening under low atmospheric pressure: From liquid foam to air-entrained cement mortar

Air bubble instability leads to the deterioration of air-void structure and threatens the frost resistance of air-entrained concrete, especially in plateau low atmospheric pressure environment. In this study, modified partially hydrophobic nano-silica (NS) particles are used to stabilize entrained air bubbles in cement mortar under low atmospheric pressure. The experimental results indicate that low atmospheric pressure has a limited effect on the bubble size distribution during the bubble generation process. However, the accelerated Ostwald ripening of the bubble system under low atmospheric pressure leads to a significant increase in the mean bubble size and air content loss. Furthermore, the modified NS shows a noticeable bubble stabilization effect. When adding 3% modified nano-silica (NS/MPS), the air-void structure of cement mortar does not show deterioration under 60 kPa. The analysis of bubble shells in foam, fresh and hardened paste system revealed that the NS/MPS with suitable wettability could strongly adsorb at the air-water interface and form adsorption layers around bubbles, thus reduce the gas permeability across the bubble films and prevent the Ostwald ripening of bubbles. This work finally provides a viable method to stabilize air bubbles and improve the air-void structure of concrete in some extreme environments.

Effect of gas injection rate on bubble generation characteristics and coal flotation

Using a high-speed motion acquisition system, the effect of gas injection rate on dynamic bubble characteristics in the presence of flotation reagents was investigated, and the force analysis during the bubble generation was performed. The corresponding coal flotation tests were conducted to discuss the relationship of the bubble characteristics and the coal recovery. According to a partition criterion of contact angle as 90° between the bubble and the capillary, the bubble generation process was divided into formation, expansion and contraction stages, and the generation time was majorly allocated to the expansion stage. As the gas injection rate increased, the bubble aspect ratio decreased, the bubble size increased and the bubble generation time decreased. The surface tension was analyzed as the main force for adhering the bubble on the capillary, and the buoyancy was deemed as the main force detaching the bubble from the capillary. Furthermore, the increase in net force was analyzed as the essential factor for the increase in bubble generation rate during the expansion stage. The combustible matter recovery increased as the gas injection rate increased, and the flotation rate constant was analyzed closely related to the bubble deformation. The results can provide valuable insights into the development of technology for mineral flotation.

Numerical investigation on spilling upward performance of hydrogen bubbles inside a delivery tube under low-gravity environment

A typical cryogenic upper stage mission requires a reliable liquid–gas phase separation prior to engine restart in space, and auxiliary engines are usually used to provide positive acceleration effect to satisfy vapor-free liquid supplement purpose. To assist thruster logic design for an upper stage, bubble distribution inside a delivery tube and bubble spilling behaviors should be known previously. In the present paper, a CFD approach is adopted to numerically study the bubble generation and bubble movement in liquid hydrogen (LH2) delivery tube under microgravity, and the bubble spilling upward behaviors under different situations are significantly analyzed. The results show that space heat from surroundings brings about liquid evaporation inside the delivery tube, and weak convection effect induced by bubble generation process could result in random bubble distribution in the whole tube range. The maximum bubble reaches the size of tube inner diameter. Under positive acceleration effect, the bubbles spill upward and bring about gas amount decrease in the delivery tube range. Moreover, the bubble spilling upward is not a continuous process, and only the big bubble discharge exerts a significant contribution to the gas amount decrease. In addition, the tube layout and the initial gas amount have slight influence on the bubble discharge performance, but the space acceleration level plays the primary effect on this process. For the present objective, the acceleration with 10−3g0 could drive the bubble to be discharged within 700 s. When the acceleration increases to 10−1g0, the required time correspondingly decreases to less than 100 s. Under the acceleration level of 10−4g0, however, the bubble could not spill upward any more but a vapor amount increase phenomenon is observed. Generally, the present work provides an opportunity to understand the bubble properties inside the cryogenic delivery tube in microgravity, and the results on acceleration influence as well as acceleration period are beneficial to the sequence design of space engine restart.

Measurement of the air bubble size and velocity from micro air bubble generation (MBG) in diesel using optical methods

In this paper, we determine the bubble size and velocity from air bubble generation (MBG) in a diesel using optical methods. A KTM Series Pump was used to generate micro air bubbles in diesel. The air bubble radius and velocity measurements can be useful parameters to optimize the bubble generation process. Two optical systems were used for measurement air bubble sizes and their velocities in diesel. First, the optical system without an objective lens was used to determine the velocity of air bubbles in diesel. Another optical system with a 10× objective lens was used to obtain the size distribution of air bubbles generated in diesel. An available optical system with a 10× objective lens can detect a bubble diameter greater than 3.3 µm that air bubble images were processed using the ImageJ program. We measured the size distribution of air bubbles generated using the ImageJ program. The micro air bubble radius measured in diesel was found to be 6.26 µm in the sample after a month from air bubble generation. In addition, the particle image velocimetry (PIV) technique was used to measure the velocity field. Then, we used the OpenPIV program for PIV image processing. The highest velocity distribution was determined to be 90 mm/s for diesel without air bubbles and 20 mm/s for diesel with air bubbles after a month of the bubble generation.

Mechanism and numerical model of bubble effect in oil-paper insulation based on microtubule model

In this paper, a numerical model of bubble evolution is established by studying the microstructure of pressboard and the physical process of bubble generation. In the model, the porous structure of fibers is equivalent to microtubules, the gases in the bubbles are classified into two categories, and the ideal gas state equation is used to describe the bubble state. Then the pressure conditions at the bubble boundary and the evaporation rate of water are taken as solution conditions to solve the physical quantities in the state equation. The numerical model provides a new way to calculate the initial temperature of bubble escape (ITBE). And the bubble evolution inside the pressboard is obtained which is hard to be observed in experiment. After that, the influences of moisture content of pressboard and cellulose aging on ITBE in the numerical model are discussed. The model results show that the expansion rate of bubble is not uniform. Bubble growth is slow in the early stage of temperature rise, when the temperature reaches a certain threshold the growth rate of bubble radius increases significantly, which is of great significance to the temperature limit on oil-immersed power transformers.

A comparative study of bubble formation characteristics in non-Newtonian and high-viscosity Newtonian fluids by a laser image technique

A laser image measurement system has been established to study the problem of dynamic bubble formation from a submerged orifice in two kinds of high-viscous fluids, namely Newtonian glycerol aqueous solution and non-Newtonian carboxymethylcellulose (CMC) aqueous solutions. The bubble generation process was measured accurately by means of He-Ne laser as light source using the beam expanding and light amplification technology. The whole formation process, including bubble expansion, stretching and detachment three stages, and the dynamic characteristics of each stage, were investigated experimentally by analyzing comparatively the evolution of the instantaneous shape, volume, surface area, and deformation rate in both fluids. The influences of solution mass concentration, gas chamber volume and orifice diameter on bubble detached volume were discussed respectively, and the results reveals that bubble begins to grow spherically due to the predominant role of surface tension, and then is elongated gradually owing to the increasing buoyancy force. Compared with Newtonian fluid, bubble formed in non-Newtonian fluid presents a slim shape as a result of the shear-thinning effect of CMC solution. Bubble detachment volume in both kinds of fluids increases with solution mass concentration and gas chamber volume, but declines with orifice inner diameter, respectively.

Comparison of Two Gas Injection Methods for Generating Bubbles in a T-junction

In this work, we compare the performance of two different methods for generating bubbles in conditions relevant to microgravity. A T-junction formed by two cylindrical (1 mm of internal diameter) and perpendicular channels is used to generate trains of non-wetting bubbles. Air and distilled water are used as dispersed and continuous phases, respectively. The difference between both methods lies in the strategy used to inject the fluids. In one case (here referred to as TJ-A configuration), the gas is injected into the side channel. In the other case (referred to as TJ-B), the gas is injected into the main channel. Bubble size dispersion is analyzed and quantified with the polydispersity index. The increase of the capillary number (based on the continuous phase) has shown a strong influence on the regularity of the bubble generation process. The TJ-A method can produce monodisperse bubbles at higher capillary numbers than configuration TJ-B. The limits of monodispersity, as far as the capillary number is concerned, are studied. These limits determine the optimal operating range of the T-junction for generating highly monodisperse bubbles. Beyond these limits, polydispersity increases, although much more abruptly in TJ-B than in TJ-A. The bubble generation frequency is also studied, with special attention to the two main regimes that characterize the frequency, i.e. the linear regime (at very low gas flow rate) and the saturation regime (at high gas flow rates). A new dimensionless expression of the bubble generation frequency is proposed in this work.

Direct Visualization of Ultra-Fast and Micro-Scale Bubble Evolutions and Electrochemical Reactions in Proton Exchange Membrane Electrolyzer Cells

The electrochemical reactions and two-phase flow that occur in energy devices could be a very important part in promoting energy efficiency of both engineering and sciences. The electrochemical reactions only occur at a micro scale at the center part of the electrochemical devices like fuel cells and electrolyzers. The objective of this research is to visualize the bubble generation and growth process inside a proton exchange membrane electrolyzer cell (PEMEC) by taking advantage of the thin/well tunable liquid gas diffusion layer (LGDL) and high-speed visualization system. The effects of temperature, current density, and flow rate on bubble dynamics in pore scale and multiphase flow in channel scale, including bubble nucleation, growth, and detachment, are investigated. The reaction sites and rapid bubble evolution phenomena are captured under various operating conditions. The number of reaction sites and bubble growth rates increases with the operating current density and temperature, while the flow rate in the channel has limited effects. The gas bubbles are observed to emerge at the edge of the LGDL pores and gradually detach from the reaction sites. This work will provide a pathway to further investigate the electrochemical reaction in energy devices.

The whole process, bubble dynamic analysis in two-phase transport of the passive miniature direct methanol fuel cells

In this paper, the force controlling bubble formation is calculated in anode reaction region by establishing a two-phase dynamic model. The whole dynamic process of bubble generation, growth, merging and detachment in porous media and microchannels are analyzed in depth. The effects of methanol concentration and discharge current on the bubble in the pores of the diffusion layer are analyzed, and the cross-section effect and hydrophobic effect of the flow channel are simulated. Besides, through visual experiments, a further understanding of cell performance, anode bubble growth characteristics and the behavior of bubbles are established to verify the simulation. Simulation and experimental results both show that the size and number of anode CO2 bubbles are highly correlated with cell operating conditions. More and smaller bubbles are easily generated when the anode reaction is more intense.

Distributor performance in a bubbling fluidized bed: Effects of multiple gas inlet jet and bubble generation

This study considers the role of the distributor effect during the bubble generation, growth and interaction within a bubbling fluidized bed. To characterize this effect, short-term and long-term dynamics are explored in time and frequency domains. Controversy occurs when inspecting what is actually known of the gas distribution mechanism in bubbling fluidized beds. To sustain discussion, classical and innovative techniques, such as Digital Image Analysis, DIA, and Wavelet Analysis WA, are used. As a result, the conclusion is that the distributor not only has no effect in the averaged long-term dynamics but also has no effect on the general bubble generation characteristic frequencies. The analysis performed on the bubble generation region concludes that the distributor design has a great influence at the short-term scale such as during the bubble generation process controlling the spatial distribution of the bubble nucleation sites over the distributor plate.