Bubble Coalescence Research Articles

Coalescence investigations in a small‐scale continuously operated setup for bubble column design

AbstractA novel micro‐channel technique for analyzing the coalescence of bubbles and obtaining relevant information for the creation of a coalescence database is presented. The micro‐channel improves the coalescence investigations by a continuously operated setup, reduces the accumulation of impurities and increases the amount of recorded data. To introduce the new setup, studies with alcoholic, electrolytic aqueous systems and liquid silicone oil as a second liquid are presented, showing the influence of different concentrations. Artificial intelligence has been successfully developed to automate data generation. This approach improves the understanding of bubble coalescence by introducing a reproducible setup. Furthermore, it facilitates the transition to a predictive column design through data‐based decisions and modelling.

Churning-Motion-Assisted Bubble Removal: A Low-Cost Approach for Enhancing PDMS Mixture Quality.

Bubble formation during mixing of the base elastomer and the curing agent for polydimethylsiloxane (PDMS) preparation presents a significant challenge, traditionally addressed through vacuum degassing or centrifugation. This study introduces a novel alternative for bubble removal in PDMS mixtures: a churning motion inspired by industrial dairy separation processes. A low-cost, manually operated, do-it-yourself (DIY) churning device has been developed for this purpose. We investigate the effectiveness of churning in eliminating bubbles across three different churning speeds and two PDMS mixtures with differing viscosities. The efficacy of this method is quantitatively assessed through the analysis of images captured during the churning process. The results demonstrate that bubble removal is notably more efficient in the PDMS mixture with a higher viscosity due to enhanced bubble coalescence. Among the tested speeds, fast churning emerges as the most effective, achieving bubble removal in less than 100 s-significantly outperforming the traditional vacuum degassing method, which requires over 3000 s. These findings highlight churning motion as a rapid, efficient, and cost-effective alternative for bubble removal in PDMS processing, promising significant advancements in material preparation techniques.

Multi-Scale Development and Integration of Granular Activated Carbon Electrodes in Multiphase Electrochemical Systems

Providing clean water remains a significant challenge in underserved areas that lack access to public water systems or are affected by extreme natural events like hurricanes. Electrochemical technologies show promise for developing affordable and effective water treatment systems due to their ability to efficiently oxidize contaminants. Particularly, the electrochemical synthesis of in-situ reagents, such as H2O2, systems can oxidize contaminants over the full range of pH with high oxidation potential, leaving only water and oxygen at the end of process, making it a safe and effective method for water treatment. This study is focused on integrating carbon-based electrodes into electrochemical water treatment systems emphasizing their scalability, cost effectiveness, and the efficient generation of reactive oxygen species (ROS). By employing multiscale computational approaches, we develop a comprehensive mechanistic understanding and optimization of ROS generation at the surface of granular activated carbon (GAC) electrodes. Using reactive molecular dynamics simulations, the thermodynamic and transport properties of water-based electrolyte (e.g., adsorption, viscosity, and diffusion) are investigated as a function of the GAC particle size and chemistry. The obtained results show that tuning the GAC particle structure at nanoscale can improve the local diffusion rates by three times. Subsequently, multiphase continuum simulations are used to integrate the GAC electrodes in full electrochemical systems through optimizing the dispersion and the solid-liquid-gas interactions in a reactive environment. The computational findings reveal that higher production of H2O2 and are observed at optimized electrode porosity values when the GAC electrodes are integrated at the system level. These observations are linked to the complex reactive solid-fluid interactions. Specifically, the electrochemical oxygen bubble splitting and coalescence are minimized by optimizing the electrode porosity, which improves the dispersion mass transfer processes by at least one order of magnitude. This leads to higher generation rates of ROS and enhanced degradation of water contaminants. These mechanistic observations will help in designing and scaling up carbon-based electrodes for developing efficient and cost-effective electrochemical water treatment reactors.

Nanocone-Modified Surface Facilitates Gas Bubble Detachment for High-Rate Alkaline Water Splitting

Alkaline water splitting (AWS) is one of the most attractive technologies for green hydrogen production. In comparison to the acidic proton exchange membrane (PEM), AWS offers greater flexibility in using non-platinum group metal catalysts and could generate higher-purity hydrogen. However, commercial AWS systems are normally operated at a lower current density (~400 mA cm-2) than PEM electrolyzers. To rapidly generate hydrogen at an industrial scale, it is critical to boost the operating current density of AWS to a level of ~1000 mA cm-2. Nevertheless, the significant amount of gas bubbles generated during high-rate AWS could be detrimental to the process. Bubbles can block the catalysts’ active site, hinder electrolyte diffusion, and increase Ohmic resistance, thus deteriorating the reaction rate and restricting the cell voltage efficiency. The detachment of gas bubbles may also damage the catalyst layer. In this study, we demonstrate a general strategy for facilitating bubble detachment by modifying the nickel electrode surface with nickel nanocone nanostructures, which turns the surface into underwater superaerophobic. The simulation and experimental data show that bubbles take a considerably shorter time to detach from the nanocone-modified nickel foil than the unmodified foil. As a result, these bubbles also have a smaller detachment size and less chance for bubble coalescence. The nanocone-modified electrodes, including nickel foil, nickel foam, and 3D-printed nickel lattice, all show substantially reduced overpotentials at 1000 mA cm-2 compared to their pristine counterpart. The electrolyzer assembled with two nanocone-modified nickel lattice electrodes retains >95% of the performance after testing at ~900 mA cm-2 for 100 hours and the surface NC structure is also well preserved, demonstrating its excellent electrochemical and mechanical stability at high current density. Our findings offer an exciting and simple strategy for enhancing the bubble detachment and, thus, the electrode activity for high-rate AWS.

Numerical simulations of particle concentration and size effects in a slurry bubble column with a CFD‐PBM coupled model

AbstractThe combined effects of particle concentration (0–20 vol%) and particle size on the hydrodynamics of a slurry bubble column were studied by computational fluid dynamics coupled with population balance model (CFD‐PBM) in a wide range of superficial gas velocity (0.02–0.20 m/s). This CFD‐PBM coupled model included multiple effects of particles, namely increased slurry viscosity, reduced drag force, promoted bubble coalescence, and attenuated liquid turbulence, among which turbulence attenuation was crucial. To quantify liquid turbulence attenuation, a mechanistic model was established by equating the energy dissipated by particle motion to the attenuated liquid turbulence energy calculated from turbulent energy spectrum. The turbulent energy dissipation rate was reduced by over 60% at a solid concentration of 20%. Using this model, the effects of particle concentration and particle size on overall gas holdup were well predicted. This model advanced our understanding of how particles affect the gas holdup in a slurry bubble column.

Bubble behavior of single-pulse discharge in EDM

The removal of machining debris in the narrow discharge gap primarily relies on high pressure bubble expansion and fluid agitation induced by the bubble pulsation in electrical discharge machining (EDM). Observing bubble behavior and elucidating the mechanism of bubble behavior is an increasing concern in EDM. The purpose of this study is to investigate the bubble behavior of single pulse discharge in EDM. Using a high-speed camera, this study observed the bubble evolution and measured the discharge duration dependent bubble characteristics. The results show that the bubble experiences rapid expansion, contraction, primary pulsation, intense pulsation in the post-discharge process, and subsequent bubble energy dissipation behavior. Furthermore, the arc plasma persists for a duration even after the discharge ends in positive polarity machining. The findings suggest that bubble behavior in single-pulse discharges is primarily dominated by the discharge energy, with the bubble behavior during the discharge significantly influenced by the jumping and oscillation of the arc plasma. Our research may introduce new ideas for improving the depth-to-diameter ratio of deep micro-holes in EDM and provide fresh perspectives on the mechanism analysis of how machining parameters affect machining performance. For instance, by synchronizing the pulse intervals with the bubble stabilization time, bubble coalescence can be reduced, thus increasing bubble removal in the small machining gap. The stability of deep micro-hole EDM may depend on bubble behavior, which is determined by the electrical parameters.

Heat transfer characteristics during bottom-blown bubble coalescence

Bubble swarm evolution and behaviors are the key to study the heat transfer between a bubble swarm and its surrounding flow field, and are of significant and systematic theoretical guidance for exploring the process reinforcement technology and processes. In this study targeted at Therminol®66 and R245fa, we analyzed the three factors affecting gas-liquid two-phase flow. The results show that the flow rate of the dispersed phase has a significant effect on the heat transfer coefficient. When the flow rate of the working medium increases from 0.4 kg s-1 to 0.8 kg s-1, the heat transfer coefficient increases by about 1.25 times. The initial temperature difference is positively correlated with the heat transfer coefficient, but the continuous phase height increases the heat transfer coefficient first and then decreases. The heat transfer coefficient model is in good agreement with the results of Kwon, and the deviation is within 20 %, accounting for 92.464 %, which can be further used to describe the heat transfer process of heat exchanger or bottom-blown melting pool. Correlation equations for the Nusselt number (Nu) is developed, with average absolute deviation of 18.49 %.

Augmentation of pool boiling heat transfer using a microstructured aluminum surface fabricated by ultrasonic cavitation modification

Micro/nanostructure is a popular and effective structure for boiling heat transfer enhancement. In this study, ultrasonic cavitation modification is employed firstly to create microstructured aluminum surface for boiling heat transfer. With the impact of ultrasonic cavitation microjet, numerous micro pits and holes are formed on the surface in one step, constituting microstructures on aluminum plate. The saturated and subcooled pool boiling experiments are conducted at atmospheric pressure using ethanol as a working fluid. It is found that boiling hysteresis is apparent on the modified plate (MP) with the porous microstructure layer, which can be eliminated by periodically fluctuating heat flux. Under the stable saturated boiling condition, the maximum heat transfer coefficient is enhanced by 208% and wall superheat is decreased by 17.6 °C compared to the bare aluminum surface. Under subcooled boiling conditions, the boiling hysteresis on MP becomes weaker and the positive effect of microstructures on heat transfer performance is enhanced. The enhanced heat transfer coefficient of MP is mainly attributed to the increase of nucleation site density and further to the facilitation of bubble coalescence and departure, while the increased critical heat flux under subcooled boiling conditions is derived from the delaying occurrence of mushroom-like bubble.

Experimental study of nucleate boiling heat transfer of refrigerant-oil mixture on various modified surfaces

As a crucial component of refrigeration and heat pump system, the evaporator performance directly affects system efficiency. Due to the lubricating oil in the compressor circulating into the evaporator, studying the nucleate boiling heat transfer (NBHT) of refrigerant-oil mixture is of greater practical and guiding significance. Surface modification technology has opened up a new avenue for enhancing NBHT of refrigerant-oil mixture. Therefore, experimental study was conducted on NBHT characteristics of R134a-POE mixture and R1234ze(E)-POE mixture on smooth surface, initial laser-ablated surface, stabilized laser-ablated surface, machined surface, and composite processed surface. The effects of oil concentration (ω), surface microstructure and surface wettability on NBHT of refrigerant were discussed and analyzed, and the underlying heat transfer mechanisms were elucidated. The research indicates that for NBHT of refrigerant-oil mixture, surface microstructure has a greater impact than surface wettability. To enhance heat transfer coefficient (HTC), it is crucial to establish a surface pattern comprising alternating regions of high and low nucleation sites to mitigate bubble interaction and coalescence. Once this objective is attained, enhancing surface wettability can be contemplated. The addition of oil promotes foaming and induces tornado-shaped bubble group, which facilitate heat transfer. However, the rise in mixture viscosity and surface tension exerts a more pronounced adverse effect on heat transfer, outweighing the positive impacts of foaming and tornado-shaped bubble group. Consequently, the presence of oil diminishes HTC. And as ω and heat flux increase, HTC further decreases. Additionally, the increase in vaporization nuclei on modified surface leads to a higher local ω, the increase in ω further weakens the enhanced heat transfer effect of modified surface, and thus narrows the difference in HTC among various modified surfaces. The research results aim to uncover the mechanisms by which surface wettability, surface microstructure, and lubricating oil affect NBHT of refrigerant.

Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures.

Bubbles and foams are often removed via chemical defoamers and/or mechanical agitation. Designing surfaces that promote chemical-free and energy-passive bubble capture is desirable for numerous industrial processes, including mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, which are textured solid topographies with interconnected gas domains. Plastrons exhibit the remarkable ability of capturing bubbles through coalescence. However, the two-step mechanics of plastron-induced bubble coalescence, namely, rupture (initiation and location) and subsequent absorption (propagation and drainage) are not well understood. Here, the influence of 1) topographical feature size and 2) gas fraction on bubble capture dynamics is investigated. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and is more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This counterintuitive behavior represents unexpected complexities. Upon rupture, the bubble's moving liquid-solid contact line influences its absorption rate and equilibrium state. These findings show the importance of rationally minimizing surface feature sizes and contact line interactions for rapid bubble rupture and absorption. This work provides key design principles for plastron-induced bubble coalescence, inspiring future development of industrially-relevant surfaces for underwater bubble capture.

Radial bubble size distributions in a rising foam column

The diameter distribution of bubbles in foam is one of the most important features in foam-based separation processes like foam fractionation and froth flotation.In this study the bubble size at different radial positions of pneumatically produced foams without coalescence and coarsening of bubbles is investigated in a cylindrical column by employing an invasive sampling probe. It is shown that pronounced differences of the local Sauter-mean diameter of the bubbles can appear in radial direction. Oftentimes a parabolic profile with the largest mean bubble diameter in the center of the column is found. The difference of the Sauter-mean diameter between wall- and center region is in the order of up to 60%. Experiments on foams produced with different spargers, gas flow rates and liquid filling levels reveal that the actual degree of the inhomogeniety depends on the specific bubble size distribution that is produced by the sparger, and becomes more pronounced if the range of bubble diameters in the foam increases. As an explanation for the observations, hydrodynamic interactions in the liquid phase, as well as the behavior of different sized bubbles close to the liquid/foam interface are proposed.The observed existence of local differences of the bubble diameters can have a strong influence the dynamic behavior, like liquid drainage, and measurement methods of pneumatic foams. In particular it can limit the applicability of surface-based bubble size measurements.

PERSPECTIVE: Interfacial stresses in thin film drainage: Subtle yet significant

Film drainage, essential in droplet and bubble coalescence and surface wetting, is influenced strongly by the stress boundary condition, in particular, when interfacial stresses are present. These stresses, caused by ubiquitous surface-active components, significantly impact the dynamics of liquid films. Through dynamic thin film balance experiments, we compare the effects of Marangoni stresses, interfacial viscosity, and interfacial viscoelasticity on the drainage of free-standing thin liquid films. These data serve to demonstrate that film deformation intricately depends on the interplay between these stresses and capillarity, resulting in widely varied drainage times. Seemingly subtle changes, especially in the local stress-carrying capacity of the interface, can lead to significant differences in film dynamics. This makes it a promising area for research into interfacial-rheologically active materials for stabilizing potentially more sustainable multiphase materials.

Microfluidic Insights into the Effects of Reservoir and Operational Parameters on Foamy Oil Flow Dynamics during Cyclic Solvent Injection: Reservoir-on-the-Chip Aided Experimental and Numerical Studies

This study examines the microfluidic characterization of foamy oil flow dynamics in heterogeneous porous media. A total of 12 microfluidic CSI experiments were conducted using reservoir-on-the-chip platforms. In addition, detailed PVT analysis was performed to characterise the heavy oil/solvent systems. Moreover, a numerical model constructed with CMG software package (2021.10) has been validated against the experimental findings in this study. A clear-cut visualization study provided by microfluidic systems revealed that factors including solvent type, pressure depletion rate, and reservoir parameters have a significant impact on foamy oil flow extension. It was found that a solvent containing a higher CO2 content demonstrated more effective performance compared with other solvent compositions, owing to its capability to reduce viscosity, enhance swelling, and offer more gas molecules due to its superior solubility. Additionally, a high pressure-depletion rate amplifies the driving force for bubble nucleation, as well as reducing the amount of time available for bubble coalescence. In addition, lower reservoir porosity impedes bubble movement and delays coalescence, thus extending the foamy oil flow. Furthermore, with the aid of a robust image analysis technique, it was discovered that utilizing 100% CO2 as a solvent resulted in a 17% increase in oil recovery over using 50% CO2 and 50% CH4. Furthermore, a 6% increase in oil recovery was achieved by applying a fast pressure depletion rate as opposed to a slow pressure depletion rate. Moreover, the numerical model constructed was found to be accurate in adjusting heavy oil recovery with an average relative error of 7.7%.

Effects of Inlet Velocity Profile on the Bubble Dynamics in a Fluidized Bed Partially Filled with Geldart B Particles

In this study, a two-dimensional computational domain featuring gas and solid phases is computationally studied for Geldart-B-type particles. In addition to the baseline case of a uniform gas-phase injection velocity, three different inlet velocity profiles were simulated, and their effects on the fluidized bed hydrodynamics and bubble dynamics have been studied. An in-house computer program was developed to track the bubbles and determine the temporal evolution of their size and position prior to their breakup. This program also provides information on the location of bubble coalescence and breakup. The gas-solid interactions were simulated using a Two-Fluid Model (TFM) with Gidaspow’s drag model. The results reveal that the bed hydrodynamics feature a counter-rotating vortex pair for the solid phase, and bubble dynamics, such as coalescence and breakup, can be correlated with the vortices’ outer periphery and the local gradients in the vorticity.

Magnetic resonance imaging of a stream of bubbles injected into liquid suspensions

Magnetic resonance imaging (MRI) is a powerful tool for characterizing opaque multiphase flows non-invasively. However, MRI has often (i) had low temporal resolution and thus not captured transient dynamics, (ii) only provided 2D slice images of 3D flows and (iii) been limited to flows in narrow (∼30 mm) tubes with significant wall effects. Here, we apply multi-band echo planar imaging (MB-EPI) with a custom-built radiofrequency coil in a full-body MRI scanner to provide fully 3D images of the dynamics of a stream of bubbles rising through a dense suspension with 151 ms resolution in a 178 mm diameter system. Image processing demonstrates that bubble rise and coalescence dynamics vary significantly with (a) initial spacing between bubbles and (b) particle volume fraction. The ability to image bubble dynamics in dense suspensions as well as in full 3D provides future opportunities to characterize complex, non-axisymmetric, multiphase flows.

Experimental study of pool boiling heat transfer on hybrid surface coupled micro-pin-finned

The performance of identical hydrophilic-hydrophobic coupled micro-pin-finned surfaces was evaluated using FC-72 as the working fluid in pool boiling experiments. Three hydrophilic and hydrophobic coupled rectangular copper column surfaces (HH4, HH9, and HH16) were constructed to analyze their boiling heat transfer properties. By integrating the micro-pin-finned surface (PF) and the smooth surface (SS), the impact of various subcoolings (0 K, 15 K, and 25 K) on the critical heat flux (CHF) was explored. Surface HH16 showed the greatest sensitivity to subcooling effects on CHF, followed by HH9 and HH4, respectively. For HH9, the CHF at ΔTsub = 0 K, 15 K, and 25 K, reached 69.6 W·cm−2, 84.2 W·cm−2, and 93.7 W·cm−2, respectively. In comparison to other surfaces (PF, HH4, HH16), the CHF of HH9 climbs by 19.32 %, 8.75 %, and 10.8 % at saturated boiling, respectively. A high-quality camera captured bubble dynamics during the experiments. The results reveal that the hydrophilic and hydrophobic coupled micro-pin-finned copper surface has a greater critical heat flux (CHF) than the standard rectangular micro-pin-finned surface, although heat transfer performance (HTC) drops marginally (both saturated and subcooled boiling). Visual monitoring demonstrates that these coupled surfaces effectively prevent bubble coalescence during subcooled boiling. Additionally, this novel surface design may serve as an effective strategy to reduce drying and delay the onset of boiling crises. The CHF improvement of the HH9 on SS surface was 313.06 %, significantly higher than the 246.17 % of PF on SS, marking a performance increase of 66.89 %. The influence mechanism of the Lattice width coefficient and the Vapor column spacing coefficient on CHF were analyzed. Fluid replenishment and bubble formation behavior were applied to clarify the strengthened heat transfer and CHF triggering mechanism on the surface of the hydrophilic and hydrophobic coupled rectangular micro-pin-finned copper column surface.

Numerical and experimental evaluation of flow boiling heat transfer in microchannels for R452B refrigerant

Energy management efficiency is crucial for the increasingly severe global energy challenges. Low global warming potential (GWP) refrigerant R452B used in air conditioning and refrigeration systems could reduce the global warming effect for applications. The objective of this paper is to conduct the numerical and experimental studies for examining the thermal fluid behaviors of flow boiling evolution of refrigerant R452B in the vertical microchannels. In the experimental approach, a gear pump and a programmable DC power supply are employed to regulate the mass flux and heat flux for determining the heat transfer and pressure drop outcomes across the microchannel at varied vapor qualities. A high-speed camera with a LED fiber optical light source is used to observe the close-up optical images over the complex flow boiling process. Theoretically, the volume-of- fluid (VOF) method built in the computational fluid dynamics (CFD) software ANSYS/Fluent® is employed to simulate the progression of bubble nucleation processes for resolving the distributions of velocity, pressure, temperature and liquid volume fraction in the microchannel. The predictions are compared against the measured heat transfer coefficients and pressure drops for the CFD model validation. The time sequences of vapor volume fraction contours are also simulated to characterize the evolving vapor-liquid interfaces for better grasping the detailed behaviors of nucleation, growth, departure, coalescence of bubbles and the transformations of dominant flow patterns. The measured results estimate the average heat transfer coefficients and pressure drops up to 14.1 kW/m2K and 76.5 kPa at a mass flux of 600 kg/m2s in the microchannel. This research further conducts the performance assessments by testing the popular correlations, and thereby reveals the effectiveness of Bertsch as well as Sun and Mishima correlations to reasonably calculate the heat transfer coefficients and pressure drops of R452B refrigerant, involving the mean absolute errors of 12.5 % and 22.6 %, respectively.

Experimental Evidence of Primary Permeability at Very Low Gas Content in Crystal‐Rich Silicic Magma

AbstractEruptive dynamics is influenced by gas escape from the ascending magma. Gas pathways form in the magma via bubble coalescence, leading to gas channeling. Magmatic crystals play a key role in gas channel formation. This work constrains experimentally decompression‐induced coalescence in high‐crystallinity silicic magmas without external deformation, focusing on low gas content and bimodal crystal size (microlites and phenocrysts). All percolating samples have permeabilities of 10−14 m2 at bulk porosities of 7–10 vol% and bulk crystallinities up to 75 vol%. Our results demonstrate the possibility of coalescence‐related outgassing at high pressure (120–350 MPa) and without external strain, which corresponds to magma stagnating deep in a volcanic conduit. Channeling at such low gas content implies that bimodal crystallinity favors effusive over explosive volcanic behavior. It may also be the missing physical mechanism explaining gas transfer across magmatic systems despite high melt viscosity and low or absent magma extrusion.

Observation of Gap Phenomena and Development Processing Technology for ECDM of Sapphire

The main purpose of this study was to develop observation techniques and processing technology for the electrochemical discharge machining (ECDM) of sapphire wafers. To measure the effect of gas-film thickness, discharge-spark conditions, and droplet sliding frequency on machining quality and efficiency in ECDM, this research utilized high-speed cameras to observe the gas film thickness and formation of the gas film during ECDM. Additionally, this study observed the machining-gap phenomena during ECDM. The formation mechanism and machining characteristics of the gas film were understood through experiments. The machining parameters included the liquid level, working voltage, rotation speed, and duty factor. This study analyzed and discussed the effect of each machining parameter on the gas-film thickness, current, electrode consumption, and droplet sliding frequency. Moreover, this study aimed to obtain optimized machining parameters to overcome the difficulty of machining sapphire. The experimental results indicated that utilizing a high-speed camera to capture the phenomena between electrodes during electrochemical discharge could effectively observe the gas-film thickness and the coverage of the gas film. A higher bubble coalescence rate enhanced the machining capability and reduced the lateral discharge. Therefore, this study could obtain better machining-hole depths through observation and analysis to improve gas-film stability and machining capability. This study demonstrated that a liquid level of 700 µm, a working voltage of 48 V, a duty factor of 50%, and a tool electrode rotational speed of 200 rpm could achieve a hole depth of 86.7 µm and a hole diameter of 129.5 µm.

Study on the violent splitting and coalescence characteristics of large-scale rising bubbles

Large-scale rising bubbles exist in some extreme marine engineering fields, such as the exploitation of combustible ice and submarine volcanism. The violent splitting and coalescence process of large-scale rising bubbles with internal pressure comparable to ambient pressure differs greatly from that of the normal small-scale bubbles and high-pressure bubbles. In this paper, the complex splitting and coalescence characteristics of large-scale rising bubbles are studied with a multi-phase smoothed particle hydrodynamics approach. The effects of the arrangement of bubbles, initial centroid distance, and the ratios of bubble radii on the bubble splitting and coalescence characteristics are investigated. The results show that the arrangement of the bubbles dictates whether or not the two bubbles coalesce and influences the coalescence mode of the bubbles further. When two large-scale bubbles are arranged horizontally, the initial spacing of bubbles has minimal influence on the coalescence features, and they always bounce off each other. When the bubbles are not placed horizontally, the bubbles can coalesce in two modes: direct-absorption coalescence and penetrating reverse-absorption coalescence. When arranged vertically, the initial centroid distance affects the coalescence extent of bubbles, and when the vertical distance exceeds five times the bubble radius, the two bubbles do not interact with each other. When the leading bubble is too large relative to the trailing bubble, the large ratios of bubble radii play negative roles in the coalescence of bubbles.