Bubble Breakup Research Articles

Experimental study on condensing flow and heat transfer characteristics in minichannels under mechanical vibration conditions

Minichannel heat exchangers are often used in the condensers of vehicle air conditioners, and vibration is inevitable in vehicle driving. However, the influence of mechanical vibration on the characteristics of minichannel condensing flow and heat transfer has been rarely studied. Therefore, the experimental study of condensing flow and heat transfer in minichannels under mechanical vibration was carried out in this paper. With R134a as the working medium, the condensation heat transfer coefficient, frictional pressure drop and flow pattern images were measured in the saturation temperature of 40 °C, mass flux density of 200 kg/(m2·s), vapor quality range of 0–1, vibration frequency of 15–50 Hz and amplitude of 0–2.4 mm. For annular flow, intermittent flow and bubble flow, the vibration can enhance the wave of gas-liquid interface, and promote the breakup of long bubbles and the polymerization of small bubbles. Vibration enhances heat transfer in most cases, but weakens heat transfer in some specific cases. The frictional pressure drops under vibration conditions are greater than those under static conditions, which indicates that the influence mechanism of vibration on heat transfer and pressure drop is different. This paper attempts to analyze the complex mechanism of vibration affecting minichannel condensing heat transfer and pressure drop, which provides theoretical support for further study of condenser optimization design under vibration conditions.

Process Intensification of Gas–Liquid Separations Using Packed Beds: A Review

The gas–liquid multiphase process plays a crucial role in the chemical industry, and the utilization of packed beds enhances separation efficiency by increasing the contact area and promoting effective gas–liquid interaction during the separation process. This paper primarily reviews the progress from fundamental research to practical application of gas–liquid multiphase processes in packed bed reactors, focusing on advancements in fluid mechanics (flow patterns, liquid holdup, and pressure drop) and the mechanisms governing gas–liquid interactions within these reactors. Firstly, we present an overview of recent developments in understanding gas–liquid flow patterns; subsequently we summarize liquid holdup and pressure drop characteristics within packed beds. Furthermore, we analyze the underlying mechanisms involved in bubble breakup and coalescence phenomena occurring during continuous flow of gas–liquid dispersions, providing insights for reactor design and operation strategies. Finally, we summarize applications of packed bed reactors in carbon dioxide absorption, chemical reactions, and wastewater treatment while offering future perspectives. These findings serve as valuable references for optimizing gas–liquid separation processes.

Relation between bubble behaviors in spray sheet and bubbly flow inside an air-induction fan nozzle: A visualization study

The present study aims to elucidate the relationship between bubble behaviors in the spray sheet and internal bubbly flow of the air-induction nozzle. An experimental work was performed using the visualization technique. Effects of the air inlet position and spray pressure were investigated. The results show that compared with the bubbles inside the air-induction nozzle, bubbles in the spray sheet have smaller volume but larger average diameter. Disturbance propagates in the horizontal air-inlet segment. When the air inlet position shifts toward the nozzle outlet, overall bubble volume inside the nozzle decreases by about 56%, while in the spray sheet, the bubble volume decreases by about 77%. Bubble breakup causes a decrease in overall bubble volume as bubbles travel from the inner flow passage of the nozzle to the environment. The coalescence and compression of bubbles induce the increase in average bubble diameter. When the spray pressure increases from 0.1 to 0.3 MPa, both the total bubble volume and average bubble diameter increase.

Two-phase flow evolution and interfacial area transport downstream of the mixing-vane spacer grid in rod bundle channels

This study investigated the axial two-phase flow evolution and interfacial area transport characteristics downstream of the mixing vane spacer grid (MVSG) in a tight lattice rod bundle channel with a subchannel hydraulic diameter of Dh = 17.27 mm. The experiment was conducted under the air-water two-phase flow at room temperature and atmospheric pressure. The phase distributions of 6 positions downstream of MVSG (Z/Dh = 9.15, 14.94, 20.73, 26.52, 32.31, and 61.26) were measured utilizing a self-developed double-layer wire-mesh sensor (WMS). 42 cases were obtained, involving the bubbly flow, cap-bubbly flow, and slug flow. Void fraction, bubble velocity, interfacial area concentration (IAC), and bubble size distribution (BSD) databases were built. Under the group-1 (G-1) flow, due to the swirling flow generated by MVSG, the G-1 bubbles were drawn from the bundle surface and the gap region into the subchannel center to form a spindle core-peak distribution. BSD results demonstrate that the swirling flow promotes bubbles’ coalescence. The one-dimensional (1-D) void fraction and IAC downstream of the MVSG decrease first and then recover, which contrasts with the non-mixing vane spacer grid (N-MVSG) effect. It was attributed to the increment of the bubbles’ velocity after MVSG due to the bubbles’ migration to the channel center and coalescence. Under the group-2 (G-2) flow, MVSG intensifies the core peak distribution of void fraction, and the void fraction profile is also spindle-shaped. The obvious bubbles’ break-up occurs at Z/Dh = 14.94 attributed to the swirling decay. The 1-D void fraction and IAC transport indicate, that with the liquid-velocity increment, the MVSG effect is different from the N-MVSG effect, which is mainly achieved by affecting the G-1 bubbles’ dynamic behaviors. The model evaluation utilizing the interfacial area transport equation (IATE) closing bubble interaction source/sink terms indicates that the advection effect dominates the IAC evolution and the contribution of the pressure effect is weak. The remarkable bubbles’ coalescence occurs under the high liquid velocity. In future studies, the additional bubble break-up and coalescence source/sink term model considering the MVSG effect should be developed based on these cases to improve the IATE prediction ability.

Unsteady numerical calculation of flow boiling heat transfer in microchannels with Kagome structures

A three-dimensional microchannel flow boiling model was developed based on the VOF model and the Lee model, and the flow and heat transfer in microchannels with Kagome structures were numerically simulated. The characteristics of bubble flow patterns, gas phase distribution, and heat transfer were analyzed under the heat fluxes of 5–8 MW/m2 and flow velocities of 0.2–0.5 m/s. The results show that at a flow velocity of 0.5 m/s, the disturbances caused by fluid impact on the Kagome structure result in bubble break-up, effectively suppressing the formation of large bubbles. However, at a flow velocity of 0.2 m/s, bubbles tend to be adhered on the surface of the Kagome structure, causing local blockage and deteriorating heat transfer. Compared to the flow velocity of 0.5 m/s, the average heat transfer coefficient in the microchannel decreases by up to 75.4 % and the pressure loss decreases by 45.38 % at the flow velocity of 0.2 m/s.

Numerical investigation of dynamic gas–liquid separator by population balance model

Dynamic gas–liquid separator (DGLS) can efficiently separate gas and liquid phases and are widely used in aerospace, chemical, and petroleum engineering. The energy loss and separation efficiency within the DGLS are studied through the combination of numerical simulations and experiments. Three-dimensional transient Reynolds-Averaged Navier–Stokes equations were solved to analyze the fluid dynamics within the DGLS. The bubble aggregation and breakup in oil were simulated by using the population balance model. Experimental data were meticulously compared with numerical results to validate the accuracy and reliability of the numerical methods. The findings revealed a direct correlation between the inlet flow rate and various performance metrics of the DGLS. Specifically, as the inlet flow rate increased, the energy loss within the DGLS escalated, resulting in higher power consumption. The degassing rate of the DGLS exhibited a decreasing trend with increasing inlet flow rate, while the de-oiling rate showed an inverse relationship. The optimal performance of the separator was observed at an inlet flow rate of 140 m3·d−1, with ηg* and ηl* reaching 0.94 and 0.99, respectively. The relationship between the Qo and the η* and Po was fitted by a second-order polynomial. Moreover, the rotational speed of the DGLS demonstrated a positive correlation with energy consumption, accompanied by an increase in power output. However, the separation efficiency of the DGLS exhibited a non-linear relationship with rotational speed, peaking at a specific value before marginally declining. The optimization of degassing and dewatering rates occurred at a rotational speed of 2500 r·min−1. These findings underscore the importance of carefully adjusting operational parameters to achieve optimal performance and energy efficiency within DGLS.

A data-driven approach to analyze bubble deformation in turbulence

Bubble deformation and breakup from turbulence is present in many engineering applications and in nature, yet the physical mechanisms still remain poorly understood. Depending on the local turbulence intensity or Weber number, a bubble may either deform without breakup, suffer a violent breakup, or exhibit a resonant behavior, where the turbulent eddies excite the bubble's natural frequencies. Recent studies have used spherical harmonic decomposition to analyze bubble interaction with turbulence, quantifying the deformation energy of each eigenmode. However, this approach is only applicable for small levels of deformation (linear regime), while the bubble shape remains close to a sphere. In the present work, we present a novel data-driven approach combining large deformation diffeomorphic metric mapping and proper orthogonal decomposition, which is more robust for large deformations. The method is tested on a set of validation cases and applied to turbulent bubble deformation cases obtained from direct numerical simulations data.

Trajectory-based breakup modelling for dense bubbly flows

A new model to predict the breakup of gaseous bubbles in a continuous liquid phase is developed. In the model each bubble is modelled as a spring–damper system, namely a Kelvin–Voigt element, while the outer force is derived by a Lagrangian analysis determining the largest stretching rate of the flow field below. The developed model is based on physical principles and no further arbitrary parameters have to be adjusted. Each bubble is observed on its way through the bubbly flow individually, taking into account its history along its respective path. With the implemented model numerical simulations in a wide range of scales are conducted, ranging from the laboratory scale of a vessel of 3L to the large industrial scale of 15m3. The simplicity of the model allows for a good cost to benefit ratio. In the present work, the achieved results are compared to experimental data obtained from optical measurements in a replica of a 200L aerated stirred tank reactor for various stirrer frequencies.

Coupled Wall Boiling and Population Balance Model for High Void Fraction Flows Under Sub-Cooled Nucleate Boiling Regime: Model Development and Validation

Abstract The subcooled flow boiling of water in a vertical annulus channel is studied numerically at low-pressure conditions. The two-fluid model is developed with flow-regime dependent interfacial transfers for mass, momentum, and energy using the algebraic interfacial area density (AIAD) framework. A discrete population balance model is used to mechanistically determine the vapor bubble diameter in the flow channel by considering the bubble aggregation and breakup effects. Energy balance at the heated wall for the subcooled nucleate boiling is handled using a suitable wall boiling model. A coupling is achieved between the discrete population balance and the wall boiling model for the nucleation and the growth rate of the vapor bubbles along the heated wall. The developed model simulates the reference experimental cases of flow boiling in a vertical channel for various flow and thermal conditions. At low wall heat flux, the wall boiling generates vapor bubbles near the heated wall and within the bubbly flow regime. With an increase in the wall heat flux, the aggregation and evaporation cause the formation of larger bubbles, which progress toward the flow channel core region, a phase that is representative of the transitional flow regime. The model's capability to predict such flow regime transition is validated with the experimental results. The bubble aggregation is found to be dominant compared to the breakup, and thus, proper choice of the aggregation factor is important for the accurate prediction of vapor parameters for the subcooled flow boiling at low-pressure conditions.

CLSVOF simulations of H2 bubble rising through liquid metal bath covered with a thick slag layer

Abstract The current study presents a fundamental understanding of the behavior of different-sized hydrogen bubbles rising in a new slag – metal system featured with a thick upper slag layer and ever-changing slag thickness and slag physical properties in the H2-based smelting process, based on newly developed CLSVOF model simulations. The evolution of bubble shape, bubble rising velocity, and the formation, collapse and detachment of metal liquid film wrapped on the bubble are predicted. The results show that decreasing the slag layer thickness or the slag viscosity delays the detachment of the metal liquid film from the bubble, increases the bubble rising velocity in the metal layer, but make no difference to the bubble terminal rising velocity in the slag layer. As a result, the bubble has shorter rising time in the bath with a higher slag viscosity. The resistance of the slag – metal interface to the bubble decreases with the increase of the bubble diameter, and small bubble therefore passes through the interface more slowly than the large one. In addition, the bubble shape evolution and bubble breakup in the slag layer are complicated with the increase of the slag thickness, bubble diameter and slag viscosity.

Research of Bubble Breakup and Influencing Factors in Flotation Machine

As the core factor of flotation, bubbles have an important effect on flotation. The rotor speed, initial gas parameters and impeller structure of the flotation machine will affect the formation and movement of bubbles. It is very important to study the influence mechanism of operating parameters of flotation machine on the bubble breakup process for flotation machine design and structure optimization. This paper takes KYF-0.2m<sup>3</sup> flotation machine as the research object, establishes a single bubble analysis model, adopts the VOF (Volume of Fluid) method to analyze the influence of different initial positions of bubbles on the bubble breakup behavior, and studies the influence of impeller speed and initial position of bubbles on the bubble breakup. Result show that the breakup of bubbles mainly occurs near the stator region. With the increase of rotational speed of the impeller, the centrifugal force and the disturbance of the convection field will become greater, the time of the bubble breakup become shorter, more bubbles breakup and generate more smaller ones. With the bubble position is closer to the rotating axis of the impeller, the impact of reflow becomes stronger and the bubble breakup effect will be better, and if the bubble initial position closer to the impeller cover, the influence of impeller on the bubbles become greater and the distribution of bubbles will be more uniform.

Spatial characteristics study of hydrodynamics and bubble dynamics in baffled stirred tank based on CLSVOF method

The limited understanding of intricate mesoscale bubble structures in stirred tank reactors under forced convection poses significant challenges to the design and optimization of related agitation systems. In this study, the coupled level set with volume of fluid (CLSVOF) method is employed to investigate bubble dynamics and macroscopic hydrodynamics in a gas–liquid stirred tank after model validation. The results demonstrate that: i) based on the dynamics of bubble clusters within the system, the stirred tank can be categorized into four distinct regions: the dispersal region, accumulation region, rising wall-region, and reflex region; ii) in the dispersal region, the rear of impeller blade generates large-scale vortex. Two merging modes are identified during the bubble rupture process: the merging of bubbles at adjacent orifices and the merging of discrete bubbles with the back of impeller blade due to negative pressure. Two bubble breakup modes are observed: bubble breakup at the back of the impeller blade due to stretching and the breakup of bubbles growing at orifices due to disturbance caused by the impeller blade; iii) the inertial forces predominantly control most bubbles in the system. A predictive correlation for bubble velocity is proposed; iv) elevating the stirring speed increases bubble number in the accumulation region. Moreover, enlarging gas flow rate and fluid viscosity leads to a discernible augmentation of overall bubble size; v) at the macroscopic scale, the radial, axial, and tangential velocities of the system exhibit an increase with enlarging stirring speed, while showing no significant change with the increase in gas flow rate. Additionally, as fluid viscosity and density increase, the flow undergoes a transition from turbulent flow to laminar flow.

Breakup regimes and heat transfer of an isolated bubble and Taylor bubble flow in the T-type microchannel

Current research on Taylor bubble flow heat transfer mainly focuses on straight channels, with less emphasis on the transport characteristics and heat transfer regimes of Taylor bubble flow in branching channels. Understanding the flow and heat transfer regimes of Taylor bubble flow in branching channels is crucial for enhancing microfluidic and microchemical applications. Numerical simulations can provide detailed flow information that experiments might miss, deepening our understanding of Taylor bubble flow for enhanced heat transfer. The research results indicate the presence of three bubble breakup regimes at the T-junction: Non-breakup (NB), Tunnel breakup (TB), and Obstructed breakup (OB). The phase diagrams were established based on Capillary number and bubble length to predict the transitions between these three breakup regimes. Pressure within the channel increases with bubble deformation but decreases after breakup. Bubble breakup converts potential energy to kinetic energy, enhancing the heat transfer coefficient in the branching microchannels. The stagnation of bubbles at the T-junction temporarily reduces the heat transfer coefficient but increases after the breakup. Up to 115 % of the best performance improvement was achieved for Taylor bubble flow when the channel width ratio was 1.2. When the channel width ratio was greater than or equal to 1, the TB regime influenced bubble breakup within the channel, resulting in asymmetric breakups and uneven heat transfer. When employing tree-like branched microchannels and Taylor flow for enhanced heat transfer, a channel width ratio of 0.8 is recommended.

Modeling of dynamic bubble deformation and breakup in T-junction channel flow

Bubble deformation and breakup due to strain-rate-induced stress is investigated for a laminar flow configuration. The bubble shape is assumed to be a prolate ellipsoid. A new model for bubble deformation under dynamic load is introduced in the form of an ordinary differential equation for the deformation energy. Breakup is identified with a critical value of the deformation. As an application case, the flow in a joining T-junction is considered with the ratio of the volume flow rate being unity and the outflow Reynolds number being 1800. Dilute, dispersed bubbles with a diameter of 0.5 mm are injected. High-speed shadowgraphy is used and bubble parameters are evaluated via image processing. The capillary number is obtained from a single-phase flow simulation providing the instantaneous shear rate at the position of the bubble. The deformation resulting from the proposed model is then compared with the measured deformation for an exemplary bubble trajectory.

Splitting behavior and breakup mechanism of bubbles in the split‐and‐recombine microchannel

AbstractSplit‐and‐recombine (SAR) microreactor is an advanced reactor for chemical process intensification. Bubble flow in the SAR microchannel is an important phenomenon that affects reaction efficiency, however drew little attention before. This study aims to explore the underlying mechanisms of bubble splitting, retraction, and breakup behaviors in a compact SAR microchannel. Two breakup flow patterns, unilateral flow and unilateral alternate flow were identified with symmetric or asymmetric splitting, respectively. Mechanism analysis indicates that the splitting symmetry issue is related to liquid slug size, viscous effect and novel retraction behavior of a splitting filament. The retraction is induced by the interconnection and unequal pressures between the splitting filaments. The correlation between normalized breakup time and Ca number confirms the Capillary‐pressure breakup mechanism for the splitting gas filaments. Two empirical correlations for the two breakup flow patterns were proposed, which illustrate the significant contribution of bubble retraction to the breakup degree.

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.

Study on the bubble breakup and internal flow characteristics in the pulsed jet ejector

To intensify the bubble breakup in the ejector, a novel pulsed jet ejector is designed in this work. The pulsed jet is introduced into the ejector by one fluidic oscillator. A high-speed camera experiment is conducted to capture the progress of bubble breakup. The intensification mechanism of pulsed jet on bubble breakup in ejector is investigated by numerical simulation. The experimental results indicate that the introduction of the pulsed jet effectively induces the occurrences of pressure fluctuations and vortex collision, prompting the deformation of bubbles to produce microbubbles. It is also found that the bubble diameter and number of microbubbles mainly depend on the process of bubble breakup in the divergent section, and the bubble diameter distribution is significantly influenced by the variation of Reynolds number. The variations of the vortex structure are induced by the pulsed jet, which is a key factor influencing mixing efficiency and bubble breakup effects. In addition, pulsed jet ejectors have superior mixing performance compared to conventional ejectors.

Bubble breakup dynamics and fluid distribution in a honeycomb microreactor with chemical reaction

The bubble breakup dynamics and fluid distribution in a honeycomb microreactor with chemical reaction were visually investigated. The bubble split and collide behavior in multistage divergent-convergent branching structures were observed and studied. It was demonstrated that the bubble breakup at bifurcations relies dramatically on the collision behavior of the bubble pairs at the downstream convergence. Four bubble flow regimes were observed at the convergence: no collision, collision-slipping, squeezing-slipping, and blockage-slipping. The non-uniformity of the fluid distribution mainly stems from uneven breakup of bubbles at the upstream bifurcation, the lack of bubble breakup, the feedback effects of downstream and the manufacturing errors. Furthermore, the variation of pressure drop in a honeycomb microreactor was explored. The overall pressure drop was found to increase by the local pressure drop, whereas reduced with the increased absorption capacity of liquid phase. Taking these impact factors into account, a modified Lockhart-Martinelli pressure drop model was developed.

Behind the Non-Uniform Breakup of Bubble Slug in Y-Shaped Microchannel: Dynamics and Mechanisms.

Bubble flow in confined geometries is a problem of fundamental and technological significance. Among all the forms, bubble breakup in bifurcated microchannels is one of the most commonly encountered scenarios, where an in-depth understanding is necessary for better leveraging the process. This study numerically investigates the non-uniform breakup of a bubble slug in Y-shaped microchannels under different flow ratios, Reynolds numbers, and initial bubble volumes. Overall, the bubble can either breakup or non-breakup when passing through the bifurcation and shows different forms depending on flow regimes. The flow ratio-Reynolds number phase diagrams indicate a power-law transition line of breakup and non-breakup. The bubble takes longer to break up with rising flow ratios yet breaks earlier with higher Reynolds numbers and volumes. Non-breakup takes less time than the breakup patterns. Flow ratio is the origin of non-uniform breakup. Both the Reynolds number and initial volume influence the bubble states when reaching the bifurcation and thus affect subsequent processes. Bubble neck dynamics are analyzed to describe the breakup further. The volume distribution after breaking up is found to have a quadratic relation with the flow ratio. Our study is hoped to provide insights for practical applications related to non-uniform bubble breakups.

Computational fluid dynamics modelling of large-scale bubble columns: from the mono-dispersed to the pure heterogeneous flow regime

This study proposes a CFD model to simulate large-scale bubble columns operating in different flow regimes. Transient 3-D simulations were performed employing a commercial code (ANSYS Fluent), and the numerical results were compared with available experimental data. The superficial gas velocity ranges between 0.0037 m/s and 0.2 m/s, covering both the mono-dispersed and pure-heterogenous flow regimes, where bubbles coalescence and breakup were modelled. The results have been critically analysed, and the discrepancies between the numerical and experimental results have been deeply commented on, setting the stage for future improvements.