Bubble Flow Characteristics Research Articles

Analysis of Bubble Flow in an Inclined Tube and Modeling of Flow Prediction

The lubricating oil system is a significant component of aviation engine lubrication and cooling, and the scavenge pipe is an essential component of the lubricating oil system. Accurately identifying and understanding the flow state of the scavenge pipe is very important. This article establishes a visualization test bench for a 45-degree inclined scavenge pipe, with upward and downward flow directions, respectively. The test temperature is 370 K, and a high-speed camera captures the changes in the two-phase flow inside the pipeline. Based on high-speed photography photos, we develop software for analyzing the flow characteristics of bubbles inside the tube and explore the influence of gas phase conversion velocity and liquid phase conversion velocity on the apparent velocity of bubbles inside the tube. Multiple algorithms were used to develop the model by combining machine learning with speed and accuracy to establish a data regression prediction model for the apparent velocity of bubbles inside the tube. Through calculation and analysis, it was found that the root mean square error of the prediction model using the BP neural network algorithm was the lowest, and the decision coefficient of the prediction model using the support vector machine algorithm was the highest.

Local characteristics of bubbly flow in rods assembly 3 × 3

This study focuses on the experimental investigation of local characteristics of upward bubble flow in a 3 × 3 square rod bundle assembly. The experiments were conducted at Reynolds numbers ranging from 4000 to 11000 and gas void fractions up to 10%. The distribution of the gas phase around the central rod of the assembly was measured using a frontal conductivity probe, while the wall shear stress on the central rod was obtained through the electrodiffusion method. It is shown that the gas phase distribution does not exhibit the symmetry characteristic of the assembly cross-section. Furthermore, as the distance from the spacer grid increases, a reconfiguration of void fraction profiles is observed. This explains the non-monotonic dependence of wall shear stress and wall shear stress fluctuations on the distance to the spacer grid.

Molecular dynamics simulation of bulk nano-oxygen-bubble fuel under high-pressure transport and sudden pressure drop process

Aiming at the injection process of bubbly flow fuel, the molecular dynamics simulation was performed to study the change of bulk nano-oxygen-bubble (BNB) fuel during high-pressure transport and sudden pressure drop process. The static processes of BNB fuel containing different oxygen molecules numbers are explored in depth at normal temperature and pressure and pressure increase as well as the dynamic process under high-flow-rate-coupled sudden pressure drop. The results show that after adding BNB, the BNB contour increases first and then decreases. Increasing the number of oxygen molecules added at normal temperature and pressure shortens the BNB stabilization time and improves BNB stability. The BNB inner density in fuel is much smaller than that in water. Under high-pressure condition, the BNB which can be stabilized in the fuel at normal temperature and pressure quickly dissolves and disappears due to the increase in solubility of oxygen, resulting in a decrease in the oxygen molecule potential energy. Increasing the number of oxygen molecules added has only a weak slowing down effect on the dissolution rate. During the high-speed jetting of fuel at the nozzle, BNB promotes fuel droplet fragmentation and increases the oxygen concentration around the fuel droplet. In this process, the oxygen molecules do work on the fuel and their kinetic energy decreases. Increasing the number of oxygen molecules added reduces fuel breakup time and improves the quality of gas mixture formed. The results of this study can provide a reference for the micro-mechanism of BNB on improving the atomization characteristics of bubbly flow fuel.

Heat transfer enhancement for flow boiling of zeotropic mixtures with regulating liquid mass transfer resistance

The mass transfer resistance is a significant element leading to heat transfer degradation of zeotropic mixtures in the flow boiling. In this paper, the heat transfer enhancement of zeotropic mixtures in the flow boiling was investigated through regulating the mass transfer resistance by constructing reasonable surface structure. The heat and mass transfer, pressure drop characteristics of stratified bubble flow for R134a/R245fa zeotropic mixtures in a rectangular channel with multi-cone staggered spoiler porous structure were studied experimentally. The bottom of channel was sintered with multi-cone staggered porous layer with thickness/height/diameter of 1/3/2 mm, respectively. The experiments were carried out under an inlet evaporation temperature of 20 ℃, and the ranges for heat and mass fluxes were 10–150 kW/m2 and 100–150 kg/(m2·s), individually. Compared to the rectangular channel with smooth porous structure, liquid mass transfer resistance of zeotropic mixtures in the rectangular channel with multi-cone staggered porous structure was decreased obviously. The decrease degree for Mixture 2 was largest, while the maximum average decrease was 89.61 % at G = 100 kg/(m2·s). The heat transfer coefficients were obviously improved and heat transfer enhancement increased gradually especially under conditions of high heat flux. The enhancement degree for Mixture 2 was the largest, while the maximum average increase was 26.05 % at G = 100 kg/(m2·s). The pressure drop increased significantly, and the increasing degree for Mixture 2 was smallest, while the minimum average increase was 38.18 % at G = 150 kg/(m2·s). Multi-cone staggered structure was helpful to reduce liquid mass transfer resistance and improve heat transfer performance for zeotropic mixture. The zeotropic mixture with higher liquid mass transfer resistance would have better heat transfer intensify and smaller increase of pressure drop with the help of multi-cone staggered structure.

Two-group drift-flux model for dispersed gas-liquid flows in annuli

Interfacial area concentration (IAC) plays a vital role in mass, momentum, and heat transfers between gas and liquid phases in two-phase flows. The IAC of group-one bubbles, including spherical and distorted bubbles, differs from the IAC of group-two bubbles, including cap, slug and churn-turbulent bubbles. The two-group interfacial area transport equation (IATE) can be used as a mechanistic model to obtain the IAC of each group. The equation demands the two-group gas velocity fields, leading to the need to solve an extra momentum equation, which is complicated. The two-group drift-flux model can prevent this complexity by providing two-group gas velocities without introducing the extra momentum equation. Despite the importance of gas-liquid flows in annuli, the drift-flux model for gas-liquid flows is not well-developed like it is for pipe flows. The main objective of this study is to develop a two-group drift-flux model for dispersed vertical flows in annuli. The two-group drift-flux model provides the flow characteristics of group-one and group-two bubbles, such as void fractions and velocities, which strongly affect the prediction of the available interfacial area concentration of the group-one and group-two bubbles for heat and mass transfer. Hence, after comprehensively investigating the available experimental data, this study first improves a one-group drift-flux model for dispersed gas and low-viscosity liquid upward flows in annuli and proposes a new two-group drift-flux model. Then, the models are evaluated using the respective group experimental data. The consistency between one-group and two-group drift-flux models is also investigated. This consistency helps to reduce the two gas momentum equations to one gas mixture momentum equation. The results of the comparison between model predictions and experimental data are satisfactory.

Characteristics of Bubbly Flow, Mass Transfer, and CO2 Absorption in a Two-Stage Internal Loop Airlift Reactor Assembly with a Diamond-Shaped Guide Baffle

Characteristics of Bubbly Flow, Mass Transfer, and CO₂ Absorption in a Two-Stage Internal Loop Airlift Reactor Assembly with a Diamond-Shaped Guide Baffle

Flow dynamics characteristics of flow boiling in minichannels with distributed pin fin structures

Bubble dynamics and flow dynamics are crucial for understanding the heat transfer mechanisms in small-scale flow boiling and can be manipulated with surface modification to enhance performance. While numerous investigations have been conducted on minichannels with pin fins, there are few research efforts on bubble dynamics and flow characteristics. Motivated by this fact, this study covers flow boiling experiments using five pin-fin structured configurations with low (1 mm) to high (3 mm) pin-fin heights and distribution (circular) to reveal the flow characteristics and bubble behavior in mini channels with pin fin structured surfaces. Pressure drops (ΔP) were obtained at effective heat flux values ranging from 18 to 175 W/cm2 and mass fluxes from 125 to 325 kg/m2s. The visually obtained images were also presented and discussed in terms of bubble growth, departure time, and departure frequency. The effect of elliptical pin fins and distributed pin fins, along with the consideration of tip clearance, was evaluated. According to the results, three different boiling patterns could be detected over the configurations: bubbly, intermittent, and wavy-annular flow. Elliptical pin fins resulted in a higher pressure drop penalty. In contrast, the distribution pin fins offered a more uniform flow distribution and decreased pressure drop, particularly for low tip clearances (<35 %).

3D simulation of pulsatile bubbly flow resembling decompression sickness conditions inside a realistic human artery

I-VED (In-Vivo Embolic Detector) is a novel diagnostic tool for non-invasive, real-time detection of bubbles in humans. Bubbles are precursors of decompression sickness (DCS), which can be encountered in astronauts, scuba divers, etc. I-VED exploits an EU patented electrical impedance spectroscopy technology, developed under the umbrella of a European Space Agency project. So far, I-VED has been calibrated and validated in vitro. In view of the forthcoming in-vivo trials, it needs to be configured for sensing bubbles in the bloodstream. For this, 3D computational fluid dynamics simulation is performed to investigate axial and radial variation of void fraction (α) and flow velocity (U) in a pulsatile bubbly flow inside a realistic human artery (diameter: 5–20 mm, implying vessel dilatation or contraction), where liquid velocity, bubble size, and void fraction resemble DCS conditions. Results show that U and α show a core-peaking profile despite the variation of artery diameter, while 3D sharp turns yield U and α non-uniformities in the angular direction that do not affect mean void fraction across the artery. Obtained knowledge allows deeper insight on the physics and spatial characteristics of bubbly flow in a real artery, which is useful in the design of measuring volume and tuning of I-VED.

Experimental and numerical investigation of flow dynamics in an upward bubbly flow in a tube undergoing oscillating rolling motion

Offshore nuclear power plants are characterized by a potential oscillatory motion depending on ocean waves. Investigating the local flow behavior in a system undergoing oscillating motion is necessary. In particular, because the local void fraction near the heating element surface significantly affects the nucleating boiling heat transfer and critical heat flux, understanding the dynamic behavior of the local void fraction is very important. Therefore, in this study, as an essential first step in predicting boiling heat transfer and departure from nucleate boiling in offshore nuclear reactors, the dynamic behavior of air–water bubbly flows has been experimentally and numerically investigated in a tube under oscillatory rolling conditions. An optical fiber Doppler probe was used to measure the local bubble parameters. The effects of the rolling period on the void fraction distributions, bubble sizes, and bubble velocities were insignificant. However, the rolling amplitude effect was significant. The void fraction was the highest at the downward-facing wall when the tube was at its maximum tilt. Moreover, the local water velocity became the highest when the tube returned to near vertical because of the combined effect of gravity and Euler force. These findings provide insights into understanding the characteristics of bubbly flow in a rolling tube.

Numerical Simulation of the Two-Phase Flow Dynamics in an Alkaline Water Electrolyzer Considering Difference in Bubble Diameter

For more efficient hydrogen production, predicting the characteristics of bubble flow inside a water electrolyzer is critical since bubbles affect ion transport, and electrode coverage by bubbles influence overpotentials and the entire electrolysis performance. Transport of bubble significantly depends on its size. In this study, a CFD model of an alkaline water electrolyzer which considers difference in sizes of generated bubbles was developed. The model treats bubbles of different sizes as different gas phases, which are independent of each other. The total bubble mass source for a given current density is divided into phases of each bubble size according to experimental visualizations. Drag, virtual mass, lift, wall lubrication, and turbulent dispersion forces are considered as interaction between gas and liquid phase.The following three sizes of bubbles were considered: diameters d = 36 μm (small), 75 μm (medium), and 208 μm (large). The entire calculation domain represents the anode side of a zero-gap monopolar alkaline water electrolyzer. Figure 1 shows the volume fraction fields of the three gas phases (i.e., bubble sizes) at a constant current density of 0.6 A/cm2. The bubbles are generated on the electrode surface at the z=0 plane. As the liquid phase forms a circulating flow inside the electrolyzer, small bubbles are transported along the circulating downstream flow due to smaller buoyancy force compared to their viscous drag. In contrast, large bubbles flow out directly to the upper outlet because buoyancy force dominates. This result points out the need to consider the difference in bubble diameter for designing flow field inside alkaline water electrolyzer.[Acknowledgements]This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Fig. 1. Total gas phase volume fraction field obtained from the sum of the volume fraction field for each size bubble, and its experimental visualization. Figure 1

VOF study of mesoscale bubble flow dynamics in the side-blown gas–liquid two-phase reactor

Understanding the flow and mixing characteristics of mesoscale bubbles within side-blown gas–liquid two-phase systems is paramount for the design and optimization of associated industrial processes. This study elucidates the gas–liquid side-blown two-phase reactor, employing the volume of fluid method coupled with the realizable k-ε turbulence model. Following the model validation through experimental measurements, the mesoscale bubble dynamics, flow characteristics, and multiscale coupling mechanisms inherent to side-blown gas–liquid reactor are explored. The key insights can be obtained: (1) the upward trajectory of bubbles originating from side-injection gas can be categorically divided into four phases with different bubble behaviors. (2) A reduction in the ratio of inertial forces to buoyancy forces during bubble ascent induces a decrease in the bubble shape factor. This transition manifests as a shift from vertical to flattened bubble shapes within the transition and plume regions. (3) Bubbles undergo transitions between states characterized by high buoyancy and low surface tension, accompanied by varying levels of viscous forces during their ascent. These dynamic states render bubbles susceptible to breakup. Under conditions of a constant nozzle diameter, the maximum bubble size within the reactor is inherently restricted. (4) Drawing upon the correlation between bubble Weber and Reynolds numbers, a predictive formula for determining the characteristic velocity of bubbles within side-blown reactors is proposed:uc=σ1-0.456(ρlLC)0.456512μl11.36Overall, this work offers meaningful insights into the fundamental mechanisms governing bubble cluster dynamics within side-blown gas–liquid reactors.

Bubbly flows in a column with submerged hollow fiber membranes

Experiments of bubbly flows in a column with submerged hollow fiber membranes are carried out to discuss the relation between bubbly flow characteristics and fouling control. Two kinds of liquids, water and xanthan gum-water solution (XG-W), are used as the liquid phases. The relaxation rates of membranes are set at 0.0 or 0.5%. The number of small bubbles with ellipsoidal shapes in water are larger than that in XG-W, while large bubbles with spherical cap shapes are remarkably observed in XG-W in spite of the presence of membranes. The rate of large bubbles is enhanced in XG-W due to bubble coalescence. The gas holdup in water is higher than that in XG-W. The gas holdup becomes higher due to the presence of membranes and membrane relaxation in both water and XG-W. For discussion on fouling control due to bubbly flows, experiments are carried out to remove pseudo sludge attached to hollow fiber membranes. The relaxation of membranes enhances the removal rate of pseudo sludge. The mean removal rate, which is the mean value of the removal rate of each membrane, increases with increasing the gas holdup. This relation is well expressed by a linear regression in the present experimental range regardless of the liquid property and membrane relaxation, that is, the gas holdup has a prospect of an indicator for estimating fouling probability.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

A simple 3D reconstruction algorithm based on laser scanning: Research on flow characteristics of bubbles rising vertically in liquid phase

Bubbly flow is frequently observed in chemical industries such as reactors and steam boilers, where flow parameters of bubble velocity, size, and void fraction are required to guide process safety. In this paper, a laser scanning-based three-dimensional reconstruction algorithm is introduced to achieve simple and effective measurements in vertical bubbly flow. A series of slice images are captured by a high-speed camera, and a modified two-dimensional cross-correlation algorithm is proposed to achieve bubble extraction and matching. Based on the reconstructed flow structures of bubbly flow, bubble velocities and their distribution in different scanning cycles are measured. Statistically, bubble sizes are mainly concentrated within 1.8–2.2 mm. The void fractions are calculated for various gas flow rates with relative errors primarily within ±10%. The relative errors of the average void fraction are less than 8% by comparing the experimental and theoretical results, indicating a higher reconstruction precision.

Lattice Boltzmann method for heat transfer in transitional flows with unified single-node curved boundary conditions

Laminar separation bubbles in transitional flows of low-Reynolds-number airfoils significantly affect aerodynamic efficiency and heat transfer characteristics, which can be dealt with using the numerical method with boundary treatment of complex geometry. This work proposes the unified single-node curved boundary conditions based on the thermal lattice Boltzmann method, and several benchmarks verify the higher accuracy and parallel scalability over conventional schemes. Multi-GPU simulations for airfoil Eppler 61 and SD8020 are carried out, and the lift coefficients CL are accurately simulated by the parametric combined momentum-exchange method. Furthermore, time-averaged pressure coefficients Cp and local Nussel number Nu of airfoil surfaces are obtained, and the unsteady flow and heat transfer characteristics of laminar separation bubbles are captured. Simulated average Nussel numbers Nu¯ for an Eppler 61 airfoil with different Reynold numbers are equivalent to and modify the Hilpert correlation. The numerical method can quickly and accurately predict transitional flows of low-Reynolds-number airfoils and guide the design and application of heat transfer with blade configuration.

Bubble behavior, flow characteristics, and mass transfer enhancement in self-priming Venturi tubes

Modulation of bubble behavior and flow field structure is an effective way to enhance mass transfer. In this study, a novel porous Venturi structure is proposed, and the bubble generation performance and mass transfer capacity of the device are evaluated. The volume of fluid model was used to reveal the velocity variation and flow field distribution inside the Venturi channel, particularly in the diverging section, and the particle image velocimetry system was used to study the motion pattern between multiple bubbles and the bubble fragmentation process. The results show that the novel self-priming Venturi tube strengthens the flow field compared with traditional self-priming Venturi tubes, Moreover the bubble diameter becomes smaller and more uniform, and the mass transfer efficiency is improved. On this basis, the effect of porous structure variables on bubble behavior was further investigated to provide a new idea for enhanced bubble fragmentation.

Numerical Simulation of Subcooled Flow Boiling in a Vertical Annulus Channel Under Near Atmospheric Pressure Conditions

In the present study, a Eulerian-Eulerian two-fluid model is developed to analyze the flow boiling phenomena under near-atmospheric pressure conditions. The required constitutive correlations for the two-fluid model are provided as flow regime dependent within the algebraic interfacial area density framework. The two-fluid model developed with Rensselaer Polytechnic Institute (RPI) wall heat flux partitioning is used to analyze the subcooled nucleate boiling of water at low pressure in three vertical annulus channels of different heated lengths over a wide range of inlet mass flux, wall heat flux, and inlet subcooling conditions. The subcooled water enters the heated annulus channel from the bottom end and is heated to near-saturation temperature. Upon reaching the saturation temperature, the wall boiling generates dispersed vapor bubbles near the heated wall. Farther along the heated length, larger bubbles can be formed by coalescence and evaporation, and the bubbles move on to the channel core region with increased vapor fraction so the flow regime changes from bubbly to transition regime. Farther along, it may turn to an annular flow regime. The benchmark experimental cases chosen are used to validate the model capability in predicting the bubbly flow and transition flow regime (slug flow regime) characteristics with the proposed methodology. Further, the low-pressure boiling model developed is successfully extended to predict the liquid sodium boiling in flow channels similar to sodium-cooled fast reactor fuel subchannels using suitable interfacial correlations.

Study on the process intensification of superaerophobic materials to improve CO2 mass transfer efficiency

Chemical absorption is the most widely used method in capturing CO2 from flue gas, in which the gas-liquid mass transfer process is the control step of the absorption rate; the spherical structure and easy flow characteristics of bubbles in the liquid phase affect the gas-liquid mass transfer efficiency. This study carefully examines the effect of superaerophobic surfaces on the mass transfer efficiency of CO2 bubbles on both dynamic and static behaviors. The results show that the 0.5 mol/L MDEA solution can absorb CO2 bubbles within 70 s when the CA (Contact Angle) of the superaerophobic surface bubbles reaches more than 150°. However, after contacting the superaerophilic substrate, the mass transfer area between the gas and liquid phases increases instantly and the CO2 bubbles burst rapidly and form a gas film within 18 ms. After that, the absorption saturation time of the absorption tower using superaerophilic packing is 25% less than that using untreated packing. Therefore, the superaerophilic packing can significantly increase the gas-liquid contact area and the mass transfer efficiency.

Experimental study on flow characteristics of cavitation bubbles in underwater vertical movement of double vehicles

The cavitation generated during the water-exit process of the vehicle will have a great influence on the water exit attitude and stability of the vehicle. At present, the research on the cavitation flow mainly focuses on the single vehicle, and there is little research on the cavitation flow during the water-exit process of the double vehicles. However, in practical applications, it often happens that the double vehicles emerge from the water at different timings. Therefore, it is necessary to study the flow characteristics of the cavitation of the vehicles. In this paper, a set of underwater vertical launching experiment device of double vehicles under decompression condition is designed. Based on this device, different time sequence water-out experiments of the vehicles are carried out, and multiple high-speed cameras are used to record the whole process of the vehicles from different angles. The effect of launch velocity, environmental pressure, and lateral and vertical distances on cavitation morphological changes and sizes are explored. Studies have shown that there will be a certain degree of interference between the two vehicles during the water-exit process. When the launch speed increases and the environmental pressure decreases, the size of the cavitation when the double vehicles exit the water will increase greatly, and gradually move closer to the single vehicle; When the lateral and vertical distances increase, the interference between the vehicles will be weakened.

Flow characterization of gas-liquid with different liquid properties in a Y-type microchannel using electrical resistance tomography and volume of fluid model

BackgroundFlow characteristics in microscale reactors are one of the key factors in fast reaction systems, such as fine chemicals, pharmaceuticals, and nanopowders. In this work, the effects of liquid properties on the bubble flow characteristics in vertical circular microchannels are studied. MethodsUsing experiments (electrical resistance tomography and high-speed camera) and CFD simulation (volume of fluid method - continuous surface force) the effects of liquid phase properties on flow pattern transitions, liquid films, were investigated. Significant findingsThe average recognition rates of ERT in air - glycerol (µ = 0.885 mPa•s–1.912 mPa•s) and air - ethanol (σ = 38.7 mN/m - 72.0 mN/m) aqueous solution are over 84%. It was found that the liquid surface tension and liquid viscosity can change the internal shear force and interfacial force in the microchannel. When Ca > 0.0093, the bubbles and the inner wall of the microchannel form a liquid film. In addition, based on the experimental data, a modified equation for the gas holdup in the microchannel is proposed, which provides a reliable reference for the application of Y-type microchannels.