Bubble Motion Research Articles

Numerical investigation of the unstart flow at off-design condition of REST inlet at a Mach of 4

Unstart flow in a rectangular-to-elliptical shape-transition (REST) inlet at the off-design operating condition has been simulated using the detached eddy simulation (DES) method. With the help of numerical simulations, the development of the unsteady inlet unstart flow at the transitional condition is obtained. The evolution patterns of unstart flow as well as the characteristics at different stages are summarized. Also, the dynamic behavior of the separation bubble is investigated. Results indicate the dynamic behavior of the separation bubble is jointly dominated by the incoming flow, spillage and the backpressure/blockage ratio. The changes in downstream flow structures can break the balance between these factors and thus lead affect the separation bubble's movement. The mechanism of the bubble's direction switch is different according to its location. It is dominated by the increased back pressure/blockage ratio at the rearward position and the shear layer instability at the forefront position. The cowl-closure leading edge (CLE) and reflected CLE shock play an important role in this process, which jointly redistribute the turbulent pulsation. It accelerates the process of separated shear layer instability and downstream flow averaging, and simultaneously causes the change of separation bubble movement behavior.

Impact of vapour bubble movement on the process of two-phase heat transfer phenomena over hybrid elliptical tubes

The use of modified surface to improve the performance of two-phase heat exchanger in terms of pressure drop and heat transfer rate is one of the current need of industries. In light of this, the current analysis compares the diabatic two-phase heat transfer performance of hybrid elliptical tubes with that of traditional circular tubes at atmospheric pressure using distilled water as a quiescent test fluid. For the purpose of this investigation, a circular tube made of AISI 304 with a diameter of 25 mm and a heating length of 130 mm is used. In order to maintain the same surface area and heating length as a circular tube, the hybrid elliptical tubes are fabricated with varying aspect ratios of 1.17, 1.31, and 1.47. The heat flux varies between 14.87 and 94.82 kW/m2. For two orientation angles, i.e. 0ᵒ and 90ᵒ, the heat transfer performance of hybrid elliptical tubes are investigated. The analysis of results confirmed that both the tubes exhibit boiling characteristics. However, hybrid elliptical tubes outperform circular tubes in terms of heat transfer rate. It is also found that the hybrid elliptical tube with 0ᵒ orientation resulted in higher heat transfer coefficient about 20.61%, 36.74% and 20.69% than that of 90ᵒ orientation for aspect ratio of 1.17, 1.31 and 1.47 respectively. Regardless of orientation angle, the hybrid elliptical tube with aspect ratio of 1.31 demonstrated the highest heat transfer coefficient of all aspect ratios. Also, a non-dimensional correlation is developed which is found to predict the heat transfer coefficient of circular as we as hybrid elliptical tubes within an error band of ±12%.

In situ studies on free-standing synthesis of nanocatalysts via acoustic levitation coupled with pulsed laser irradiation

Acoustic levitation is a distinctive and versatile tool for levitating and processing free-standing single droplets and particles. Liquid droplets suspended in an acoustic standing wave provide container-free environments for understanding chemical reactions by avoiding boundary effects and solid surfaces. We attempted to use this strategy for the production of well-dispersed uniform catalytic nanomaterials in an ultraclean confined area without the addition of external reducing agents or surfactants. In this study, we report on the synthesis of gold and silver nanoparticles (NPs) via acoustic levitation coupled with pulsed laser irradiation (PLI). In situ UV–Visible and Raman spectroscopic techniques were performed to monitor the formation and growth of gold and silver NPs. The PLI was used for the photoreduction of targeted metal ions present in the levitated droplets to generate metal NPs. Additionally, the cavitation effect and bubble movement accelerate the nucleation and decrease the size of NPs. The synthesized Au NPs with ∼ 5 nm size showed excellent catalytic behavior towards the conversion of 4-nitrophenol to 4-aminophenol. This study may open a new door for synthesizing various functional nanocatalysts and for achieving new chemical reactions in suspended droplets.

Mechanism study of bubble dynamics under the buoyancy effects

The motion of a single bubble in a 2-D vertical channel filled with stagnant or moving fluid is simulated for various sets of conditions using the computational fluid dynamics(CFD) method. The volume of fluid (VOF) model is applied to track the interface of the bubble. Corresponding results are compared with experimental and theoretical studies, and good agreement is achieved. Two-phase flows composed of various fluids are simulated to check the impact of physical properties on bubble dynamics, and buoyancy is found to have a key influence on the bubble rising behavior, including both the trajectory and the terminal rising velocity. The dimensionless number Bu is introduced to characterize this effect, and a larger Bu leads to a larger bubble rising velocity. Three other dimensionless numbers Eo, We, and Mo are introduced to study the bubble deformation which can be characterized by aspect ratio E, and an approximately linear relationship is found between E, Eo, and bubble size. Inertial force is proved to influence the bubble motion significantly in moving fluid, whose velocity dominates the bubble terminal speed. The simulation of motion for two bubbles is also performed and investigated detailedly.

An experimental investigation of subcooled pool boiling on downward-facing surfaces with microchannels

Upward-facing pooling boiling enhancement employing micro-structure has been extensively studied, yet it is rarely applied in the cooling system in microgravity or In-Vessel Retention (IVR) in nuclear industry. The main challenge lies in effective removal of the vapor columns and replenishment of the fresh liquid without the aid of the buoyancy force. To solve the issue, additional pressure induced by microchannel is introduced to yield spontaneous bubble removal and liquid replenishment in the present work. Downward-facing pool boiling of subcooled FC-72 are conducted on Plain, Straight- and Expanding-Channeled Surfaces (PS, SCS and ECS) with visualization experiments. The superheat at the onset of nucleate boiling (ONB) on channeled surfaces was decreased about 3 K compared to that of the PS, and the maximum heat transfer coefficient (HTC) of the SCS and ECS were improved by 149 % and 217 %, respectively. Compared to PS, the channeled surfaces are more independent from saturated pressure, displaying high and relatively consistent heat transfer coefficients under various pressure conditions. Additionally, with the increasing subcooling degree, channeled surfaces showed greater enhancement in heat transfer performance. Proved by the bubble dynamics, the ECS enabled a directional movement and effective removal of vapor bubbles even though the buoyancy force cannot be utilized under downward-facing condition. The proposed enhancement mechanism attributed to separated vapor–liquid flow paths is independent on gravity and achieves a satisfied pool boiling performance, thus is expected to provide a promising solution for thermal management systems in aerospace and nuclear industry.

Growth and stability of bubbles in a yield stress fluid

Experiments are reported that explore the onset of motion of bubbles in a model yield stress fluid, Carbopol gel. Starting from a trapped spherical bubble in a gel, the yielding limit for the bubble motion is obtained by gradually expanding the bubble via a stepwise decrease in pressure. Our results show that at the yielding limit bubbles are longer and thinner when they are in a higher concentrated gel. This is suggestive of a link between the shape and size of the bubbles at the onset of motion and the rheology of the material, in particular elastic behaviour below the yielding point. Particular attention has been paid to investigating the dynamic response of gel during the bubble growth. Subjecting the bubble to a periodic change in the pressure confirms the irreversibility of the gel deformation and its hysteresis, which are hallmarks of nonlinear viscoelastic behaviour of the gel before yielding. In this context, the periodic expansion and contraction of the bubbles leave residual deformation (stresses) in the gel which facilitates the liberation of bubbles.

Significance of recirculating micro-bubbles in oxygenation from poly-dispersed plumes

Poly-disperse bubble plumes are simulated to explore the effect of interactions between multiple bubbles of varying sizes on the liquid flow, plume dynamics, and oxygen transfer in aeration processes. An Eulerian-Lagrangian model based on the multiphase particle-in-cell method (MPPIC) implemented in OpenFOAMTM has been extended to account for bubbly flow. Bubbles have been approximated as spherical particles whose diameters change in response to pressure and nitrogen and oxygen mass dissolution and transfer. The mass transfer model relies on a Sherwood number correlation which accounts for the individual bubble size and relative velocity and is parameterized against the liquid phase temperature. A Large Eddy Simulation (LES) approach based on a transport equation for the turbulent kinetic energy has been adopted to model the effect of the sub-grid scales (SGS) of turbulence on the liquid flow and bubble motion. The bubble injection is defined in terms of the air mass transfer and the bubble size distribution. The model has been compared against experimental measurements of the evolution in oxygen concentration in a 1m deep water tank in response to bubble plumes generated from a porous ceramic plates at two different flow rates. The interaction between milli- and micro-bubbles over a wide range of sizes has been shown to create significant differences in the aeration process. A method to correct for bubble recirculation has been proposed and allows for good agreement between measured and simulated volumetric transfer rates. The simulation has shown the important role that the interplay between bubble sizes takes in determining the oxygen transfer efficiency. Larger standard deviations from Gaussian bubble size distributions have been shown in particular to reduce recirculation from smaller bubbles thereby reducing the aeration efficiency at an equivalent flow rate. An analysis of the age distribution by bubble size plumes suggest that the largest bubble have an important role in increasing the entrainment of smaller bubbles limiting their recirculation and reducing their retention time.

Influence of Bionic Waveform Leading Edge Blade on Drag Reduction Characteristics of Mixed Pump

Because the helical axial flow gas-liquid mixing pump has the great advantage of conveying gas-liquid two-phase mixed medium, it has become the main core equipment for deep-sea oil and natural gas exploitation. The gas phase aggregation and bubble movement trajectory in the impeller channel have been widely studied, but the increase of medium flow resistance caused by flow separation has not been deeply discussed. Combined with the Euler multiphase flow model and the SST k-ω turbulence model, the numerical calculation of the helical axial flow gas-liquid mixed pump is carried out. Under design flow conditions Q = 100 m3/h, head H = 30 m, speed n = 4500 r/min, specific speed ns =213.6 r/min, and under different inlet gas content conditions, the influence of the bionic waveform leading edge blade on drag reduction characteristics of the helical axial flow gas-liquid mixed pump was investigated. By designing the blade with a leading-edge structure with different heights and pitches, the separation of the mixed medium and the suction surface is effectively suppressed, and the flow resistance of the medium in the 1/10 area of the inlet end of the blade is reduced. The results show that when the height A is 0.25%L and the pitch λ is 12.5%h, the maximum drag reduction rate in this region is 52.6%, the maximum increase in efficiency of the mixed pump is 2.2%, and the maximum increase in head is 4.8%. This study can provide technical support for flow drag reduction in gas-liquid mixed pump.

Oriented intra-granular bubble transport due to coupling of pinned bubble growth and dislocation climb

The intra-granular fission gas release during post-irradiation annealing tests cannot be predicted if bubble motion is not taken into account. Indeed, without irradiation, the equilibrium between trapping and re-solution is completely shifted in favor of trapping, the dynamic resolution due to fission spikes no longer existing. Moreover, the sink strength of the population of intragranular bubbles is greater by a factor ∼1000 than the sink strength of the grain surface. Trapped in supposedly immobile bubbles, the gas would not escape from the grain, which is in contradiction with experimental observations. Several alternative scenarios involving bubble movement emerged to explain the observed fission gas release. The purpose of our work is to assess these scenarios using simulation. In a previous article, it was demonstrated that neither the movement of bubbles in a vacancy gradient, nor the Brownian movement of bubbles, nor the combination of them, could explain the large fission gas release obtained during post-irradiation annealing in our reference experiment. This demonstration was performed using a mesoscale model, called BEEP, where individual bubbles are described, along with the diffusion of vacancies from each bubble to the other, as well as from the free surface. In this paper, we extend the BEEP model to assess the role of dislocations in interaction with highly pressurized bubbles. It is concluded that a mechanism of dislocation climb coupled with the growth of highly pressurized pinned bubbles may explain the large intra-granular fission gas release in annealing conditions.

Spatial and temporal regulation of homogeneous nucleation and crystal growth for high-flux electrochemical water softening

Electrochemical softening is an effective technology for the treatment of circulating cooling water, but its hardness removal efficiency is limited because that nucleation and growth of scale crystals depended on cathode surface. In this study, a novel method was proposed to break through this limit via spatiotemporal management of nucleation and growth processes. A cube reactor was divided into cathodic chamber and anodic chamber via installing a sandwich structure module composed of mesh cathode, nylon nets, and mesh anode. Using this continuous-flowing electrochemical reactor, OH ̄ generated by water electrolysis was rapidly pushed away from cathode surface by water flow and hydrogen bubbles movement. As a result, a wide range of strongly alkaline regions was rapidly constructed in cathodic chamber to play a nucleation region, and homogeneous nucleation in liquid phase replaced heterogeneous nucleation on cathodic surface. Furthermore, the growth process of scale crystals in alkaline regions was monitored in situ. It took only 150 s of residence time to grow to 500 nm, which may be easily separated from water by a microfiltration membrane. With this new method, the precipitation rate was 290.8 g/(hˑm2) and corresponding energy consumption was 2.1 kW·h/kg CaCO3, both were superior to those reported values. Therefore, this study developed an efficient electrochemical softening method by spatial and temporal regulation of homogeneous nucleation and crystal growth processes.

Review of bubble dynamics on charged liquid–gas flow

When a fluid is subject to an electric field, it usually processes unique features compared to the conventional fluid that arises from coupling between charged particles and fluid interface. Based on this commonality, we defined the concept of “charged multiphase flow” and constructed a generalized charged multiphase flow system using the “Tai Chi Diagram” to analyze the properties and features of different study objects, with an emphasis on the bubble dynamics on the charged liquid–gas flow object, covering the processes of bubble generation, motion, and interaction, as well as the important dynamic behaviors, involved such as bubble deformation, coalescence, and breakup. Furthermore, in light of the special plasma–liquid interface phenomenon formed by the ionization of the gas/vapor phase in the liquid phase in strong electric fields, the traditional gas–liquid–solid three-phase flow system is expanded into a broader range of multiphase flow systems involving plasma, which enriches the theoretical and frontier scientific problems of the multiphase flow. In addition, technical innovations, remaining work, and future trends in the development of the charged liquid–gas flow, and their potential applications are discussed.

Examination of credit balloons during the Covid-19 pandemic process: a study on the Turkish banking sector

Purpose- Banks are one of the most important players in the financial system, which collects the idle funds of the savers and gives loans to the people in need of financing. Banks contribute to economic development and employment by supporting consumption, investment and production with the loans they give. The covid-19 pandemic, which emerged as of the beginning of 2020, has affected all countries socially, culturally and economically. The measures taken to stop the spread of the pandemic caused deterioration in the country's economies. The pandemic caused the supply and demand balance in the economies to deteriorate and increased the liquidity risk of the sectors. This situation experienced by the sectors especially affected banks and credit processes. The aim of this study is to investigate whether the domestic credit volumes of the Turkish banking sector during the covid-19 pandemic period, in the period of March/2020-November/2022, contain a bubble. Methodology- The weekly frequency data set was used in the study. Since the Turkish banking sector loan disbursement volumes are published weekly by the BRSA, the data set is handled on a weekly basis. In the study, the GSADF method was applied to detect the bubbles in the domestic credit volume of the Turkish banking sector. GSADF tests are iterative right-tailed unit root tests. In the study, commercial and other loan disbursement volumes and consumer loan volumes of the banking sector were used as variables. Housing, vehicle and consumer loans are included in consumer loan volumes. Findings- According to the results of the study, 2 bubble movements were detected in the commercial loan and other loan variable and 3 bubble movements in the consumer loans variable at the 1% significance level. It is observed that the third bubble in consumer loans still continues as of November/2022. Conclusion- It is thought that the precautionary packages taken by the economy management to limit the economic and financial effects of covid-19 create credit bubbles. In addition to the support loans provided by public banks to the real sector and exporting companies, interest-free loans to low-income segments on the individual side fueled the formation of a bubble. In this situation, it is thought that the need for cash of people has increased rapidly with the prolongation of the pandemic, and the unemployment of individuals in this process or the decrease in their monthly income may have increased their loan demands. In addition, it is assumed that the successive interest rate cuts made by the Central Bank cause the formation of foam, as the companies borrow in TL and return to foreign currency and turn to commodities to protect themselves against price increases. Keywords: Banking sector, credit balloons, bank credits, GSADF test JEL Codes: G00, G21, C58

Numerical Studies of Bubbles in Swirling Channel Flows

Abstract The motion of bubbles in upflow in a vertical rotating closed channel is examined numerically, using a front tracking/finite volume method. The flow is driven upward by a constant pressure gradient. The Reynolds number is low enough so that the flow remains laminar and the Eötvös number is sufficiently low so the bubbles remain nearly spherical. A bubble is placed between the centerline and the walls and for low rotation rate the bubble moves to a wall, due to the lift force and the fluid shear near the walls, but for higher rotation rate the bubble moves to the center of the channel, due to the radial pressure gradient established by the rotation. For intermediate rotation rates, we find bubbles where the lift force and the pressure gradient balance and the bubbles remain between the centerline and the walls. We also examine the collective motion of a few bubbles and show that their dynamics are similar to what is observed for a single bubble.

A stabilised finite element framework for viscoelastic multiphase flows using a conservative level-set method

The development of stable numerical schemes for viscoelastic multiphase flows has implications for many areas of engineering applications. The principal original contribution of this paper is the implementation of a conservative level-set method to define implicitly the interface between fluid phases, fully integrated into the mathematical framework of viscoelastic flow. The governing equations are discretized using the finite element method and stabilisation of the constitutive equation is achieved using either the discontinuous Galerkin (DG) or streamline upwinding (SU) method. The discrete elastic viscous stress splitting gradient (DEVSS-G) formulation is also employed in the Navier-Stokes equations to balance the hyperbolic characteristics of the polymeric stress tensor. The numerical scheme is validated with reference to several benchmark problems and excellent quantitative agreement with published data is found for Newtonian and viscoelastic fluids, for both single and multiphase flows. The motion of a gas bubble rising in a viscoelastic fluid is studied in detail. The influence of polymer concentration, surface tension, fluid elasticity and shear-thinning behaviour, on flow features such as the development of filaments and cusps and the generation of negative wakes is explored.

考虑表面张力的球泡运动的理论解研究

考虑表面张力的球泡运动的理论解研究

Unsteadiness characterisation of shock wave/turbulent boundary-layer interaction at moderate Reynolds number

A direct numerical simulation of an oblique shock wave impinging on a turbulent boundary layer at Mach number 2.28 is carried out at moderate Reynolds number, simulating flow conditions similar to those of the experiment by Dupont et al. (J. Fluid Mech., vol. 559, 2006, pp. 255–277). The low-frequency shock unsteadiness, whose characteristics have been the focus of considerable research efforts, is here investigated via the Morlet wavelet transform. Owing to its compact support in both physical and Fourier spaces, the wavelet transformation makes it possible to track the time evolution of the various scales of the wall-pressure fluctuations. This property also makes it possible to define a local intermittency measure, representing a frequency-dependent flatness factor, to pinpoint the bursts of energy that characterise the shock intermittency scale by scale. As a major result, wavelet decomposition shows that the broadband shock movement is actually the result of a collection of sparse events in time, each characterised by its own temporal scale. This feature is hidden by the classical Fourier analysis, which can only show the time-averaged behaviour. Then, we propose a procedure to process any relevant time series, such as the time history of the wall pressure or that of the separation bubble extent, in which we use a condition based on the local intermittency measure to filter out the turbulent content in the proximity of the shock foot and to isolate only the intermittent component of the signal. In addition, wavelet analysis reveals the intermittent behaviour also of the breathing motion of the recirculation bubble behind the reflected shock, and allows us to detect a direct, partial correspondence between the most significant intermittent events of the separation region and those of the wall pressure at the foot of the shock.

Numerical simulation of film boiling heat transfer in the presence of a uniform electric field using front tracking

Film boiling has many applications in various industries, such as industrial furnaces, heat exchangers in nuclear and fossil power plants. In this paper, film boiling on a horizontal plate in the presence of a uniform electric field is simulated in two dimensions by a finite difference/front tracking method with constant heat flux and constant wall temperature boundary conditions. Applying an electric field is an active method that is widely used to enhance heat transfer. Simulation of this phenomenon is of great importance in industry to predict the growth and movement of steam bubbles produced. The electric field is applied by an electric potential difference to the upper and lower boundaries. It is assumed that the working fluid is a completely dielectric fluid. The electric force is a volumetric force that is added directly to the Navier-Stokes equations. The present numerical method is validated by simulation of film boiling in the presence and absence of electric field and comparing with available experimental and numerical results. The effect of the electric field on the Nusselt number for constant heat flux and constant wall temperature boundary conditions are investigated. The effect of dimensionless numbers is also studied. Generally, the Nusselt number increases by applying an electric field. The bubble growth time, bubble thickness and vapor film thickness are reduced by applying an electric field. Small bubbles are formed on the surface with increasing electric field for constant heat flux boundary condition. The number of bubble growth sites increased with increasing electric field in the constant wall temperature boundary condition, or in other words, the instability wavelength decreased.

Simulations of Bubble Formation and Oscillation Behavior in Micro-Leakage Measurement System

In the process of measuring micro-leakage based on the bubble method, the physical parameters of the liquid phase have a great impact on the formation and movement of bubbles, thus affecting the measurement accuracy of the system. Therefore, it is of great significance to study the physical parameters of the liquid phase. At present, it is difficult to independently study the parameters of the liquid phase by experimental methods. In this paper, based on the Cahn-Hilliard phase field model, a gas-liquid two-phase flow model for the expression of leakage is established to independently study the influence of different physical parameters on the formation and movement of bubbles in the measuring system. Firstly, the effects of liquid medium density, dynamic viscosity coefficient and surface tension on the initial shape (aspect ratio), the generation period and the shape oscillation of bubbles after they leave the nozzle are analyzed. Then, an experimental platform for visualization of bubbles in two-phase flow based on machine vision is built, through which the morphological parameters of bubbles in the liquid phase can be obtained in real time. Finally, the results of visualization experiment and numerical simulation are compared. The results show that the initial aspect ratio error of bubbles obtained by visual experiment and numerical simulation is 1.8%, and the average oscillation period error is 12.3%. The results are basically consistent, which proves the rationality and correctness of the mathematical model established. It is found that the liquid with larger dynamic viscosity coefficient μ and density ρ and smaller surface tension σ can be selected as the liquid medium of the measurement system. It is more conducive to the accurate expression of micro-leakage in the measurement system, and the bubble shape can quickly reach a stable state. It provides theoretical support for improving the accuracy and stability of the micro-leakage measurement system.

Numerical investigation of the translational motion of bubbles: The comparison of capabilities of the time-resolved and the time-averaged methods.

In the present study, the accuracies of two different numerical approaches used to model the translational motion of acoustic cavitational bubble in a standing acoustic field are compared. The less accurate but less computational demanding approach is to decouple the equation of translational motion from the radial oscillation, and solve it by calculating the time-averaged forces exerted on the bubble for one acoustic cycle. The second approach is to solve the coupled ordinary differential equations directly, which provides more accurate results with higher computational effort. The investigations are carried out in the parameter space of the driving frequency, pressure amplitude and equilibrium radius. Results showed that both models are capable to reveal stable equilibrium positions; however, in the case of oscillatory solutions, the difference in terms of translational frequency may be more than three fold, and the amplitude of translational motion obtained by the time-averaged method is roughly 1.5 times higher compared to the time-resolved solution at particular sets of parameters. This observation implies that where the transient behaviour is important, the time-resolved approach is the proper choice for reliable results.

Deep learning-based bubble detection with automatic training data generation: Application to the PEM water electrolysis

Various fields, such as the paper industry, chemical engineering, and renewable energy, are faced with gas-liquid two-phase flows and are being studied by visualization and observation. Although it is necessary to quantitatively evaluate the characteristics of bubbles, there is a limitation in the amount of labor required for detection and measurement by human observation of images. There are no examples for bubbles in polymer electrolyte membrane water electrolysis (PEMWE), where the bubbles in PEMWE have heterogeneous backgrounds, unpatterned patterns, and unclear bubble contours. Existing methods for detecting these bubbles are not expected to be accurate enough. In this study, a deep learning-based bubble detection method using convolutional neural networks (CNN) was developed for bubbles in PEMWE. Our method has two novel approaches: first, we developed an algorithm that automatically draws a pseudo-bubble image based on an actual bubble image on an arbitrary background as a method for automatically generating training data for the CNNs. Second, we developed a CNN-based bubble detection method that uses the motion of bubbles, specifically, the difference between the bubble image and the image from one frame ago, as input. Finally, we tested the algorithm on bubble images in a PEMWE and achieved an F1 score of 0.83 or better for all current densities of 0.5, 1, 2, and 3 A/cm2, and an F1 score of 0.844 for the entire detection.