Two-phase Bubbly Flow Research Articles

Determination of the velocity and size of bubbles in two-phase flows by using a laser doppler anemometer

Natural oscillations of a spherical interface between a gas and a liquid in a bubble are registered. A possibility of measuring the geometric parameters of stationary and moving particles of the disperse phase by a laser Doppler anemometer is demonstrated. A method for simultaneous determination of the size and velocity of a bubble or a droplet in a two-phase flow is developed. The mean sizes of a group of bubbles settled on a ruler are compared: the results are obtained by two independent methods, i.e., by analyzing the image and by processing the Doppler signal containing information about the natural oscillations of the spherical interface between the media. A possibility of using a laser Doppler anemometer for simultaneous measurements of the velocity and size of bubbles or droplets in a two-phase flow is confirmed.

Ultrasonic pulse echography for bubbles traveling in the proximity of a wall

The behavior of a bubbly two-phase flow in the vicinity of a wall affects heat, mass, and momentum transfer; therefore, there is great interest in developing a quantitative technique to monitor this behavior. Herein we propose a new method based on ultrasound echo signal processing that it feasible for industrial applications where the boundary layer is modified by traveling bubbles. By introducing time-resolved direct waveform analysis at 100 MHz, we have succeeded in the spatio-temporal detection of bubble surfaces at echographic profiling frequencies in the range of 15–20 kHz. Unlike conventional approaches, which use short pulses, a relatively long pulse length is applied to allow ultrasound Doppler velocimetry in the liquid phase. Examination of the horizontal bubbly two-phase turbulent channel flows demonstrated the feasibility of this method; spatio-temporal echography of moving bubble surfaces is successfully achieved as the bubbles travel on length scales smaller than the spatial ultrasonic pulse length near the wall. The applicable range of parameters (e.g. bubble size and shape, and flow speed) was determined by 3D numerical analysis of the wave equation and its application to bubbles flowing beneath a flat-bottom model ship.

Hydrodynamics characteristics of hydrogen evolution process through electrolysis: Numerical and experimental studies

A reliable numerical procedure using the control volume formulation has been built up for predicting the hydrogen-generation process. The hydrogen production is due to the flow of an electrolyte between cathode and anode at different current densities. A bubbly two-phase flow process has been considered and a mathematical model based on Eulerian–Eulerian two-fluids has been adopted. The transport equations have been solved for both phases with allowance of interfacial transfer of mass and momentum. The conservation equations have been discretized using a finite volume method and solved by the SIMPLE algorithm. Measurements have been carried out along a tested cell gap at different current densities to visualize the hydrogen generation process. New insights into the model of hydrogen bubble-size variation in the computations are considered. Comparisons of numerical results based on the model with both experimental measurements and results available in the literature have been performed. The results indicate that the developed numerical model accurately predicts the hydrogen production process. The study shows also that the best production process is reached by decreasing the main flow velocity. Increasing the current density and reducing the gap distance between the cathode and the anode of the electrochemical cell helps improving the hydrogen production process. The bubble-diameter formulation of the dispersed hydrogen gas considerably influences the local and global characteristics of two-phase stream.

An Assessment of the Virtual Mass Force in RELAP5/MOD3.3 for the Bubbly Flow Regime

One-dimensional two-fluid models used for the simulation of large, industrial-scale problems require many simplifying assumptions to make a closed model that is tractable, applicable to a wide variety of flow regimes, and computationally efficient. Of particular interest here is the virtual mass force and the simplified form used in the RELAP5/MOD3.3 model. Comparison of the characteristics of the simplified model with a more complete two-fluid model for bubbly two-phase flow shows a remarkable similarity. Comparison to experimental data is also surprisingly favorable—provided that the flow conditions are determined appropriately. Namely, the characteristic analysis determined that a drift velocity for distorted bubbly flow, rather than for churn-turbulent flow, matches the data more accurately. The study is concluded by implementing a distorted bubbly drift velocity correlation into the RELAP5/MOD3.3 code. A comparison of the void wave speeds with the data confirms the results of the characteristic analysis.

Bubble stimulation efficiency of dinoflagellate bioluminescence.

Dinoflagellate bioluminescence, a common source of bioluminescence in coastal waters, is stimulated by flow agitation. Although bubbles are anecdotally known to be stimulatory, the process has never been experimentally investigated. This study quantified the flash response of the bioluminescent dinoflagellate Lingulodinium polyedrum to stimulation by bubbles rising through still seawater. Cells were stimulated by isolated bubbles of 0.3-3 mm radii rising at their terminal velocity, and also by bubble clouds containing bubbles of 0.06-10 mm radii for different air flow rates. Stimulation efficiency, the proportion of cells producing a flash within the volume of water swept out by a rising bubble, decreased with decreasing bubble radius for radii less than approximately 1 mm. Bubbles smaller than a critical radius in the range 0.275-0.325 mm did not stimulate a flash response. The fraction of cells stimulated by bubble clouds was proportional to the volume of air in the bubble cloud, with lower stimulation levels observed for clouds with smaller bubbles. An empirical model for bubble cloud stimulation based on the isolated bubble observations successfully reproduced the observed stimulation by bubble clouds for low air flow rates. High air flow rates stimulated more light emission than expected, presumably because of additional fluid shear stress associated with collective buoyancy effects generated by the high air fraction bubble cloud. These results are relevant to bioluminescence stimulation by bubbles in two-phase flows, such as in ship wakes, breaking waves, and sparged bioreactors.

English

A computational study of the flow field characteristics and the performances of two-phase bubbly flow in water jet of a ramjet propulsion system with a diverging-converging nozzle has been performed.  The analysis use a finite volume approach for the main flow coupled numerically with the Lagrangian equations for the bubbles motion including the change of bubbles radii with time.  Computed results for an experimental two-phase flow in a ramjet propulsion system with a converging-diverging nozzle are presented to validate the numerical model.  The performance of the ramjet for different initial velocities of the incoming water and for various locations of the bubbles injections has been studied.  Results reveal that the best location for the bubbles injection is at the beginning of the nozzle section and produces a higher thrust.   Key words: Bubble dynamic, water ramjet, thrust, computational fluid dynamics (CFD), two-phase flows.

Measurement of Bubbly Two-phase Flow in Vertical Pipe Using Multiwave Ultrasonic Pulsed Dopller Method and Wire Mesh Tomography

Abstract Two measurement methods which are multiwave ultrasonic pulsed Doppler (multiwave UVP) method and wire mesh tomography (WMT) have been applied to the measurement of bubbly two-phase flow. Velocity profiles of bubbles and liquid have been measured using multiwave UVP. Simultaneously, cross-sectional void fraction distribution has been measured using WMT. As the result, a combination method has been set up. The measured parameters are indispensable in order to obtain detailed structures of two-phase flow. Multiwave UVP method exploits two basic ultrasonic frequencies which are 2 MHz and 8 MHz. Simultaneous measurement of velocity profiles of bubbles (using 2 MHz frequency) and those of liquid (using 8 MHz frequency), at the same position, is enabled. A multiwave ultrasonic transducer (multiwave TDX) which is able to emit and receive the two ultrasonic frequencies along the measurement line at the same time has been applied. The signal processing is based on the pulsed Doppler method. Using the combination method, first, instantaneous velocity profiles and void fraction distribution have been measured simultaneously for air-water counter-current bubbly flow in a vertical pipe. Flow structure has been clarified. Effect of initial condition on void fraction distribution has been confirmed. Next, measurements have been carried out for subcooled boiling bubbly flow in a vertical pipe. For measurement of subcooled boiling flow, a high temperature wire mesh sensor (WMS) has been developed. A method for separation of velocity profiles of bubbles with different sizes and velocities has been suggested.

Visualized Flow Structure inside an Odor-Removing Basin System with Multi-Stacked Porous Baffles

Baffles are frequently used to reduce flow speed and to change fluid passages by directly blocking oncoming flow. This study employed multi-stacked porous baffle obstructions to enhance the flow relaxations of water-soluble buoyant gases inside an odor removing basin system. We performed qualitative flow visualization by using tracing particles in working fluid to evaluate the effects of porous baffles. Particle image velocimetry technique was also employed to quantitatively analyze the flow field around the baffle regime. As a result, oncoming two-phase bubbly flow from the basin bottom regime was bent, passing the edge of a porous baffle plate. The main stream speed and momentum were effectively reduced by the installed porous flow obstruction devices.

Enhancing Numerical Stability of a Two-Fluid Model by the Use of Interfacial Pressure Terms

Analytical and numerical investigations were carried out to show that the characteristics and the numerical stability of the two-fluid model are improved by the use of the interfacial pressure terms that express the pressure difference between bubbles and continuous liquid phase in bubbly two-phase flow. In particular, it was demonstrated that the numerical stability is enhanced not only in the simulation of adiabatic two-phase flow but also in the simulation of subcooled flow boiling.

Two-phase bubble flow and convective mass transfer in water splitting processes

When hydrogen or oxygen is produced from water splitting by electrolysis, thermochemical cycles or solar-based photocatalytic methods, bubble flow and vapor transfer into the gas phase occur during phase transition. This undesirable vapor transfer requires the use of more energy input to compensate for the evaporation heat requirement as well as for subsequent gas purification in the downstream unit. In this paper, both experimental and modeling studies are performed to examine the dynamics of bubble flows and kinetics of water vapor transfer, particularly related to processes of hydrogen production. Experimental data are obtained using an advanced laser-based shadow imaging system and on-line vapor monitoring system. The bubble dynamics and water vapor transfer kinetics are modeled with non-dimensional parameters involving the bubble diameter, velocity and trajectories so that the water vapor transfer rate can be quantified under different operating conditions for various hydrogen production methods. Also, a predictive model is developed to simulate the physical processes of bubble transport in a vertical liquid column, as it occurs in water splitting processes such as oxygen generation in the thermochemical copper–chlorine cycle, as well as hydrogen generation in electrolytic and photocatalytic processes.

Hydrodynamic Performances of Air-Water Flows in Gullies with and without Swirl Generation Vanes for Drainage Systems of Buildings

As an attempt to improve the performances of multi-entry gullies with applications to drainage system of a building, the hydrodynamic characteristics of air-water flows through the gullies with and without swirl generation vanes (SGV) are experimentally and numerically examined. With the aid of present Charge Coupled Device (CCD) image and optical systems for experimental study, the mechanism of air entrainment by vortex, the temporal variations of airflow pressure, the trajectories of drifting air bubbles and the self-depuration process for the gullies with and without SGV are disclosed. The numerical simulations adopt Flow-3D commercial code to attack the unsteady two-phase bubbly flows for resolving the transient fields of fluid velocity, vorticity and pressure in the gullies with and without SGV. In the twin-entry gully without SGV, air bubbles entrained by the entry vortex interact chaotically in the agitating bubbly flow region. With SGV to trip near-wall flows that stratify the drifting trajectories of the air bubbles, the air-bubble interactions are stabilized with the discharge rate increasing more than 7%. The reduction of the self-depuration period by increasing discharge rate is observed for the test gullies without and with SGV. Based on the experimental and numerical results, the characteristic hydrodynamic properties of the air-water flows through the test gullies with and without SGV are disclosed to assist the design applications of a modern drainage system in a building.

Spherical bubble dynamics in a bubbly medium using an Euler–Lagrange model

For applications involving large bubble volume changes such as in cavitating flows and in bubbly two-phase flows involving shock and pressure wave propagation, the dynamics, relative motion, deformation, and interaction of bubbles with the surrounding medium play crucial roles and require accurate modeling. We present in this paper a fundamental study of the dynamic oscillations of a “primary” bubble in a bubbly mixture using a two-way coupled Euler–Lagrange model. It addresses a simplified spherical configuration while using the full three-dimensional code. A main objective of the study is to investigate how the dynamics of a “primary” bubble is affected by the presence of a surrounding bubbly medium and how it differs from its behavior in a pure liquid. This helps elucidate the physics at play for this relatively simple configuration. The model simulates the mixture as a continuum and solves the corresponding Navier Stokes equations with grids moving with the interface of the primary bubble wall. The surrounding microbubbles are tracked in a Lagrangian fashion accounting for their volume evolution. The two-way coupling between the bubbly medium and the primary bubble dynamics is realized through the local density of the mixture obtained from the tracking of the microbubbles and the determination of their volumes and spatial distribution.The simulations clearly indicate that the surrounding microbubbles absorb energy emitted from the primary bubble during its oscillations. This results in a reduction of the maximum radius and the period of oscillations of the primary bubble as compared to the dynamics in the pure liquid. Also, accounting for the dynamics of the field bubbles brings out the presence in the two-phase medium of a phase shift between density and pressure distributions. Such a shift is not captured by two-phase homogeneous medium models. These effects increase with increase in the mixture void fraction and in the initial bubble sizes in the mixture. The numerical observations are found to be in good qualitative agreements with previously published experimental data (Jayaprakash et al., 2011) investigating spark generated bubble dynamics in a bubbly medium.

0403 超音波を用いた気液二相流における気泡速度の時間変化計測

0403 超音波を用いた気液二相流における気泡速度の時間変化計測

187 花弁状二重管を流れる気液二相気泡流の流動・伝熱特性

187 花弁状二重管を流れる気液二相気泡流の流動・伝熱特性

F233 2×2ロッドバンドル内気液二相気泡流の液相速度分布(OS9 熱・流動(6))

F233 2×2ロッドバンドル内気液二相気泡流の液相速度分布(OS9 熱・流動(6))

Bubble entrainment and liquid–bubble interaction under unsteady breaking waves

AbstractLiquid–bubble interaction, especially in complex two-phase bubbly flow under breaking waves, is still poorly understood. In the present study, we perform a large-eddy simulation using a Navier–Stokes solver extended to incorporate entrained bubble populations, using an Eulerian–Eulerian formulation for a polydisperse bubble phase. The volume-of-fluid method is used for free-surface tracking. We consider an isolated unsteady deep water breaking event generated by a focused wavepacket. Bubble contributions to dissipation and momentum transfer between the water and air phases are considered. The model is shown to predict free-surface evolution, mean and turbulent velocities, and integral properties of the entrained dispersed bubbles fairly well. We investigate turbulence modulation by dispersed bubbles as well as shear- and bubble-induced dissipation, in both spilling and plunging breakers. We find that the total bubble-induced dissipation accounts for more than 50 % of the total dissipation in the breaking region. The average dissipation rate per unit length of breaking crest is usually written as $b{\it\rho}g^{-1}c_{b}^{5}$, where ${\it\rho}$ is the water density, $g$ is the gravitational acceleration and $c_{b}$ is the phase speed of the breaking wave. The breaking parameter, $b$, has been poorly constrained by experiments and field measurements. We examine the time-dependent evolution of $b$ for both constant-steepness and constant-amplitude wavepackets. A scaling law for the averaged breaking parameter is obtained. The exact two-phase transport equation for turbulent kinetic energy (TKE) is compared with the conventional single-phase transport equation, and it is found that the former overpredicts the total subgrid-scale dissipation and turbulence production by mean shear during active breaking. All of the simulations are also repeated without the inclusion of a dispersed bubble phase, and it is shown that the integrated TKE in the breaking region is damped by the dispersed bubbles by approximately 20 % for a large plunging breaker to 50 % for spilling breakers. In the plunging breakers, the TKE is damped slightly or even enhanced during the initial stage of active breaking.

Numerical study of the influence of geometrical characteristics of a vertical helical coil on a bubbly flow

In this article, turbulent single-phase and two-phase (air-water) bubbly fluid flows in a vertical helical coil are analyzed by using computational fluid dynamics (CFD). The effects of the pipe diameter, coil diameter, coil pitch, Reynolds number, and void fraction on the pressure loss, friction coefficient, and flow characteristics are investigated. The Eulerian-Eulerian model is used in this work to simulate the two-phase fluid flow. Three-dimensional governing equations of continuity, momentum, and energy are solved by using the finite volume method. The k-ɛ turbulence model is used to calculate turbulence fluctuations. The SIMPLE algorithm is employed to solve the velocity and pressure fields. Due to the effect of a secondary force in helical pipes, the friction coefficient is found to be higher in helical pipes than in straight pipes. The friction coefficient increases with an increase in the curvature, pipe diameter, and coil pitch and decreases with an increase in the coil diameter and void fraction. The close correlation between the numerical results obtained in this study and the numerical and empirical results of other researchers confirm the accuracy of the applied method. For void fractions up to 0.1, the numerical results indicate that the friction coefficient increases with increasing the pipe diameter and keeping the coil pitch and diameter constant and decreases with increasing the coil diameter. Finally, with an increase in the Reynolds number, the friction coefficient decreases, while the void fraction increases.

Prediction of hydrodynamic entrance length for single and two-phase flow in helical coils

The hydrodynamic entrance length in helical coils for single and two-phase bubbly flow was studied experimentally and numerically. Development region length and the detailed characteristics of fluid flow have been investigated by varying helical coil parameters such as tube diameter, coil diameter and void fraction. For the CFD simulation of the two-phase fluid flow, the Eulerian–Eulerian model was employed. To calculate the turbulent fluctuations, the SSTk−ω turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that the hydrodynamic developing length (L/D) increases when Reynolds number increases in the single and two-phase flows. The obtained results show that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decreases while curvature ratio of helical coil and void fraction increase. A correlational equation has been suggested to predict the hydrodynamic entrance length as functions of various parameters of the helical coil.

Evaluation of interfacial area transport equation in vertical bubbly two-phase flow in large diameter pipes

The introduction of interfacial area transport equation (IATE) has greatly improved the overall performance of two-fluid model due to the fact that the IATE can predict the interfacial area concentration (IAC) more accurately. However, models in the source and sink terms of IATE that are developed for predicting bubble coalescence and breakup are mainly based on small diameter pipe flows. Since the two-phase flow in a large diameter pipe is characterized by its multi-dimensional nature, the IATE should be confirmed before being applied to large pipe flows. From this point of view, local measurements in air–water bubbly flow systems in a large pipe with 101.6mm diameter were performed by using four-sensor optical probes. The radial profiles of the void fraction, IAC, bubble Sauter mean diameter and interfacial velocity were obtained at two axial locations of z/D=2 and 29. The simplified one-dimensional, steady-state, adiabatic one-group IATE with eight sets of bubble coalescence and breakup models was evaluated against the present experimental data and that from the literatures. The evaluation results showed that the Sun et al. (2004a) model is the best one for large diameter pipes. This model can well reflect the interfacial transfer mechanisms and can provide a reasonable prediction in general. Besides, the models of Smith et al. (2012a) and Ishii and Kim (2001) are also recommended due to their relative good performance in the prediction of IAC. Further work should be undertaken to develop a new bubble coalescence and breakup model with extra consideration of the multi-dimensional flow behavior in large pipes, if necessary.

Bubbly Two-Phase Flow: Part I- Characteristics, Structures, Behaviors and Flow Patterns

An important topic in fluid dynamics is multiphase flows. Multiphase flows can be found in numerous fields in engineering, e.g. aerospace, biomedical, chemical, electrical, environmental, mechanical, materials, nuclear and naval engineering. There is an enormous variation in applications, e.g. rocket engines, chemical reactors, contamination spreading, multiphase mixture transport, cavitation, sonoluminescence, ink-jet printing, particle transport in blood, crystallization, multiphase cooling, fluidized beds, drying of gases, air entrainment in oceans/rivers and anti-icing fluids. The number of papers on multiphase flow in the field of fluid dynamics is enormous and still growing. The diversity of flow types makes a general description almost impossible. This makes fundamental research necessary. Especially, controlled experiments are needed for a better physical understanding and as test cases for numerical and theoretical work. The purpose of this paper is the desire to demonstrate, review and summarize the major finding of the previous research of bubbly two-phase flow characteristics, structures, behaviors and flow patterns. Moreover, to elucidate some important models and techniques to measure the two-phase bubbly flow parameters such as bubble motion, flow regime, bubble shape which play a considerable role in many engineering applications.