Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

Investigation on Effect of Gravity Level on Bubble Distribution and Liquid Turbulence Modification for Horizontal Channel Bubbly Flow

Bubbly flows in the horizontal channel or pipe are often seen in industrial engineering fields, so it is very necessary to fully understand hydrodynamics of horizontal bubbly flows so as to improve industrial efficiency and to design an efficient bubbly system. In this paper, in order to fully understand mechanisms of phase distribution and liquid–phase turbulence modulation in the horizontal channel bubbly flow, the influence of gravity level on both of them were investigated in detail with the developed Euler–Lagrange two–way coupling method. For the present investigation, the buoyance on bubbles in both sides of the channel always points to the corresponding wall in order to study the liquid–phase turbulence modulation by bubbles under the symmetric physical condition. The present investigation shows that the gravity level has the important influence on the wall–normal distribution of bubbles and the liquid–phase turbulence modulation; the higher the gravity level is, the more bubbles can overcome the wall–normal resistance to accumulate near the wall, and the more obvious the liquid–phase turbulence modulation is. It is also discovered that interphase forces on the bubbles are various along the wall–normal direction, which leads to the fact that the bubble located in different wall–normal places has a different wall–normal velocity.     

Effect of the size of air bubbles on enhancement of heat transfer in an impinging liquid jet

The numerical modeling of heat transfer in a bubbly impinging jet is carried out. The axisymmetric system of RANS equations that take into account the two-phase nature of the flow is resolved based on the Euler approach. The turbulence of the liquid phase is described by the Reynolds stress transport model with taking into account the effect of bubbles on modification of the turbulence. The effect of the gas volumetric flow rate ratio and the bubble size on the flow structure and the heat transfer in a gas–liquid impact stream is studied. It is shown that the addition of the gas phase in a turbulent fluid causes an increase up to 1.5-fold in heat transfer. The comparison of the simulation results with experimental data showed that the developed model enables the simulation of turbulent bubbly impinging jet with heat transfer with the pipe wall in a wide range of gas fraction.     

Effects of Reduced Gravity Conditions on Bubble Dispersion Characteristics in the Bubble Column

Bubble-liquid turbulent flow has an excellent heat and mass transfer behaviors than single gas or liquid flow. In order to analyze the effects of normal and reduced gravity on cold bubble-liquid two-phase turbulent flow in bubble column a second-order moment cold bubble-liquid two-phase turbulent model was developed to disclose the bubble dispersion characteristics. Under the reduced gravity condition, volume fraction caused by the decrease of buoyance force is larger than normal gravity level due to bigger bubble solid volume. In addition, bubble frequency is also decreased by in decrease of buoyance force. Normal and shear stresses have strongly anisotropic characteristics at every directions and have larger values under normal gravity than reduced gravity. The liquid turbulent kinetic energy has the two-peak bimodal distribution and weaker than bubble turbulent kinetic energy with one peak unimodal, which is caused by vigorous wake fluctuations. The correlation of fluctuation velocities between bubble and liquid has clearly anisotropic behaviors Under reduced gravity, the bubble motion has a little impact on liquid turbulent flow caused by slight buoyancy force, however, it will greatly reduce the liquid turbulent intensity due to energy cascade transport, which was transformed into bubbles or dissipated by interface friction. Bubble formation and detachment mechanisms affected by gravity conditions lead to the different levels of bubble dispersion distributions.     

Frequency characteristics of the noise of R600a refrigerant flowing in a pipe with intermittent flow pattern

The acoustic characteristics of a long-shaped cylindrical bubble for slug or churn flow in a pipe are different from those of a freely rising spherical bubble in infinite liquid. In this research, the theoretical estimation of the natural frequency of the long-shaped cylindrical bubble was derived using the energy conservation law for a single bubble in a pipe. The acoustic characteristics of bubbles in a pipe were also investigated with the R600a refrigerant, which is widely used in refrigerators when the flow pattern in a pipe is slug or churn flow. In order to make slug and churn flow artificially, refrigerant-supplying equipment was designed and developed. Using this test equipment, the frequency characteristics of the long-shaped cylindrical bubble in 2-phase flow were investigated experimentally.     

Two-phase flow characteristics of liquid nitrogen in vertically upward 0.5 and 1.0 mm micro-tubes: Visualization studies

Application of liquid nitrogen to cooling is widely employed in many fields, such as cooling of the high temperature superconducting devices, cryosurgery and so on, in which liquid nitrogen is generally forced to flow inside very small passages to maintain good thermal performance and stability. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical two-phase flow patterns of liquid nitrogen in vertically upward micro-tubes of 0.531 and 1.042 mm inner diameters. It was found from the experimental results that the flow patterns were mainly bubbly flow, slug flow, churn flow and annular flow. And the confined bubble flow, mist flow, bubble condensation and flow oscillation were also observed. These flow patterns were characterized in different types of flow regime maps. The surface tension force and the size of the diameter were revealed to be the major factors affecting the flow pattern transitions. It was found that the transition boundaries of the slug/churn flow and churn/annular flow of the present experiment shifted to lower superficial vapor velocity; while the transition boundary of the bubbly/slug flow shifted to higher superficial vapor velocity compared to the results of the room-temperature fluids in the tubes with the similar hydraulic diameters. The corresponding transition boundaries moved to lower superficial velocity when reducing the inner diameter of the micro-tubes. Time-averaged void fraction and heat transfer characteristics for individual flow patterns were presented and special attention was paid to the effect of the diameter on the variation of void fraction.     

液氢加注系统竖直管道内Taylor气泡的行为特性

针对液氢加注系统竖直管道内气液两相流实验化困难的问题,运用建模仿真的方法建立了竖直管道内Taylor气泡的运动模型,对Taylor气泡的形成过程、大小以及充分发展的Taylor气泡上升速度进行了研究.采用VOF方法对气液两相的交界面进行追踪,并引入CSF模型对两相间的表面张力进行计算.仿真结果表明:Taylor气泡是由...     

Specificity of laminar-turbulent transition in upward monodispersed microbubbly flow

Results of experimental investigation of the wall shear stress in the upward monodispersed bubbly flow in a vertical tube are presented. The bubble generator based on the flow focusing technique has been developed for monodispersed submillimeter bubbles production. The results of investigation prove that submillimeter bubbles significantly increase the flow mass transfer with the wall. Some peculiarities of the inherent liquid turbulence interaction with pseudoturbulence induced by submillimeter bubbles in transitional flow regime have been detected.     

Time series analysis of gamma densitometry signals

Single, narrow-beam densitometry has been developed as a method for determining the flow regime and void fraction for industrial liquid gas experiments at high pressures and temperatures in a vertical, thick-walled, steel vessel. To develop suitable techniques, the experimental conditions were simulated using a transparant air/water column. In the transition region from bubbly to slug flow, a time sequence of four regimes, viz. annular flow, partially developed annular flow, coalescing bubble flow and bubbly flow were visually identified in a given cross section. Gamma rays were used to interrogate a column diameter, and digital time series analysis methods were applied. Amplitude spectral densities were used to determine any periodicity in the gas phase flow. The average void fraction for periodic gas flows was obtained by analysis of probability density distributions (PDD). The time sequence of the flow regimes was obtained from the signal magnitude of the diametral void fractions and the time spent in each regime was measured by the associated probability. The results compared well with those obtained from other methods. In the bubbly flow region, the standard deviation of the PDD exceeded that expected for nuclear counting. This bubble noise was assessed with respect to bubble properties.     

Gravity Influence on Heat Transfer Rate in Flow Boiling

The aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed with parabolic flights. The paper will show the analysis of differences between the heat transfer coefficients at normal and at zero gravity, and the study of the effects of mass flux, heat flux, and tube diameter on boiling phenomena at microgravity. Three tube diameters are tested: 6.0, 4.0, and 2.0?mm. With respect to terrestrial gravity, both heat transfer rate enhancement (up to 15?C20%) and deterioration (up to 35%) have been observed. Heat transfer differences for the two gravity conditions may be related to the different bubble size in each of them. The size of a bubble in flow boiling is generally affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heating wall. Heat transfer enhancements at low gravity, are observed in those conditions where the flow pattern is bubbly flow at normal gravity and intermittent flow at low gravity. The results are presented in a flow boiling gravity influence map, which can be considered a useful tool for designing boiling systems for space applications.     

Numerical Investigation on Turbulence and Bubbles Distribution in Bubbly Flow Under Normal Gravity and Microgravity Conditions

Two-phase flows of gas and liquid are increasingly paid much attention to space application due to excellent properties of heat and mass transfer, so it is very meaningful to develop studies on them in microgravity. In this paper, gas-phase distribution and turbulence characteristics of bubbly flow in normal gravity and microgravity were investigated in detail by using Euler–Lagrange two-way model. The liquid-phase velocity field was solved by using direct numerical simulations (DNS) in Euler frame of reference, and the bubble motion was tracked by using Newtonian motion equations that took into account interphase interaction forces including drag force, shear lift force, wall lift force, virtual mass force and inertia force, etc. in Lagrange frame of reference. The coupling between gas–liquid phases was made with regarding interphase forces as a momentum source term in the momentum equation of the liquid phase. Under the normal gravity condition, a great number of bubbles accumulate near the walls under the influence of the shear lift force, and addition of bubbles reduces turbulence of the liquid phase. Different from the normal gravity condition, in microgravity, an overwhelming majority of bubbles migrate towards the centre of the channel driven by the pressure gradient force, and bubbles have little effect on the turbulence of the liquid phase.     

Bubbly Jet Impingement in Different Liquids

The impingement of bubbly jets in distilled water and ethanol has been experimentally studied on ground. An experimental apparatus for the study of jet impingement on ground and in microgravity has been designed. The opposed-jet configuration with changeable orientation is used in order to study which is the better disposition to achieve an efficient mixing process. The impact angle between jets that can be changed from 0° (frontal collision) up to 90° (perpendicular collision). The impinging jets are introduced into a test tank full of liquid by means of two bubble injectors. The bubble generation method, insensitive to gravity level for low Bond numbers, is based on the creation of a slug flow inside a T-junction of capillary tubes of 0.7 mm of diameter. Bubble velocities at the injector outlet and generation frequencies can be controlled by changing gas and liquid flow rates. Individual bubble properties and coalescence events, as well as the whole jet structure are analyzed from the images recorded by a high speed camera. Bubble velocities are compared with the velocity field of a single-phase jet. Rate of coalescence between bubbles is found higher in ethanol than in water, creating a higher dispersion in bubble sizes.     

Two-phase Flow in Fuel Cells in Short-term Microgravity Condition

A visual observation of liquid–gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.     

A numerical study of periodic slug flow at zero gravity and normal gravity

This paper reports numerical simulations of slug flow at zero and normal gravity. The particular experimental results chosen for validation were obtained at microgravity under conditions which resulted in evenly-spaced and evenly-sized Taylor bubbles facilitating a simulation with periodic boundary conditions. The numerical technique was a free-surface method which explicitly tracked the motion of the gas-liquid interface using a volume-of-fluid specification and a finite volume discretisation of the solution domain. The large scale features of the bubble such as the classic bullet-shaped nose were well predicted by the model. Unsteady features of the bubble shape such as waves in the film and fluctuations of the bottom surface were also predicted but are harder to compare quantitatively to the experiments. The velocity field predictions reveal several interesting features of the flow. When viewed by an observer moving with the bubbles, the liquid slug is dominated by a large recirculating region with the flow travelling from the leading to the trailing bubble along the tube centreline. In this frame of reference, the near-wall region features a jet of fluid issuing from the film of the leading bubble which entrains fluid in the slug. As the film of the trailing bubble begins to form, the entrained fluid must be ejected since the flowrate in the film of each bubble must be the same. It appears to be this process that drives the main recirculation.     

Design of an Experiment for the Study of Bubble Jet Interactions in Microgravity

A new experimental setup for the study of bubble coalescence and bubble jet interactions in microgravity conditions is presented. The section consists of a cavity full of liquid containing two bubble injectors whose separation distance and relative orientation angle can be controlled. Injection of bubbles is based on the generation of a slug flow in a capillary T-junction, which allows a control of bubble size and velocity by means of liquid and gas flow rates. Individual and collective behaviour of bubbles injected in the cavity has been studied. On ground results on the individual trajectories, maximum distance reached, and the delimitation between turbulence and buoyancy regions are presented. The influence on these results of the inclination angle of one injector with respect to gravity has also been considered. A good knowledge of bubble jets behaviour in microgravity will enhance the development of space technologies based on two-phase systems.     

Effect of electrostatic charge of particles on hydrodynamics of gas-solid fluidized beds

The aim of this work was to investigate effect of electrostatic charge of particles on the fluidization hydrodynamics. Behavior of bubbles in beds of polyethylene particles was studied through analysis of pressure fluctuations in the frequency domain. Fluidized beds of uncharged, pre-charged and bed-charged particles were used in the experiments. Results revealed that in the bed of pre-charged particles, compared to uncharged experiments, particle-particle repulsive force increases the bed voidage and reduces equilibrium bubble size while the transition velocity to turbulent fluidization is decreased. In the case of bed-charged particles, at low gas velocities bubble fraction is greater compare to the other cases due to faster bubble coalescence in the presence of particle-wall attractive electrostatic force. Electrostatic charge of bulk increases by increasing the gas velocity. At high gas velocities, the repulsion force between highly charged particles overcomes the particle-wall effect on bubble formation and reduces the bubble size to less than in uncharged experiments. Accumulation of particles near the wall in the bed od bed-charged particles affects the hydrodynamics in two ways: first it accelerates bubble growth via bubble coalescence at low gas velocities, second it limits the bubble growth and reduces the transition velocity to turbulent regime to a value less than for pre-charged particles.     

Numerical Investigation of Bubble Induced Marangoni Convection: Some Aspects of Bubble Geometry

Thermocapillary or Marangoni convection is a surface tension driven flow that occurs when a gas–liquid or vapor–liquid interface is subjected to a temperature gradient. In the past, the contribution to local heat transfer arising from Marangoni convection has been overlooked as insignificant since under earth gravity it is overshadowed by buoyant convection. This study numerically investigates some aspects of bubble size and shape on local wall heat transfer resulting from Marangoni convection about individual bubbles on a heated wall immersed in a liquid silicone oil layer (Pr = 110) of depth 5 mm. It was found that increasing bubble volume causes an increase in the area over which Marangoni convection has affect. Heat transfer therefore increases with bubble size. Over the effective area, the surface averaged hot wall heat transfer is not affected greatly by bubble shape. The surface averaged heat transfer over the effective area on both the hot and cold walls is affected dramatically by bubble size, but the increase is more profound on the cold wall.     

气泡群振荡及噪声数值模拟研究

针对水中气泡群振荡及辐射噪声问题,基于均质混合流模型,考虑气泡动力学作用,建立数学模型;并采用拉格朗日有限体积法,对气泡群振荡过程及辐射噪声进行数值模拟。重点研究了初始气泡半径、含气率及气泡群尺度等对振荡辐射噪声的影响。结果表明,随着初始含气率、气泡群尺度增大,气泡辐射噪声频率逐渐减小、声压幅值逐渐增大即声强增大;初始气泡半径对辐射噪声频率影响较小,但声压幅值随其增大而减小。     

Rise of a Single Bubble in Ascending Laminar Flow: Slip Velocity and Wall Friction

An experimental study has been made of the motion of single bubbles in ascending laminar flow in a vertical pipe. An electrodiffusion procedure has been used for visualization of wall friction in passage of a single bubble. Time realization of friction stress is considered as the structure frozen-in into the flow and moving along the flow together with the bubble. The experiments have revealed the complex structure of wall-friction stress, which corresponds to different components of interaction of the bubble with the wall. The evolution of these components as a function of the Reynolds numbers of the bubble and the pipe is discussed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 4, pp. 129–135, July–August, 2005.