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. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

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.     

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.     

Experimental Study of a Microchannel Bubble Injector for Microgravity Applications

We perform a quantitative characterization of a microbubble injector in conditions relevant to microgravity. The injector pregenerates a slug flow by using a capillary T-junction, whose operation is robust to changes in gravity level. We address questions regarding the performance under different injection conditions. In particular we focus on the variation of both gas and liquid flow rates. The injection performance is characterized by measuring bubble injection frequency and bubble sizes. We obtain two distinct working regimes of the injector and identify the optimal performance as the crossover region between them.     

圆管流动的二次转捩

利用直接数值模拟方法求解N-S方程来研究空间发展的圆管转捩,计算中雷诺数Re选定为3000。在局部壁面引入的周期性吹吸(PSB)扰动作用下,圆管中首先出现了塞流结构,并向下游迁移,同时有稳定的流向条带结构形成;在塞流结构离开计算域后,随着扰动的不断发展,流向条带结构逐渐破裂失稳,圆管流动出现了第二次转捩,这是一种新的转捩形式,我们称之为“二次转捩”。     

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.     

Influence of gravity state upon bubble flow in the deep penetration molten pool of vacuum electron beam welding

The governing equations of two-dimensional bubble flow model for gas–liquid two-phase system in deep penetration molten pool of vacuum electron beam welding were developed according to the laws of mass and momentum conservation. The separation models of gas and liquid convections in bubble flow were formed by regarding the gas phase in molten pool as a particle phase, and the vacuolar fraction, velocity slip, pressure gradient and other factors were introduced into the models. The influences of the gravity state upon the convection of bubble flow and the distribution of cavity-type defects in molten pool of AZ91D magnesium alloy were studied by the method of numerical simulation based on the mathematical models. The results showed that the gravity is an important factor to drive the convection of the bubble flow in the deep penetration molten pool during vacuum electron beam welding. The gravity has an impact on the gas distribution in molten pool, thus affects the distribution of cavity-type defects in weld. Because of the gravity contributing to driving the convection of bubble flow, it is conducive to the escape of gas phase in molten pool and reducing the air rate. A larger convection velocity of gas phase is helpful to the escape of gas phase, thus reduce the tendency of cavity-type defects.     

超重力场下球形炸药水下爆炸实验及数值模拟

常重力条件下,Mach数和Froude数无法同时满足相似。采用离心模型实验,研究水下爆炸冲击波特性及气泡脉动规律,并采用数值模拟方法重现了超重力场下的水下爆炸过程。实验及数值模拟的结果表明,重力的改变基本不影响冲击波的峰值,COLE理论在超重力环境下仍然适用,而超重力场下,小当量炸药可以模拟气泡的脉动迁移过程,其结果可以用于预测大当量炸药深水爆炸的特性。此外,为减少计算时间并提高计算精度,采用将二维冲击波作为初始条件映射到三维模型的建模方法。对于冲击波计算,网格尺寸宜取药包半径的1/20¹/10;对于气泡脉动,宜取药包半径的1/21/10;对于气泡脉动,宜取药包半径的1/2¹。     

Microgravity performance of micro pulsating heat pipes

A micro pulsating heat pipe made of a thin clear Teflon tube of 1.6 mm ID was used to observe the pulsating flow inside a heat pipe under different gravity levels using parabolic flights. More vigorous pulsating flow was observed under microgravity, compared to the depressed movements under hypergravity. Two metallic micro pulsating heat pipes made of an aluminum plate with small internal channels were also tested to investigate the effect of gravity on their heat transfer characteristics. Reduced gravity experiments were performed aboard Falcon 20 aircraft flying parabolic trajectories. Under normal and hypergravity conditions, both the orientation of the pulsating heat pipe and locations of the heated and cooled sections affected the heat transfer performance. Under reduced gravity, however, the heat pipes showed better operating and heat transfer performance than that under normal and hypergravity. These experiments have for the first time confirmed that pulsating heat pipes are capable of operating under reduced gravity and thus are suitable for deployment in space applications such as satellites.     

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...     

Gas–Liquid Interfacial Friction Factor for the Transition from Stratified to Slug Flow

Calculations based on the slug stability model and simplified stratified flow model provide predictions of the critical liquid height and the critical superficial velocities of a stratified flow for the transition to a slug flow in a horizontal pipe. Since slug flow derives from different interfacial waves patterns, previous interfacial waves model in stratified gas–liquid flows brings about the discrepancy between theoretical prediction and experimental data. A partial analysis for this behavior is given, which recognizes that the values of gas–liquid interfacial friction factor at the onset of slug flow have been underestimated, especially at high gas flows, and they should be obtained indirectly from other measured variables. Modified correlations for the interfacial friction factor are presented and better agreement between predicted and measured critical superficial velocities has been obtained.     

超重力场下球形炸药水下爆炸实验及数值模拟

常重力条件下,Mach数和Froude数无法同时满足相似。采用离心模型实验,研究水下爆炸冲击波特性及气泡脉动规律,并采用数值模拟方法重现了超重力场下的水下爆炸过程。实验及数值模拟的结果表明,重力的改变基本不影响冲击波的峰值,COLE理论在超重力环境下仍然适用,而超重力场下,小当量炸药可以模拟气泡的脉动迁移过程,其结果可以用于预测大当量炸药深水爆炸的特性。此外,为减少计算时间并提高计算精度,采用将二维冲击波作为初始条件映射到三维模型的建模方法。对于冲击波计算,网格尺寸宜取药包半径的1/20~1/10;对于气泡脉动,宜取药包半径的1/2~1。     

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.     

R134a-DMF垂直管内鼓泡吸收特性研究

为研究垂直管内R134a-DMF(二甲基甲酰胺)鼓泡吸收过程的热、质传递特性,本文搭建了垂直管内鼓泡吸收实验测试装置,构建了管内R134a被R134a-DMF混合溶液鼓泡吸收过程的热、质传递数学模型.进一步通过模型分析了当吸收压力为0.35 MPa,蒸气入口温度为5℃,稀溶液入口质量流量为12.0 kg/h时,吸收过程...     

Influence of particles on the transition to turbulence in pipe flow

We investigate by experiment the influence of suspended solids upon the transition to turbulence in pipe flow. The particles are monodisperse and neutrally buoyant with the liquid. The role of the particles on the transition depends upon both the volume fraction, phi, and particle size. Below a critical particle diameter, particles alter the transition to larger critical Reynolds numbers for all phi. In contrast to this, larger particles move the transition to smaller Reynolds numbers for small phi, but they delay the transition at larger concentration.     

Solids Friction Factors for Fluidized Dense-Phase Conveying

There have been numerous correlations proposed for determining a solids friction factor ( λs ) for fully suspended (dilute phase) pneumatic conveying. Currently, there are no equivalent correlations that predict λs in nonsuspension dense-phase flows. In dense-phase conveying there are two basic modes of flow: plug/slug flow, which is predominantly based on granular products, and fluidized dense-phase flow, which is more suited to fine powders exhibiting good air retention capabilities. In plug/slug type flow, the stresses between the moving plug of material and the pipe wall dominate the solid-phase frictional losses. In fluidized dense-phase flow the frictional losses are characterized as a mixture of particle-wall and particle-particle losses but are heavily influenced by the gas-solid interactions. In this paper, a series of calculations were performed on experimental data in order to estimate λs for four types of material conveyed in the fluidized dense-phase flow regime. The solids frictional factors were found to be relatively independent of particle properties for varying air and solid mass flow rates and pressure drops. The resultant pressure drop from the empirical model showed good agreement with the experimental data.     

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.