Dynamics of bubbles in field access devices studied using a high speed optical sampling microscope

Dynamics of bubbles in field access devices operated at 100 kHz and 200 kHz have been studied using a high speed optical sampling microscope capable of making a 10 nsec single exposure photograph at a known sampling time with respect to a rotating field orientation. By scanning the sampling time through one rotating field cycle, a series of time-sequenced transient domain configurations of bubbles have been recorded on 16 mm film. The devices studied were commonly used permalloy propagating structures, T-bar, T-X, X-bar, and chevron fabricated on an epitaxial garnet film. From each frame of the 16 mm film, plots of the position of the leading and trailing edges of a bubble as a function of rotating field angle were made. These position plots revealed very nonuniform bubble motion in these circuits. The instantaneous velocities of the leading and trailing edges of the bubble were calculated from the position plots. The results show a large velocity variation in all circuits, the ratio of the maximum velocity to the average velocity ranging from 3.5 for the X-bar to 4.8 for the chevron. Maximum vetocities in excess of that predicted on the basis of the critical velocity for dynamic conversion were observed.     

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 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.     

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.     

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

Sliding and sticking vapour bubbles under inclined plane and curved surfaces

In flow boiling heat is transferred by the combined effects of nucleate boiling, with local generation of bubbles, and evaporative and convective cooling by the passage of bubbles generated elsewhere. In this study, nucleate boiling was eliminated by measuring the heat transfer near injected steam bubbles sliding under an inclined plate heated to low superheats, using liquid crystal thermography combined with high speed video recording and computerised image analysis. Heat was transferred by evaporation of the thin liquid film between the bubble and the wall and by enhanced convection in a wake region wider than the bubble and many bubble diameters long. Evaporation was the dominant mechanism for large, easily deformed, slow-moving bubbles. For small, faster-moving bubbles the reduction in evaporation was offset by an improvement in convection.     

Analytic determination of overwrite capability in magnetic recording systems

Analytic design criteria are provided to determine if a digital magnetic recording system can overwrite under worst-case conditions. The worst-case condition is taken to be a bubble of reversed magnetization in an otherwise saturated medium, written by applying current to a stationary recording head. A leading and a trailing transition are formed, creating a large demagnetizing field opposing the head field. Although the leading transition is commonly thought to be unimportant in saturation writing, its demagnetizing field can significantly hamper the writing of the trailing transition. First, self-consistent numerical calculation shows the characteristics of the bubble and its associated fields. Then the bubble is approximated analytically by a biquadratic form, and the demagnetizing field at the bubble center is compared with the field necessary to saturate the medium. A rapid loss of overwrite ability with decreasing gap-length is demonstrated. The importance of including image fields for thin-film heads is discussed. Graphical representations are given for the minimum gap-length necessary for overwrite. A relation between media parameters, bit density, and magnetic energy/bit in commercial disk drives is discussed.     

CFD study of the thermochemical characteristics of mesoscale bubbles in a BFB gasifier

Using the computational fluid dynamic model based on the multiphase particle-in-cell approach, reactive gas-solid flows in a three-dimensional bubbling fluidised bed gasifier regarding coal gasification is simulated. The predicted results of gas components are firstly validated with experimental measurements. Subsequently, the dynamics (e.g. chord length, aspect ratio, velocity, volume) together with the essential thermal features (e.g. temperature, pressure, density, viscosity, conductivity, specific heat capacity) of mesoscale bubbles are explored. Bubbles in the central part of the gasifier have a large lateral chord length and volume. The restriction effect of the gasifier wall elongates the vertical chord length and enlarges the aspect ratio of bubbles close to the wall. The density and pressure of bubbles decrease along with the bed height, and the thermochemical properties of bubbles are strongly related to bubble temperature. A large content of the CO₂ exists for those bubbles close to the bed surface. The gas velocity, operating pressure and gasifying agent obviously affect the bubble property, while the effects of inlet gas temperature and coal flow rate are relatively weak. The results obtained provide meaningful insight regarding the mesoscale structures involved in fluidised bed coal gasification.     

Bubble growth rate and phenomenon of degenerate boiling of a fluid in the form of film vaporization

An analysis of results of an investigation of vaporization of a thin fluid film under vacuum is presented. The vaporization process has basic features characteristic of bubble boiling of a fluid, except for formation of bubbles. This makes it possible to classify the phenomenon as degenerate boiling of a fluid in the form of film evaporation during which local thinning of the layer takes place and funnel- and crater-shaped structures are formed. Funnels and craters are vapor sources with different powers. Dependences of the bubble growth rate above vapor sources of both types that generalize available experimental data are obtained.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

Combined momentum, heat and mass transfer in vertical slug flow absorbers

Numerical solutions have been obtained for the system of equations of momentum, heat and mass transfer describing the absorption of a refrigerant vapour from a Taylor bubble into the refrigerant-absorbent solution film around the bubble. The numerical results are compared with Nusselt's solution of the energy equation and with the penetration theory solution of the mass diffusion variation. Experimental data have been collected in vertical tubular absorbers in the slug flow region with the systems ammonia-lithium nitrate and ammonia-sodium thiocyanate. Four different absorber tubes have been tested with internal diameters of 10, 15, 20, and 25 mm. These data are compared with the numerical and theoretical results. The effect of the bubble nose on mass transfer is studied. Typical temperature profiles during the absorption process in absorption cooling/heating systems are shown.     

壁面与自由液面联合作用下气泡动态特性实验研究

壁面与自由液面联合作用下，气泡的动态特性与单独一个边界条件作用时相比要复杂很多，难于从理论角度分析探讨。为深入探究壁面与自由液面联合作用下的气泡动态特性，采用安全性高且实验效果理想的200V直流电压电火花气泡发生装置生成气泡，依次改变气泡发生位置距壁面及自由液面的距离，同时采用高速运动分析系统对不同工况下的实验结果进行采集与分析，得到了壁面与自由液面联合作用下的气泡形态变化，同时通过对大量实验数据的总结归纳，得到壁面与自由液面联合作用对气泡脉动周期、迁移轨迹及气泡内射流方向的影响规律，从而为相关的理论与数值研究提供参考。     

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...     

基于多相流的LCM工艺气泡生成仿真

分析了液体模塑成型工艺（LCM）下织物预成型体中双尺度流动以及由此造成的空气裹入,进而产生细观及微观气泡的现象。基于多相流（VOF）方法建立了树脂空气两相流体在单胞内部流动的数学模型,并确定了该模型中多孔介质阻力源项和毛细力源项的具体形式。基于Fluent软件的UDF功能实现了上述两相流模型的数值求解,研究了平纹织物单胞中的两相流动以及2种气泡的生成过程。对Rovcloth 2454织物的气泡生成仿真结果显示,毛细数 Ca 对气泡的产生有决定性作用：当毛细数接近临界毛细数 Ca^c时,气泡产生量最低,而当Ca小于Ca^c时,容易产生细观气泡,反之容易产生微观气泡。通过与文献中的理论预测和实验数据对比,验证了本文算法的正确性。     

竖直管路内流动液氢条件中弹状氢气泡运动速度

低温加注系统中的弹状流可能引发各种非稳现象,造成低温液氢传输管路的破坏.针对垂直管路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路中弹状流气泡的运动模型,模拟了弹状氢气泡在流动液氢中的上升过程,并对其运动速度进行了研究.仿真结果表明:弹状氢气泡在流动液氢中运动时,其速度模型中的速度系数与流体的速度有关,...     

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.     

An experimental study of bubble behavior in a temperature gradient near a heated wall under low-gravity and microgravity conditions aboard NASA DC9 airplane

The behavior of a single bubble and a pair of bubbles under microgravity conditions has been investigated using the NASA-DC9 aircraft in order to understand the effects of various parameters and to control the bubble behavior in space. Silicone oil was used as the test liquid, and a nitrogen gas bubble was injected from the top wall under different experimental conditions. In an isothermal case, two different microgravity conditions were achieved by either fixing the experimental apparatus to the aircraft floor or freely floating the apparatus in the aircraft cabin. The bubble behavior was found to be clearly influenced by the quality of the microgravity environment, and variations of the bubble aspect ratio with the Bond number were presented. The results indicate that there is a critical Bond number of the order of 10⁻¹ which determines the bubble shape deformation. In the free-floating experiments, a temperature gradient was imposed on the liquid around the bubble near the heated top wall. Marangoni convection was expected to occur around the bubble and the bubble behavior was studied under various temperature gradients. The bubble aspect ratio was found to decrease with an increase in the Marangoni number. A theoretical model for the relation between the Marangoni flow around the bubble and the aspect ratio is proposed based on simple assumptions. Visualization of Marangoni convection around the bubbles using the photochromic dye activation method was successfully performed. The aspect ratios predicted by the model agreed with the experimental results reasonably well. Direct measurements of surface velocity are, however, necessary to further evaluate the validity of the model.