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.     

可调式引射器流量调节特性研究

引射器在石油、化工、天然气输气过程、气波制冷、天然气脱水工艺、热能工程等领域中得到广泛应用.为了适应工况变化,需要研制流量可调式引射器.本文探讨了调节锥形状、尺寸对可调式引射器流量调节特性的影响.结果表明:对于圆锥型调节锥,随着锥角的增大,调节锥轴向可移动距离明显减小;流量与调节锥轴向移动距离呈二次曲线关系,即流量调节具有非线性性质.提出了一种可使引射器流量实现线性调节的调节锥型线方程.为具有良好调节性能的可调式引射器的研制打下基础.     

Numerical Simulation of Fluid Flow and Mixing in Gas-Stirred Ladles

In this work air injection into a water physical model of an industrial steel ladle was simulated. Calculations were developed based on a multiphase Eulerian fluid flow model involving principles of conservation of mass, momentum, and chemical species for both phases to predict turbulent flow patterns and mixing times for centric and eccentric injections. Effects of gas flow rate, injector position, number of injectors, and geometry of ladle on mixing time were analyzed. Optimum injection conditions are: single injector at 2/3 of radius and high gas flow rates. Quantitative correlation of mixing time as a function of main process variables was obtained.     

由振动信号估计柴油机喷油参数

喷油器振动的激振源主要是高压燃油的冲击,针阀开启和关闭引起的机械撞击;对应地,喷油器的振动响应由三个瞬态序列组成。本文用小波分析的方法在不同尺度下近似测得的振动信号,并由近似信号估计了喷油正时的参数。为实现柴油机燃油系的自动诊断提出了一种新方法。     

Effect of the geometric parameters on a flexible fiber motion in a tangentially injected divergent swirling tube flow

A particle-level simulation method is employed to study the effects of some geometric parameters, such as the injector diameter, the injection angle and position on fiber motions in a 3D compressible decaying swirling flow in a divergent tube. The flexibility of the fiber is defined by the bending and twisting displacements. The fiber-wall collision and compressible effects are considered here. The fiber with different complex configurations moves as spiral orbit with stream-wise direction. With the increase in the injector diameter, the deformation degree of the fiber increases. For a larger injector diameter, the fiber appears to be closed coil loops or asymmetric S-loopturn. The larger the injection angle is, the earlier the fiber starts to swirl and the larger the number of ‘turn’ is. The complex coiled configurations with entanglement can be observed under small injection angle. Especially, a ‘zigzag’ configuration is formed for larger injection angle and injection position.     

Influence of wettability on bubble formation in liquid

This paper presents experimental results of the surface phenomena effect on bubble formation from a single orifice in water and at nozzle in liquid aluminium with gas blowing at small flow rates. The usage of coated orifice in water and nozzles of different materials in the melt realized wide range of contact angles. The meaningful stages, termed (1) nucleation period, (2) under critical growth, (3) critical growth and (4) necking, were identified during bubble formation in a regime referring to simultaneous forced flow and surface tension control. It was revealed that bubble formation is substantially dominated by hysteresis of contact angle. Evolution of interface equilibrium and force balance conditions distinctive for bubble formation is clarified. X-ray fluoroscope was used to carry out in-situ observation of bubble formation in the melt. It was shown that bubble volume increased with wettability worsening both for aqueous and metallic systems. A further insight into the mechanism of the bubble formation was obtained by comparison of the bubble behaviour at the tip of the injection devices in liquid aluminium and at the orifice in water.     

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.     

Effect of contact angle on bubble formation at submerged orifices

This paper presents a theoretical model and experimental results of the generation of bubbles due to the injection of a constant flow rate of gas through an orifice submerged in liquid. The bubble formation process can be identified into three distinct stages termed (1) nucleation stage, (2) growth stage, and (3) detachment stage by analyzing the evolution of interface equilibrium and force balance conditions. Influence of contact angle on bubble formation at each stage is quantitatively elucidated. Experimental investigations in the preparation process of aluminum foams by gas injection method were conducted, and the generated bubble size was measured. The theoretical prediction of the present model suits well with the experimental results when a contact angle of 30° is introduced. The present model covers a wide range of contact angle (θ < 90°) at different gas flow rates and orifice radii in both aqueous and metallic systems.     

Simulation and experimental studies on plasma temperature, flow velocity, and injector diameter effects for an inductively coupled plasma

An inductively coupled plasma (ICP) is analyzed by means of experiments and numerical simulation. Important plasma properties are analyzed, namely, the effective temperature inside the central channel and the mean flow velocity inside the plasma. Furthermore, the effect of torches with different injector diameters is studied by the model. The temperature inside the central channel is determined from the end-on collected line-to-background ratio in dependence of the injector gas flow rates. Within the limits of 3% deviation, the results of the simulation and the experiments are in good agreement in the range of flow rates relevant for the analysis of relatively large droplets, i.e., ～50 μm. The deviation increases for higher gas flow rates but stays below 6% for all flow rates studied. The velocity of the gas inside the coil region was determined by side-on analyte emission measurements with single monodisperse droplet introduction and by the analysis of the injector gas path lines in the simulation. In the downstream region significantly higher velocities were found than in the upstream region in both the simulation and the experiment. The quantitative values show good agreement in the downstream region. In the upstream region, deviations were found in the absolute values which can be attributed to the flow conditions in that region and because the methods used for velocity determination are not fully consistent. Eddy structures are found in the simulated flow lines. These affect strongly the way taken by the path lines of the injector gas and they can explain the very long analytical signals found in the experiments at low flow rates. Simulations were performed for different injector diameters in order to find conditions where good analyte transport and optimum signals can be expected. The results clearly show the existence of a transition flow rate which marks the lower limit for effective analyte transport conditions through the plasma. A rule-of-thumb equation was extracted from the results from which the transition flow rate can be estimated for different injector diameters and different injector gas compositions.     

A nanoinjector for microanalysis

We describe a simple miniature injection device that can be used for introduction of nanoliter sample volumes in microfluidic systems. The hybrid microstructure consists of two hydraulically connected parts, a pulse micropump, and a multilevel cross-flow injector. Sample injection is accomplished by creating a transient pressure pulse in the sample line by means of the solenoid-based micropump. The sample line is aligned at right angles to the main carrier flow line. The two flow channels are located in two different parallel planes. The cross section of the two channels is defined by a self-sealing aperture in an elastomer. During the pressure pulse, the sample is introduced through this aperture directly into the main flow stream. Fast impulse-based injection causes rapid mixing of the injected sample with the main flow stream. This permits simple single-line manifold micro flow injection (MFI) systems. The deformation/relaxation of the elastomer is fast and repeatable; as such, rapid serial actuations essentially result in a larger injected sample volume without significantly affecting the peak shape. In the present form, 2-40-nL samples are easily injected by single injection, and the injected volume can be chosen by system parameters. The injection repeatability as observed by a photometric detector is better than 1.2% (n = 100).     

Design and characterization of poly(dimethylsiloxane)-based valves for interfacing continuous-flow sampling to microchip electrophoresis

This work describes the fabrication and evaluation of a poly(dimethyl)siloxane (PDMS)-based device that enables the discrete injection of a sample plug from a continuous-flow stream into a microchannel for subsequent analysis by electrophoresis. Devices were fabricated by aligning valving and flow channel layers followed by plasma sealing the combined layers onto a glass plate that contained fittings for the introduction of liquid sample and nitrogen gas. The design incorporates a reduced-volume pneumatic valve that actuates (on the order of hundreds of milliseconds) to allow analyte from a continuously flowing sampling channel to be injected into a separation channel for electrophoresis. The injector design was optimized to include a pushback channel to flush away stagnant sample associated with the injector dead volume. The effect of the valve actuation time, the pushback voltage, and the sampling stream flow rate on the performance of the device was characterized. Using the optimized design and an injection frequency of 0.64 Hz showed that the injection process is reproducible (RSD of 1.77%, n = 15). Concentration change experiments using fluorescein as the analyte showed that the device could achieve a lag time as small as 14 s. Finally, to demonstrate the potential uses of this device, the microchip was coupled to a microdialysis probe to monitor a concentration change and sample a fluorescein dye mixture.     

Sample dispersion for segmented flow in microchannels with rectangular cross section

Hydrodynamic dispersion in microchannels can be significantly reduced by segmentation with a second immiscible phase. We address the effect of microchannel cross section on the dispersion of analytes in a segmented gas-liquid flow of alternating bubbles and liquid segments. Channels of square or nearly square cross section are considered. A significant fraction of the liquid surrounds the bubbles and wets the channel walls in the form of films or menisci. This stagnant fraction of the liquid remains when gas and liquid segments flow by, and it is connected to the liquid within the liquid segments by diffusion only and it effectively increases dispersion. We design and fabricate a microchip with integrated analyte injection and detection to investigate the effects of the influence of the stagnant liquid in segmented flow through square microchannels on the analyte bandwidth. The measured data and a corresponding model confirm the experimental trends and suggest operating conditions at which the unwanted effect of dispersion in segmented microchannel flow is minimized. Dispersion is least when the liquid flow rate is greater than the gas flow rate, and the optimum ratio of the two flow rates slightly increases with increasing bubble velocity.     

Visual experimental research on the effect of nozzle orifice structure on R124–DMAC absorption process in a vertical bubble tube

A visual experimental platform for R124–DMAC bubble absorption in a vertical tube absorber was designed and built for this research. The bubble behaviors, flow pattern characteristics and distributions are observed and the bubble absorption heights (BAHs) were measured when the two kinds of different structure nozzles (single-orifice or multi-orifice nozzle) were applied in the absorber. The results showed that the BAH will heighten with increases of vapor flow rate and nozzle flow area. Based on visual experimental observations, the BAH or bubble absorption performance was significantly affected by the velocity of vapor from the nozzle rather than by the nozzle structure. The proportion of slug flow in BAH or the BAH can be decreased by using a multi-orifice nozzle in the absorber under the same flow area condition. However, the flow resistance of the vapor through the nozzle will increase, which has a negative action on the performance of absorption refrigeration systems. So, using multi-orifice nozzle does not improve the absorption performance of the bubble absorber under the same nozzle flow resistance condition.     

The effects of surfactant on the multiscale structure of bubbly flows

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1mm size can be much smaller than that on a clean bubble.     

高超声速激波/边界层干扰及MVG阵列流动控制研究

高超声速飞行器流场中通常会伴随激波/边界层干扰(SWBLI),其引发的流动分离将导致进气道性能下降。该文采用湍流分离涡(DES)方法、结合有限体积离散方法对来流马赫数为7流场中SWBLI诱导的分离气泡进行数值研究,模拟结果清晰地显示了分离气泡从产生到充分发展的具体过程,揭示了分离气泡的产生机理。利用微型涡流发生器(MVG)阵列对其进行控制,讨论了流场结构、壁面静压力、壁面剪切力及总压损失等参数变化对SWBLI控制效果的影响。结果表明:MVG阵列可显著改变高超声速流体边界层,使得分离气泡尺寸减小,分离激波强度减弱,分离气泡内及其下游流体的流向速度梯度增加,总压损失降低可达1.9%。     

提升管管径对导流式气泡泵性能影响的理论与实验研究

基于两相流分相模型,构建气泡泵性能实验系统,以水为工质,对大气压下采用不同提升管内径的导流式气泡泵性能进行理论和实验研究。研究了加热功率100~650 W,沉浸比0.2~0.4,提升管内径7 mm、9 mm、11 mm、13mm、16 mm,提升管长600 mm工况下的气泡泵性能。结果表明,沉浸比的大小对液体提升量的多少起着关键作用;其他条件不变时,一定范围内提升管径的增加能够显著提升气泡泵的液体提升量,超过管径的临界值,效果相反,不但降低了液体提升量,气泡泵的效率也大幅减少,如加热功率300 W时,采用11 mm和16 mm管径的气泡泵液体提升量相差10.23 g/s,管径增加了5 mm,提升量减少了61.15%。     

Multi-dimensional modelling of spray,in-cylinder air motion and fuel-air mixing in a direct-injection engine

In this work, three-dimensional fuel-air mixing inside a conventional spark ignition engine cylinder is simulated under direct injection conditions. The motivation is to explore retrofitting of conventional engines for direct injection to take advantage of low emissions and high thermal efficiency of the direct injection concept. Fuel-air mixing is studied at different loads by developing and applying a model based on the Lagrangian-drop and Eulerian-fluid (LDEF) procedure for modelling the two-phase flow. The Taylor Analogy Breakup (TAB) model for modelling the hollow cone spray and appropriate models for droplet impingement, drag and evaporation are used. Moving boundary algorithm and two-way interaction between both phases are implemented. Fuel injection timing and quantity is varied with load. Results show that near-stoichiometric fuel-air ratio region is observed at different locations depending on the load. The model developed serves to predict the fuel-air mixing spatially and temporally, and hence is a useful tool in design and optimization of direct injection engines with regards to injector and spark plug locations. Simulations over a range of speed and load indicate the need for a novel ignition strategy involving dual spark plugs and also provide guidelines in deciding spark plug locations.     

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.     

The two options for sample evaporation in hot GC injectors: thermospray and band formation. optimization of conditions and injector design.

Although classical split and splitless injection is more than 30 years old, we only start to understand the vaporization process in the injector. Solvent evaporation determines much of the process and is the first obstacle to overcome. Videos recorded on devices imitating injectors showed that sample (solvent) evaporation is often a violent process which is poorly controlled and might well explain many of the puzzling quantitative results often obtained. We do not adequately take into account that two vaporization techniques are in use. Partial solvent evaporation inside the syringe needle (optimized as "hot needle injection") produces thermospray: the sample liquid is nebulized upon leaving the needle. The resulting fog is rapidly slowed and moves with the gas. Solute evaporation largely occurs from microparticles suspended in the gas phase. Empty liners are most suitable. Fast autosamplers suppress vaporization in the needle, i.e., nebulization, and shoot a band of liquid into the chamber that must be stopped by a packing or obstacles suitable to hold the liquid in place during the 0.2-5 s required for solvent evaporation. Solute evaporation largely occurs from the surfaces onto which the sample is deposited. Insights into these mechanisms help optimize conditions in a more rational manner. Methods should specify whether they were optimized and validated for injection with thermospray or band formation. The insights should also enable a significant improvement of the injector design, particularly for splitless injection.     

Miniaturization of frit inlet asymmetrical flow field-flow fractionation

A miniaturized frit inlet asymmetrical flow field-flow fractionation (mFI-AFlFFF) channel has been constructed and tested for the separation of proteins. By scaling down the geometrical channel dimension of a conventional FI-AFlFFF system, flow rate ranges that can be manipulated were decreased to 20-30 microL/min, which reduces the injection amount of sample materials. The end effect contribution to plate height was evaluated by varying the inner diameter of the connection tubing between the injector and the channel inlet at various injection flow rates, and the results showed that the use of silica capillary tubing of the shortest possible distance is essential in reducing the initial band broadening prior to the sample injection to the microscale channel. The capability of the microFI-AFlFFF system was demonstrated with the separation of protein standards, polystyrenesulfonates, and ssDNA strains and for the characterization of replication protein A-ssDNA binding complex regulated by redox status.