Preparation of JEREMI Experiment: Development of the Ground Based Prototype

This study has been performed in the frame of preparing the space experiment JEREMI (Japanese and European Research Experiment on Marangoni Instabilities). The use of forced coaxial gas flow is proposed as a way to stabilize the Marangoni convection in liquid bridges, which might have important technological applications in the floating zone technique. A new set-up is under development and all sub-systems have passed severe tests. Here we present the design of this set-up and preliminary results of experiments for shear-driven two-phase flows in a confined volume of liquid under conditions of normal gravity. The geometry corresponds to a cylindrical liquid bridge concentrically surrounded by an annular gas channel with external solid walls. Gas enters into the annular duct, flows between solid walls and upon reaching the liquid zone entrains initially quiescent liquid. The test liquids are ethanol, n-decane and 5?cSt silicone oil, which have different degrees of viscosity and of volatility. The gas flow along the interface strongly enhances the evaporation and, correspondingly, affects the interface shape. Silhouette measurements are used for optical determination of the interface shape. From the digital images the variation of the liquid volume as a function of flow rate is calculated.     

Thermovibrational Convection in Microgravity: Preparation of a Parabolic Flight Experiment

This work describes the preparation of the future experiments on thermovibrational convection in microgravity during parabolic flights. The experimental setup for observing thermovibrational flows is designed. It consists of a cubic cell with liquid, which is subjected to controlled vibration, and equipment for registering velocity and temperature fields with a help of optical digital interferometry. The question of choosing working liquid and control parameters of the experiment is addressed. A 3D numerical simulation of thermovibrational convection in a cubic cavity is performed for real parabolic flight conditions. The study is aimed at estimating the values of physical quantities that manifest the presence of thermovibrational flows and can be experimentally measured during short microgravity time (20 s).     

Ions and Atoms in Superfluid 4Helium: Motion Studies and Optical Transitions

In this paper systematic studies of the ion production mechanism are reported. The use of laser ablation inside the liquid is compared to the laser sputtering technique which produces the ions in the gas phase. For the first time impurity ions produced directly in the liquid could be separated from other sputtering products in the laser focus and identified unambigously by their characteristic emission light during the recombination process with electrons emitted from a field emission tip. This technique is important for future, pressure dependent measurements of the ionic mobility in liquid helium when ion production in the gas phase is impossible. Furthermore, optical transitions of various elements implanted into liquid helium are reported which became observable due to an improved set-up used in this experiment.     

Effects associated with bubble formation in optical trapping

Abstract

Heating of absorbing particles in a liquid medium by an optical trapping beam may lead to bubble formation. Powerful currents, which we identify as due to Marangoni convection, can be observed in the vicinity. At the micron size scale such surface tension effects can be very powerful, whereas normal thermal convection is negligible. Similar effects cause bubbles to be attracted to regions of higher temperature, providing a very powerful means of trapping bubbles, which are repelled by optical forces in a Gaussian beam. Measurements of the temperature required for bubble formation show that it occurs above the boiling point of the surrounding liquid, in reasonable agreement with nucleation theories.     

The Effect of the Surrounding Medium and Its Pressure on Data Obtained in Thermal Diffusivity Measurements Using the Flash Method

This work describes experimental measurements made with a high temperature–high pressure flash thermal diffusivity instrument, using nitrogen, argon, and helium as environment. Data was generated using pressures from vacuum to 30 bar in the temperature range of ambient to 1000°C. NIST SRM 8425 (Poco AXM 5Q, fine grain graphite) was used for the tests. A total of 2.970 data points were obtained, showing a clear and prominent shift in the data, depending on the pressure and the thermal properties of the surrounding gas. Preliminary conclusions drawn from the work indicate the influence of heat conduction, convection, and diffusion through the environmental gas, on the thermal diffusivity results.     

Nanoscale alignment and optical nanoimaging of a birefringent liquid

An anisotropic nanopatterning method, based on a technique of atomic force microscopy (AFM) scribing of a thin polyimide film, is used to generate an alignment layer whose topography depends on the writing direction. Detailed experimental measurements are presented for the topographical anisotropy that arises when the polyimide alignment layer is scribed parallel and antiparallel to the AFM cantilever orientation. By means of a novel nanotomographic approach, the optical retardation δ of an alignable birefringent liquid that covers the scribed substrate is measured with unprecedented resolution of only a few tens of nanometers. In this technique a thin optical fiber is raster-scanned at several fixed heights inside the birefringent liquid, and the transmitted polarized light is collected downstream. The optical retardation δ from the fiber's tip to the polyimide interface was measured as a function of position x,y,z, with the results reflecting the spatially varying depth of the medium due to the polymer film surface topography. Theoretical calculations for δ are in excellent agreement with both the topographical and the high resolution nanoimaging experimental results obtained.     

Heat Transfer by Thermo-Capillary Convection Sounding Rocket COMPERE Experiment SOURCE

This paper describes the results of a sounding rocket experiment which was partly dedicated to study the heat transfer from a hot wall to a cold liquid with a free surface. Natural or buoyancy-driven convection does not occur in the compensated gravity environment of a ballistic phase. Thermo-capillary convection driven by a temperature gradient along the free surface always occurs if a non-condensable gas is present. This convection increases the heat transfer compared to a pure conductive case. Heat transfer correlations are needed to predict temperature distributions in the tanks of cryogenic upper stages. Future upper stages of the European Ariane V rocket have mission scenarios with multiple ballistic phases. The aims of this paper and of the COMPERE group (French–German research group on propellant behavior in rocket tanks) in general are to provide basic knowledge, correlations and computer models to predict the thermo-fluid behavior of cryogenic propellants for future mission scenarios. Temperature and surface location data from the flight have been compared with numerical calculations to get the heat flux from the wall to the liquid. Since the heat flux measurements along the walls of the transparent test cell were not possible, the analysis of the heat transfer coefficient relies therefore on the numerical modeling which was validated with the flight data. The coincidence between experiment and simulation is fairly good and allows presenting the data in form of a Nusselt number which depends on a characteristic Reynolds number and the Prandtl number. The results are useful for further benchmarking of Computational Fluid Dynamics (CFD) codes such as FLOW-3D and FLUENT, and for the design of future upper stage propellant tanks.     

Multipass capillary cell for enhanced Raman measurements of gases

A simple Raman multipass capillary cell (MCC) is described that gives 12- to 30-fold signal enhancements for non-absorbing gases. The cell is made by coating the inside of 2-mm inner diameter silica capillary tubes with silver. The device is very small and suitable for remote and in situ Raman measurements with optical fibers. Application of the MCC is similar to previously described liquid core waveguides but, unlike the latter devices, the MCC is generally more applicable to a wide range of non-absorbing gases.     

Thermo-hydrodynamic transport phenomena in partially wetting liquid plugs moving inside micro-channels

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas–liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a ‘unit cell’ flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated.     

Thermo-capillary and residual natural convection in an orbiting spherical liquid system

The influence of a given temperature distribution applied at the surface of a spherical liquid system in a circular orbit has been determined analytically. Three basic convectional flows inside the liquid have been investigated: Marangoni convection, residual gravity natural convection and the natural convection due to self-attraction. For small drops thermocapillary convection is dominant, while for large liquid spheres the convection due to self-attraction is predominant for orbits around the sun, while for orbits of smaller diameter, as for instance around the earth, the natural convection due to the residual gravity is dominant for large spherical drops.     

连续式高速风洞液氮降温供配气系统设计与实现

研制低温高雷诺数风洞对我国国防工业的发展具有重要战略意义和工程应用价值。通过喷注液氮的方式,建成了国内第一套适用于连续式高速风洞的降温系统。介绍了NF-6连续式高速风洞降温系统的总体方案和主要技术指标,重点论述了其中供配气系统结构和技术原理,并给出了运行调试结果。测试结果表明:NF-6风洞降温系统的液氮需求量计算方法正确,液氮存储装置工作稳定,液氮存储量和驱动气源的能力满足降温实验要求;配气系统设计合理,预增压装置工作稳定,喷前压和挤推压控制平稳;供配气系统与整体降温系统匹配良好,总温、总压、马赫数及运行时间等关键指标达到设计要求,风洞实验雷诺数提高近50%。     

R404A在水平强化管外的冷凝实验及数据处理方法

针对一种双侧强化换热管,实验测试和分析了制冷工质R404A在管外凝结与水在管内对流的传热规律,采用"Wilson图解法"和"Gnielinski法"两种不同的方法对实验数据进行了处理。经理论分析和实验研究表明,Wilson图解法对于双侧强化换热管管内、管外表面传热系数实验容易产生较大误差,"Gnielinski法"是更合适的方法。实验得出了管内对流传热和管外凝结传热的计算关联式及传热的强化倍率。对于制冷剂R404A,在强化管外凝结的表面传热系数随着壁面过冷度的增加而增大,呈现出与纯工质光滑管外冷凝时不同的变化趋势。     

Numerical Simulation of Heat Conduction to Liquids from a Thin Vertical Cylinder

The paper presents numerical simulations of heat conduction around a circular vertical cylinder immersed in liquids. A finite volume formulation is used, and the numerical analysis is performed in unsteady state with an explicit scheme. The numerical predictions are compared with experiments performed on liquids to find the temperature inside the cylinder, where a thermocouple is located, and at the wall of the insulated coaxial container, where the liquid is poured. The cylinder is immersed vertically. The numerical results are in good agreement with the temperature at the wall of the container. The experimental temperature measurement of the thermocouple located inside the probe is intermediate between the numerical temperatures on the axis and on the surface of the probe. The natural convection phenomenon is evidenced in the experiments, after a certain time from the beginning of heating, in some of the liquids used, except glycerol. Natural convection is not considered in the present numerical simulations, which solve only the heat conduction equation.     

Image-plane measurements of coexistence curve diameters

The diameter of the coexistence curve has been measured for several fluids to determine differences from linear form. The experimental method consists of optical interference measurements in a Mach-Zehnder interferometer supplemented by measurements of deviation in a hollow prism. The fluid sample is contained in a temperature-stabilized cell in one branch of the interferometer. A variation in cell temperature causes the density profile within the cell to change, resulting in a change of the interference pattern, which is monitored photographically. From the relation of this pattern at any temperature to the pattern at the critical temperature, information on the refractive indices of the liquid and vapor phases is obtained. This information is combined with measurements of the Lorentz-Lorenz coefficient to obtain liquid and vapor densities along the coexistence curve. The average of liquid and vapor densities is analyzed in terms of RG theory. The results of the experiment yield information on three body interactions. Studies have been completed on ethane, ethylene, hydrogen, and fluoroform.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

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.     

Flow Structure and Surface Deformation of High Prandtl Number Fluid Under Reduced Gravity and Microgravity

The numerical simulation has been conducted to investigate the flow structure and surface deformation in a liquid bridge of high Prandtl number fluid under reduced gravity and microgravity. The Navier–Stokes equations coupled with the energy conservation equation are solved on a staggered grid, and the mass conserving level set approach is used to capture the free surface deformation of the liquid bridge. The effect of reduced gravity and thermocapillary convection on the surface deformation of the liquid bridge is investigated, and the results show that the amplitude of the surface horizontal vibration decreases gradually, and the thermocapillary convection inside the liquid bridge starts to turn into a steady state after the initial period. Moreover, the shift of the center of the recirculating flow inside the liquid bridge under horizontal external acceleration and zero gravity is also studied, and the results indicate that the vortex centers move initially toward the cold disk and reach an equilibrium position, and then the vortex centers vibrate around the equilibrium position periodically.     

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.     

Marangoni convection in a rectangular container

If the free liquid-gas interface of a liquid in a rectangular Container is subjected to a temperature gra-dient the shear stress on the free liquid surface being temperature dependent transmits by viscous traction a thermo-capillary convection into the bulk of the liquid. For constant temperature T ₁ at one wall and T ₂ at the other a steady Marangoni convection takes place while for time-oscillatory temperatures of the walls a time-dependent thermo-capillary convection appears, which will create wave patterns on the free liquid surface. They shall, depending on the forcing frequency of the temperature, exhibit resonance peaks. The velocity distribution, the response magnitude inside the Container, the forced free surface displacement and the influence of the Prandtl number have been investigated.     

Multi-channel Optical Fiber Thermometer for PEM Fuel-Cell Applications

Miniature, durable, and fast-responding temperature sensors are needed for proton exchange membrane fuel cells (PEMFCs). When embedded in a single cell or in a cell stack, they can provide useful information both at the design stage for optimizing the cell efficiency and during operation for monitoring the working conditions and thus preventing failures. Optical fiber sensors are especially promising in this field because they are small, rugged, and inexpensive. In addition, they can provide safe temperature measurements in an electrically hostile environment. A four-channel optical fiber thermometer, based on intensity-independent fluorescence lifetime thermometry was developed at INRIM. It consists of a photonic unit for the excitation/detection of the fluorescence signals and a set of custom optical fiber probes based on a temperature-sensitive fluorescent material attached to the distal end of an optical fiber. The system was characterized in the range from room temperature to about 100 °C in order to point out its metrological features. A temperature repeatability to within 0.06 °C with a response time lower than 1 s to a step temperature change was obtained. A preliminary investigation inside a PEMFC stack using the optical fiber fluorescence thermometer was also performed. In order to check the temperature uniformity along the stack, temperatures within an adjacent membrane electrode assembly (MEA) of a six-fuel-cell stack were measured during the unit operation. The system design, the probe construction, and its laboratory testing are presented in this article together with an assessment of the overall system performance. The application of such a system in a fuel-cell test rig is also described. The experimental results demonstrate the suitability of the system in real-time temperature mapping in operating fuel cells.