Drop Tower Experiments and Numerical Modeling on the Combustion-Induced Secondary Flow in Standing Acoustic Fields

From the microgravity experiments on single fuel droplet combustion and on spark-ignited premixed flame in acoustic field, that have been done in this decade, secondary flow has been confirmed. The flow occurs only when combustion occurs and always in the direction to the neighboring node of velocity oscillation in standing acoustic field. This flow is thought to be driven by a kind of acoustic radiation force that arises from density non-uniformity due to combustion. According to the hypothesis, the driving force is expected to be very similar to buoyancy, being a volumetric force proportional to density difference. Since the mechanism tells that only the density difference due to combustion is the requirements for the occurrence of the secondary flow, a simpler system is employed for the hypothesis validation. A hot wire in a standing acoustic field gives a local temperature rise and the secondary flow is successfully reproduced. To avoid buoyancy that covers the radiation force, microgravity conditions are used. Through direct- and modeled numerical simulations and with drop tower experiments, the validity of the hypothesis is checked out. As a result, the supposed radiation force is found to be able to explain well the characteristics of the secondary flow. It is found that adding an acoustic radiation force term determined from acoustic parameters into conventional CFD calculation, one can sufficiently reproduce and predicts the behavior of the secondary flow numerically.     

Bubble Dynamics in Turbulent Duct Flows: Lattice-Boltzmann Simulations and Drop Tower Experiments

Lattice-Boltzmann simulations of a turbulent duct flow have been carried out to obtain trajectories of passive tracers in the conditions of a series of microgravity experiments of turbulent bubble suspensions. The statistics of these passive tracers are compared to the corresponding measurements for single-bubble and bubble-pair statistics obtained from particle tracking techniques after the high-speed camera recordings from drop-towers experiments. In the conditions of the present experiments, comparisons indicate that experimental results on bubble velocity fluctuations are not consistent with simulations of passive tracers, which points in the direction of an active role of bubbles. The present analysis illustrates the utility of a recently introduced experimental setup to generate controlled turbulent bubble suspensions in microgravity.     

Drop Tower Setup to Study the Diffusion-driven Growth of a Foam Ball in Supersaturated Liquids in Microgravity Conditions

The diffusion-driven growth of a foam ball is a phenomenon that appears in many manufacturing process as well as in a variety of geological phenomena. Usually these processes are greatly affected by gravity, as foam is much lighter than the surrounding liquid. However, the growth of the foam free of gravity effects is still very relevant, as it is connected to manufacturing in space and to the formation of rocks in meteorites and other small celestial bodies. The aim of this research is to investigate experimentally the growth of a bubble cloud growing in a gas-supersaturated liquid in microgravity conditions. Here, we describe the experiments carried out in the drop tower of the Center of Applied Space Technology and Microgravity (ZARM). In few words, a foam seed is formed with spark-induced cavitation in carbonated water, whose time evolution is recorded with two high-speed cameras. Our preliminary results shed some light on how the size of the foam ball scales with time, in particular at times much longer than what could be studied in normal conditions, i.e. on the surface of the Earth, where the dynamics of the foam is already dominated by gravity after several milliseconds.     

Temperature Increase of Highly-Polarized Fermi Liquids in Spin-Echo Experiments

We show that there are restrictions on the maximum tipping angle that can be used without significantly raising the temperature of the ³ He distribution in high B/T spin-echo experiments with pure liquid ³ He and ³ He- ⁴ He solutions. The temperature increase occurs during the diffusion process as quasiparticles in mixed-spin states are scattered and converted into thermal excitations at the spin-up and spin-down Fermi surfaces. This temperature increase can mimic the effects of zero temperature attenuation, leading to a higher values of the measured anisotropy temperature T_a. We analyze the dependence of the increase on polarization, initial temperature, ³ He concentration, and tip angle, and estimate the size of the effect in recent experiments.     

A Fast and Self-Acting Release-Caging-Mechanism for Actively Driven Drop Tower Systems

Today’s and future scientific research programs ask for high quality microgravity conditions of 10^?6 g on ground combined with high repetition rates of 100 flights per day or more. Accordingly, a new type of drop tower, the GraviTower Bremen, (GTB), has been suggested and is currently under development. As a first stage of development, a GTB-Prototype (GTB-Pro) has been designed which uses an active rope drive to accelerate a slider/drag shield and an experiment therein on a vertical parabola. During the free fall phase, the experiment is decoupled from the slider by a self-acting Release-Caging-Mechanism (RCM). Our prototype will provide 2.5 s of microgravity for experiments of up to 500 kg for at least 100 times per day. In this article, the final concept of the engineering of the active rope drive and the RCM are presented in detail. Based on extensive simulations aiming at an optimization of the whole system we developed a hydraulic rope drive system with minimized vibrational amplitude and low number of eigenfrequencies. The RCM achieves a very fast (≤ 0.1 s) self-acting release of the experiment from the slider by making use of the dynamics of the hydraulic rope drive. Furthermore, passive hydraulic stop dampers in the RCM build a passive and self-acting recoupling mechanism. This system is optimized for a fast decoupling to compensate for the time limitation posed by the chosen drive technology. The simulations included a comparison of different drive technologies, physical effects like the Coriolis force, and the dynamics of the RCM system itself.     

Compound Capillary Flows in Complex Containers: Drop Tower Test Results

Drop towers continue to provide unique capabilities to investigate capillary flow phenomena relevant to terrestrial and space-based capillary fluidics applications. In this study certain ‘capillary rise’ flows and the value of drop tower experimental investigations are briefly reviewed. A new analytic solution for flows along planar interior edges is presented. A selection of test cell geometries are then discussed where compound capillary flows occur spontaneously and simultaneously over local and global length scales. Sample experimental results are provided. Tertiary experiments on a family of asymmetric geometries that isolate the global component of such flows are then presented along with a qualitative analysis that may be used to either avoid or exploit such flows. The latter may also serve as a design tool with which to assess the impact of inadvertent container asymmetry.     

Drop Tower Setup for Dynamic Light Scattering in Dense Gas-Fluidized Granular Media

Investigation of dynamics in dense granular media is challenging. Here we present a setup that facilitates gas fluidization of dense granular media in microgravity. The dynamics is characterized using diffusing wave spectroscopy. We demonstrate that agitated granular media reach a steady state within fractions of a second in drop tower flights. The intensity autocorrelation functions obtained in microgravity show a remarkable dependence on sample volume fraction and driving strength. A plateau in correlation emerges at low volume fractions and strong driving, while correlation decays only very slowly but continuously at high packing fractions. The setup allows to independently set sample volume fraction and driving strength, and thus extends the possibilities for investigations on dynamics in dense granular on ground.     

ESA Parabolic Flights,Drop Tower and Centrifuge Opportunities for University Students

“Fly Your Thesis!—An Astronaut Experience” is an educational programme launched by the ESA Education Office that aims to offer to European students the unique opportunity to design, build, and eventually fly, a scientific experiment as part of their Master or Ph.D. thesis. Selected teams accompany their experiments onboard the Zero-G aircraft for a series of three flights, each consisting of 30 parabolas, with each parabola providing about 20 s of microgravity. ESA Education Office financially supports the flights and part of the hardware development, as well as travel and accommodation for participants. For the first edition of this programme, four student teams were selected to participate in the 51st ESA Microgravity Research Campaign in November 2009. The 2010 edition of the programme was launched in April 2009 and the final selection was announced in January 2010. In parallel, ESA Education Office is setting up two new hands-on activities to provide European university students with access to drop tower (up to 9.3 s of microgravity) and centrifuge (from 1 to 20 times Earth’s gravity) facilities. Through ELGRA, the selected student teams are working in close contact with renowned European scientists working in gravity-related research. This paper will introduce the three new educational programmes and present the selected experiments, as well as give information to students interested in the future editions.     

Drop Tower Microgravity Improvement Towards the Nano-g Level for the MICROSCOPE Payload Tests

The Bremen drop tower at the Center of Applied Space Technology and Microgravity (ZARM) provides high quality micro-g condition needed for many high precision tests. This is even more improved by the development of a free flyer technology. This new technology is used for a free fall test of the MICROSCOPE differential accelerometers which only can work with a residual acceleration disturbance level below 25 nano-g in the high resolution mode. The French MICROSCOPE space mission for testing the Weak Equivalence Principle is scheduled for 2012/2013. The free fall accelerometer test campaign at ZARM is an important part of the pre-mission test program. In this article the new free flyer technology, its performance as well as the accelerometer tests are described.     

Sub-Kelvin Thermal Conductivity and Radioactivity of Some Useful Materials in Low Background Cryogenic Experiments

We present measurements of the thermal conductivity between 0.05 and 1 K, and radioactive contamination levels, for some thermally isolating materials. TIMET Ti 15-3-3-3, Mersen grade 2020 graphite, Vespel SP-1, Vespel SP-22, Vespel SCP-5000, Vespel SCP-5050, Graphlite CFRP, and a Kapton/epoxy composite are all investigated. Thermal conductivities were measured using a single-heater longitudinal heat flow method. Material radioactivity was determined for the materials at a low background counting facility using a high-purity gamma detector and GEANT4 Monte Carlo simulations.     

AlN微粉非等温氧化动力学研究

以d50为5μm的A1N微粉为原料,在气氛组成为4/5N2+1/5O2、流量为55mL/min、升温速率为10℃/min的条件下,采用TG-DTA和XRD分析对AlN微粉的非等温氧化反应过程和动力学参数进行研究.结果表明:d50为5μm的AlN微粉明显氧化增重过程约为900℃之后,其反应过程受反应机制和扩散机制交替控制...     

r-PTFE对PBT非等温结晶动力学的影响

以回收聚四氟乙烯(r-PTFE)为成核剂,对聚对苯二甲酸丁二醇酯(BPT)进行成核结晶改性,采用差示扫描量热仪(DSC)研究了PBT/r-PTFE复合材料的非等温结晶行为,考察了不同降温速率对PBT/r-PTFE共混物结晶行为的影响,并对其DSC扫描数据采用Jeziorny法和Liu-Mo法进行处理,采用Kissiinger法计算体系结晶活化能。结果表明:r-PTFE对PBT具有异相成核作用,可明显提高PBT的结晶度及其结晶速率,且加入r-PTFE并不会改变PBT的成核和增长方式。由Kissinger法计算得出的结晶活化能结果表明,加入r-PTFE树脂能够明显降低PBT的结晶活化能,进一步验证r-PTFE对PBT具有成核促进作用。     

双峰聚乙烯非等温结晶行为的研究

采用DSC方法研究了双峰聚乙烯(BPE)和茂金属聚乙烯(mPE)的非等温结晶过程,分别采用Ozawa方程和莫志深方法对所得的数据进行了处理。结果表明:茂金属聚乙烯结晶速率高于双峰聚乙烯。在实验范围内,两种聚乙烯的非等温结晶动力学均符合莫志深方程。     

Drop Tower Experiment to Study the Capillary Flow in Symmetrical and Asymmetrical Channels: Experimental Set-up and Preliminary Results

Owing to the development of the space exploration activities, the in-orbit management of fluids such as the transportation of propellant liquid in microgravity becomes the important direction of microgravity fluid research, and one of main problems is the stability behaviors of free surface flow in capillary channel of PMD. In the present study, an experiment set-up of the fluid transport with two different capillary channels has been developed on the Beijing Drop Tower platform. Both symmetrical and asymmetrical flow channels, with the same cross-sectional areas and lengths and different cross-sectional geometries were used and HFE-7500 is chosen as test liquid. 10 times of the drop-down experiments were performed for investigation of the capillary flow characters in different volumetric flow rates, and the three main patterns of capillary flows: subcritical flow, critical flow and supercritical flow were found in experiments, these patterns are distinguished by the movement of the point of lowest surface over time. Meanwhile, the critical flow rates at which free surface becomes instable observed in our experiments are (1) 2.7 ±0.2ml/s for the critical flow rate of asymmetrical channel; and (2) 2.2 ±0.2ml/s for symmetrical channel flow, respectively.     

铜合金的深冷处理研究

本文主要综述了金属深冷处理的发展历史,简要介绍了铜合金经深冷处理后性能的变化,国内外对深冷处理提高铜合金性能及其机理的研究。结合本人所阅读的文献综述了国内有关铜合金的深冷处理现状以及相关机理研究,就目前这个领域存在的问题和发展趋势进行了讨论。