Linear response of a viscous liquid to translational excitation

Forced oscillations of a viscous liquid with a free liquid surface in a circular cylindrical container have been treated by observing various boundary conditions. The response of the velocity distribution, the free surface elevation and response force of the liquid due to translational excitation of the tank have been determined. Three cases have been treated and compared with each other. One is the case of slipping liquid at the side wall, where the contact line is free to slip, while the other case considers an anchored contact line. Finally all adhesion conditions at the side wall of the container have been satisfied, while at the tank bottom only the normal velocity condition is satisfied. The first two cases are valid for small liquid filling rates, while the last treatment renders good results for large aspect ratio containers. It was found that a new phenomenon appears for low filling rates, where the liquid is only capable to perform an aperiodic motion.     

Effect of gravity on pool boiling heat transfer on thermal spray coating

This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m² and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.     

Performance analysis of no-vent fill process for liquid hydrogen tank in terrestrial and on-orbit environments

Two finite difference computer models, aiming at the process predictions of no-vent fill in normal gravity and microgravity environments respectively, are developed to investigate the filling performance in a liquid hydrogen (LH₂) tank. In the normal gravity case model, the tank/fluid system is divided into five control volume including ullage, bulk liquid, gas–liquid interface, ullage-adjacent wall, and liquid-adjacent wall. In the microgravity case model, vapor–liquid thermal equilibrium state is maintained throughout the process, and only two nodes representing fluid and wall regions are applied. To capture the liquid–wall heat transfer accurately, a series of heat transfer mechanisms are considered and modeled successively, including film boiling, transition boiling, nucleate boiling and liquid natural convection. The two models are validated by comparing their prediction with experimental data, which shows good agreement. Then the two models are used to investigate the performance of no-vent fill in different conditions and several conclusions are obtained. It shows that in the normal gravity environment the no-vent fill experiences a continuous pressure rise during the whole process and the maximum pressure occurs at the end of the operation, while the maximum pressure of the microgravity case occurs at the beginning stage of the process. Moreover, it seems that increasing inlet mass flux has an apparent influence on the pressure evolution of no-vent fill process in normal gravity but a little influence in microgravity. The larger initial wall temperature brings about more significant liquid evaporation during the filling operation, and then causes higher pressure evolution, no matter the filling process occurs under normal gravity or microgravity conditions. Reducing inlet liquid temperature can improve the filling performance in normal gravity, but cannot significantly reduce the maximum pressure in microgravity. The presented work benefits the understanding of the no-vent fill performance and may guide the design of on-orbit no-vent fill system.     

Influence of particle size on spin switching properties and magnetoelectric coupling in SmFeO<Subscript>3</Subscript>

Controlling the magnetic properties of a material is of great importance for spintronics and magnetoelastic devices. We studied effect of reduced particle size on structural, dielectric and magnetic properties of SmFeO₃ nanoparticles prepared by co-precipitation method (SFO-C) and by combustion (SFO-S). Reduced particle size modified interesting magnetic features of SmFeO₃. Temperature dependent magnetic study reveal significant enhancement in magnetization reversal temperature and drop in spin reorientation transition temperature. The signature of spin reorientation transition for SFO-C (~?300 nm) is marked at ~?450 K, while this temperature drops down to ~?400 K for SFO-C (~?50 nm). The magnetization reversal temperature is achieved at 30.5 K for SFO-C, much higher than 4 K, reported for the single crystal and bulk SmFeO₃. The presence significant anomalies in the temperature dependent dielectric behavior of SmFeO₃ samples across spin reorientation transition temperature indicate magneto electrical coupling. Strong exchange–bias effect is observed at low temperature for both the samples. The lowering of spin reorientation/switching transition temperature due to reduction in particle size and the signature of magnetoelectric coupling at this temperature are useful for room temperature devices. The observed experimental results establish that the spin switching properties of SmFeO₃ can be modified for practical applications in devices.     

Numerical Investigation of Bubble Induced Marangoni Convection: Some Aspects of Bubble Geometry

Thermocapillary or Marangoni convection is a surface tension driven flow that occurs when a gas–liquid or vapor–liquid interface is subjected to a temperature gradient. In the past, the contribution to local heat transfer arising from Marangoni convection has been overlooked as insignificant since under earth gravity it is overshadowed by buoyant convection. This study numerically investigates some aspects of bubble size and shape on local wall heat transfer resulting from Marangoni convection about individual bubbles on a heated wall immersed in a liquid silicone oil layer (Pr = 110) of depth 5 mm. It was found that increasing bubble volume causes an increase in the area over which Marangoni convection has affect. Heat transfer therefore increases with bubble size. Over the effective area, the surface averaged hot wall heat transfer is not affected greatly by bubble shape. The surface averaged heat transfer over the effective area on both the hot and cold walls is affected dramatically by bubble size, but the increase is more profound on the cold wall.     

Wetting and reaction of molten La with poly- and mono-crystalline MgO at 1323 K

Wetting of poly- and mono-crystalline MgO substrates by molten La was investigated at 1323 K in a high vacuum using a modified sessile drop method. The wettability seems to depend mildly on the substrate orientation but strongly on the surface roughness. The initial contact angles on the smooth (100), (110), and (111) surfaces are 63° ± 1°, 69° ± 1°, and 69° ± 1°, respectively, while on the rough polycrystalline surfaces they are much larger (104° ± 3°). The wetting behavior is dictated by the disruption of the oxide film covering the La surface, the extent of the interfacial reaction and the evolution of the reaction product. A thick layer of La₂O₃ phase formed at the interface and then enwrapped the liquid surface, leading to the recession and warping of the triple line and finally the deterioration in the wettability. On the other hand, magnesium was displaced by the reaction and its evaporation provided additional impetus for the movement of the triple line. Due to different reaction intensities, the wetting behavior of La on the different orientations of the MgO surfaces also showed some discrepancies.     

微重力环境低温流体无排气加注过程数值研究

针对加注系统受注贮箱,采用CFD方法就液氮贮箱无排气加注过程开展数值仿真,对比了不同重力水平下的无排气加注性能,分析了加注口结构、壁面初始温度、加注流体温度和加注流量等因素对微重力无排气加注性能的影响规律。所构建的二维轴对称模型将流体区与固壁区一起作为求解区域并划分网格,并通过植入用户自定义程序(UDF)计算加注口液体闪蒸过程及气液之间的热质交换。经过实验数据验证,该模型能够合理展示箱内温度场分布和相分布情况,并获得贮箱压力等参数变化信息。数值计算结果表明:(1)加注条件相同时,微重力工况较常重力工况体现出更好的无排气加注性能。(2)微重力条件下,无排气加注性能几乎不受加注口结构的影响,壁面初始温度和加注流体温度越高,贮箱压力越高,加注流量仅对加注时间有显著影响。     

Producing Towed Grid Quantum Turbulence in Liquid 4He

To produce turbulence in a pure quantum liquid, a superconducting linear motor has been built to tow a grid through a channel of superfluid helium at 20 mK. The design was developed using a computer simulation which considered the critical aspects of the cryogenic and electrical environment. This resulted in a single superconducting solenoid motor with an armature moving through its center. This light insulating armature is constructed of 3 phenolic tubes separated by two hollow cylindrical niobium cans placed 26 mm apart, with the turbulence-producing grid attached to one end. A conducting section on the armature, composed of one of the Nb cylinders and silver paint coating part of the phenolic rod, is inside a closely fitting capacitor made of two semi-cylindrical copper sheets. This capacitor, coupled to a bridge circuit, measures the armature position. When driven with the properly shaped current pulse (also determined by simulation), a magnetic field is produced that accelerates the rod (and grid) quickly, moves the rod and grid at near constant speed for at least 10 mm, and then quickly decelerates it. With LabView, complex pulse shapes are applied to the superconducting solenoid to produce the desired motion. The decay of turbulence is detected by the calorimetry technique in the isolated cell after the grid is pulled. Results of tests at 4.2 K (normal, classical liquid) and in superfluid down to 1.3 K, show that the motor preforms adequately in this temperature regime.     

Axisymmetric viscous liquid oscillations in a cylindrical container

The axisymmetric natural damped frequencies (m=0) of a viscous liquid in a cylindrical container are obtained for slipping and anchored contact line at the container wall. The results may also be applied to viscous liquid in a micro gravity environment, if the contact angle of the contact line to the container wall is in the vicinity of π/2, indicating that the free liquid surface remains a plane surface in equilibrium. It was found that there exists, in contrast to frictionless liquid, a range where only aperiodic motion of the liquid is possible. This appears for small liquid height ratios h/a.     

Liquid oscillations in a circular cylindrical container with “sliding” contact line

Depending on the magnitude of the excitation response of the free liquid surface, the liquid and solid properties, the surface tension and the roughness or smoothness of the solid the contact line of the liquid surface to the solid container wall may slip, slide or be anchored. For an ideal liquid in a cylindrical tank the natural frequencies and response of the liquid have been determined for the cases. They are depending on the magnitude of the “sliding friction” coefficient larger than those of the freely slipping edge. The results for the anchored contact line exhibit the largest natural frequencies.     

Iron Burning in Pressurised Oxygen Under Microgravity Conditions

An investigation of cylindrical iron rods burning in pressurised oxygen under microgravity conditions is presented. It has been shown that, under similar experimental conditions, the melting rate of a burning, cylindrical iron rod is higher in microgravity than in normal gravity by a factor of 1.8 ± 0.3. This paper presents microanalysis of quenched samples obtained in a microgravity environment in a 2.0 s duration drop tower facility in Brisbane, Australia. These images indicate that the solid/liquid interface is highly convex in reduced gravity, compared to the planar geometry typically observed in normal gravity, which increases the contact area between liquid and solid phases by a factor of 1.7 ± 0.1. Thus, there is good agreement between the proportional increase in solid/liquid interface surface area and melting rate in microgravity. This indicates that the cause of the increased melting rates for cylindrical iron rods burning in microgravity is altered interfacial geometry at the solid/liquid interface.     

Room temperature ferromagnetism in triple perovskite Sr3CrFeMoO9

Triple perovskite Sr₃CrFeMoO₉ was designed and prepared in reducing atmosphere. The ceramics are single phase with homogeneous, porous-like microstructure. The valences of the interior Cr, Fe, and Mo cations are +3, +2/+3, 0/+6, whereas that of the surface cations are +3, +3, and +6, respectively. The ceramics show well-saturated magnetization-magnetic field hysteresis loop, the saturation magnetizations are 1.0 and 0.6 μ_B per formula unit at 10 K and room temperature, respectively. The ferromagnetic Curie temperature is determined to be 385 K. Furthermore, semiconductor behavior in the measuring temperature range (10–300 K) and large negative magnetoresistance of ?30.7 % at 10 K are observed. Our results may stimulate further works on room temperature ferromagnetism in triple perovskite.     

Überhöhungsfaktor für erzwungene Konvektion auf kleinen Heizelementen

The convective heat transfer at small flush mounted elements is important for microelectronics and thermal sensing applications. The maximum heat transfer appears at the upstream area of the heating element with a length l*. The heat transfer coefficient h* averaged along the length l* of the heating element is larger than the heat transfer coefficient h at the same position of a uniformly heated part like e.?g. a flat plate or cylinder. An increase factor K = h*/h is introduced for the characterization of this relation. The present study compares K values derived from boundary layer calculations, heat transfer measurements, calibrations of thermal wall shear stress sensors and from numerical calculations for l* = 0.01 to 4?mm. The increase factor K is dependent on the flow length l*, the heat transfer coefficient h and the thermal insulation.     

Pseudogap and Superconducting Energy Gap in Single Crystals of URu2Si2 by Point Contact Spectroscopy

We performed point contact spectroscopy measurements in the superconducting and normal state of a single crystal of the heavy fermion URu₂Si₂. The differential resistance as a function of bias voltage shows the features of the superconducting energy gap up to 1.37 K, above T _C we observe another feature that we identify as a pseudogap that persists up to 2 K. The superconducting gap does not fit a BCS behavior.     

Description of the Sounding Rocket Experiment—SOURCE

A SOUnding Rocket Compere Experiment (SOURCE) is prepared for launch in spring 2008 and shall deliver approximately 360 s of microgravity time. The experiment is intended to partially fulfill the scientific objectives of the European Space Agency Microgravity Applications Program project AO-2004-111 (Convective boiling and condensation). One of the tasks of this experiment is the investigation of capillary dominated flow at a heated wall. The SOURCE experiment will also serve the needs of the COMPERE research group whose mandate is to investigate the behavior of propellant in spacecraft tanks. SOURCE is a benchmark type of experiment on fluid behavior in tanks to test hypotheses and numerical predictions (quantitative results on a tank scale). Several work packages have been distributed to the Center of Applied Space Technology and Microgravity (ZARM) to manage a part of the preparation of this experiment. Since ZARM acts also as a principal investigator, the subject of surface tension driven flows is one of the main topics. It includes the design of the experimental setup to study free surface behavior as well as the numerical predictions to quantify the heat transfer at a non-isothermal boundary condition in the absence of gravity.     

Thermal Response of a Two-Phase Near-Critical Fluid in Low Gravity: Strong Gas Overheating as Due to a Particular Phase Distribution

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF₆ in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase ³He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.     

Effect of rapid thermal annealing on Zn/ZnO layers

Zn/ZnO layers were deposited on SiO₂/Si substrate by magnetron sputtering at room temperature, and then these layers were annealed at various temperatures from 200 to 400 °C in nitrogen atmosphere for 1 min. The structural and electrical properties of the Zn/ZnO layers before and after annealing are systematically investigated by X-ray diffraction, scanning electron microscopy, current–voltage measurement system, and Auger electron spectroscopy. Current–voltage measurements show that the Zn/ZnO layers exhibit an Ohmic contact behavior. It is shown that, initially, the specific contact resistivity decreases with the increase of the annealing temperature and reaches a minimum value of 9.76 × 10^?5 Ω cm² at an annealing temperature of 300 °C. However, with a further increase of the annealing temperature, the Ohmic contact behavior degrades. This phenomenon can be explained by considering the diffusion of zinc interstitials and oxygen vacancies. It is also shown that Zn-rich ZnO thin films can be obtained by annealing Zn on the surface of ZnO film and that good Ohmic contact between Zn and ZnO layers can be observed when the annealing temperature was 300 °C.     

The shape and stability of wall-bound and wall-edge-bound drops and bubbles

The behavior of wall-bound drops and bubbles is fundamental to many natural and industrial processes. Key characteristics of such capillary systems include interface shape and stability for a variety of gravity levels and orientations. Significant solutions are in hand for axisymmetric pendent drops for a variety of uniform boundary conditions along the contact line with gravity acting normal to a planar wall. The special case of a wall-bound drop or bubble that is also pinned at an edge (i.e. a ‘wall-edge-bound’ drop) is considered here where numerical solutions are obtained for interface shape and stability as functions of drop volume, contact angle, fluid properties, and uniform gravity vector. For a semi-infinite zero-thickness planar wall (plate), a critical contact angle is identified below which wall-edge-bound drops are always stable. The critical contact angle is computed as a function of the gravity vector. The numerical procedure, which makes no account for contact angle hysteresis, predicts that such wall-edge-bound drops are unconditionally unstable for any gravity field with a component that is tangent to the wall while inwardly normal to the edge. Select experiments are conducted that support the conclusions drawn from the numerical results.     

Spin-Reorientation and M?ssbauer Study of Orthoferrite TbFe0.75Mn0.25O3

The crystallographic and magnetic properties of TbFe_0.75Mn_0.25O₃ powder were characterized by x-ray diffraction (XRD), Mössbauer spectroscopy, and vibrating-sample magnetometry (VSM). The crystal structure was found to be orthorhombic (space group Pbnm) with lattice constants a ₀=5.317 Å, b ₀=5.604 Å, and c ₀=7.598 Å, respectively. Mössbauer spectra of TbFe_0.75Mn_0.25O₃ have been taken at various temperatures ranging from 4.2 to 550 K. For Mössbauer spectra, we have fitted the spectra to a model based on a random distribution of Fe and Mn ions on the octahedral sites. The magnetic hyperfine fields of the four pattern (B ₀,B ₁,B ₂,B ₃) at 4.2 K are found to be H _hf=553, 544, 535, and 527 kOe, respectively. Isomer shift at room temperature is 0.25?C0.26 mm/s, which means that the valence state of Fe ions is ferric (Fe³⁺). A sudden change in both the magnitude of magnetic hyperfine field and its slope between 150 and 220 K suggests that magnetic phase transition related to the spin ordering takes place abruptly. The Néel temperature was determined to be T _N=550±5 K. The inflection points arising from a spin reorientation in the temperature dependence of the magnetic moment is observed. Its spin-reorientation transition is 70 K lower than that of 250 K for pure TbFeO₃.     

Two-phase Flow in Fuel Cells in Short-term Microgravity Condition

A visual observation of liquid–gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.