Japan Microgravity Center Research Articles

Microgravity droplet combustion: effect of tethering fiber on burning rate and flame structure

Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with published experimental data. The simulations were conducted using a transient one-dimensional spherosymmetric droplet combustion model, where the effect of the supporting fiber was implicitly taken into account. The model applied staggered convective flux finite volume method combined with high-order implicit time integration. Thermal radiation was evaluated using a statistical narrow band radiation model. Chemical kinetics and thermophysical properties were represented in rigorous detail. Tether fiber diameter, droplet diameter, ambient pressure and oxygen concentration were varied over a range for n-decane droplets in the simulations. The results of the simulations were compared to previously published experiments conducted in the Japan Microgravity Center (JAMIC) 10 second drop tower and the NASA Glenn Research Center (GRC) 5.2 second drop tower. The model reproduces closely nearly all aspects of tethered n-decane droplet burning phenomena, which included droplet burning history, transient and average burning rate, and flame standoff ratio. The predictions show that the presence of the tethering fiber significantly influences the observed burning rate, standoff ratio, and extinction.

Melting and phase-separation of lead borate glasses in low gravity drop shaft

A drop shaft experiment was conducted to melt and solidify a glass of 3.5PbO–96.5B 2O 3 (mol%) composition adhered to a small platinum heating coil (2–3 mm i.d., 5–6 mm long) at the Japan Microgravity Center (JAMIC). The temperature of glass melt recorded during the drop experiment indicates that the phase-separation of this glass mainly happened during the high gravity period of the experiment. A similar experiment was conducted on the ground to explore the effect of gravity level on the melting and phase-separation of lead borate glass. The homogeneity of different samples was compared through EDS analysis and the microstructure of the samples were observed by SEM. The lead borate glasses separated into two different glass phases, the separated phase (Pb-rich phase) and the continuous phase (B-rich phase), regardless of the gravity level the phase-separation happened. The size of separated Pb-rich phase in the top of sample from drop shaft experiment is much smaller than that in the bottom of sample, while the sizes of separated Pb-rich phase are almost the same in different locations in the ground sample. The homogeneity of the drop shaft sample is much worse than that of ground sample.

Melting and phase separation of lead borate glasses in low gravity drop shaft

Abstract A drop shaft experiment was conducted to melt and solidify a glass of 3.5PbO–96.5B2O3 (mol%) composition adhered to a small platinum heating coil (2–3 mm i.d., 5–6 mm long) at the Japan Microgravity Center (JAMIC). The temperature of glass melt recorded during the drop experiment indicates that the phase-separation of this glass mainly happened during the high gravity period of the experiment. A similar experiment was conducted on the ground to explore the effect of gravity level on the melting and phase-separation of lead borate glass. The homogeneity of different samples was compared through EDS analysis and the microstructure of the samples were observed by SEM. The lead borate glasses separated into two different glass phases, the separated phase (Pb-rich phase) and the continuous phase (B-rich phase), regardless of the gravity level the phase-separation happened. The size of separated Pb-rich phase in the top of sample from drop shaft experiment is much smaller than that in the bottom of sample, while the sizes of separated Pb-rich phase are almost the same in different locations in the ground sample. The homogeneity of the drop shaft sample is much worse than that of ground sample.

Convection in Weld Pool under Microgravity and Terrestrial Conditions

Electron beam (EB) welding and tungsten inert gas (TIG) welding were performed under both microgravity and terrestrial conditions in order to investigate the effects of gravity and surface tension on the convection in a molten pool. The microgravity conditions were achieved using the drop-shaft at the Japan Microgravity Center (JAMIC). A small-sized EB or TIG welding system was loaded into the drop capsule, and then the capsule was dropped 710m below ground level. The system attains a microgravity level of 10-5 G for a duration of 10 seconds. Pure iron and an iron-tungsten alloy (SKD4) were used for the iron samples, while pure aluminum and an aluminum-copper alloy (A2219) were used for the aluminum samples. The cross sections of the specimens were analyzed by EPMA after the welding to investigate the distribution of the minor elements. During the EB welding, the surface tension and the buoyancy determine the convection. Under microgravity, only the surface tension causes the convection because the buoyancy is considered to be negligible. As a result, it was found that the convection due to the surface tension is dominant for the iron alloys, but it is very weak for the aluminum alloys.

In-Situ Observation of the Soot Deposition Process on a Solid Wall with a Diffusion Flame along the Wall

Experiments at the Japan Microgravity Center (JAMIC) have investigated the interaction between diffusion flames and solid surfaces placed near flames. The fuel for the flames was C2H4 and the surrounding oxygen concentration 35%, with surrounding air temperatures of Ta=300 and 600K. The effects of these parameters on soot distribution in diffusion flames and soot deposition on solid walls were studied. Direct images of the whole flame and shadow images of the flame with back light were recorded and used to calculate the soot volume fraction by the Abel transformation method. Results show that at the higher surrounding air temperature the soot particle distribution region is closer to the wall and results in more deposition. Numerical simulation was also performed to determine the motion of soot particles in the flames and the soot deposition characteristics. The results are in good agreement with the observed soot behavior in microgravity.

Microgravity experiments of nano-satellite docking mechanism for final rendezvous approach and docking phase

Laboratory for Space Systems (LSS), Tokyo Institute of Technology (Tokyo Tech) conducted three-dimensional microgravity environment experiments about a docking mechanism for mothership-daughtership (MS-DS) nano-satellite using the facility of Japan Micro Gravity Center (JAMIC) with Hokkaido Institute of Technology (HIT). LSS has studied and developed a docking mechanism for MS-DS nano-satellite system in final rendezvous approach and docking phase since 2000. Consideration of the docking mechanism is to mate a nano-satellite stably while remaining control error of relative velocity and attitude because it is difficult for nano-satellite to have complicated attitude control and mating systems. Objective of the experiments is to verify fundamental grasping function based on our proposed docking methodology. The proposed docking sequence is divided between approach/grasping phase and guiding phase. In the approach/grasping phase, the docking mechanism grasps the nano-satellite even though the nano-satellite has relative position and attitude control errors as well as relative velocity in a docking space. In the guiding function, the docking mechanism guides the nano-satellite to a docking port while adjusting its attitude in order to transfer electrical power and fuel to the nano-satellite. In the paper, we describe the experimental system including the docking mechanism, control system, the daughtership system and the release mechanism, and describe results of microgravity experiments in JAMIC.

FLAME PROPAGATION IN SPRAYS AND PARTICLE CLOUDS OF LESS VOLATILE FUELS

ABSTRACT Flame propagation mechanisms in spray and particle clouds of less volatile condensed fuels are discussed, based on microgravity experimental results obtained in tests using the Japan Microgravity Center with a microgravity test duration of 10 seconds and its microgravity quality of 10−5 g. First, n-decane sprays were tested under microgravity using a flame propagation tube with an inner diameter of 62 mm and a length of 605 mm. A flame front was observed in which there were many envelope flames surrounding droplets. It was suggested that the whole spray flame proceeded by a droplet-to-droplet propagation of envelope flames. The flame propagation speed had a maximum at a certain Sauter mean diameter (SMD) when the total equivalence ratio was kept constant. All experimental flame propagation speeds could be arranged on a curve when the mean droplet spacing divided by the mean radius of envelope flames surrounding droplets was employed as an abscissa. Second, to examine the fundamental mechanism of spray combustion mentioned above, we analyzed the flame spread rate of droplet arrays under various conditions. The flame spread rate had a maximum at a certain droplet spacing, and so the spread rate decreased when the spacing was large or small. This supports the observation of the above-mentioned flame propagation of spray. Third, to confirm the mechanism of flame propagation of dispersed less volatile fuels, we rearranged our previous experimental data of Polymethylmethacrylate (PMMA) particle clouds in microgravity. Although we were not able to arrange the data by means of flame radii since the measurements were difficult, the flame propagation speeds had a maximum at a certain mean particle spacing divided by the mean particle diameter.

超小型衛星用ドッキング機構によるスラスタ搭載衛星の進入把持３次元微小重力実験

This paper describes 3-dimensional microgravity experiments of a new docking mechanism for nano-satellite at Japan Microgravity Center (JAMIC). The proposed docking sequence is divided between approaching/grasping phase and guiding phase. In the approaching/grasping phase, the docking mechanism grasps the nano-satellite even though the nano-satellite has some control errors in a docking space. In the guiding phase, the docking mechanism guides the nano-satellite to a docking port while adjusting its attitude in order to transfer electrical power and fuel to the nano-satellite. We design and fabricate an experimental model of the docking mechanism which has the grasping and guiding functions for each phase. The objective of the experiments is to verify the grasping function because 3-dimensional microgravity environment is very important in the approaching/grasping phase. In this paper, we introduce the docking mechanism and the daughtership satellite model and explain the microgravity experimental setup and its results.

In Situ Observation of Liquid-Liquid Phase Separation of Glass under Microgravity

We succeeded in the in situ observation of liquid-liquid phase separation of glass under microgravity at the Japan Microgravity Center (JAMIC), which provides a 10 s microgravity duration. The microgravity level is 10-5 g (g≡gravitational field at Earth's surface). We constructed an apparatus for the observation of phase separation suitable for the experimental facility. The melt of 3BaO•97B2O3 (mol%) was held as a film on a Pt wire loop. The sample was initially heated above the immiscibility temperature and held at that temperature. After the homogenization stage, the melt was cooled. Liquid-liquid phase separation of the melt was observed directly with a video camera. It was found that the nucleation and growth of phase-separated particle under microgravity are slower than in the case of 1-g, and the delay time of phase separation between the case of microgravity and 1-g depends on the flow rate of the melt under microgravity. These observations could be interpreted in terms of Marangoni convection and a change of diffusion.

Microgravity Experiments on Phase Change of Self‐Rewetting Fluids

A series of microgravity experiments on self-rewetting fluids has been conducted at the 10-second drop shaft of the Japan Microgravity Center (JAMIC). In all the experiments, 1.5 wt% of 1-butanol aqueous solution were employed as a self-rewetting fluid. The objective of the first experiment was to observe the boiling behavior of two-dimensional adjacent dual vapor bubbles with the aid of a two-wavelength interferometer and tracer particles. A significant difference was observed between a self-rewetting fluid and a normal fluid (CFC-113 in this experiment) in bubble interaction and flow developed along vapor/bubble interface. The second experiment focused on the flow at the bubble/heater contact area and around the three-phase interline, visualized with tracer particles. Differing behavior among three fluids, 1-butanol aqueous solution, CFC-113, and ethanol aqueous solution, was observed. The last microgravity experiment was a demonstration of wickless heat pipes containing three different fluids as a working fluid, 1-butanol aqueous solution, water, and ethanol aqueous solution. The temperature variation of working fluid in the heat pipe was monitored, and the liquid flow returning from the condensation region to the evaporation region was visualized by tracer particles. In addition to microgravity experiments, the performance of conventional heat pipes with 1-butanol aqueous solution was evaluated on the ground, and compared with water heat pipes. Our preliminary results are presented.

Effect of Magnetic Field on the Crystalline Structure of Magnetostrictive TbFe2 Alloy Solidified Unidirectionally in Microgravity

We performed unidirectional solidification experiments on TbFe(2) alloy in a static magnetic field in microgravity of 10(-4) g for 10 sec obtained by a 490 m free fall of the Japan microgravity center (JAMIC). When the magnetic field strength was increased from zero to 4.5 x 10(-2) T during unidirectional solidification in microgravity, a [1 1 1] crystallographic alignment dominated, and the maximum magnetostriction constant increased from 1,000 ppm to 4,000 ppm. For unidirectional solidification in normal gravity, the maximum magnetostriction constant remained at 2,000 ppm with increasing magnetic field. The columnar structure grows and orients along the magnetic field. TbFe(2) crystals grow in microgravity predominantly in the same direction as the magnetic field.

Sooting behavior of ethanol droplet combustion at elevated pressures under microgravity conditions

Liquid ethanol is widely used in practical fuels as a means to extend petroleum-derived resources or as a fuel additive to reduce emissions of carbon monoxide from spark ignition engines. Recent research has also suggested that ethanol and other oxygenates could be added to diesel fuel to reduce particulate emissions. In this cursory study, the combustion of small ethanol droplets in microgravity environments was observed to investigate diffusion flame characteristics at higher ambient pressures and at various oxygen indices, all with nitrogen as the diluent species. At the NASA Glenn Research Center 2.2-second drop tower, free ethanol droplets were ignited in the Droplet Combustion Experiment (DCE) apparatus, and backlit and flame view data were collected to evaluate flame position and burning rate. Profuse sooting was noted above 3 atm ambient pressure. In experiments performed at the Japan Microgravity Center 10-second (JAMIC) drop shaft with Sooting Effects in Droplet Combustion (SEDC) apparatus, the first data that displayed a spherical sootshell for ethanol droplet combustion was obtained. Because of the strong sensitivity of soot formation to small changes in an easily accessible range of pressures, ethanol appears to be a simple liquid fuel suitable for fundamental studies of soot formation effects on spherical diffusion flames. The results impact discussions regarding the mechanism of particulate reduction by ethanol addition to fuels in high-pressure practical combustors.

Combustion Behaviour Over ETFE Insulated Wire in Slow External Flow under Microgravity

Opposed flame spreading over ETFE (ethylene-tetrafluoroethylene co-polymer) insulated wire, which is used for actual wire in space, in low air flow velocity has been investigated under microgravity environment. The experiments were performed with Japan Microgravity Center (JAMIC) 10 s dropshaft and NASA's KC 135 parabolic flight. Experiments were performed with changing O2 concentration, 35 and 40%, external air flow velocity, 0 (quiescent) -22 cm/s and dilution gas, N2, CO2, He. The sample of 0.32 mm inner core diameter with 0.15 mm insulation thickness was used. The results showed that flame spread rate over wire insulation was strongly affected by air flow velocity and the dilution gases. Flame spread with N2 dilution had maximum value in a low flow velocity region. However, CO2 dilution resulted in monotonic increase in flame spread rate with decrease in air flow velocity and the maximum flame spread at Ve=0 cm/s (quiescent). The different dependency of the spread rate on flow velocity with different dilution gas was explained by the reabsorption effect of CO2 gas ahead of flame front. Reabsorption of radiation heat with CO2 gas recovers the radiation heat from the flame and prevent the flame temperature decrease even in very low flow velocity region, which is known as radiation control region for optically thin gas.

Formation and disappearance of pores in aluminum alloy molten pool under microgravity

With the start of construction of the international space station, a welding technique in space is becoming essential for the construction and repair of space structures. We now describe the results obtained under microgravity by electron beam welding, which is thought to be the most probable candidate for space welding. The microgravity environments were achieved using the drop-shaft system at the Japan Microgravity Center. The system can produce a microgravity of 10–25 G and a duration of 10 s. The sample was an aluminum–copper alloy, 2219, which is commonly used in the aerospace industry. The effective diameter of the electron beam was set at approximately 3 mm in order to prevent the keyhole formation. A significant decrease in the number of the pores in the Al alloys was observed under microgravity. All of the pores smaller than 100 μm disappeared for both microgravity and terrestrial environments. These results indicate that there exists a new bubble formation mechanism, consisting of a reaction between the molten Al and Al2O3 forming Al2O gas, and that the convection due to the Marangoni flow is negligible in an Al molten pool.

The Effect of Core Material on Combustion Behaviour over Polyethylene Insulated Wire under Microgravity

Combustion of wire insulation is important for fire safety in Space. Insulated wire has two unique features, one is sample geometry and another is existence of wire core. In this paper we investigate the effect of core materials on combustion phenomena of insulated wires. An experimental study of flame spread phenomena over polyethylene insulated wires with two different core materials, copper and nichrom, in different O2 concentrations was performed in microgravity. The experiments were performed at the Japan Microgravity Center (JAMIC) 10 s dropshaft. Experiments were performed with different O2 concentration and core material. The results show that the core material strongly affects the flame shape irrespective of gravity level. The flame spread rate is also strongly affected by the core material. Under normal gravity, the flame spread rate with copper cores is faster than that with nichrom cores in any of the tested oxygen concentrations. In microgravity, the effect of the core materials on the flame spread rate changes by oxygen concentration. In 35% O2, flame spread rate with copper wire was faster than that with nichrom core wire, as also seen in the normal gravity case. In 21% O2, the flame spread rate with the nichrom core wire is faster than that with copper core wire. The core heat conduction analysis shows that the thermal conductivity of the core material has an important effect on the flame spread rate. The case of high flame temperatures, the core acts as a heat source and high conductivity material supplies more heat to the insulation. At low temperatures, the core becomes heat sink and high conductivity material causes heat removal from the insulation near the flame front.

Thermal Conductivity of Molten Al, Si and Ni Measured under Microgravity

The thermal conductivities of molten Si, Ni, Al and their alloys have been measured by means of the non-stationary hot wire method under 1 G and microgravity of 10 -5 G. A Mo wire was used as the heating wire and coated with alumina by the electrophoretic deposition to prevent an electric current leakage through the melts. Natural convection in the melts was suppressed by measuring the thermal conductivity under the microgravity of 10 -5 G using the drop shaft facility of the Japan Microgravity Center. Moreover, the free surface of the melts was covered with a ceramics plate to prevent Marangoni flow. The thermal conductivity of molten Si is about 10 Wm -1 K -1 , and that of molten Al is 47 Wm -1 K -1 at 1260 K. The thermal conductivity of molten Al-30 mass% Si alloy is about 30 Wm -1 K -1 , and that of molten Ni is 5 Wm -1 K -1 at 1773 K. The thermal conductivities of some molten metals are discussed on the deviation from the Wiedemann-Franz law.

Unidirectional solidification of magnetostrictive materials using a magnetic field in microgravity.

Unidirectional solidification experiments of magnetostrictive materials of Tb(0.3)Dy(0.7)Fe(2-x), (Terfenol-D) alloy in a static magnetic field and microgravity were performed using the Japan Microgravity Center (JAMIC) free-fall facilities. When the magnetic field strength was increased from 0T to 3.7 x 10(-2) T during unidirectional solidification in microgravity, a <111> crystallographic alignment of the grains dominated, and the maximum magnetostriction constant increased from 500 ppm to 2,000ppm. For unidirectional solidification in normal gravity, the maximum magnetostriction constant remained at 1,000 ppm for increases in the magnetic field.

Effect of low external flow on flame spread over polyethylene-insulated wire in microgravity

An experimental study of flame spread phenomena over polyethylene-insulated wires has been performed in opposed flow under microgravity. The experiments were performed at the Japan Microgravity Center (JAMIC) 10 s drop shaft. Two samples with different insulation thicknesses, 0.075 and 0.15 mm, and with the same inner core diameter, 0.5 mm, were used. Experiments were performed with O 2 concentrations of 21%–50% and external flow velocities 0 (quiescent) to 30 cm/s. The results show that the rate of flame spread is affected by the flow velocity and that the effect is much stronger at high oxygen concentrations. According to the results, flame spread phenomena of wire insulation can be classified into four different regimes based on the flow velocity: (1) an oxygen transport control regimes, (2) a geometrical effect regime, (3) a thermal regime, and (4) a chemical kinetic control regime. A special feature of the flame spread over wire insulation is the existence of the geometrical effect regime and a maximum spread rate between the oxygen transport control and geometrical effect regimes. The mechanism that gives rise to the unique features is discussed based on changes in preheat length, standoff distance, and flame temperature. The importance of the three effects and their relation to sample geometry, enhancement of diffusive oxygen supply, reduction of standoff distance, and logarithmic effect for the heat transfer are discussed.

Effect of Kampo herbal medicines on murine water metabolism in a microgravity environment.

To determine the possibility of new applications of Oriental medicines, we examined the changes in water metabolism of mice that underwent microgravity and were treated with Kampo medicines. Male ICR mice were used in this experiment. Eight extracts of Kampo herbal medicines were dissolved in water and added to the drinking water administered to mice at 1 g/kg body weight for two days. The microgravity experiment was performed at the Japan Microgravity Center. We used a drop-shaft type microgravity experimental system with a free fall of 490 m. Before the drop, 7 ml of physiological saline was injected intraperitoneally. Under fasting and dehydration, body weights were measured and loss of body weight was calculated as urine. Blood samples were collected 24 hours after the microgravity experiment, and the antidiuretic hormone (ADH) in plasma related to water metabolism was measured by the radioimmunoassay (RIA) method. Heat shock protein in the spleen was measured by the enzyme-linked immunosolvent assay (ELISA) method. In the Hachimi-jio-gan and Hochu-ekki-to groups in microgravity, a decrease of urine was observed, which significantly suppressed the increase of ADH due to microgravity. Hachimi-jio-gan reduced the content of heat shock protein (HSP) 70 in the spleen. It is suggested that Hachimi-jio-gan and Hochu-ekki-to could be used as water metabolism adjustment reagents in a space environment. Furthermore, it is suggested that Hachimi-jio-gan could ease the stresses caused by microgravity. The physiological changes resulting from a microgravity environment are serious problems for space flight. Pre-treatment with Kampo medicines is expected to prevent, ease and treat these problems.

Processing sodium tellurite melts in low gravity drop shaft: Part II Melt solidification and glass formation

The effect of gravity on glass formation and crystallization of the Na2Ocdot;8TeO2 (NT8) and Na2Oċ4TeO2 (NT4) melts were investigated using the low gravity drop shaft at the Japan Microgravity Center (JAMIC). This drop shaft produces a low gravity of <10−3 g for ∼10 s during free-fall and about 8 to 10 g for ∼5 s during deceleration of the capsule. The glass initially adhered to a small platinum heating coil was re-melted in low gravity. The melt detached from the heating coil during the high-g period and solidified after being splattered on a plate (substrate) located ∼4 cm below the heating coil. The parameters that were varied for the drop shaft experiments were the melt temperature and the substrate material on which the melt splattered. Like what was observed at 1-g (ground), the NT8 splatters from the drop shaft experiments always formed glass, being independent of the melt temperature and the substrate material used. The splatters from the NT4 melts partially crystallized in all the drop shaft experiments, even though this melt is an excellent glass former at 1-g. The splatter on a substrate of higher cooling ability such as copper had a smaller amount of crystals than the splatter on a substrate of smaller cooling ability such as glass or alumina. The glass transition temperature, heat capacity in the glass transition region, activation energy for crystallization and the infrared (IR) spectra for the drop shaft splatters were not significantly different from those for the similar splatters prepared at 1-g. However, the crystallization temperature of all the drop shaft splatters was 5 to 10°C lower than that of their 1-g counterparts. This result suggests that the NT8 and NT4 melts solidified under drop shaft conditions are less resistant to crystallization than the similar melts solidified at 1-g.