Mitsuru Ohnishi

High-speed observation of the piston effect near the gas-liquid critical point

We investigated adiabatic changes in a near-critical fluid on acoustic time scales using an ultrasensitive interferometer. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10(-7) g/cm(3) and a temperature change of order 10(-5) K. Very small heat inputs at a heater produced short acoustic pulses with width of order 10 micros, which were broadened as they moved through the cell and interacted with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions agree well with our data.

Space experiment on the instability of Marangoni convection in large liquid bridge - MEIS-4: effect of Prandtl number -

Microgravity experiments on the thermocapillary convection in liquid bridge, called Marangoni Experiment in Space (MEIS), are carried out in KIBO of ISS. Three series of experiments, MEIS-1, 2, and 4, have been conducted so far. This paper reports the results obtained from MEIS-4, in which 20cSt silicone oil (Pr = 207) is used to generate large liquid bridges. They are suspended between coaxial disks that are 50mm in diameter, with their maximum length equal to 62.5mm. MEIS-4 aims at (1) determining the critical temperature difference for the onset of oscillatory flow; (2) realizing high Marangoni number conditions for high Pr fluid; (3) clarifying the effects of volume ratio, heating rate, hysteresis, and cooled disk temperature; and (4) observing whether the hydrothermal wave with azimuthal mode number m = 0 appears or not. The main results are presented and compared with those obtained in MEIS-1 and 2, which utilized liquid bridges of 5cSt silicone oil (Pr = 67).

Ultra-sensitive high-speed density measurement of the “piston effect” in a critical fluid

In order to conduct a detailed investigation of the piston effect, the peculiar heat transportation phenomenon in critical fluids, an ultra-sensitive high-speed density measurement system was developed using a very thin heater, a sensitive interferometer and a large-capacity high-speed data acquisition system. As the first step of the investigation, an experiment was conducted to measure the velocity of sound in CO2 near its critical point. Short heat pulses suppressing the disturbance of natural convection were applied to a small cell filled with almost-critical CO2 fluid and were detected by the measurement system. Because the pulses propagated through the cell at the velocity of sound and were reflected several times between the cell walls, the velocity could be precisely determined by measuring the time intervals between successive reflections. The resulting velocity profile versus temperature showed good agreement with theoretical prediction and numerical simulations. This result validated the density measurement system, and it is considered that the measurement system will be a very effective tool for further studies on critical phenomena with the aid of numerical simulation.

Air Revitalization in Orbit Demonstration for a Future Long-Duration Manned Mission

Various space agencies are currently considering the moon and asteroids as way stations for astronauts bound for Mars. Under JAXA’s long-term vision, JAXA2025, the agency will by 2015 finalize its development plans for the next period of manned space activity. To establish supporting technologies, JAXA is researching systems that will be indispensable for manned space activities beyond low earth orbit, including water and air recycling. Three types of assemblies are necessary for recycling air: a carbon dioxide separator to remove exhaled carbon dioxide from a spacecraft’s atmosphere, a carbon dioxide reduction assembly to produce water from the separated carbon dioxide and added hydrogen, and water electrolysis equipment for oxygen production. This paper describes a carbon dioxide removal assembly and a water electrolysis assembly designed for operation in microgravity.

Air Revitalization Demonstration on the JEM (KIBO) for a Manned Space Exploration

To further advance manned space exploration, a critical issue that must be addressed is recycling of resources, especially air revitalization and water reclamation. Japan is already a leader in terrestrial environmental technologies, and aims to apply this know-how to develop air and water recycling technologies for space applications. To support proposed post-ISS missions such as manned lunar or asteroid exploration and an Earth-Moon Lagrange point (EML1) space station, JAXA is developing an air revitalization system for an on-orbit demonstration on the International Space Station (ISS) early in the extended ISS operation period (2015–2020). The requirements for this technical demonstrator have been investigated and its specifications established. Regenerative life support functions include oxygen recovery from carbon dioxide using a combination of CO2 reduction by a Sabatier process and O2 generation by electrolysis. Each air and water re-vitalization subsystem is planned to be transported to the ISS separately by successive H-II Transfer Vehicle (HTV) launches in the sequence Water Reclamation → Water Electrolysis → CO2 Reduction. This paper presents the air re-vitalization system developed by JAXA, and gives details of water electrolysis in microgravity.

Study for the Reduction of Carbon Dioxide Using Sabatier Reaction

The Sabatier reaction with a titanium dioxide (TiO2)-supported ruthenium (Ru) catalyst (Ru/TiO2) was investigated with the goal of scaled-up carbon dioxide (CO2) methanation at lower temperatures than conventional catalysts. The catalytic reaction of stoichiometric amounts of CO2 and hydrogen (H2) without any diluting was conducted over a 0.75 wt% Ru/TiO2 (Ru/TiO2(0.75)) at several flow rates. The CO2 reduction was achieved with catalyst temperatures less than 300°C in good to excellent conversions. Transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) images of Ru/TiO2(0.75) revealed highly dispersed Ru nanoparticles having one to a few nanometers’ diameter on the TiO2 supports. The physical properties of Ru metals are considered to contribute the decrease of the reaction temperature.

Study of Regenerative Fuel Cells for Aerospace Missions of Japan

A fuel cell (FC) /regenerative fuel cell (RFC) for space utilization has special system requirements, such as operational conditions and designs, because of its isolated low gravitational and closed environment, which differs greatly from that for terrestrial use. Over 1,000 h continuous and stable operation of PEFC using the developed prototype system was achieved. During conditions of no thermal control, no external humidifier, and low current density operation under low pressure, the voltage was maintained at a constant level under such low temperatures. No cross-leak or flooding phenomena were observed from these results: good performance was also obtained. These results suggest that the polymer electrolyte and other components used in this study were stable in such conditions.