Akihiro Nakano

Application of laser holography interferometer to heat transport phenomena near the critical point of nitrogen

A laser holography interferometer is applied to investigate heat transport phenomena near the critical point of nitrogen. The very high thermal compressibility and small thermal diffusion near the critical point of fluids affect thermal energy propagation, lead to the formation of weak acoustic waves as carriers of thermal energy. This heat transfer phenomenon is called the piston effect. The piston effect in supercritical nitrogen is investigated using a laser holography interferometer. Natural convection due to gravity interferes with the piston effect under terrestrial conditions. Therefore, we attempt to suppress the generation of natural convection by creating stable density stratification in the cell. The experiment consisted of two different procedures. In the first, heat was added from the bottom of the experimental cell, while in the second, heat is added from the top of the cell. From the two results, we successfully extracted the heat transfer phenomenon, the piston effect, which is considered to be the fourth mechanism of heat transfer.

An experimental study of heat transfer characteristics of a two-phase nitrogen thermosyphon over a large dynamic range operation

Heat transport characteristics of a cryogenic two-phase nitrogen thermosyphon have been experimentally investigated in this study. The thermal resistance and the maximum heat transfer rate were mainly investigated over a wide dynamic range from near the triple point to the critical point. The experimental data suggests that the nominal thermal resistance does not have pressure dependence in the high pressure and high temperature region. The present experimental result is well explained by the theoretical prediction. From the experimental result of the operating limit of the thermosyphon, it is found that the maximum heat transfer rate is governed by the interaction between the vapor flow and the returning liquid film flow along the wall in the evaporator section, even near the critical point.

He II liquid/vapour phase separator for large dynamic range operation

A phase separator, which separates helium vapour from liquid superfluid helium (He II), is an indispensable device for space cryogenics. The most recent approach to the Space Infrared Telescope Facility (SIRTF) uses a new design concept in which only the detector package is cold at launch, the remainder of the telescope being subsequently cooled to operating temperature on orbit. Therefore, a large dynamic operational range is required of the cryogen system. This is a report of initial laboratory test results with candidate porous plugs as phase separators. Mass flow rates and pressure and temperature differences across a porous plug were measured in this experiment. Relatively large mass flow rates were observed even at small pressure differences. In the high mass flow rate region, hysteresis was observed with increases and decreases in the pressure difference. A linear theory is proposed and compared with experimental data to explain several phenomena observed in this system.

Helmholtz resonance technique for measuring liquid volumes under micro-gravity conditions

The Helmholtz resonance technique has been investigated for measuring liquid volumes under micro-gravity conditions. We proposed a theoretical model of the Helmholtz resonance phenomenon induced in a closed container, and carried out a micro-gravity experiment to confirm the applicability of the measurement technique using water as a test liquid. A swept frequency signal was added to a loudspeaker, and together, a microphone and the loudspeaker picked up the sound of the Helmholtz resonance. The liquid volume was calculated from the Helmholtz resonance frequency using an equation derived from the theoretical model. The liquid volume obtained from the micro-gravity experiment agreed well with the liquid volume obtained under terrestrial conditions. Thus, it was confirmed that the Helmholtz resonance technique was applicable for measuring liquid volume under micro-gravity conditions.

Flow structure of thermal counterflow jet in He II

Abstract The flow structure of a thermal counterflow jet is investigated by direct measurement of the normal fluid velocity with a laser Doppler velocimeter (LDV). The temporal and spatial variation of the normal fluid velocity is measured to investigate the detailed properties of a He II thermal counterflow jet. The velocity profile and the variation of the centre-line velocity with the axial distance are also obtained to determine the flow structure in the downstream region. It is found that the thermal counterflow jet in the far-field behaves like a fully developed turbulent jet in an ordinary fluid. However, it seems that the mode of development differs from that of an ordinary fluid jet.

Velocity measurement of He II thermal counterflow jet accompanied by second sound Helmholtz oscillation

Abstract A laser Doppler velocimeter (LDV) is applied to the measurement of a thermal counterflow jet in He II. It is capable of direct measurement of the normal fluid velocity. Detailed analysis of the velocity measurement results suggests that the normal fluid component entrains the surrounding superfluid component to result in a zero relative velocity flow between the normal and superfluid components, even very close to the nozzle exit. It is demonstrated that the second sound Helmholtz oscillation measurement is of use for diagnosis of the superfluid turbulent state in a jet. The variation in frequency and the decay of the oscillation as a result of tangled vortices-flow interaction were investigated for this purpose. It is also suggested that a weak superfluid turbulent state, apparently different from the Gorter-Mellink type superfluid turbulent state, may be reached in the case of small heat input. On the other hand, for large heat input, the turbulent state was identified as the Gorter-Mellink type superfluid turbulent state.

Effect of PTFE contents in the Gas Diffusion Layers of Polymer Electrolyte-based Unitized Reversible Fuel Cells

AbstractPolymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In this study, the influence of hydrophobic agent in the gas diffusion layer (GDL) of URFC was investigated. The titanium (Ti)-felt GDL which treated with different percent of PTFE emulsion was intensively tested with various humidification temperature conditions in the single cell of URFC. I-V performance curves and divided three overpotentials were compared and analyzed in the fuel cell mode. The electrolyzer performances were also evaluated with the I-V curves. Experimental results showed that the GDLs with high PTFE contents have better performance in the dry conditions. On the other hand, the increased PTFE contents accelerate flooding problem in the wet condition and it is related with rising concentration overpotential. The electrolyzer performances are almost same with different PTFE contents of GDL.

Visualization of oscillatory flow phenomena in tapered pulse tube refrigerators

An oscillating flow in a pulse tube refrigerator with a tapered pulse tube was observed by a smoke wire flow visualization method. Effects of a tapered tube on flow phenomena were investigated from the view point of suppressing secondary streamings. Two kinds of tapered pulse tubes, a converging and a diverging tapered tube, were tested and the result was compared to that of cylindrical pulse tubes with the same interior volume as the tapered ones. It was observed in the converging tube that the velocity profile of the streaming is much more flattened compared to that in the cylindrical tube, and gas moves not only in the axial direction but in the radial direction substantially. It is also found that the performance of the converging tapered pulse tube is slightly improved compared to that of the cylindrical one, but it is not clearly distinguished from the observation whether the improvement is due to the suppression of the secondary streaming or not. On the other hand, the secondary streaming in the diverging tube is enhanced and the performance is deteriorated compared to those of both cylindrical and converging tubes.

Investigation of technique for measuring liquid volume under micro-gravity conditions

Measurement of liquid volume under the micro-gravity condition is an important aspect of space technology. In an attempt to create an effective and low-cost measurement system, a Helmholtz resonance technique was applied. Two different liquids were tested. We used water and liquid nitrogen as test liquids, the latter to confirm the applicability of this measurement technique to cryogens. It was found that the measurement technique using Helmholtz resonance is applicable under micro-gravity conditions. However, it was also found that obtaining accurate velocity of sound is important for the measurement. The measurement of spatial temperature distribution in the container is necessary to apply this measurement technique to cryogens because the velocity of sound has strong temperature dependency.

The Development of the Totalized Hydrogen Energy Utilization System for Commercial Buildings

We have proposed the Totalized Hydrogen Energy Utilization System(THEUS) for applying to commercial buildings. THEUS consists of fuel cells, water electrolyzers, metal hydride tanks and their auxiliaries. We evaluated the energy saving of the basic THEUS in view of an actual operation using thermal demand data of actual buildings. The THEUS have ability for achieving energy saving, in comparison with an ice storage system for cooling demand. We have developed the hydrogen storage system using metal hydride and unitized reversible cell (URC) combine the functionality of a fuel cell and an electrolyzer in a single device, these are important components of THEUS. The metal hydride tank designed and manufactured to evaluate that we assumed actual operation in the experiments. We developed the numerical simulation code, and simulation results are in good agreement with experimental data. The simulation about two tanks system was done by using this code. The heat utilization can be improved greatly. To achieve URC commercial use, improvement of durability was examined. It confirmed that our URC had durability for several thousand hours.