小林 弘明

The critical heat flux in liquid hydrogen is ten times higher than that in liquid helium and is approximately half of that in liquid nitrogen. Since the resistivity of pure metal such as copper or silver at 20 K is less than one-hundredth of that at 300 K, HTS magnets immersed in liquid hydrogen are expected to satisfy the fully cyostable condition or to be stable against high resistive heat generation enough for quench detection at a practical current density. In order to examine cryostability of HTS magnets in liquid hydrogen, a pool-cooled Bi2223 magnet with a 5 T magnetic field at 20 K has been designed, fabricated and tested in liquid nitrogen prior to excitation tests in liquid hydrogen. The magnet consists of six outer double pancake coils with the inner diameter of 0.20 m and four inner double pancake coils with the outer diameter of 0.16 m. The resistive voltage to initiate thermal runaway in the coil as-sembly in liquid nitrogen was higher than 1 V that is sufficient high for quench detection.

We focused on the liquid hydrogen (LH2) cooled field winding superconducting generator and conducted a test to develop a liquid hydrogen cooling system for a rotor of superconducting generator. The LH2 supply and exhaust system for the rotating tank which simulated the generator rotor was designed and fabricated. The rotation test was actually performed at a speed of 1800 rpm successfully and safely in a state where LH2 was stored and continuously kept liquid level in the rotating tank. The rotating tank equipped with eight thermometers, two MgB2 superconducting liquid level meters to know the state of liquid hydrogen in the tank. A rotation speed variation test was performed, and the behavior of the temperature and the liquid level in the rotating tank was observed. The vibrations of the rotor shaft throughout the test were within the allowable value. The experiments demonstrated the LH2 cooling system of the superconducting generator rotor.

The world's first Liquefied Hydrogen (LH2) Carrier which will transport LH2 from Australia to Japan has been built. At Japanese port, reducing the tank pressure will be required for the safe tank operation. However, pressure reduction will cause flashing, leading to an excess of the venting capacity or liquid leakage. Hence, the purpose of this research is to clarify the pressure reduction rate and liquid behavior through experimental and numerical approaches respectively. Pressure reduction experiment under high liquid level condition was conducted by using 30 m3 tank filled with saturated LH2. The experiment showed that the pressure recovery occurred at the beginning of depressurization, and then the pressure decreased based on the equilibrium state. From numerical analysis by VOF based in-house CFD code, it was found that the pressure recovery was caused by the boiling delay, and the tank pressure followed the saturation pressure after the liquid fully stirred.

Understanding the thermal-fluid characteristics of boiling hydrogen is of great significance for applications of liquid hydrogen, such as alternative clean energy and space vehicles. The boiling temperature of liquid hydrogen under atmospheric pressure is 20.3 K; thus, it is easy to boil to form a gas-liquid two-phase flow. Fuel transfer under the boiling state has been avoided in the space industry because of its unstable flow characteristics; precise control of the fuel, including the boiling flow, is necessary to improve the space-vehicle performance. This study aims to understand the flow-regime transition characteristics of boiling hydrogen through experimental investigation. The experimental conditions were as follows: the flow direction was horizontal, the inner diameter of the heating pipe was 15 mm, the mass flux ranged from 50 to 110 kg/m(2)s, and the pressure ranged from 250 to 300 kPa A. The flow regime transition characteristics were obtained by a high-speed camera. Fully liquid phase (LP), dispersed bubbly flow (DB), intermittent flow (IN), and annular flow (AN) were observed during the experiment. Each flow-regime boundary model is constructed using two dominant forces from the experimental result based on a Taitel-Dukler model. For the DB/IN boundary, a large-bubble sustainable condition is derived by the balance between the shear and buoyancy forces acting upon the bubble; for the IN/AN boundary, a droplet-sustainable condition is derived in terms of the force balance between the drag and gravity acting on the droplet. The semi-theoretical model predicts the experimental data with 96.7% accuracy. (c) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Heat transfer from inner surface of a vertical heated pipe to subcooled or saturated liquid hydrogen flowing upward was measured for quasi-steadily increasing heat input up to fully developed film boiling regime and decreasing the heat input through the film boiling regime. The experiments were carried out at the inlet pressures of 400, 700 and 1100 kPa and subcoolings from 0 K to 11 K. Three test pipe heaters made of SS310S with inner diameters of 6 and 8 mm and lengths of 100 and 200 mm were used. Experimental data from non-boiling to developed film boiling and developed film boiling down to minimum film boiling was obtained with the record of mass flow rate by continuously increasing and decreasing the heat input. It was observed that though the mass flow rate decreases with the increase of the heat generation rate, the heat transfer coefficient increases. Discussions on heat and mass transfer in inverted annular flow, dispersed droplet flow, single-phase vapor flow regimes and their changing conditions from one by one were carried out to clarify the phenomena. A calculation code of heat transfer characteristics was developed based on the discussions. The calculated results are in good agreement with the experimental results.

The thermal behavior of cryogenic propellant tanks is crucial issue in the operation of cryogenic propulsion systems. Herein, ground experiments were conducted in a 600-mm-diameter cryogenic tank filled with liquid nitrogen. As pre-pressurants, gaseous helium and gaseous nitrogen (of the same species as the liquid), were used to investigate its effect in accordance with actual propulsion systems. The tank was sealed after pre-pressurization to observe the self-pressurization. The evaporation rate and heat flow in the tank were estimated based on pressure and temperature measurements. In addition, the axial liquid temperature distribution was obtained through the liquid draining from the tank bottom, and a thermal stratification model was developed. These results demonstrated that the type of pre-pressurant significantly affected the thermal behavior in the tank. A higher evaporation rate and higher liquid internal energy rise rate with a thicker thermal layer were observed in the helium pre-pressurization case. The lower nitrogen partial pressure in the helium case enhanced the vaporization and growth of the thermal layer. Estimation of the power balance in the tank demonstrated that not only the ullage but also the heat mass of the tank provided heat for the evaporation and thermal layer. The evaporation occurs mainly at the contact point between the tank skin and liquid surface. The nitrogen vapor rises in a thin layer along the tank skin because of buoyancy under nitrogen pre-pressurization; however, buoyancy is lower under helium pre-pressurization, and a radial vapor flow is probably produced from the contact point instead, leading to a higher heat flux to the liquid.

Our research group has been researched for developing MgB2 superconducting energy apparatus in liquid hydrogen immersion cooling, such as superconducting generators. In this study, two-pole MgB2 race-Track coil, which was a model for a few tens kVA superconducting generator field magnet, was designed and made. This coil consists of two pieces of a similar MgB2 race-Track coil which has 529 turn with straight section of 150 mm, bending diameter in the end of section of 100 mm and thickness of 34 mm. We carried out excitation tests of the coil immersed in liquid hydrogen at the temperature of 21 K to 32 K, and the load line of the coil was obtained. The critical current under self-field was obtained for various temperatures. The normal zone propagation behavior of the coil at the quench was also investigated using several potential taps installed in the coil.

This paper presents a hydrogen ignition experiment conducted to establish safety standards for high-pressure hydrogen handled at the hydrogen stations for fuel cell vehicles (FCV). In the experiment, cryogenic hydrogen pressurized to over 80 MPa was leaked from a pinhole nozzle, and the blast pressure at the ignition and the flame length during steady combustion were measured. The hydrogen supply equipment used in the experiment has a maximum flow rate of 100 kg/h, a maximum discharge pressure of 90 MPa, and a temperature adjustment range of 50 K-300 K. Four types of pinhole nozzles with different outlet diameters, viz. 0.2 mm, 0.4 mm, 0.7 mm, and 1.0 mm were used to leak the hydrogen. In the experiment, the effects of the pinhole nozzle diameter, hydrogen supply pressure and temperature, and an igniter location on the blast pressure and flame length were evaluated. The igniter being appropriately positioned, once a steady flame was formed, combustion continued even if the ignition source was turned off, which necessitated the stopping of hydrogen supply to extinguish the fire. As a result of the experiment, it was found that the blast pressure and the flame length can be expressed as the correlation equations of the hydrogen leakage flow rate. However, even if the leakage flow rate was the same, we found that the flame length increases with decreasing the hydrogen supply temperature. We presented a correlation equation for the cryo-compressed hydrogen flame length that is about 30% longer than the previously presented equations for 300 K hydrogen flame. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

In the next generation orbital propulsion systems, cryogenic propellant recirculation will be applied to reduce engine chill-down consumption by recirculation chill-down and to achieve efficient propellant utilization by active propellant cooling. To understand tank-to-tank recirculation, a recirculation test campaign was conducted using liquid nitrogen as a working fluid. An electrically-driven recirculation pump developed for this experiment has bleed holes on the impeller to bleed vapor rather than liquid so that it works under two-phase flow operation. In addition to this pump, the test facility includes a 600 mm-diameter cryogenic tank, void fraction meters, an ultrasonic flow meter, and temperature and pressure sensors. The tank has a double layered window on the top to visually observe the return flow inside. The void fraction meters and ultrasonic flow meter were used to evaluate the two-phase mass flow rate. The tests were carried out in two different return port positions in the tank, the side and bottom return port cases, and the results indicated that the pressure and temperature behaviors in the tank were affected by the return flow rate, phase of the return flow and the return port position in the tank. In the side return port case, the flow returned to tank ullage and less disturbance of liquid in the tank was observed at a low flow rate while strong mixing of the liquid was observed at a high flow rate. The tank pressure was affected by the phase of return flow rather than the ullage temperature. In the bottom return port case, less disturbance was observed in vapor return flow, while in two-phase or liquid return flow, strong disturbance was observed even at lower flow rate. The tank pressure in this case was similar to that in the side return port case: the vapor return increased the pressure while the two-phase or liquid return decreased the pressure. The bleed holes on the impeller helped the inlet flow to recover to subcooled liquid condition and to keep the impeller working under saturation condition at the inlet although the recirculation system suffered decreased pump head, loss of liquid and unsteady oscillation of pump head and flow rate.

We have been developing a liquid hydrogen (LH2) cooled superconducting energy apparatus, such as superconducting generator, SMES, and so on. An MgB2 superconductor whose critical temperature is 39 K is now developing for a practical use. It can he cooled by LH2 with a sufficient temperature margin. An MgB2 wire is expected to be used for superconducting equipment due to the low production cost and material cost. In order to design equipment using the MgB2 wire, we have carried out measurement tests of critical superconducting properties of MgB2 under LH2 cooling and investigation of heat transfer characteristics of LH2. There are few reports on the experimental results of a superconducting coil using a long MgB2 wire under the LH2 immersion cooling. In this study, we have carried out an excitation test of a small MgB2 coil immersed in LH2 under an external magnetic field. Tanaka et al proposed that the test coil is a 529 turn solenoid coil with an inner diameter of 120 mm, an outer diameter of 190 mm, and a height of 41 mm, which was produced by the Wind and React method using a 300-m multifilament MgB2 wire (Hitachi Ltd., Ibaraki, Japan). The temperature of LH2 was changed from 21 K to 30 K and the external magnetic field was also applied up to 4.5 T. In the experiment, the critical characteristic of the solenoid coil was measured and a coil load line was obtained.

Developing applications of liquid-hydrogen (LH2)cooled superconducting devices is a challenging issue. Since the boiling point of LH2 is 20.4 K, MgB2, whose critical temperature is 39 K, can be cooled with a sufficient temperature margin. Furthermore, MgB2 wire is expected to be used for superconducting equipment due to the low production and material cost. Therefore, in order to design MgB2 superconducting equipment, the knowledge of normal zone propagation phenomena is important for the thermal stability and the quench protection. In this study, normal zone propagation and minimum quench energy (MQE) with a multi-filamentary MgB2 superconducting wire produced by Hitachi, Ltd. were observed under immersed in LH2. In the experiment, heat pulse, which initiates a normal zone, was injected to the center area of 200 cm long MgB2 wire. Then, the MQE and the normal zone propagation velocity (NZPV) were measured under specific conditions. NZPV was in the order of several cm/s and MQE was in the order of a few J at 30 K under LH2 cooling. In order to clarify temperature distribution along the wire during the normal zone propagating phenomena, the simulation model of MgB2 wire cooled by LH2 was created and analyzed using a finite element method simulation software.

The void fraction and vapor quality are important parameters for characterizing the gas-liquid two-phase flow. However, neither an established void fraction measurement method nor a verified void fraction - vapor quality interconversion model is available for the two-phase hydrogen flow. The object of this study is the development of a void fraction measurement technique and the investigation of the void fraction-quality correlations. A capacitive void fraction sensor was developed using the electric field analysis (EFA) and design of experiment (DOE), and it was applied in a boiling hydrogen experimental facility. The experimental conditions were as follows: the inner diameter of the heating pipe was 15 mm, the mass flux was ranged from 50 to 110 kg/m(2)s, and the static pressure was ranged from 250 to 300 kPaA. Further, the correlation between the thermal equilibrium quality (chi(ac) = - 0.03-0.14) and void fraction (alpha = 0-70%) was compared with that obtained in previously proposed models, and the void fraction - actual quality - thermal equilibrium quality interconversion models applicable to the boiling hydrogen flow were investigated. It was observed that the combination of the Sekoguchi model for thermal equilibrium quality - actual quality conversion and the Steiner drift-flux model for actual quality - void fraction conversion agreed well with the experimental results. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

© 2019 Published under licence by IOP Publishing Ltd. The knowledge of heat transfer properties of liquid hydrogen is important for designing and developing superconducting devices. In this study, film boiling heat transfers of liquid hydrogen flowing inside of heated pipe were measured under saturated conditions at the absolute pressures of 700 kPa for various mass flow rates. Test pipe heater made of SS310S with a diameter of 8 mm and a length of 200 mm was used. The test heater was once heated up to the film boiling regime with exponential heat generation rate. And then, while the heat generation rate was decreased exponentially down to the minimum heat flux, the film boiling heat transfer coefficient, mass flow rate per unit area and the degree of superheat of pipe length direction were measured. It was observed that though the mass flow rate decreased according to increase of the heat generation rate, the heat transfer coefficient increased. Discussions on the experimental results of various conditions were carried out to clarify the phenomenon of film boiling of liquid hydrogen flowing inside of heated pipe. It was considered that since the void fraction in the flow path was high, the effect of improving the heat transfer rate was also observed in the film boiling region due to the acceleration of the liquid phase. © Published under licence by IOP Publishing Ltd.

© 2019 Published under licence by IOP Publishing Ltd. Film boiling heat transfer coefficients were measured for the test wires of 0.5 and 0.7 mm in diameters and 200mm in length located at the centre of 8 mm diameter conduit with upward flow of liquid hydrogen. Pressures are ranged from 0.4 to 1.1 MPa, liquid subcoolings from 0 to 11 K and flow velocities up to 5 m/s. The experimental data were compared with the authors' correlation for forced flow film boiling already presented based on the data for 1.2 mm diameter test wire. The experimental data are higher than the predicted values by the correlation. Discussion is made on the effect of wire diameter observed and a new correlation was presented. © Published under licence by IOP Publishing Ltd.

The aim of this study is a characterization of boiling hydrogen flow in horizontal circular pipe flow. The most important parameters for boiling flow are a void fraction and flow quality. Although the void fraction is measurable in some way, there is no established method for cryogenic fluid. The authors developed a capacitive void fraction sensor and applied it for boiling hydrogen flow experimental facility. The correlations between the void fraction and flow qualities are investigated by comparing the previously proposed models. The conversion model of the combination of Sekoguchi simple model and the Steiner model agrees very well with the experimental result.

<p>Many space vehicles are powered by liquid hydrogen and liquid oxygen. Such fuel are cryogenic fluids, so they are easy to boil and become gas-liquid two phase flow. The LE-5B-3 engine has the capability of the idle mode firing same as the LE-5B-2 engine. Assessment of flow condition at the inlet of fuel turbo pump is important to operate the engine, because the fuel may flow in saturated condition under the idle mode in principle. In a two-phase flow state, void fraction is one of the most important parameters to assess the flow. Although many types of void fraction sensors were proposed, the capacitive technique has advantages to mount on the engine from the viewpoint of size, weight, toughness. In this study, plural circular electrodes capacitive void fraction sensor is developed for LE-5B-3 engines' ground firing test. The sensor was designed based on electric field analysis, and the specification was assessed prior to the ground test. The sensor was used in qualification test, and it was succeeded in achieving stable measurement and it helped to understand the fluid state during the engine operation. The sensor design technique, the assessment results and the ground test results are discussed in this paper.</p>

To realize high-performance cryogenic propulsion systems, the chilldown sequence has to be improved. Because the chilldown is carried out under low gravity, the effect of gravity on the two-phase flow, especially at low flow rate, should be investigated. To understand the physics under low gravity, an experiment was conducted using a sounding rocket. Two identical test sections with different mass flow rates simulated part of a turbopump, each of which has a complex flowpath including slits and a dead end. Using liquid nitrogen, the flight experiment obtained data of temperatures, pressures, void fractions, and video frames of liquid motion. Then, the flight experiment data were compared to the ground data taken under normal gravity, revealing that the slits played an important role in the chilldown process and that the test sections were quickly chilled down under low gravity. The slits of the test sections formed liquid jets, and their behaviors were different from those in the ground experiment. In the flight experiment, the jets easily reached the dead end of the test sections and cooled down the whole walls due to the increase in inertia and wettability; however, such behaviors were hardly observed in the ground experiment. The difference between the ground and flight is significant at lower flow rate.

This manuscript describes the work performed on void fraction measurements a cryogenic flow by means of a customized capacitive sensor. In a preceding activity, described in Part I, the instrument was developed and validated at room conditions. In the current study, the probe is exploited to detect the gaseous content during liquid nitrogen chilldown experiments. The sensor performances are evaluated both numerically and experimentally. The numerical simulations lead to the development of a new calibration formula improving the sensor measurement accuracy down to +/- 6.0%FS, within 99% confident interval. The experimental campaign mainly reveals a dependency of the sensor performance on the pressure and temperature variations during the cooldown of the test section. The so-called "thermal effect" therefore modeled and two compensation equations are derived. The void fraction results accordingly corrected, match the single-phase flows reference conditions within 2% discrepancy. Background light visualizations are also performed allowing the optical verification of the flow regimes. For a specific flow condition, a correlation between the recorded light intensity and the capacitive measurements is obtained. By means of the high-speed movies, the capacitive sensor response time is also evaluated to be 100 Hz.

This manuscript presents the design of a capacitive void fraction sensor for cryogenic LN2 two phase flows and its validation at room conditions. The capacitive void fraction sensor is first designed by means of Electric Field Analysis (EFA) simulations taking into account specific technical constraints coming from the test section in which it should be accommodated. Then it is manufactured and validated using a proper combination of fluids (Polydimethylsiloxane (PDMS) and air) having a dielectric constant ratio similar to the one encountered in LN2/GN2 two phase flows. The validation is performed through comparison with void fraction measured by means of optical visualizations and shows how the capacitive measurement technique robustness allows obtaining reasonable accurate values of void fraction also for the substitute fluid case. The sensor presented in this manuscript was used to evaluate the void fraction during LN2 chilldown of a rectangular cooling channel and the results are presented in the second part of this work.

To improve safety regulations for fuel cell vehicles and hydrogen infrastructures, experiments on cryo-compressed hydrogen leakage diffusion were conducted. The experimental apparatus can supply 90 MPa hydrogen at various temperature conditions (50 K-300 K) at a maximum flow rate of 100 kg/h. The hydrogen leakage flow rate was measured using pinhole nozzles with different outlet diameters (0.2 mm, 0.4 mm, 0.7 mm, and 1 mm). It was confirmed that the hydrogen leakage flow rate increases as the supply temperature decreases. To evaluate the hydrogen flow rate including the cryogenic condition, the orifice equation for liquid was found to be appropriate. The orifice flow coefficient converged to a constant value of 0.6 on the high-density condition side. The hydrogen concentration distribution was measured by injecting high-pressure hydrogen from the 0.2-mm pinhole for 10 min under a constant pressure/temperature condition. The axial hydrogen concentration distribution obtained by the ambient temperature (similar to 300 K) hydrogen injection test well agreed with the experimental formula based on previous research studies. In addition, as the hydrogen injection temperature decreased, it was found that the hydrogen concentration increased, and an empirical formula of the 1% concentration distance for the cryogenic hydrogen system was newly presented. Additional tests were conducted using pinholes of different diameters, and a 1% concentration distance was confirmed to be proportional to the hydrogen leakage flow rate to the 0.5th power. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

For assessing the risks of liquid hydrogen pump facilities, we conducted a test in which liquid hydrogen was pressurized to a maximum of 85 MPa and steadily released through a pinhole nozzle. In order to quantitatively evaluate the cooling effect of the released cryogenic hydrogen on the surrounding environment, the temperature of the hydrogen jet was measured while changing the supply pressure and temperature parameters. We applied shadowgraph flow visualization to understand the diffusion mechanism of the low-temperature hydrogen jet escaping the pinhole nozzle. The results showed that no liquid phase appeared during the cryo-compressed hydrogen leakage and that the hydrogen jet temperature could be accurately predicted using the Joule-Thomson expansion equation. The shadowgraph showed that a dense potential core was formed in the hydrogen jet even under a high-temperature condition far from the critical point (Tr = 2.4) and was characterized by a supercritical jet. In addition, it was confirmed that the boundary where the hydrogen jet becomes visible exists near the Widom line. Further consideration is required regarding the consistency of these results with the conclusions of existing studies that pseudo-boiling becomes negligible in the region wherein Pr > 10. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Space propulsion systems use screen mesh devices as filters to block contaminants and as propellant management devices to settle the propellants. The bubble point pressure indicates the basic capillary performance for liquid acquisition of screen meshes. Actively controlling the bubble point pressure can result in flexible and efficient operation of the propulsion systems. High-performance cryogenic propellants, such as liquid hydrogen and oxygen, exhibit magnetic properties. Therefore, a method to actively control the bubble point pressure of cryogenic propellants by applying a magnetic field is proposed in this study. The magnetic pressures affect the pressure balance around the gas-liquid interface separated by the screen mesh, which can thereby control the bubble point pressure. To demonstrate the concept and theoretical basis, a bubble point experiment is conducted using a ferrofluid and solenoid. This experiment proves that the magnetic field actively controls the bubble point pressure and performs both suppression and enhancement of the liquid acquisition performance of the screen mesh. The theory related to magnetic pressures is observed to successfully predict the experimental results. The feasibility of the active control of the bubble point pressure of liquid oxygen is discussed based on the validated theory, and two applications of this technique in cryogenic propulsion systems are depicted.

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was measured at the pressures of 0.4, 0.7 and 1.1 MPa for wide ranges of flow rate and liquid temperature. Nine test heaters with different inner diameters of 3, 4, 6 and 9 mm and the lengths of 50, 100, 150, 200, 250 and 300 mm were used. The DNB (departure from nucleate boiling) heat fluxes in forced flow of liquid hydrogen were measured for various subcoolings and flow velocities at pressures of 0.4, 0.7 and 1.1 MPa. Effect of L/d (ratio of heater length to diameter) was clarified for the range of L/d <= 50. A new correlation of DNB heat flux was presented based on a simple model and the experimental data. Similar experiments were performed for liquid nitrogen at pressures of 0.5 MPa and 1.0 MPa by using the same experimental system and some of the test heaters. It was confirmed that the new correlation can describe not only the hydrogen data, but also the data of liquid nitrogen.

&lt;p&gt;Reducing the amount of propellant for re-cooling is an important issue for the rocket propulsion system using cryogenic fuel. Immediately after the start of the engine, the liquid fuel boils and becomes two-phase flow. In the state of two-phase flow, the void fraction, which is the gas-liquid ratio, is one of the important value for flow control. For above problem, we are developing void fraction measurement system for the cryogenic fluid. These devices were attached to the S310-43 sounding rocket for the purpose of &quot;measuring two-phase flow behavior and heat transfer characteristics during coasting flight.&quot; These devices withstood the vibration shock test of 40G and succeeded to measure the void fraction of liquid/gas nitrogen two phase flow under vacuumed and microgravity circumstance. This report explains development and experiment results of the void fraction sensor and a capacitance amplifier. &lt;/p&gt;

&lt;p&gt;超電導推進システムを使用する水素航空機の概念検討を行った．従来のターボファンエンジンに比べてバイパス比を向上させて低騒音化と推進効率向上を図るため，超電導モータで駆動する分散ファンを備えた推進システムを想定し，設計仕様を設定した．超電導を維持するための冷媒としては，極低温の液体水素燃料を使用することを想定した．過去の超電導発電機/モータの開発事例を参考にして諸元を設定するとともに，設計回転数と分散ファン個数を変更して，質量の変化を評価した．結果として，回転子の周速を変えずに設計回転数を上昇させることで，超電導発電機/モータの寸法と質量を大幅に低減できる可能性があることを確認した．また，分散ファンについては，超電導モータの質量を低減するために，減速機と組み合わせて設計回転数をさらに向上させる必要があることを確認した．さらに，超電導発電機/モータの出力と質量の関係を整理した．&lt;/p&gt;

© Published under licence by IOP Publishing Ltd. Liquid hydrogen has excellent physical properties, high latent heat and low viscosity of liquid, as a coolant for superconductors like MgB2. The knowledge of Departure from Nucleate Boiling (DNB) heat flux of liquid hydrogen is necessary for designing and cooling analysis of high critical temperature superconducting devices. In this paper, DNB heat fluxes of liquid hydrogen were measured under saturated and subcooled conditions at absolute pressures of 400, 700 and 1100 kPa for various flow velocities. Two wire test heaters made by Pt-Co alloy with the length of 200 mm and the diameter of 0.7 mm were used. And these round heaters were set in central axis of a flow channel made of Fiber Reinforced Plastic (FRP) with inner diameters of 8 mm and 12 mm. These test bodies were vertically mounted and liquid hydrogen flowed upward through the channel. From these experimental values, the correlations of DNB heat flux under saturated and subcooled conditions are presented in this paper.

© Published under licence by IOP Publishing Ltd. The knowledge of forced flow heat transfer characteristics of liquid hydrogen (LH2) is important and necessary for design and cooling analysis of high critical temperature superconducting devices. However, there are few test facilities available for LH2 forced flow cooling for superconductors. A test system to provide a LH2 forced flow (∼10 m/s) of a short period (less than 100 s) has been developed. The test system was composed of two LH2 tanks connected by a transfer line with a controllable valve, in which the forced flow rate and its period were limited by the storage capacity of tanks. In this paper, a liquid hydrogen recirculation system, which was designed and fabricated in order to study characteristics of superconducting cables in a stable forced flow of liquid hydrogen for longer period, was described. This LH2 loop system consists of a centrifugal pump with dynamic gas bearings, a heat exchanger which is immersed in a liquid hydrogen tank, and a buffer tank where a test section (superconducting wires or cables) is set. The buffer tank has LHe cooled superconducting magnet which can produce an external magnetic field (up to 7T) at the test section. A performance test was conducted. The maximum flow rate was 43.7 g/s. The lowest temperature was 22.5 K. It was confirmed that the liquid hydrogen can stably circulate for 7 hours.

A method for reducing the time and total mass of cryogenic fluid required fora chilldown process in piping was experimentally investigated in this study. The inner wall of a pipe with an outer diameter of 1/4" (=6.35 mm) was coated with Polytetrafluoroethylene, which has a low thermal conductivity. Liquid nitrogen (LN2) was supplied to the pipe at a constant tank pressure of 120-170 kPa. The fluctuations of the two-phase flow, which were composed of LN2 and gas phase nitrogen, were observed. A pipe without an insulating layer and three other pipes with insulating layers of thicknesses 23 m, 63 Am, and 91 mu m, respectively, were used in the experiment. The results indicated that the temperature of the minimum heat flux point (MHF) was higher for the pipe with the insulating layer. This increased temperature caused earlier transition to nucleate boiling. Furthermore, the total mass of LN2 consumed in the chill down process could be retrenched up to a maximum of 64%. The heat flux decreased after reaching the MHF point; however, heat flux after MHF point is not dominant to overall chilldown time. The effect of the layer to increase the temperature of MHF point is dominant to overall chilldown time, which results in the decrease in the chilldown time and the total mass of LN2 consumed in the chilldown process. (C) 2017 Elsevier Ltd. All rights reserved.

<p>本研究では，水素社会の普及に対応する将来の航空機システムについて，主に推進系の比較検討を行った．水素は単位質量あたりの発熱量が高く，搭載燃料を軽量にできる利点があるが，密度が低くタンク容積が大きくなることが欠点となっている．また，燃料電池自動車等に用いられている固体高分子形燃料電池（PEFC）等による電力変換が可能なこと，極低温の液体水素は冷熱源として利用しやすく，電動機器を超電導としやすいことも特徴としてあげられる．これらを踏まえ，化石燃料を使用し燃焼により動力を得る現在の推進システムをベースに，推進の電動化を含む将来の推進系候補を17ケース選定し，推進系を構成する要素の効率，重量見積もりから航続距離の簡易的な比較を行った．この結果，飛行速度の低い小型機において，液体水素を利用するケースが将来技術レベルにおいて競争力を得やすい結果となった．</p>

The payload capacity of launch vehicles must be increased in order to extend space exploration and development beyond low-Earth orbit into the solar system. A propellant system using a cryogenic fluid such as liquid oxygen or liquid hydrogen must reduce the amount of unusable propellant due to evaporation and boiling. However, in space exploration and development, where the safety and reliability of missions are critical, predictions of boiling heat transfer using existing technology are not sufficiently reliable for thermal management design, given the lack of pertinent knowledge and relevant research. Therefore, the objective of this research is to understand and accurately predict boiling heat transfer by developing numerical simulation tools for two-phase flows that consider phase change. This paper presents a recent research activity toward the development of chill-down process simulation technology. The cryogenic chilldown experiment conducted in a vertical pipeline and in a complicated channel was verified via simulation to show the effectiveness of the simulation tool under development.

<p>JAXA has constructed an experimental facility to pressurize and supply liquid hydrogen at a maximum pressure of 90 MPa to conduct experimental research on the injection of high pressure liquid hydrogen into the atmosphere. Liquid hydrogen has a property that its density greatly changes depending on pressure despite being a liquid phase. In addition, the high pressure hydrogen gas is in a supercritical state and has an intermediate property between a gas and a liquid. Therefore, it is a difficult question whether to treat the injection of high pressure liquid hydrogen as a gas phase phenomena or as a liquid phase phenomena. As a result of the experiment, it was found good to apply the liquid orifice equation to predict the discharge flow rate of high pressure liquid hydrogen.</p>

© 2002-2011 IEEE. Liquid hydrogen (LH2) has excellent properties such as large latent heat, low viscosity, and so on. Because of these properties, LH2 is expected to be used as a coolant for a high-critical-temperature superconductor (HTS), including MgB2. The over-critical-current properties of an HTS are important for the stable designs of HTS devices cooled by LH2; however, the temperature change of a sheathed superconducting wire cannot be revealed only by current-voltage properties beyond critical current, since the current-sharing ratio among MgB2 and sheath materials is not unknown. In this study, transient over-critical-current tests were performed on a short MgB2 sample cooled by LH2 under magnetic field, and the sample temperatures were calculated from test data. In addition, the test sample temperature based on the test was compared with that of the computer simulation result. It was found that the temperature change of the wire just after the transport current exceeds the critical current is affected by external magnetic fields.

A capacitance-based void fraction sensor has been developed for the rocket or airbreathing engines, which is simple and do not disturb the flow. Typical conventional sensors usually have two concave electrodes mounted on the outer wall of the dielectric tube. They are relatively low accuracy if they have a noise shield; the maximum measurement error is over 30% in our research. The aim of this study is to improve the measurement accuracy while keeping the advantage of simplicity, mountability and non-intrusive characteristics. A theoretical formulae and electromagnetic field analysis, EFA, are used to design the sensors and are compared to an experiment using air/silicon-oil mixture flow. As the result, a newly developed asymmetrical type sensor which consists of asymmetric flat electrodes with side walls shows good performance; the inaccuracy between true void fraction and measured void fraction is 6% for the stratified flow.

<p>The payload capacity of launch vehicles must be increased in order to allow the exploration and development of space to be extended from low-Earth orbit into the solar system. A propellant system using a cryogenic fluid must reduce the amount of unusable propellant due to evaporation and boiling. However, in space exploration and development, where safety and reliability of missions are critical, predictions of the boiling heat transfer of current technology are not sufficiently reliable for thermal management design due to a lack of knowledge and relevant research. Therefore, the objective of this research is to understand and accurately predict boiling heat transfer by developing numerical simulation tools for two-phase flows that consider phase change. In this paper, recent research activity toward the development of chill-down process simulation technology is presented.</p>

© 2016 The Authors. High-Tc (HTC) superconductors including MgB2 will show excellent properties under temperature of Liquid Hydrogen (LH2:20K), which has large latent heat and low viscosity coefficient. In order to design and fabricate the LH2 cooled superconducting energy devices, we must clear the cooling property of LH2 for superconductors, the cooling system and safety design of LH2 cooled superconducting devices and electro-magnetic property evaluation of superconductors (BSCCO, REBCO and MgB2) and their magnets cooled by LH2. As the first step of the study, an experimental setup which can be used for investigating heat transfer characteristics of LH2 in a pool and also in forced flow (circulation loop with a pump), and also for evaluation of electro-magnetic properties of LH2 cooled superconductors under external magnetic field (up to 7 T). In this paper, we will show a short sketch of the experimental set-up, practical experiences in safety operation of liquid hydrogen cooling system and example test results of critical current evaluation of HTC superconductors cooled by LH2.

The research and development conducted regarding fully superconducting motors for liquid-hydrogen transfer pumps using MgB&lt;sub&gt;2&lt;/sub&gt; monofilamentary wires are gathered into this paper. Synchronous rotation without slip can be realized using squirrelcage-type rotor windings composed of quasi-superconducting loops soldered between MgB&lt;sub&gt;2&lt;/sub&gt; wires, thereby enabling superconducting motors with low loss and high efficiency to be achieved. The use of MgB&lt;sub&gt;2&lt;/sub&gt; wires for stator windings also enables the primary winding loss to be reduced drastically compared to conventional copper winding. The transfer of liquid hydrogen from a metal cryostat containing the fabricated MgB&lt;sub&gt;2&lt;/sub&gt; motor to a glass Dewar vessel for visual monitoring is demonstrated successfully.&lt;tt&gt; &lt;/tt&gt;

High-Tc (HTC) superconductors including MgB2 will show excellent properties under temperature of Liquid Hydrogen (LH2:20K), which has large latent heat and low viscosity coefficient. In order to design and fabricate the LH2 cooled superconducting energy devices, we must clear the cooling property of LH2 for superconductors, the cooling system and safety design of LH2 cooled superconducting devices and electro-magnetic property evaluation of superconductors (BSCCO, REBCO and MgB2) and their magnets cooled by LH2. As the first step of the study, an experimental setup which can be used for investigating heat transfer characteristics of LH2 in a pool and also in forced flow (circulation loop with a pump), and also for evaluation of electro-magnetic properties of LH2 cooled superconductors under external magnetic field (up to 7 T). In this paper, we will show a short sketch of the experimental set-up, practical experiences in safety operation of liquid hydrogen cooling system and example test results of critical current evaluation of HTC superconductors cooled by LH2. (C) 2016 The Authors. Published by Elsevier Ltd.

Film boiling heat transfer coefficients in liquid hydrogen were measured for the heater surface superheats to 300 K under pressures from 0.4 to 1.1 MPa, liquid subcoolings to 11 K and flow velocities to 8 m/s. Two test wires were both 1.2 mm in diameter, 120 mm and 200 mm in lengths and were made of PtCo alloy. The test wires were located on the center of 8 mm and 5 mm diameter conduits of FRP (Fiber Reinforced Plastics). Furthermore film boiling heat transfer coefficients in liquid nitrogen were measured only for the 200 mm long wire. The film boiling heat transfer coefficients are higher for higher pressure, higher subcooling, and higher flow velocity. The experimental data were compared with a conventional equation for forced flow film boiling in a wide channel. The data for the 8 mm diameter conduit were about 1.7 times and those for the 5 mm conduit were about 1.9 times higher than the predicted values by the equation. A new equation was presented modifying the conventional equation based on the liquid hydrogen and liquid nitrogen data. The experimental data were expressed well by the equation.

Transient heat transfers from Pt-Co wire heaters inserted into vertically-mounted pipes, through which forced flow subcooled liquid hydrogen was passed, were measured by increasing the exponential heat input with various time periods at a pressure of 0.7 MPa and inlet temperature of 21 K. The flow velocities ranged from 0.3 to 7 m/s. The Pt-Co wire heaters had a diameter of 1.2 mm and lengths of 60 mm, 120 mm and 200 mm and were inserted into the pipes with diameters of 5.7mm, 8.0 mm, and 5.0 mm, respectively, which were made of Fiber reinforced plastic due to thermal insulation. With increase in the heat flux to the onset of nucleate boiling, surface temperature increased along the curve predicted by the Dittus-Boelter correlation for longer period, where it can be almost regarded as steady-state. For shorter period, the heat transfer became higher than the Dittus-Boelter correlation. In nucleate boiling regime, the heat flux steeply increased to the transient CHF (critical heat flux) heat flux, which became higher for shorter period. Effect of flow velocity, period, and heated geometry on the transient CHF heat flux was clarified.

Forced flow heat transfer of hydrogen from a round wire in a vertically-mounted pipe was measured at pressure of 1.5 MPa and temperature of 21 K by applying electrical current to give an exponential heat input (Q=Q0exp(t/τ),τ=10 s) to the round wire. Two round wire heaters, which were made of Pt-Co alloy, with a diameter of 1.2 mm and lengths of 54.5 and 120 mm were set on the central axis of a flow channel made of FRP with inner diameter of 5.7 and 8.0 mm, respectively. Supercritical hydrogen flowed upward in the channel. Flow velocities were varied from 1 to 12.5 m/s. The heat transfer coefficients of supercritical hydrogen were compared with the conventional correlation presented by Shiotsu et al. It was confirmed that the heat transfer coefficients for a round wire were expressed well by the correlation using the hydraulic equivalent diameter.