Kenji Nakamichi

Study on the development of a capacitance-type flowmeter for slush hydrogen

Abstract Slush hydrogen is a two-phase solid–liquid cryogenic fluid consisting of solid hydrogen particles in liquid hydrogen, various applications for which are anticipated, including fuel for reusable space shuttles. Noting the slight fluctuation in the distribution of solid hydrogen particles in slush hydrogen, i.e., slush hydrogen density, the authors of the current study placed capacitance-type densimeters at two locations along the piped flow to measure the density and to detect density fluctuations. The flow velocity was also calculated from the densimeter distance and delay time when the cross correlation function of the two density signals was at a maximum. This capacitance-type flowmeter has no moving mechanical parts and no probe in the flow stream. This paper reports the development of a prototype slush hydrogen capacitance-type flowmeter, together with the fabrication of required test equipment, and confirmation of flowmeter accuracy.

Development of a microwave-type densimeter for slush hydrogen

Abstract Slush hydrogen is a two-phase solid–liquid cryogenic fluid consisting of solid hydrogen particles in liquid hydrogen, various applications for which are anticipated, including fuel for reusable space shuttles. The authors of the current study have measured the density of slush hydrogen by using the phase shift that takes place when microwaves are propagated through slush hydrogen, i.e., using the change in the specific dielectric constant. This new technique, unlike the conventional method using a waveguide and horn antenna, features a coaxial cable and patch antenna that can be used at cryogenic temperatures, leading to the development of a slush hydrogen densimeter with a high accuracy of within ±0.5%.

Fundamental investigation of a superconducting level sensor for liquid hydrogen with MgB2 wire

The feasibility study of a superconducting level sensor for liquid hydrogen with a magnesium-diboride (MgB2) wire is carried out from an experimental point of view. The sample wire consists of a mono-cored MgB2 superconductor and a cupronickel sheath, and several potential taps are attached to it at even intervals in order to understand the position of a threshold between the superconducting and resistive states roughly. The fabricated sensor is vertically located in a glass dewar vessel with an infill of liquid hydrogen, and the position of a preselected potential tap is adjusted by eye and hand to liquid level before starting a new measurement. Simulated operations with constant currents finally yield the future possibilities as the level sensor for liquid hydrogen with MgB2 wire although the fabricated sensor has a few problems at present. In order to improve the performance of the sensor, the specifications required for MgB2 wires will be reported elsewhere by applying the stability theory in superconductor composites and by simulating the operation with a numerical code.

Experimental Study on Magnetic Refrigeration for the Liquefaction of Hydrogen

An experimental study of magnetic refrigeration has been conducted for the liquefaction of hydrogen. The magnetic material selected is GGG (Gd3Ga5O12). The experimental magnetic refrigeration system principally consists of a superconducting pulsed magnet, a Gifford-McMahon type refrigerator as heat expelling portion and a thermal siphon type heat pipe as a thermal switch on the low temperature side. In the GGG heat-pipe system, gaseous hydrogen coming up from liquid hydrogen reservoir condenses on the GGG surface, then falls down to the reservoir. Liquefaction rate is measured from the input power of the electrical heater in the hydrogen bath. The authors try to find the suitable gap width of the GGG heat-pipe system considering heat capacity around GGG and interaction between liquefied and gaseous hydrogen. Subsequently we conducted a hydrogen liquefaction test and obtained a liquefaction rate of 3.55 g/h (50 cc/h) or refrigeration power of 0.4 W at 20.3 K, and these experimental results agreed with the calculated results of cycle simulation.