Tetsuji Okamura

Magnetohydrodynamic power generation experiments with Fuji-1 blowdown facility

Experimental results of nonequilibrium plasma magnetohydrodynamic (MHD) power generation from the Fuji-1 blowdown facility are presented. The Fuji-1 experiments have been conducted since 1983, and the disk-shaped MHD generators called Disk-F3a, F3r, and F4 have been tested during the last decade. In the experiment with the newest Disk-F4 generator, which was designed based on the results of previous experiments with the Disk-F3a and F3r generators, an output power of 506 kW and an enthalpy extraction ratio of 18.4% for thermal input of 2.75 MW were simultaneously obtained. During the experiment, however, the deposition of seed material on the electrodes and the insulator walls was revealed, and large amount of impurity (water vapor) contamination in the working gas was detected. In the last experiment the stainless-steel-coated anodes were used instead of copper anodes to prevent the seed material from depositing on the generator walls, and the impurity contamination was reduced by increasing the bottom temperature of the heat exchanger. Consequently, an output power of 544 kW for thermal input of 3.38 MW and an enthalpy extraction ratio of 18.9% for thermal input of 2.17 MW were successfully demonstrated. Each value is the highest among those achieved with the Fuji-1 facility.

Measurements of properties concerning isentropic efficiency in a nonequilibrium MHD disk generator

The isentropic efficiency and the effective Hall parameter in a nonequilibrium disk MHD generator have been successfully evaluated on the basis of the experiments under high enthalpy extraction conditions. Special attention is devoted to measuring the exit total pressure and the Faraday current. The maximum isentropic efficiency achieved in the present experiments was 46% with the enthalpy extraction ratio of 31.6%. The experimentally obtained values of the effective Hall parameter covered a range of 2-3.

Improvement of enthalpy extraction over 30% using a disk MHD generator with inlet swirl

Abstract Performance of a disk-type CCMHD generator has been investigated. In the present power generation experiments, special attention has been devoted to the effect of inlet swirl on generator performance. In order to clarify this effect, one-dimensional numerical simulation has also been performed. The results showed that the enthalpy extraction ratio was successfully improved up to 32.5% by employing inlet swirl with the swirl ratio S(= U 0 U r ) of 1. Static pressure and Hall potential distributions were measured and showed that the inlet swirl reduced the steep increase in static pressure owing to the strong Lorentz force at the inlet of the disk channel. Hence, relatively low static pressure and, thus, high Hall parameter were kept throughout the channel, and therefore, the remarkable increase in output performance was achieved. Numerical simulation can qualitatively confirm these experimental results well. Realization of high Hall parameter and high enthalpy extraction with smaller pressure ratio of inlet to exit for the case with inlet swirl led to improvement of the isentropic efficiency.

Heat transport in He II channel with phase transition

Abstract Experimental and numerical investigations have been carried out on the heat transport characteristics in a channel where atmospherically pressurized He II coexists with He I. The channel has a rectangular cross-section and both ends are open to the bath. One of the side walls of the channel is heated uniformly. The channel is placed vertically or horizontally in the bath. The experimental results are analysed by two-dimensional time-dependent numerical calculations. An explicit finite element scheme is applied to the two-fluid equations which include the Gorter-Mellink mutual friction term. It is found that the applied heat is transported by one-dimensional internal convection through the channel filled with single phase He II. Once He I is generated in the channel, however, two-dimensional flow occurs due to natural convection and this influences the transport of heat.

Experimental studies of closed cycle MHD power generation with FUJI-1 blow-down facility

Abstract Recent results of power generation experiments with an improved heat exchanger system in the FUJI-1 facility were described. One of the main purposes was to study the effect of working gas temperature on generator performance. The results with argon working gas showed that the gas temperature of 1850 K is enough to eliminate the effect of inlet relaxation under the present experimental conditions and that gas temperature does not greatly affect the output performance so long as the inlet relaxation is not significant. The radial component of velocity was successfully measured with high time resolution by means of the cross-correlation method. The effect of seed fraction on the measured velocity was discussed. For the case of helium working gas, the voltage drop owing to an inlet relaxation was remarkably decreased, and improvement in both output power and enthalpy extraction can be observed by the increase of gas temperature. The voltage drop still existed at the inlet of the channel, and therefore, higher gas temperature and higher seed fraction are required in order to achieve higher generator performance.

High enthalpy extraction experiments with Fuji-1 MHD blow-down facility

Abstract Recent experimental results of closed cycle MHD electrical power generation with the “Fuji-1” blow-down facility are presented. In the experiment with Disk-F4 MHD generator, which was conducted with a modified seed injection system in 1997, an enthalpy extraction ratio of 18.4% was successfully demonstrated with a large output power of 506 kW. This enthalpy extraction ratio is the highest among those achieved with the Fuji-1 facility. The experimental results also revealed the electrical characteristics of the generator installed in the blow-down facility. The decline in the output power and its recovery were observed at the early stage of the power generation run. This fact could be attributed to the attachment of seed material to the generator walls and to its detachment, related to the relatively slow rise in temperature on the wall surface. It was verified for the first time in the Fuji-1 experiment that the reduction of impurity contamination resulted in improvement in the generator performance.

Results of the Excitation Test of the LHD Helical Coils Cooled by Subcooled Helium

Large helical device, the largest superconducting stellarator, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is produced by a pair of pool-cooled helical coils, in the innermost layers of which a normal-zone had been induced several times at the bottom of the coil at higher currents than 11.0 kA. Since the field is not the highest there, the local cooling conditions are probably deteriorated by bubbles gathered by buoyancy. In order to improve the cryogenic stability by subcooling, an additional cooler with two-stage cold compressors was installed at the inlet of the coil in 2006. The inlet and outlet temperatures of the coils were successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. In spite of a half charging rate to reduce AC losses, a normal-zone was induced near the top of the coil at 11.45 kA. It propagated to one side and stopped near the inner equator, where the field is the highest. In comparison with the stability tests with a model coil, the local temperatures of the innermost layers near the top is considered to have been raised up to almost the saturated temperature of 4.4 K by charging. The excitation method was revised to waiting cool-down at 11.0 kA, and the excitations up to 11.5 kA have been attained.

Performance improvement of a two-stage GM cryocooler by use of Er(Ni0,075Co0.925)2 magnetic regenerator material

Abstract A newly developed magnetic regenerator material Er(Ni 0.075 Co 0.925 ) 2 , having high specific heat at temperatures from 10 to 20 K, has been tested inside the second regenerator of a two-stage GM cryocooler. Different working conditions have been examined: four Er(Ni 0.075 Co 0.925 ) 2 /Pb material ratios, three displacer reciprocating speeds, and two cryocooler stroke lengths. In the best working conditions, the experimental results show a cooling power improvement up to 15% over the whole temperature range from 10 to 20 K compared to that of the same GM cryocooler employing only lead.

Effect of load segmentation on the performance of a nonequilibrium disk MHD generator

Power generation experiments of a nonequilibrium disk MHD generator have been conducted under segmented load conditions. Effects of the load segmentation on the plasma fluid behavior and the generator performance are made clear. The MHD channel is divided into upstream and downstream parts. Although an MHD interaction along the fluid flow is divided by a middle electrode, each individual power generating part influences each other electrically and fluid dynamically. A higher loading condition in the upstream part is preferable in comparison with that in the downstream one to obtain higher electrical conductivity of the plasma. An excessive enthalpy extraction from the upstream part deteriorates the isentropic efficiency of the generator. It is possible for the segmented loading to maintain a comparable performance to that of the single-loading case by controlling each electrical efficiency in the segmented generation channels.

PERFORMANCE OF UPGRADED COOLING SYSTEM FOR LHD HELICAL COILS

Helical coils of the Large Helical Device (LHD) are large scale superconducting magnets for heliotron plasma experiments. The helical coils had been cooled by saturated helium at 4.4 K, 120 kPa until 2005. An upgrade of the cooling system was carried out in 2006 in order to improve the cryogenic stability of the helical coils and then it has been possible to supply the coils with subcooled helium at 3.2 K, 120 kPa. A designed mass flow of the supplied subcooled helium is 50 g/s. The subcooled helium is generated at a heat exchanger in a saturated helium bath. A series of two centrifugal cold compressors with gas foil bearing is utilized to lower the helium pressure in the bath. The supplied helium temperature is regulated by rotational speed of the cold compressors and power of a heater in the bath. The mass flow of the supplied helium is also controlled manually by a supply valve and its surplus is evaporated by ten heaters at the outlet above the coils. In the present study, the performance of the cooli...