Tetsuya Suekane

Numerical studies on the nonequilibrium inductively coupled plasma with metal vapor ionization

Nonequilibrium inductively coupled plasma (ICP) where cesium metal atoms contained in argon gas as a seed material are dominantly ionized is investigated with two-dimensional (2-D) numerical calculations which are based on a fully time-dependent (FTD) model and sinusoidal approximation (SA) model. Calculation results with the FTD model indicate that the amplitude of electron temperature oscillation over one radio frequency cycle is below about 120 K for the mean value of about 5600 K, and that of the electron number density is negligible due to its long relaxation time. Results with the SA model coincide with that with the FTD model, and it is valid to predict the plasma properties with the SA model. Under the suitable operating conditions, the region where electron number density is kept constant is formed and extended in the ICP, since the electron temperature ranging from 4000 to 6000 K is realized, and cesium atoms are fully ionized while the ionization of argon atoms is not significant. Since no current is induced at the center axis of ICP, the density profile around the axis is almost determined by the diffusion of electrons from the region of full seed ionization.

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.

The effects of boundary layer phenomena on the performance of disk CCMHD generator

Two dimensional calculations were carried out to clarify the behavior of boundary layer and its effects on performance of closed cycle MHD (CCMHD) generator and to investigate the relation between enthalpy extraction ratio and adiabatic efficiency. Calculation results suggest that the large Lorentz force causes propagation and separation of boundary layer where reverse current flows, because of small electromotive force. For large load resistance boundary layer becomes very thick and the eddy current arises in broad region. The push work of working gas against Lorentz force is effectively converted into electric energy under the condition at which the Lorentz force decelerates the working gas to Mach number in the range between 1.0 and 1.5 in this case of the generator. Stagnation pressure loss increases with load resistance until enthalpy extraction ratio takes maximum value. The entropy production due to Joule heating and viscosity increases with load resistance. The difference between the load resistances for which the enthalpy extraction ratio and the adiabatic efficiency take maximum value can be explained with the entropy production of Joule heating and viscosity. >

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.

Microtomography of Imbibition Phenomena and Trapping Mechanism

Water imbibition during the waterflooding process of oil production only sweeps part of the oil present. After water disrupts the oil continuity, most oil blobs are trapped in porous rock by capillary forces. Developing an efficient waterflooding scheme is a difficult task; therefore, an understanding of the oil trapping mechanism in porous rock is necessary from a microscopic viewpoint. The development of microfocused X-ray CT scanner technology enables the three-dimensional visualization of multiphase phenomena in a pore-scale. We scanned packed glass beads filled with a nonwetting phase (NWP) and injected wetting phase (WP) in upward and downward injections to determine the microscopic mechanism of immiscible displacement in porous media and the effects of buoyancy forces. We observed the imbibition phenomena for small capillary numbers to understand the spontaneous imbibition mechanism in oil recovery. This study is one of the first attempts to use a microfocused X-ray CT scanner for observing the imbibition and trapping mechanisms. The trapping mechanism in spontaneous imbibition is determined by the pore configuration causing imbibition speed differences in each channel; these differences can disrupt the oil continuity. Gravity plays an important role in spontaneous imbibition. In upward injection, the WP flows evenly and oil is trapped in single or small clusters of pores. In downward injection, the fingering phenomena determine the amount of trapped oil, which is usually in a network scale. Water breakthrough causes dramatic decrease in the oil extraction rate, resulting in lower oil production efficiency.

Behaviour of fully ionized seed plasma excited by microwave

Nonequilibrium plasmas with cesium metal vapor ionization in helium and argon gases at moderate pressures are excited with microwave power. The structures and behaviour of the seeded plasmas are experimentally examined, particularly under the condition of Full seed (cesium atoms) ionization. By cesium seeding, the minimum power sustaining the plasma is reduced markedly, and both a broad plasma observed in pure helium and unsteady filament-like plasmas in pure argon change to the steady and broad plasma locating close to the inner surface of a discharge tube, it is revealed from the electron temperature measurements that the plasma can be in the regime of full seed ionization for suitable microwave powers, where the electron density is kept almost constant. The thickness of the fully ionized seed (FIS) plasma decreases with increasing the mole fraction of cesium vapor, and is almost independent of noble gas pressure. The thickness almost coincides with the skin depth determined from the electrical conductivity almost uniform in the FIS plasma. These facts suggest that the FIS plasma will be easily produced and maintained as long as the microwave power is consumed to the electron heating.

High-enthalpy extraction demonstration with closed-cycle disk MHD generators

Recent results of high-enthalpy extraction experiments with closed-cycle MHD disk generators are described. Power generation experiments were carried out with two types of facility: the shock tube facility with the Disk-IV generator and the FUJI-1-blow-down facility. In the shock tube experiments, the effect of channel shape on generator performance was studied using helium seeded with cesium as working fluid. A more divergent channel shape was effective in sustaining a high Hall field throughout the channel and achieving high generator performance even under the strong MHD interaction. High-enthalpy extraction of 27.3% was achieved. Furthermore, these experimental results agree well with the result of one-dimensional calculations. In the FUJI-1 blow-down experiments, the effect of stagnation gas pressure on performance was studied with a working gas of seeded argon. The highest enthalpy extraction ratio of 15.7% was achieved with the lower stagnation pressure of 0.46 MPa, whereas the largest output power of 516.7 kW and power density of 70 MW/m{sup 3} were extracted with the nominal stagnation pressure of 0.6 MPa. This suggested the possibility of a part-load operation without significant degradation of generator performance by reducing stagnation pressure.

Velocity measurement of clay intrusion through a sudden contraction step using a tagging pulse sequence

Magnetic resonance imaging (MRI) with a spatial tagging sequence was used to measure the velocity distribution of clay that was forced past a sudden contraction. A spatial tagging sequence provided magnetic resonance images of clay that allowed measurement of the velocity distribution in the clay, which can provide profound insights on the deformation process of clay during the intrusion process. The experiments were conducted using a specially-designed vessel that could operate at up to 30 MPa. The vessel offers a rectangle test section with a sudden contraction step that had a ratio of contraction of 2:1. The vessel was installed into a commercial magnetic resonance imaging equipment and then the fluid motion of clay flowing into the narrow contracted channel was quantitatively investigated to examine behaviors of flowing clay as non-Newtonian fluid. MRI results are compared with those obtained by computational fluid dynamics (CFD) calculation. Velocity distributions obtained from each tag displacement did not well agree with those predicted by CFD results near the contraction step where the fluid accelerated rapidly. However, a post-processing on calculation results, in which virtual tag displacement is calculated, gave better agreement with experiment and enabled us to compare MRI results with CFD results.

Effect of Buoyancy on Pore-Scale Characteristics of Two-Phase Flow in Porous Media

Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. The global atmospheric concentration of CO2 has increased from a pre-industrial value of approximately 280 ppm to 379 ppm in 2005. The warming of the climate system is unequivocal, as is now evident from observations of increasing global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level (IPCC, 2007a). To stabilize the concentration of CO2 in the atmosphere, emissions need to peak and then decline thereafter. In the long term, energy conservation, efficiency improvements in energy conversion, lower carbon fuels such as natural gas and renewable energy sources are the most promising alternatives. For lower stabilization targets, scenarios put more emphasis on the use of low-carbon energy sources, such as renewable energy and nuclear power, and the use of CO2 capture and storage (CCS; Pacala & Socolow, 2004; IPCC, 2007b); however, the transition from the current dependence on fossil fuels would take many decades. The capture of CO2 from fossil fuel power plants and other large-scale stationary emission sources and storage in geologic formations is the only option that permits a transition from current high-intensity carbon-based energy sources to low-carbon energy sources. The safety of geologic storage of CO2 is obviously a central concern in planning carbon sequestration on a large scale. The current concept of geologic storage involves the injection of CO2 into deep formations, which typically contain brine. CO2 is supercritical at temperatures and pressures above the critical values of 304 K and 7.38 MPa. In typical geologic formations, the critical condition of CO2 is reached at a depth of approximately 740 m. Because of geologic pressure, the density of CO2 dramatically increases with depth; however, the density of CO2 is approximately 0.9 times that of water, so when CO2 is injected into the subsurface, buoyancy tends to bring CO2 upward in geologic formations. On the other hand, CO2 will be retained by physical and geochemical mechanisms, such as physical trapping (IPCC 2005), capillary trapping (Suekane et al. 2008, 2010a; Al Mansoori et al. 2010; Pentland et al. 2010; Zhou et al. 2010; Wildenschild et al. 2010; Saadatpoor 2010), solubility trapping (Lindeberg & Wessel-Berg 1997; McPherson & Cole 2000; Ennis-King et al. 2003; Gilfillan et al. 2009; Iding & Blunt 2010) and mineralization (Gunter et al. 1993). Capillary trapping is sometimes referred to as residual gas trapping or relative permeability hysteresis trapping. When CO2 is injected into the subsurface, it spreads in geologic