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In an actual boiling channel, the circumferential heat flux is not uniform. Thus, the understanding of the heat transfer characteristics of non-uniformly heated tube becomes an important design factor for conventional boilers, esp...
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In an actual boiling channel, the circumferential heat flux is not uniform. Thus, the understanding of the heat transfer characteristics of non-uniformly heated tube becomes an important design factor for conventional boilers, especially for the compact water-tube boiler with tube-nested combustor. The small compact boiler is operated at low-pressure and low-mass flux condition compared with the large scale boiler, so that the non-uniformity in heat flux may strongly affect the heat transfer characteristics. In this investigation, non-uniform heat flux distribution along the circumferential direction was generated by using Joule heating of SUS304 eccentric tubes. The heated length of test-section was 900mm, inner diameter was 20mm, and outer diameter was 24mm. The eccentricity of the tube was s=0, 0.5, 1.0 and 1.5mm. Corresponding maximum/minimum heat fluxes rations were 1.0, 1.7, 3.0 and 7.0, respectively. Experiments were conducted for upward and downward flows, with system pressures 0.3 and 0.4MPa, mass flux range 10-100kg/m~2s and inlet temperatures 30 and 80deg.C. The eccentricity resulted in an increase in the CHF of the upward flow, while the effect was hardly observed in the downward flow. The effect of eccentricity on the CHF was well interpreted by the liquid film redistribution. In this paper, the local heat transfer coefficient along the circumferential direction was also presented.
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To further our understanding of the solar wind entry across the magnetopause under northward IMF, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortex event on 24 March 1995. We have found that the protons consist of...
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To further our understanding of the solar wind entry across the magnetopause under northward IMF, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortex event on 24 March 1995. We have found that the protons consist of two separate (cold and hot) components in the magnetosphere-like region inside the KH vortical structure. The cold proton component occasionally consisted of counter-streaming beams near the current layer in the KH vortical structure. Low-energy bidirectional electron beams or flat-topped electron distribution functions in the direction along the local magnetic field were apparent on the magneto sphere side of the current layer. We discuss that the bidirec tionality of the electrons and the cold proton component implies magnetic reconnection inside the KH vortical structure. In addition, we suggest selective heating of electrons inside the vortical structure via wave-particle interactions. Comparing temperatures in the magnetosphere-like region inside the vortical structure with those in the cold plasma sheet, we show that further heating of both the electrons and the cold proton component is taking place in the cold plasma sheet or on the way from the vortices to the cold plasma sheet.
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As part of basic research on the flow characteristics of a two-phase mixture pool under severe accident of fast breeder reactor (FBR), visualization and measurement of nitrogen gas-molten lead/bismuth two-phase flow in a rectangul...
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As part of basic research on the flow characteristics of a two-phase mixture pool under severe accident of fast breeder reactor (FBR), visualization and measurement of nitrogen gas-molten lead/bismuth two-phase flow in a rectangular pool were performed by using the neutron radiography technique. Measurements of drag coefficient of a single bubble and bubble shape regime showed that the relationship between the shape, size and the rising velocity of a single isolated nitrogen bubble in the molten lead/bismuth was not much different from that for an ordinary one. Appropriate correlation for drift velocity and drag coefficient between phases were recommended based on the drift flux correlation of measured pool void fraction. One- and two-dimensional analyses were performed by using a next generation computational code for safety analysis of severe accident of FBRs, SIMMER-III with various drag coefficient models. It was revealed that Kataoka-Ishii's equation was suitable basically for estimation of drift velocity, namely, drag force between phases.
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We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substor...
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We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)-Z(proxy) coordinates.
The results show that the Poynting flux toward the plasma-sheet center starts at least 10Omin before the substorm onset, and is further enhanced at X～-12 R_E (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X～- 11 R_E and X～-19 R_E and a duskward flow around X=-10 R_E were enhanced. The total pressure starts to decrease around X=-16 R_E about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward flows commence, characterised by southward magnetic fields in the form of a plasmoid.
We confirm various observable-parameter variations based on or predicted by the relevant substorm models; however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic recon-nection. After magnetic reconnection is induced, it plays a significant role in driving the substorm.
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Power spectra of a spin-transfer nano-oscillator, in which a reflection layer is included, are measured for current densities up to 1.2 × 10~7 A/cm~2. A clear oscillation peak, which is slightly asymmetric with respect to the pea...
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Power spectra of a spin-transfer nano-oscillator, in which a reflection layer is included, are measured for current densities up to 1.2 × 10~7 A/cm~2. A clear oscillation peak, which is slightly asymmetric with respect to the peak frequency, is observed at 5 GHz under the external field of 700 Oe. The large red shift of the peak frequency on increasing the current, which has been reported in nano-contact devices, is not observed. The relatively low oscillation frequencies as well as the absence of the large red shift indicate the reduction in the demagnetizing field in the free layer. The linewidth decreases with the current and is inversely proportional to the square of the precession amplitude of magnetization in accordance with the recent linear theories.
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With two electron beam sources, we have tested two new Hamamatsu [Hamamatsu Photonics K.K., Shizuoka,Japan ] avalanche photodiodes (APDs) of spl 3988 and spl 6098 to detect electron beams up to 100 keV. Though our previous results...
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With two electron beam sources, we have tested two new Hamamatsu [Hamamatsu Photonics K.K., Shizuoka,Japan ] avalanche photodiodes (APDs) of spl 3988 and spl 6098 to detect electron beams up to 100 keV. Though our previous results showed the effectiveness and the advantage of an APD to measure 2-40 keV electrons, its upper limit was not high enough to detect so-called medium-energy electrons. In addition to the limitation of its detectable range, the response at different energies was also not linear. These newly developed APDs, which have thicker depletion-layers, provide full coverage of this missing range along with a good linearity. The depletion-layer thickness was increased to 140 μm for both APDs, the dead-layer of spl 3988 became 10 times thicker than that of spl 6098. The thin-surface dead-layer and thick depletion-layer of spl 6098 allows the detection of electrons from 3keV up to 100 keV with a good linearity and with an excellent energy resolution of 4 keV at 100-keV electrons. The wide dynamic range from 3 keV to 100 keV of those APDs will increase their appeal in detecting electrons for space plasma research.
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The silicon core and carbon mantle particles were produced by using the advanced carbon coating method which enables direct covering with the carbon layer using an electron microscope. The growth of SiC crystal was observed upon h...
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The silicon core and carbon mantle particles were produced by using the advanced carbon coating method which enables direct covering with the carbon layer using an electron microscope. The growth of SiC crystal was observed upon heating at 500℃ in vacuum. The growth process of SiC on both the carbon layer and silicon particles was directly observed by in situ observation. The inward movement of carbon into silicon began at the twinned part. The growth rate of SiC on the carbon mantle layer was estimated from in situ images and found to be 8 times faster than the growth rate of silicon carbide in silicon particles.
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GaN films were grown at various V/III ratios on sapphire (0001) substrates with and without nitridation process by radio-frequency plasma-excited molecular beam epitaxy. Based on the results of X-ray diffraction pole-figure and φ...
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GaN films were grown at various V/III ratios on sapphire (0001) substrates with and without nitridation process by radio-frequency plasma-excited molecular beam epitaxy. Based on the results of X-ray diffraction pole-figure and φ scan measurements, the GaN film grown without nitridation process has a tendency to form the metastable in-plane rotation domain by ～11 deg from the substrate, in addition to the main domain rotated by 30 deg from the substrate. In suppressing the formation of the metastable in-plane rotation domain, it is found to be very effective to perform the nitridation process. In addition, even in GaN films grown without nitridation process, decrease in V/III ratio enables us to suppress the formation of the metastable domain.
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We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak pr...
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We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 x 10~(46) ergs s~(-1) and that the total energy was 4.3 x 10~(44) ergs s~(-1) (E approx> 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806—20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.
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摘要 :
We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak pr...
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We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 × 10~(46) ergs s~(-1) and that the total energy was 4.3 × 10~(44) ergs s~(-1) (E approx> 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806-20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.
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