Mixing in multiple-impeller stirred tank reactors with boiling liquids

Mixing in a boil-off mechanically stirred tank reactor with multiple impellers was examined. Power consumption and gas hold-up were measured in boiling water in a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow disk turbines. Vapour was generated from both the immersed ring heater and the impellers. At low vapour generation rates, vapour was mainly generated from the impellers rather than from the heater, whereas nucleation occurred at the heater instead of the impeller at higher vapour generation rates. The mechanical power consumption decreased due to vapour generation. The change in boiling-to-non-boiling mechanical power ratio with varying impeller rotational speed and boiling rate was complicated and not monotonous except at higher impeller speeds and boiling rates. The gas hold-ups increased with increasing vapour generation rate but were rather small as compared to those in cold gas dispersing systems. Empirical correlations for power consumption and gas hold-up in boiling liquids were developed using the present experimental data.     

Switching characteristics of vacuum gap switch employing UV‐laser triggering

The switching time and arc‐ignition probability of a sustained main discharge in a laser trigger vacuum gap were measured. The third harmonic beam of an Nd:YAG pulse laser (wavelength 355 nm, energy 5 mJ/pulse, power density 4 × 10⁷ W/cm²) was used to trigger the gap. The main electrodes 85 mm in diameter were made of oxygen‐free copper. The gap length was 1.6 mm and the experimental chamber was evacuated to about 1.3 × 10⁴ Pa by a turbomolecular pump. The UV beam from the laser was focused at normal incidence onto the grounded cathode. The switching time decreased and the arc‐ignition probability increased with increasing applied voltage. The switching time was 326 ns at 15 kV with a circuit inductance of 107 μH. Its jitter was 24.8 ns. The advantageous results obtained with the UV beam are compared to the previously reported data on triggered vacuum gaps. © 2000 Scripta Technica, Electr Eng Jpn, 132(3): 8–13, 2000     

RELAP5 analyses on the influence of multi-dimensional flow in the core on core cooling during LSTF cold-leg intermediate break LOCA experiments in the OECD/NEA ROSA-2 Project

Test-2 and Test-7 of the second Organization for Economic Co-operation and Development / Nuclear Energy Agency (OECD/NEA) Rig-of-Safety Assessment (ROSA-2) Project were performed with the Large Scale Test Facility (LSTF), which simulated thermal-hydraulic responses during a pressurized water reactor cold-leg intermediate break loss-of-coolant accident (IBLOCA). Test-2 simulated 17% cold-leg break with single failure of an emergency core cooling system (ECCS). The core liquid level decreased to the bottom at loop seal clearing (LSC), causing high cladding temperature excursion. Test-7 simulated 13% cold-leg break with full injection of the ECCS. Compared to Test-2, the cladding surface temperature in Test-7 was quite low due to greater liquid level recovery after the LSC. To well understand the observed phenomena and to improve the best-estimate code predictive capability, RELAP5 post-test analyses were performed. The RELAP5 analyses employed two core models: one is a single-channel core model that simulates the whole core with one channel of a vertical stack of nine equal-height volumes, and the other is a multiple-channel core model that is composed of three channels in which adjacent vertically stacked volumes are horizontally connected with cross-flow junctions. The analyses with the multi-channel core model predicted better than with the single-channel core model for such parameters as core-collapsed liquid level and cladding surface temperature for both Test-2 and Test-7, by more realistically representing multi-dimensional flow in the core. Such a practical method for better representation of multi-dimensional flows turned out to be important to improve the IBLOCA analysis.     

Determination of time to contact using a compound eye sensor

Recently, many control methods for autonomous robots based on biological mechanisms have been studied. In particular, the concept of time to contact, called the tau-margin, which is studied in ecological psychology, has attracted considerable attention. In conventional studies, various approaches have been used for the determination of the tau-margin, and the tau-margin has been used for the timing control of mobile robots. However, in these studies, robots fitted with a light bulb have been used, and direct light from the bulb has been used to determine the tau-margin. It is impossible to apply conventional methods to robots that use indirect light for determining the tau-margin. Therefore, we have developed a compound eye sensor comprising photodiodes. We employ the framework of optical flow to detect objects, and determine the tau-margin from the apparent size of the detected objects and the temporal changes in size. To demonstrate the effectiveness of the developed compound eye sensor, we control a mobile robot using this sensor, and conduct experiments in which the robot has to avoid an oncoming object. The timing of the robot is controlled by using the tau-margin. We conducted experiments by using an actual robot, and found that the robot could avoid an oncoming object successfully. We concluded that it is possible to determine the tau-margin by using the compound eye sensor which we have developed.     

A new low-temperature bonding technology between large-area,high-power devices and Mo electrodes using Au-Al films

In order to realize large-area, high-power devices with high-performance, low-temperature diffusion, bonding technology between Al electrodes on both cathode and anode sides of Si devices and Mo electrode foils has been investigated, The 100 mm diameter power devices and the same size Mo foils could be joined by the formation of Au-Al intermetallic compounds below 573 K. The compounds were formed by solid state diffusion with activation energy of about 1.0 eV. Substantial reduction of the mounting force while keeping uniform contact was possible after low-temperature bonding. Reliability of the bonded large area, high-power device was predicted to be sufficient from a metallurgical viewpoint     

On the texture of γFe2O3particles in magnetically and mechanically oriented magnetic tape

The degree of preferred orientation in magnetic recording tape was investigated by an X-ray pole figure technique and by measuring magnetic properties. Acicular γFe2O3particles dispersed in PVA film were oriented by mechanical stretching and magnetic orientation. The pole figures of these samples indicated the texture with [110] to the orientating direction. The degree of orientation in stretched film was much higher than in magnetically oriented film. Further, it was observed that the particles tend to align uniaxially in mechanical orientation, and biaxially in magnetic orientation.     

A new buried-gate GTO structure having a large safe operating area

A novel structure of a buried-gate GTO (gate turn-off thyristor) was proposed for expanding the safe operating area (SOA) of unit-GTOs. The SOA of unit-GTOs in a test sample GTO and the spike voltage at the limit of turn-off of the test sample were investigated experimentally. The SOA was calculated by means of a simple model in order to study the mechanism of the expansion of SOA in the structure. The SOA was expanded due to the reduction of the sheet resistance of the p-base layer by the fine mesh pattern of the buried layer. Corresponding to the increased size of the SOA, the spike voltage increased to 1000 V