Hideaki Nishihara

Some characteristics of gas-liquid flow in narrow rectangular ducts

Abstract Flow regime, void fraction, slug bubble velocity and pressure loss were measured for rectangular ducts with a narrow gap and a large aspect ratio. The neutron radiography technique was used to visualize the flow and the void fraction was obtained by image processing. The void fraction was well-correlated by the drift flux model with the existing correlation for the distribution parameter, which was about 1.35. Similar results were obtained for the slug bubble velocity, however the distribution parameter was in the range 1.0–1.2. The frictional pressure loss was well-correlated by the Chisholm-Laird correlation. In collaboration with previously obtained data, it was found that the Chisholms parameter C , however, changed from 21 to 0 as the gap decreased.

The effect of flow direction and magnitude on CHF for low pressure water in thin rectangular channels

Abstract Critical heat flux (CHF) at low flow condition can become important in an MTR-type research reactor under a number of accident conditions. Regardless of the initial stages of these accidents, a condition which is basically the decay heat removal by natural convention boiling can develop. Under such conditions, burnout may occur even at a very low heat flux. In view of this, the CHF at low-flow-rate and low-pressure conditions has been studied for water flowing in thin rectangular channels. Experiments were carried out with two types of rectangular test sections, namely, the one heated from one wide side and the other heated from two opposite sides. In order to observe the effects of gravity, CHF was measured both in upflow and downflow. The CHF at complete bottom blockage was also studied. The results indicate that burnout can occur at a much lower heat flux than pool-boiling CHF or than predicted by the conventional correlations. There was observed a minimum CHF at complete bottom blockage and at very low downflow. The low CHF at very low downflow appears to be due to the stagnation of the bubble in the heated section. This fact indicates that special care should be taken in analyzing the boiling phenomenon which occurs when the coolant flow is very low in a low pressure system.

Effect of channel geometry on critical heat flux for low pressure water

Abstract The main difficulty in interpreting critical heat flux (CHF) at low velocity and pressure conditions arises from the fact that the burnout phenomenon under such conditions is vulnerable to the effect of buoyancy and flow instabilities. This study is intended to provide some systematic understanding on CHF at low velocity and pressure conditions. Data obtained in the previous experiments for water in an annulus, rectangular ducts and a round tube are briefly reviewed and augmented in collaboration with existing data and correlations to extract more generic information. The effect of channel geometry on CHF is then discussed. The effect of channel geometry turned out to be remarkable at intermediate mass velocities. The difference in CHF at these mass velocities between a round tube and the other channel geometries was attributed mainly to the existence of an unheated wall which cause a non-uniform distribution of liquid film.

Visualization and measurement of two-phase flow by using neutron radiography

Abstract To apply neutron radiography (NR) technique to fluid research, high frame-rate NR with a steady thermal neutron beam has been developed in the present research program by assembling up-to-date technologies for neutron source, scintillator, high-speed video and image intensifier. This imaging system has many advantages such as a long recording time (up to 21 min), high-frame-rate (up to 1000 frames s −1 ) imaging and no need for triggering signal. Visualization of air-water two-phase flow in a metallic duct was performed at the recording speeds of 250, 500 and 1000 frames s −1 . The qualities of those consecutive images were good enough to observe and measure the flow structure and the characteristics. It was demonstrated also that some characteristics of two-phase flow could be measured by using the present imaging system. Image processing technique enabled measurements of various flow characteristics in two ways. By utilizing geometrical information extracted from NR images, data on flow regime, rising velocity of bubbles, and wave height and interfacial area in annular flow were obtained. By utilizing attenuation characteristics of neutrons in materials, measurements of void profile and average void fraction were performed. It was confirmed that this new technique may have significant advantages both in visualizing and measuring high-speed fluid phenomena when the other methods such as an optical method and X-ray radiography cannot be applicable.

Visualization and measurement of gas–liquid metal two-phase flow with large density difference using thermal neutrons as microscopic probes

Abstract In a core melt accident of a fast breeder reactor, there is a possibility of boiling of the fuel–steel mixture in the containment pool. In relation to safety evaluation on severe accident, it is indispensable to evaluate the possibility of re-criticality of melted core. Gas–liquid two-phase flow with a large liquid-to-gas density ratio is formed due to the boiling of fuel–steel mixture. Although it is anticipated that the large density ratio may affect the basic characteristics of two-phase flow, little work has been performed so far on two-phase flow with a large liquid-to-gas density ratio. In this study, visualization and void fraction measurement of gas–liquid metal two-phase flow were performed by using neutron radiography and image processing techniques. Then, the effect of large density difference between gas and liquid phases on the basic flow characteristics of two-phase flow was clarified.

A new correlation method for transit-time estimation

Abstract A new correlation method for transit-time estimation is proposed. The transit-time of a propagating quantity between two points along the propagation path could be measured by noise analysis technique when this quantity conveys some measurable stochastic signal with it. This is usually performed by observing directly the peak in the cross-correlation function of the stochastic signal measured at two points in the propagation path, or indirectly by reading the slope of the phase-angular frequency curve. The proposed method presents the transit-time information directly in the same way as in the ordinary cross-correlation technique, but with a better resolution when the conveyed noise signal is band-limited. It also rejects extraneous noise contamination when such noise appears in a narrow frequency band. The usefulness of this method was tested in an air-water two-phase flow channel to estimate the transit-time of the bubbles.

Consumer attitudes to radiation and irradiated potatoes at radiation fair in Osaka, Japan

Abstract “Radiation Fair” has been held in summer vacation season in August for more than 10 years in Osaka, the largest city of western Japan, for the purpose of public education and information transfer of radiation and radiation-related technology. We distributed questionnaires to the visitors for recent 3 years to inquire their status toward radiation and irradiated products including irradiated potatoes as well as impression toward the displays. According to the survey results, more than 60% of the kids visitors were satisfied with this exhibition as informative, more than half of the older visitors (16 years old and upward) indicated that they recognized the word of “radiation” when they were at elementary school and the most significant sources of this information were school lessons and the mass media. Consumers image toward radiation seems to shift to more “positive” when correct knowledge about radiation is given. More than half of consumers did not know “irradiated potatoes” but the percentage indicating that irradiated potatoes was definitely hazardous was less than 10%.

Measurement of Temperature Fluctuations in a Natural Convection Water Loop

Investigations of reactor noise in water-cooled research reactors show that the power spectral density rises in the low frequency domain. The cause of this phenomenon is often attributed to fluctuations in the coolant temperature, but this has never been proved experimentally. The present experiment is an attempt in this direction. The temperature fluctuation in a natural convection heat transfer loop decoupled from neutronics was measured and analyzed in the frequency domain. The test section of the loop had a rectangular channel measuring 5 mm × 50 mm in cross section and 500 mm in length. This configuration simulated a coolant channel of the MTR-type fuel element used in swimming-pool reactors. The power spectral density of the temperature fluctuation at the channel exit showed a shape similar to the power spectral density of the noise-equivalent source obtained in the Kyoto University Reactor at comparable power levels.

Public status toward radiation and irradiated potatoes at “Youngster’s Science Festival” in several cities including Tokyo, Osaka, and Hiroshima, Japan

Abstract “Youngster’s Science Festival” has been held in several big cities in various districts in Japan for the purpose of induction of young students’ interests in science and scientific experiments. On the basis of the survey results from the participants of the “Radiation Fair” in Osaka, Japan, which was presented at the last IMRP, we expanded the area of survey and distributed questionnaires to the visitors of the above event to inquire their status toward radiation and irradiated products including irradiated potatoes. The survey results indicated the same trends as that of the “Radiation Fair” survey. That is, more than half of the older visitors (16 years old and upward) indicated that they recognized the word of “radiation” when they were at elementary school and the most significant sources of this information were school lessons and the mass media. We will discuss the relationship between consumer’s image toward radiation and the description of radiation related topic in school textbooks.

Acoustic Emission in Subcooled Nucleate Pool Boiling

Intensity and power spectral density of acoustic emission in subcooled nucleate pool boiling of water from boiling inception to the heater burnout was investigated experimentally. Platinum and Ni-Cr wires with dia. 1.0 mm or smaller were used as the heater. The measurements were performed in containers at atmospheric pressure and in a dam at pressures up to 3 atm. In the former case photographic observations of boiling bubbles were also made. The acoustic emission in boiling had frequency components up to and sometimes beyond 50 kHz; the shape of power spectral densities behaved differently for heat fluxes above and below a certain value at which the overall acoustic intensity assumed a maximum value. With the increase in heat flux the acoustic emission increased in high frequencies below this heat flux, while it became eminent in low frequencies above the same heat flux. This phenomenon is related to the transition from the region with single bubbles to the region with coalesced bubbles. Resonance-like p...