Terushige Fujii

Application of fast neutron radiography to three-dimensional visualization of steady two-phase flow in a rod bundle

Abstract Three-dimensional void fraction distribution of air–water two-phase flow in a 4×4 rod-bundle near a spacer was visualized by fast neutron radiography using a CT method. One-dimensional cross sectional averaged void fraction distribution was also calculated. The behaviors of low void fraction (thick water) two-phase flow in the rod bundle around the spacer were clearly visualized. It was shown that the void fraction distributions were visualized with a quality similar to those by thermal neutron radiography for low void fraction two-phase flow which is difficult to visualize by thermal neutron radiography. It is concluded that the fast neutron radiography is efficiently applicable to two-phase flow studies.

Three-dimensional visualization of void fraction distribution in steady two-phase flow by thermal neutron radiography

Abstract Three-dimensional void fraction distributions of a steady air–water two-phase flow in a 4×4 rod-bundle with circular ferrule type spacers were measured by neutron radiography using a CT method. The high flux thermal neutron radiography system at JRR-3M in JAERI was used. Two-phase flow was visualized with a SIT tube camera and time-averaged one-dimensional cross sectional averaged void fraction distributions were calculated. Visualization with high spatial resolution up to 0.18 mm was carried out by using a cooled CCD camera. Projections in 250 directions were obtained and were reconstructed by a filtered back projection method after using some image processing techniques. Animations were made to show the three-dimensional distributions. One-dimensional and three-dimensional void fraction distributions of the steady state two-phase flow in the rod bundle near the spacer were clearly visualized.

A Study of Gas-Liquid Two-Phase Flow in a Horizontal Tube Under Microgravity

Abstract: A better understanding of the effect of gravity level on two‐phase flow characteristics is crucial for designing two‐phase heat transport systems and thermal control systems for space. Hence, an experiment was conducted aboard MU‐300 aircraft, as well as at normal gravity using the same horizontal tube, 10mm internal diameter and flow conditions. The experimental results, obtained under Earth, hypergravity (2g), and microgravity environments were compared with previous models. Flow pattern data were compared with data from models for predicting microgravity flow pattern transitions. The mean void fraction under μg and 1g conditions was also compared with the Inoue‐Aoki model for vertical upward annular flow at normal gravity. Frictional pressure drop fitted well with the Lockhart‐Martinelli model, slightly influenced by the change in gravity levels (μg, 1g, and 2g); however, the effect of changing gravity on the pressure drop was insignificant for turbulent flow.

A METHOD FOR QUANTITATIVE MEASUREMENT BY THERMAL NEUTRON RADIOGRAPHY

Abstract A quantitative measurement method by thermal neutron radiography was proposed for two-dimensional void fraction measurement in two-phase flow. The umbra method was modified for two-dimensional measurement by using a neutron absorber grid. Image processing methods to compensate for the effects of neutrons scattered in the object and optical rays scattered in camera were developed. A step made from acrylic resin was tested with a B4C grid 3mm in width and 3mm in interval. It was shown that two dimensional quantitative measurement was possible with this method in sacrifice of the spatial resolution.

Flow pattern transition and heat transfer of inverted annular flow

Abstract The characteristics of flow pattern transitions and heat transfer of a steady inverted annular flow (IAF) in a vertical tube are studied experimentally and analytically using freon R-113 as a working fluid. The heat transfer characteristics are divided into three regions and a heat transfer characteristic map is proposed. Flow patterns along the tube change from IAF to dispersed flow (DF) through inverted slug flow (ISF) for lower mass fluxes and through agitated inverted annular flow (AIAF) for higher mass fluxes. A flow pattern map is proposed and the boundaries of the flow patterns agree well with those of the heat transfer characteristic map. A turbulent boundary layer analysis is applied to IAF using modified Reichardt equations. Radial velocity and temperature profiles of both the gas and liquid phases are obtained. The predicted heat transfer coefficients agree well with the experimental results. The flow pattern transition criterion from IAF to AIAF is predicted by the analytical results and the Kelvin-Helmholtz instability and that from AIAF to DF is predicted by a previous drift flux model and a void fraction of 0.7. The predicted criteria agree well with the experimental results.

Flow characteristics of gas–liquid two-phase flow in plate heat exchanger: (Visualization and void fraction measurement by neutron radiography)

Abstract In order to clarify the gas–liquid two-phase flow characteristics in a plate heat exchanger, gas–liquid two-phase flows in simulated heat exchangers with a single channel placed in a vertical plane were visualized by a neutron radiography method. Air–water adiabatic two-phase flows and chlorofluorocarbon R141b boiling two-phase flows were visualized, and two-dimensional void fraction distributions were measured from visualized images via some image processing techniques. Moreover, total pressure loss of adiabatic air–water two-phase flows through the test section was measured, and the frictional pressure loss was calculated by using measured results of average void fraction. As a result, it was shown that the phase distributions in heat exchangers were strongly affected by the inlet configuration and the inlet conditions. When the incoming flow was a gas–liquid two-phase flow, liquid stagnation at the test section inlet caused unsymmetrical phase distribution. For high gas volumetric flux, both phases tended to flow straight separately. Moreover, it was shown that the average void fractions and the frictional pressure loss of adiabatic air–water two-phase flows in the ribbed channel could be correlated by the drift-flux model and the L–M method, respectively.

Three-dimensional void fraction measurement of two-phase flow in a rod bundle by neutron radiography

Abstract Three-dimensional void fraction distribution and one-dimensional cross-sectional averaged void fraction distribution of air-water two-phase flow in a 4 × 4 rod bundle near a spacer was measured by neutron radiography using a CT method. Two-phase flow behavior in the rod bundle near the spacer, which is important to study from the safety considerations of nuclear reactors and which is difficult to observe by other methods, was clearly visualized.

Visualization and void fraction measurement of gas-liquid two-phase flow in a commercial plate heat exchanger by thermal neutron radiography

Adiabatic air-water two-phase flows in a commercial plate heat exchanger were visualized by thermal neutron radiography method as a nondestructive test in order to clarify liquid distributions. Liquid distributions in a single channel and a multichannel plate heat exchanger were investigated. From the results of a single channel, it was shown that gas phase tended to flow at the center of the net-like conduit and liquid fraction at the both side was higher. However, in the case of higher gas volumetric flux above 19 m/s, liquid distributions seemed to be homogenous. Measured average void fractions were lower than those by the homogenous model. On the other hand, from the results of a multichannel, it was shown that liquid distribution into the each channel strongly depended on inlet conditions. In the case of higher liquid flow rate and lower gas flow rate, liquid fraction became higher with the deeper channel due to larger liquid momentum. However, in the case of lower liquid flow rate and higher gas flow rate, the opposite tendency of liquid distribution was observed, that is to say, liquid fraction was lower with the deeper channel.

VOID FRACTION DISTRIBUTION MEASUREMENT IN TWO-PHASE FLOW BY REAL-TIME NEUTRON RADIOGRAPHY AND REAL-TIME IMAGE PROCESSING

Abstract A real-time two-dimensional void fraction distribution measurement of gas-liquid two-phase flow was carried out by real-time neutron radiography and real-time image processing. The JRR-3M real-time thermal neutron radiography system and a Musashi dynamic image of processing system were used. Image processing methods to calculate two-dimensional and cross-sectional void fraction distributions were proposed. The void fraction distribution was calculated by non-linear processing of the neutron radiography image and displayed by pseud-color in real-time. A simple gas-liquid two-phase flow induced by injecting gas through needles at the bottom of water pool in a rectangular vessel was tested.

Performance and flow characteristics of nozzles for initially subcooled hot water (influence of turbulence and decompression rate)

Abstract The purpose of this study was to improve the performance characteristics of a convergentdivergent nozzle for flashing expansion of initially subcooled hot water. The slip between the vapor and liquid along the divergent passage and the maximum nonequilibrium pressure drop at the nozzle throat (this is related to the thermal nonequilibrium) cause a decline in the nozzle efficiency. To decrease the maximum pressure drop, a nozzle with thin wires just upstream of the throat and a nozzle with a smaller convergent angle were manufactured and tested. Initially subcooled hot water at an inlet pressure of 0.47 MPa and inlet temperatures of 409.9 to 421.8 K (inlet subcoolings of 0.9–12.9 K) were used with the nozzle back pressures ranging from 5 to 101.3 kPa. The following results were attained: (1) the maximum nonequilibrium pressure drop at the throat decreased and the thrust coefficient increased by 10%; (2) the thrust coefficient was found to be independent of the inlet subcooling and exit stream-expansion ratio; (3) a correlation between the ratio of the exit pressure to the inlet pressure and the appropriate exit stream-expansion ratio was obtained empirically; and (4) the critical flow rate was found to be lower than that for an ordinary convergent-divergent nozzle.