Yoshiyuki Kozawa

Studies on two-phase cross flow. Part I: Flow characteristics around a cylinder

Abstract A two-phase flow around a body has scarcely been studied until now, though the flow is used in many industrial components. The cross flows around a spacer in a fuel assembly of light water reactors (LWR) and tube supports in a steam generator are closely related to the long-term reliability and the safety. The present study has been planned to clarify the two-phase flow and heat transfer characteristics around a body including the unknown complicated flow behavior. In the first report, the flow characteristics near and behind a cylinder which was located in a vertical upward air-water bubbly flow were investigated. From the observation of the flow patterns and the measurements of the distributions of void fraction, liquid velocity and static pressure, it is revealed that the vortex flow and the change of the static pressure and liquid velocity distribution around the cylinder resulted in the large distortion of the void fraction distribution around the cylinder. The most noticeable phenomena in the wake were that the peaks of the local void fraction appeared in the vicinity of the cylinder surface near the separation point and in the wake behind the cylinder.

Studies on two-phase cross flow. Part II: Transition Reynolds number and drag coefficient

Abstract A reduction of the transition Reynolds number from laminar separation to turbulent one of cross flow around a body is anticipated in a gas—liquid bubbly flow, since there exists intensive turbulence in the main flow. A decrease in the drag coefficient of the body can also be expected. This report was aimed to classify flow patterns of the two-phase wake flow behind a cylinder and to investigate quantitatively the change of the drag coefficient corresponding to the transition of the flow patterns. From measurements of the static pressure distribution on the cylinder surface and the drag coefficient of the cylinder, it was found that the flow pattern was sure to change finally into a new one similar to the transcritical type in the single-phase flow with an increase of the mean void fraction in the main flow. It was concluded that a large reduction of the upper transition Reynolds number occurred in a two-phase flow, because the transition could be realized by a little increase of the void fraction even below the lower transition Reynolds number in the case of a large cylinder diameter.

Studies on two-phase cross flow. Part III: Characteristics of unsteady flow behavior

Abstract Basic characteristics in a two-phase wake flow, in particular, fluctuations of bubble number density, were investigated experimentally using mainly flat plates. New measuring procedures of the periodicity of the bubble number density and of the spatial distribution of the phase-averaged void fraction were developed. The experimental results indicated that two kinds of flow patterns appeared alternately in the wake. Moreover, it was confirmed that one of them corresponded to the Karman vortex type in a single-phase flow and the other was similar to the twin eddies type which was observed under low Reynolds number ( Re 2 ) in the flow. The alternation process of the two flow patterns depended on the shape of obstacles in the cross flow. In case of a cylinder, numbers of the Karman vortices formed continuously were very few.

Development of general methodology of safety analysis and evaluation for fusion energy systems (GEM-SAFE)☆

Abstract A synthesized methodology of safety analysis and evaluation for fusion systems has been developed to concretely assess the adaptability of fusion systems to the environment from the earliest stages of system development. The methodology objective was to summarize both the safety design requirements and achieve rational safety in fusion systems. The framework of the methodology was constructed to clarify its logical consistency. The safety characteristics of fusion systems were then investigated in detail paying attention primarily to potential hazards, so that a fusion system was identified as a distributed system in regard to energy sources and radioactive materials. Based on this recognition, a General Descriptive Model (GDM) of a fusion system has been constructed which is a highly generalized and integrated expression. The safety ensuring principle, on the other hand, set up items to be protected and categorized events for a fusion system. The development of the safety ensuring principle was a key to the practical performance of safety analysis and its evaluation in a general fusion system. Finally, by using the Function-Based Safety Analysis (FBSA) on the GDM, abnormal events were summarized into 16 typical events, according to the safety ensuring principle. Consequently, 20 design based events for the general fusion system were selected to envelope all credible abnormal events.

Application of the general methodology of safety analysis and evaluation for fusion energy systems (GEMSAFE) to the FER design

Abstract A safety analysis for the FER has been carried out along the framework of the GEMSAFE. First, the safety characteristics of the FER were investigated so that the RI and energy sources were identified and the General Descriptive Model (GDM) was defined to be consistent with the conceptual design of the FER. Second, the radioactive materials and the boundaries of subsystems were classified based on the definition of the proposed event categorization. Finally the Function-Based Safety Analysis (FBSA) has been performed to select 19 Design Basis Events (DBEs), i.e., seven Category-1 events, eight Category-2 events and four Category-3 events. These DBEs not only elucidate the safety requirements to be satisfied with the event categorization, but also suggest crucial items for the safety evaluation of the FER. It is noted that, for establishing the rational safety assurance for the FER, future studies should be performed upon (1) spectra of event propagations which may occur following plasma disruption and coolant inleakage into the vacuum area, and (2) technological feasibility of safety features which can prevent or mitigate these event propagations.

Abstracts of the papers to be presented at the 3rd world conference on experimental heat transfer, fluid mechanics and thermodynamics Sheraton Waikiki Hotel, Honolulu, Hawaii, USA October 31–November 5, 1993 contributed papers gas-liquid flowsCharacteristics of flow and heat transfer in air-mercury two-phase stratified flow under a vertical magnetic field

Abstract For cooling of the first wall and blanket in a magnetic confinement fusion reactor, the use of a helium-lithium annular-mist flow is proposed to mitigate the effects of a strong magnetic field on magnetohydrodynamic (MHD) pressure drop and the deterioration of heat transfer due to laminarization. In annular-mist flow, the magnetic field has mainly an effect on film flow of liquid lithium near the wall region. Instead of the helium-lithium annular-mist flow at high temperature, an air-mercury stratified flow in a horizontal rectangular duct in a vertical magnetic field was investigated to get more detailed information on the characteristics of interfacial waves, MHD pressure drop, and heat transfer. Below 0.4 tesla, when the magnetic flux density increases, the pressure drop decreases due to laminarization of mercury flow and damping of the interfacial waves. However, it increases above 0.5 T due to both the Hartmann flow effect and the increment in the mercury layer thickness. A new correlation was obtained for pressure drop multipliers in two-phase MHD flow. As the magnetic flux density increases, the heat transfer coefficient decreases monotonously and becomes about half of its initial value in the high magnetic flux density. however, when the gas Reynolds number is increased, the heat transfer deterioration is suppressed until the magnetic flux density becomes high enough. Criteria of Nusselt number deterioration under a magnetic field were obtained in various flow conditions. A computational analysis using the κ-ϵ turbulence model including magnetic effect terms was tried and compared with the experimental values of temperature profiles and heat transfer coefficients. The computational results agreed well with the experimental values in the condition with negligible interfacial waves.