威尼斯娱人城官网

    Title：STUDY ON WAVY INTERFACE BEHAVIOR AND DROPLET ENTRAINMENT OF ANNULAR TWO-PHASE FLOW IN ROD-BUNDLE GEOMETRY WITH SPACER

    Speaker：Dr. Tomoaki KUNUGI，Professor，Department of Nuclear Engineering, Kyoto University，C3-d2S06, Kyoto-Daigaku Katsura, Nishikyo-Ku, Kyoto, 615-8540 JAPAN

    Time：14：00-15：30, Oct 22th

    Place：F307, School of Mechanical Engineering

  
     BACKGROUND: The annular two-phase flow exists near the top of the BWR fuel core and keeps an important role in the efficiency as well as the safety of the system. For the rod-bundle geometry of the core, the annular flow consists of wavy liquid films flowing on the fuel rod surfaces and liquid droplets flying in the vapor core between the rods. Among the fundamental phenomena of this flow regime which have not been clarified, the generation of liquid droplets strongly affects the momentum, mass and heat transfers in the system.

    OBJECTIVES: The aim of this study is to provide the visualization data which can describe the mechanisms behind the droplet generation. The concrete subjects are as follows: 

    (1) To investigate the wavy interface behaviors and the droplet generation occurring at the liquid film on the rod bundle

    (2) To measure the liquid film quantities on the rods

   
     EXPERIMENTAL METHODS: The detail description of the annular two-phase flow loop was presented in [1]. The test-section simulating 3x3 BWR fuel rod-bundle consists of a square duct made of transparent acrylic resin and nine steel rod of OD 12 mm which are fixed by three circular ferrule-type spacers. The image data are recorded by the system of the high speed camera (Phantom V7.1 - Vision Research Inc.) and the tele-microscope (Cassegrain - Seika Corporation). A high speed camera technique with the backlight arrangement has been applied to study the annular two-phase flow on a 3x3 simulating BWR rod bundle test-section.

   
     CONCLUSIONS:  The image data at micro-scale of time and space helped to evaluate other measuring techniques and provided the real-time description of the gas–liquid interfaces’ behaviors which improve the understanding of the following important processes:

    (1) Detailed observations of wavy interface behaviors

    - Singlet disturbance-crests are formed near the inlet. A mechanism of this process based on the turbulent characteristic of the gas flow was proposed.

    - Entrainment phenomena occur under the bag break-up, ligament break-up and droplet impingement mechanisms. 

    (2) Measurements of liquid film and droplets of annular two-phase flow

    - The average liquid film thickness, wave amplitude, and wave velocity measured at the corner rod are higher than those at the side rod.

    - At upstream of the spacer, the droplet’s population and size distribution directly relate to the magnitude of waves existing on the liquid films because most of them are generated through the entrainment processes.

    - The spacer generates a huge number of small-size droplets and, in the case of low gas superficial velocity, several droplets whose diameters exceed the maximum droplet diameter at upstream.

    (3) Close-up visualization conducted near the spacer

    - The close-up visualization near space successfully provided detailed information of the phenomena happening at right up- and downstream of this structure.

    - At both low and high gas superficial velocities, the spacer region lets large-amplitude waves enter smoothly instead of enhancing the liquid entrainment processes. 

    - The obtained images of the generation of droplets and their behaviors right downstream of this structure explain well the mechanisms behind the size distribution, which consists of several large droplets and a huge number of small-size ones.