Core-Plenum interaction experiments; Investigation on onset condition depth penetration flow

Kobayashi, Jun ; Ohshima, Hiroyuki; Kamide, Hideki ; Ieda, Yoshiaki 

In the case that the Direct Reactor Auxiliary Cooling System (DRACS) is adapted as a decay heat removal system of FBR, cold fluid coming from the heat exchangers immersed directly in the hot plenum may penetrate into some subassemblies under a certain natural circulation condition. This phenomenon is important from the viewpoint of the coolability of the core, because it affects the temperature distribution in the subassemblies and the natural circulation head in a primary cooling system. The objective of this study is to obtain the fundamental understanding of the penetration phenomenon with water experiments. The test section is composed of a plenum, a cooled wall and a vertical channe1, which simulate the hot plenum, the heat exchanger and lower power subassembly, respectively. The inlet temperature of water is kept constant during a run of experiments. The velocity and temperature fields in the vertical channel are measured simultaneously with two instruments: Ultrasonic velocity Profile monitor (UVP) for an axial profile of the vertical velocity components and T-type (copper-constantan) thermocouples for the temperature fields. Inlet temperature and flow rate are selected as experimental parameters. Results of the tests are follows: (1)The transient axial profiles of velocity in the vertical channel were compared with those of temperature under the condition that the penetration flow was observed. The trend of temperature profiles agreed well with that of velocity profiles. It was, therefore, considered that the depth of the penetration flow could be estimated with the temperature data as well as the velocity data. (2)The penetraion depth is determined based on buoyancy, inertia, conduction and entrainment effects. With dimensional analysis, Gr/Re and Pr number were derived as relevant parameters for the depth of penetration flow, z: z=f(Gr/Re, Pr). In our experiments, the depth of penetration flow could be correlated with Gr/Re as ...