Basic study on flow separation phenomenon in cooling system piping in fast reactors. Clarification of complex flow structure in a multi-elbow in a high Reynolds number regime. Joint research report in JFY2008 and JFY2009

In Tohoku University, different scale model experiments were carried out in order to clarify unsteady flow fields and pressure fluctuation characteristics in the cold leg piping by means of visualization experiment and pressure measurement, respectively, and to investigate the scale effect. The visualization experiments were done by using 1/15 and 1/7 scale models while 1/15 and 1/7 scale models were used for the pressure measurement. In JFY2008, the visualization experiment by PIV measurement for the 1/15 scale piping was carried out using a double elbow geometry, in which Reynolds number was set at 50,000 and 100,000. Besides, shakedown tests of the 1/7 scale model were conducted for the case of a single elbow. To check the soundness of the experimental loop, such as natural frequency, the inlet flow condition and total pressure loss coefficient, the visualization experiment for the Reynolds number of 320,000 has been initiated. In JFY2009, visualization experiments for the 1/7 scale models with the single and a three-dimensionally connected double elbow geometries were carried out by varying Reynolds number up to 1,000,000, and detailed flow fields were observed under the inlet flow condition of almost fully developed turbulence. From these obtained data, effects of the multiple elbows on the flow field were assessed. In addition, the pressure measurements for single elbow flows by using 1/15 and 1/7 scale models were performed by varying Reynolds number up to 400,000 for the 1/15 scale and 1,000,000 for the 1/7 scale, respectively. Pressure fluctuation characteristics were examined in terms of power spectrum density of the pressure fluctuation, which have been provided to investigate the scale effect on the characteristics. In JAEA, a numerical analysis method is being developed to evaluate the feasibility of the reactor piping design. For this development, in this study, the validation of the flow analysis method has been carried out using the experimental data obtained in Tohoku University. Numerical analyses for the 1/7 scale double elbow experiments were performed using the commercial thermal-hydraulics code STAR-CD. The analyses showed that the calculated velocity distributions were in good agreement with the visualization experimental results. It was confirmed, from these analyses, that the URANS analysis method validated for the single elbow flow has applicability to the flow in a double elbow. (author)