Study on effects of pulsating flow upon thermal-hydraulic characteristics of a marine reactor

In order to investigate effects of the pulsating flow on thermal-hydraulic behaviors in passive safety marine reactors, the following studies were conducted: (1) a series of flow visualization experiments of pulsating flow obstructed by an array of square or circular cylinders; (2) numerical simulations of pulsating flow obstructed by an array of square cylinders; (3) arrangement of natural convection heat transfer data in the core of a model marine reactor during rolling motion. In the flow visualization experiments, PTV (Particle Tracking Velocimetry) was applied to investigate the flow qualitatively. In the steady flow obstructed by an array of square rods, Karman vortices shed regularly from each rod, when the pitch ranges from 6.67 to 13.3. Similar behavior was observed in the steady flow obstructed by an array of circular cylinders, when 3.33 ≤ L/dc ≤ 13.0. When the pulsating amplitude is small (Q2/Q1 approx. = 0.15), any noticeable differences between the pulsating flow obstructed with an array of square/circular cylinders and the steady flow were not observed. When the pulsation amplitude is relatively large (Q2/Q1 ≥ 0.43) and pulsation period is long compared with the shedding period (St* approx. = 0.08), flow acceleration stabilizes wakes behind the cylinder and the flow deceleration destabilizes it. When the pulsating period becomes shorter with the same pulsation amplitude, the flow pulsation controls the generation and breakdown of the Karman vortices. A numerical simulation code was developed by applying the periodic boundary condition along the streamwise direction, and calculated results were compared with the experimental results. Calculated results showed a little dependency on the range of the calculation domain when Re=1000 and L/d=6.67, 10.0. In spite of that, the agreement between the calculated results and the experimental results was satisfactory and the simulation code was demonstrated to be valid for computing the pulsating flow. Heat transfer coefficients in the core during the rolling motion were obtained from the natural circulation tests data with a simulated marine reactor mounted on a rolling bed. Heat transfer with an upright attitude is well correlated with the Rayleigh number. Heat transfer in the core is not affected by the inclination angle within the range of the present experiment (|α| ≤ 22.5deg.). Heat transfer in the core is enhanced by the rolling motion and is classified into three regimes: namely (1) 0.05 < RiR ≤ 0.3 where heat transfer is dominated by the inertial force due to the rolling motion; (2) 0.3 < RiR ≤ 2 where heat transfer is affected by the combined effect of inertial force and natural convection; and (3) RiR > 2 where heat transfer is affected only by natural convection. (author)