Experimental study of turbulent coherent structures under different flow mixing devices in lower plenum of a scaled-down reactor pressure vessel

Turbulent flow mixing in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is important for design of flow mixing devices. In this study, turbulent flow in a scaled-down RPV model with three designs of mixing devices, including vortex suppression plate, flow skirt and semi-spherical perforated drum, was measured by time-resolved particle image velocimetry (TR-PIV) with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. Turbulent coherent structures were studied based on the experimental data. The time-averaged flow structures in lower plenum include flow separation at lower support plate, fluid bifurcation at the flow mixing devices, horizontal jets at holes of flow skirt, flow turning horizontally below the flow mixing devices and flow turning upward through the flow mixing devices. The strong turbulent mixing induced by flow mixing devices improves Reynolds stresses in lower plenum and introduces large spatial-temporal length scales in lower plenum.