CFD investigation of bypass flow in HTR-PM

Predictions of various kinds and distributions of bypass flows are important to the thermal hydraulic design of the HTR-PM (High Temperature gas-cooled Reactor-Pebble-bed Module). In the reactor core, gaps among graphite reflectors are widely distributed and connected, which become typical bypass flow paths interacting with the main flow through the pebble bed. In previous researches, simplified flow network was employed to present the main flow through the pebble bed, and a great many interconnected bypass flow paths, while some irregular gaps were modeled by the computational fluid dynamic (CFD) tools to clarify the resistance coefficients. In present paper, a full CFD model was used to further investigate the bypass flow in the HTR-PM reactor core, which could reveal more local helium flow and heat transfer phenomena if vertical gaps close to pebble bed existed. With separately verified CFD models, the vertical gap bypass flow problem was simulated by the full three-dimension symmetrical model including the helium cooling channel, the cold helium plenum, the cavity above the pebble bed, the bottom reflector, the hot helium plenum and the vertical gap from top to bottom reflectors. Flow distributions in the whole height of active core, as well as radial flow directions along the interface of pebble bed and gap, were focused to find out that the bypass flow rate varied in the vertical gap. For horizontal helium flows, the cold helium mainly flowed into the cavity above the pebble bed while hot helium flowed out of pebble bed. In the full power operation condition, the bypass flow rate ratio was between 0.33% and 1.94% of the total helium flow rate when the gap was consistent with 1.6 mm in width. More gap sizes could be modeled in the future to study the effect to the bypass flow rate ratio.