Experimental and CFD studies of the bypass flow in a prismatic core of VHTR using a small-scale model

The bypass flow in a prismatic very high temperature reactor (VHTR) core is an important parameter in reactor design, which has not been well assessed in publications to date. To rectify this deficit and evaluate the bypass flow fraction in a VHTR core, experiments were conducted using a small-scale model for a portion of a prismatic core of VHTR with air as the working fluid. Bypass flow simulations were performed using STAR-CCM + software to validate the code by comparing basic physical quantities related to bypass flow to each other. Specifically the bypass flow fraction, pressure drop, coolant channel Reynolds number, and bypass gap Reynolds number were assessed. Three bypass gap widths, 2.7 mm, 4.4 mm and 6.1 mm, were tested, and two downstream flow conditions through prismatic blocks were employed to examine characteristics of the bypass flow. It was found that the bypass flow fraction measured could reach up to about 38% for the 6.1 mm gap when the flow meters were removed in this experiment. The pressure drop in the bypass gap was a strong function of the Reynolds number in the gap and the bypass gap size. CFD simulation results matched very well with experimental data except that the CFD results overestimated the bypass flow fraction for 2.7 mm case due to the fact that both laminar and turbulent flow existed in the narrowest gap.