Numerical analysis of the effects of different outlet sizes on pebble flows in HTR-10 pebble beds

The effects of outlet size on the flow characteristics in the pebble bed are studied by means of Discrete Element Method (DEM). Two ways of changing the outlet are considered: fixed hopper angle (‘D’ cases) and fixed hopper height (‘d’ cases). The pebble flow inside pebble beds shows the characteristics of mass flow in the upper part and funnel flow in the lower part, therefore, the changes in the flow pattern are studied first to find out the special transition region by critical time and critical height. Results show that the transition of flow pattern will occur earlier when the opening is smaller, and the critical height of transition increases with the decrease of outlet size for ‘d’ cases. By contrast, there is almost no difference in the critical time and height in ‘D’ cases, indicating that the hopper angle is an important factor affecting the overall flow pattern rather than the hopper height. In the second part, from the perspective of core safety, the influences of outlet size on the retention, flow uniformity and velocity characteristics inside the core are analyzed. The increase of outlet size can improve both the retention and flow uniformity. Changing the outlet by adjusting the hopper angle has a more significant influence on the flow performance. From the analysis of pebbles in different radial positions, it can be found that although pebbles in the middle region flow out slower when the opening is larger, it is opposite in terms of pebbles near the wall, where less retention is observed. Overall, a larger proportion of pebbles are in motion with relatively small but similar speed, showing a more uniform movement. Interestingly, simply changing the hopper height to adjust the opening has no effect on the flow of pebbles within 1d range from the wall.