Effect of Multisized Packing on the Heat Transfer Characteristics of a Face-Centered-Cubic-Structured Pebble Bed

Inserting small spheres of a fixed size into a pebble bed can enhance heat transfer, lower the surface temperature of fuel elements, and reduce the risk of local hot spots. However, does a multisized pebble bed outperform a double-sized pebble bed in terms of heat transfer? To address this question, numerical simulations were conducted to evaluate the heat transfer characteristics of face-centered-cubic-structured pebble beds with two and three types of small spheres, maintaining a constant solid volume. The results showed that the average heat transfer coefficients of the multisized pebble bed were nearly identical to those of the double-sized bed. This suggests that using two types of small spheres does not necessarily provide better heat transfer performance than using a double-sized bed. Additionally, the number, size, and placement of the spheres influenced the pressure drop. These findings offer insights into the heat transfer behavior of high-temperature reactor cores and provide a useful reference for the design of future pebble bed reactor cores.