Analysis on heat transfer and flow performance of pebble-bed fuel in three regular arrangements with multi-layers

Abstract

The fuel pebbles in the core of the high-temperature reactor (HTR) are randomly arranged, making it difficult to analyze the heat transfer and flow characteristics quantitatively. Traditionally, three regular fuel pebble arrangements—simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC)—have been used in quantitative studies to determine the possible HTR heat transfer characteristics. However, most currently established arrangement models focus on a few layers, making them inadequate for revealing the variations in heat transfer and flow characteristics with different arrangements and pebble layers. This study established multilayered pebble-bed models for the three arrangements and analyzed the variations in the heat transfer coefficients with increased pebble layers. The reliability of the numerical computational approach was validated through a comparative analysis with experimental data. The study revealed that with the increase of layers, the heat transfer coefficient of SC and FCC arrangement basically stabled at 1480 and 4000 W/(m2·K). The heat transfer coefficient of BCC pronouncedly increased from 2500 to 3500 W/(m2·K). The flow velocity distribution suggested that the increase was caused by a rotation of the helium flow, which became apparent in the ninth layer and stabilized around the thirteenth layer. An analysis of the turbulent kinetic energy showed that this rotation was caused by an increase in turbulence. As the physical properties of helium changed, the flow became unstable and finally entered a new, stable state. Analysis of the boundary conditions at the inlet further supports this view.