LAUSR

Abstract The lower plenum of a High-Temperature Gas-Cooled Reactor (HTGR) consists of numerous jets from the core mixing together across a wide range of temperatures (differences as high as 300–400 K) and velocities (50–100 m/s). The flow from these jets enters the lower plenum and changes direction by 90° and is then forced to navigate through a hexagonal array of support posts. Modeling this level of complexity is a significant computational challenge due to the high velocities and the small length and time scales at which mixing could occur. Efforts aimed at verification and validation of modeling approaches for the lower plenum are underway and will be aided by both experimental and computational studies. The current work focuses on non-isothermal Large Eddy Simulations motivated by a scaled experimental facility whose design is a small subset of the lower plenum, consisting of six jets in the presence of a crossflow, and seven support posts. This “unit cell” serves to mimic flow conditions in different regions of the lower plenum by adjusting velocities, temperatures, and orientations. The primary purpose of this LES-based effort is to provide a much-needed foundation for more applied modeling and experiments by providing insight into the turbulent mixing in the lower plenum of the HTGR. Results in this work reveal that each post in the lower plenum can experience entirely different thermal fluid signatures depending on its location within the unit cell and the jet/crossflow velocities and temperatures. As the jet to crossflow velocity ratio is increased, the penetration of the jets becomes more significant and the variance of the temperature signal on the surface of the posts can become quite large. A significant recirculation region just downstream of the unit cell is also apparent and can begin to impact the temperature signal near the furthest downstream support posts. Regions of the domain where detailed experimental data would be of great value are identified for three separate unit cell configurations representing a wide range of locations within the lower plenum.