An enhanced factor model for coupled conductive - radiative heat transfer at the particle scale in pebble beds

In nuclear pebble beds, heat transfer is a complex physical process involving three primary mechanisms: particle-particle conduction, conduction through the fluid film between contact pairs (particle-fluid-particle conduction), and particle-particle thermal radiation. The enhanced factor model, a novel approach for particle-scale simulations, was developed to efficiently address these combined phenomena. The model incorporates the contribution of thermal radiation directly into the particle conduction equation for contacting particles. This is achieved through an enhanced factor that is approximated using a Taylor polynomial. A key advantage of this method is its ability to bypass complex and computationally expensive procedures such as thermal ray tracing for view factor calculations. This efficiency, combined with the use of a sub-cell radiation model (SCM), allows for the performance of particle-scale simulations of conduction and radiation heat transfer in large-scale pebble beds. The enhanced factor model has been validated against experimental data, showing general agreement and demonstrating its effectiveness as a tool for analyzing heat transfer in these complex nuclear systems.