Study on Fuel Element Contact Models and Conjugate Heat Transfer Characteristics in Regularly Packed Pebble-Beds

Pebble-bed high-temperature gas-cooled reactors (HTGRs) utilize helium gas as their primary operational coolant, an engineering choice providing substantial technological advantages like exceptionally robust inherent safety and highly elevated heat-transfer efficiency. Despite these numerous benefits, the intricate flow dynamics and the complex heat-transfer behaviors of the helium coolant as it continuously navigates through the complex interstitial channels between spherical fuel elements remain exceedingly complicated in actual engineering practice. Consequently, within current nuclear engineering research, there remains a highly significant scarcity of high-precision numerical simulation studies specifically analyzing the sophisticated fluid dynamics within these pebble-bed reactor core flow channels. To rigorously investigate the detailed distribution characteristics of thermal-hydraulic parameters governing helium flow within these channels, this study established a highly precise numerical computational model. This advanced model was meticulously designed to evaluate complex helium flow and heat-transfer mechanisms within a regularly packed pebble-bed core. Initially, rigorous sensitivity analysis was conducted to critically evaluate the performance and accuracy of various theoretical fuel sphere contact models. Subsequently, relying strictly upon the optimized contact model, advanced numerical simulations were executed. These calculations specifically target simulating intricate helium flow and heat-transfer processes occurring within a structurally defined, eight-layer body-centered cubic (BCC) architectural arrangement of fuel sphere channels. Comprehensive empirical research findings unequivocally demonstrate several critical thermodynamic fluid phenomena. First, when compared directly to standard 1-millimeter and 2-millimeter spatial gap models, the advanced bridge-column connection model is proven to authentically reflect localized physical effects within the reactor core. This is particularly evident in its ability to accurately model localized temperature variations and thermal parameters precisely at physical contact positions between adjoining fuel spheres. Furthermore, during continuous flow of the helium gas coolant through complex multi-layer channels, a significant and distinct multi-stable Coanda effect is explicitly observed. Consequently, distinctly localized thermal hot spots are definitively formed at the exact physical contact positions where fuel spheres intersect. Quantitatively, the highly rigorous numerical analysis mathematically reveals that within the designated eight-layer fuel sphere heat-transfer channel, the absolute maximum temperature elevation experienced by the circulating helium coolant reaches a specific peak of 34.71 K. Concurrently, the maximum temperature differential observed exclusively within the structural solid domain rapidly expands to 63.04 K, while the highest absolute temperature recorded within this particular solid configuration reaches 868.60 K. Finally, detailed hydrodynamic analysis mathematically indicates that the localized turbulent kinetic energy achieves its maximum peak concentration specifically within the fluid flow recirculation zone situated in the fifth-layer flow channel.