System-level thermal-hydraulic models provide bulk quantities but cannot directly resolve rod-wise temperatures or hotspot migration during local accidents. This study develops an FMI-based multiscale workflow that couples a Modelica primary-loop FMU with an OpenFOAMderived reduced-order model of a 3×3 lead-bismuth eutectic rod bundle through a Python driver. Four scenarios are evaluated: baseline operation, single-rod local overpower, and local overpower combined with mild or moderate loss of flow. The coupled model predicts bundleaverage outlet temperature, nine rod temperatures, hotspot temperature, and limiting-rod location. The hotspot-tooutlet temperature difference increases from 0.803 K in the baseline case to 18.274 K under local overpower with moderate flow reduction, while the hotspot migrates to the south-east rod. OpenFOAM calculations at 15 s and 40 s identify the same limiting rod. All four 60 s scenarios require 0.319 s of online execution time, demonstrating faster-than-real-time local accident assessment.

Printed circuit heat exchangers (PCHEs) are key components in supercritical carbon dioxide (S-CO2) Brayton cycles because of their compact structure and high heat transfer capability. However, existing studies have mainly focused on steady-state structural optimization or isolated component analysis, while the influence of geometric parameters on thermal-hydraulic responses under system-level transient disturbances remains insufficiently understood. In this study, a Modelica-based dynamic simulation framework is developed for the system-level analysis of recuperators in an S-CO2 Brayton cycle. A one-dimensional dynamic PCHE model is established and validated against benchmark data, with a maximum relative error of 0.87%. A continuously varying heat sink temperature ranging from 22 ° C to 26 ° C is introduced as a representative offdesign disturbance. The effects of channel diameter and channel length on heat transfer coefficient and pressure drop are investigated under both design reference and transient operating conditions. The results show that smaller channels and longer flow paths enhance heat transfer performance but introduce larger pressure-drop penalties under the design reference condition. Under transient heat sink temperature disturbances, smalldiameter and long-channel configurations exhibit stronger thermal-hydraulic response variations than under steadystate conditions. These results indicate that parameter trends obtained under steady-state conditions may not directly represent transient thermal-hydraulic behavior under off-design operation. The proposed framework provides an efficient approach for preliminary parametric assessment and system-level transient analysis of recuperators in S-CO2 Brayton cycles.

To be come