Comparative analysis of RELAP5 and ANSYS-CFX simulations for microchannel heat exchangers. A case study on the VLF primary heat exchanger

Micro-channel heat exchangers represent a significant innovation in heat transfer technology, offering high thermal efficiency and compact designs potentially suitable for advanced nuclear systems. Despite their potential, limited numerical analyses and experimental results are available in literature fully characterizing their performance, especially under prototypical operating conditions found in nuclear reactors. For this purpose, the Versatile Loop Facility (VLF) was designed and built to test the key components which will be part of the reactor coolant system of the Westinghouse LFR, focusing on the Primary Heat Exchanger (PHE). The PHE is a hybrid microchannel heat exchanger manufactured using a diffusion bonding process offering a high heat transfer area-to-volume ratio, resulting in exceptional compactness, a significant advantage for the design of the primary reactor pool as it minimizes required space allocation. This paper presents a comparative study between RELAP5 and ANSYS-CFX for modelling microchannel heat exchangers, using the PHE of the VLF as a case study with the aim to focus on the temperature distribution, pressure drops and heat transfer coefficients and subsequently to improve the accuracy and reliability of thermal-hydraulic system codes and related modelling methodologies for design assessment and safety analyses. The results show that the simulations from both codes are in good agreement, indicating that RELAP5 provides a satisfactory system-level prediction for this specific case study, which represents the main objective of this work. The extension of the model validation to a broader set of scenarios, including both operative and accidental conditions, will be the subject of future work, pending the availability of reliable experimental data