Calculation and Analysis of High Temperature Microchannel Heat Exchanger

Abstract: Background: High Temperature Reactors (HTRs) represent a significant advancement in fourth-generation nuclear technology, with core outlet temperatures exceeding 900°C. The Intermediate Heat Exchanger (IHX) in HTRs is crucial for transferring heat from the primary to the secondary circuit. However, the technology for IHX in HTRs is still in its developmental stages, posing a significant challenge in the advancement of HTRs. Microchannel heat exchangers (MCHEs) are emerging as a promising solution for IHX applications due to their compact design and high thermal efficiency. Operating at temperatures above 750°C, MCHEs are classified as Class A components under the ASME III-5 standard, necessitating rigorous structural safety and integrity assessments. Purpose: The aim of this study is to evaluate the safety performance of high-temperature MCHEs under cyclic conditions. The research focuses on developing a systematic approach for stress assessment, strain assessment, and fatigue damage and life prediction, adhering to the ASME III-5 standard and Code Cases N-898. Methods: The study employs a comprehensive finite element modeling approach to analyze the macroscopic and microscopic behavior of a custom-designed MCHE. The models consider the effects of argon arc welding and diffusion welding, defining weld regions in the MCHE. The analysis involves establishing ideal elastoplastic finite element models, calculating the relationship between macro and micro plastic strain, and yield stress. Stress classification lines are established in critical areas such as the shell and nozzles, and stresses are linearized and categorized accordingly. The study also incorporates strain assessment and fatigue damage evaluation based on ASME Code Cases N-898, defining allowable strain limits and calculating equivalent plastic strain. Results: The results demonstrate that the studied MCHE meets the ASME III-5 and Code Cases N-898 requirements. The maximum fatigue damage value is obtained, allowing for the prediction of the MCHE's maximum safe service life. Quantitative results include the calculation of stress and strain in critical areas, such as the weld regions and the core body. For instance, the study finds that the maximum equivalent plastic strain in the D region core body is 0.00074, which is below the allowable limit of 0.00110. The fatigue damage values at key points, such as the A point in the hot side inlet manifold and the B point in the cold side outlet manifold, are calculated to be 9.3×10-8 and 5.1×10-7, respectively, indicating that the MCHE can operate safely for more than three years under the specified conditions. Conclusion: The research concludes that the proposed safety performance analysis method, based on ASME III-5 and Code Cases N-898, provides a robust framework for evaluating the safety of MCHEs. The method effectively considers the microchannel structure in the core body, enabling a detailed assessment of stress, strain, and fatigue damage. The findings confirm that the MCHE design is safe and reliable for operation in high-temperature reactors, with a predicted service life exceeding the design life of three years. This study contributes to the development of safer and more efficient heat exchanger technologies for nuclear applications, providing valuable insights for future design and operational strategies.