Transient thermal–hydraulic modeling of a leakage in a horizontal concentric heat exchanger with RELAP5 and ANSYS-CFX

• Numerical study of a coaxial tube heat exchanger under transient leakage. • RELAP5 and Ansys-CFX were used to simulate global behavior and local flow phenomena. • Both tools showed strong agreement in predicting accidental transients. • Leakage disrupts flow dynamics and reduces thermal efficiency. • Study offers valuable insights for safety, design, and operation of heat exchangers. Heat exchangers are a fundamental component in several plants due to their compact design, high efficiency and significant economic role, as most thermal energy produced or transferred within an installation passes through them. However, unexpected transient conditions, such as fluctuations in flow rates or temperatures, can significantly affect system performance, safety and efficiency. Therefore, understanding their thermal–hydraulic behavior under accidental transient conditions is crucial for process control and to assess their impact on plant components. This study aims to analyze the thermal–hydraulic response of a horizontal concentric tube heat exchanger during a leak scenario. A combined modeling approach was employed, using Relap5 for global thermal–hydraulic analysis and Ansys-CFX for local flow and heat transfer phenomena.The heat exchanger model was developed and validated against experimental data from the literature under both steady-state and transient conditions in counter-current and co-current configurations, showing strong agreement. Simulation results demonstrate that the codes effectively capture the dynamic evolution of key parameters and reveal the significant impact of leakage on flow structure and heat transfer performance. Moreover, this study provides valuable insights for safety assessment, operational control, and system optimization in thermal engineering applications.