Multi-physics analysis and structural integrity evaluation of helically coiled tubes under thermohydraulic instability

In advanced compact heat exchangers, helically coiled tubes are subjected to severe two-phase flow instabilities, such as density wave oscillations (DWO), which create a harsh operating environment. Understanding the material-environment interactions under these conditions is critical for mitigating degradation and extending component life. To evaluate the structural integrity and material lifing potential of these components, this study introduces a comprehensive multi-physics numerical framework. The analysis focuses on an Alloy 690 tube characterized by a complex geometry, comprising a central helical region and arbitrarily curved transient regions at both ends. To accurately capture the harsh operational environment, spatially non-uniform thermal-hydraulic loads were derived from steady-state evaluations and subsequently integrated into the structural model via a multi-physics, one-way fluid-structure interaction (FSI) approach. The structural response was evaluated using a nonlinear finite element analysis based on the curved Timoshenko beam theory, formulated within the local Frenet-Serret frame. Dynamic evaluations confirm that the tube's fundamental natural frequency provides a sufficient margin against low-frequency flow excitations, effectively minimizing resonance risk. However, the simulation highlighted that the unsupported transient regions are highly vulnerable to localized displacement and stress. Furthermore, sensitivity analysis demonstrated that although the pressure differential determines the overall magnitude of deformation, periodic expansion and contraction are primarily driven by cyclic wall temperature oscillations. Ultimately, this multi-physics methodology supports the rapid assessment of fatigue and wear potential during the conceptual design phase, providing strategies for extending material life by suggesting that future designs of supports and heat transfer tubes must be refined to distribute stress more effectively.