Study Note on Thermal-Fluid-Solid Coupled Analysis and Structural Optimization of Eccentric Reducer Tee

Literature Overview

The paper by Duan Chenghong, Wu Gangben, Luo Xiangpeng, and Wang Bo from Beijing University of Chemical Technology, published in Chemical Engineering and Machinery (2022, Vol. 49, No. 3, pp. 441–444), presents a thermal-fluid-solid coupled finite element analysis of an eccentric reducer tee (异径三通) and its structural optimization using response surface methodology. This work addresses the critical issue of stress concentration in reducer tees under combined thermal and hydraulic loading conditions.

Core Technical Findings

Using ANSYS Workbench for multiphysics simulation, the authors established that the maximum stress occurs at the orthogonal transition zone inside the branch of the eccentric reducer tee. This stress level exceeds the allowable stress intensity requirements, indicating a potential failure mechanism under operational conditions. The study then employs response surface optimization (RSM) with the transition fillet radius as the design variable to achieve compliance with stress intensity criteria.

Technical Interpretation of Multiphysics Coupling

The thermal-fluid-solid coupling analysis captures the sequential interaction between temperature distribution, fluid flow patterns, and structural stress response. In a reducer tee operating under hot process conditions, the temperature field creates thermal stresses that superimpose on the mechanical stresses from internal pressure and flow-induced forces. The eccentric geometry introduces additional complexity because the branch offset creates asymmetric stress distributions that cannot be resolved through simple analytical methods.

The maximum stress at the orthogonal transition zone is consistent with stress concentration theory. The abrupt geometric change from the branch to the run creates a stress concentration factor (Kt) that amplifies the nominal stress. The fillet radius directly modulates this concentration factor, with larger radii generally reducing peak stresses but potentially introducing other geometric or manufacturing constraints.

Engineering Practice Integration

In pressure piping design per ASME B31.3 and B31.4, the allowable stress at fittings is governed by the material's stress intensity at operating temperature. For carbon steel fittings (e.g., ASTM A234 WPB, A403 WPB), the allowable stress decreases with increasing temperature, making thermal analysis particularly important for high-temperature service. The coupled analysis approach demonstrated in this paper is increasingly necessary for modern piping systems where:

• Temperature gradients are significant due to process variations
• Flow-induced vibration and fatigue are concerns
• Corrosion reduces effective wall thickness over time
• Composite materials with different thermal expansion coefficients are used

The response surface optimization approach is efficient for identifying optimal fillet dimensions without exhaustive parametric studies. In practice, the optimized fillet radius must also satisfy manufacturing requirements, including forging or welding feasibility, and inspection accessibility per applicable standards.

Key Reflections and Study Insights

This paper exemplifies the growing importance of multiphysics simulation in pressure piping design. Traditional hand calculations based on code formulas often overlook the coupling effects between thermal, fluid, and structural domains. The finding that the orthogonal transition zone is the critical stress location reinforces the need for careful geometric design at branch connections. The response surface optimization methodology provides a practical framework for engineers to systematically improve fitting designs while maintaining compliance with stress intensity requirements. This approach is particularly valuable for custom reducer tees where standard geometries may not adequately address the specific thermal and hydraulic conditions of the application.