Fluid–Thermal Characteristics and Thermo-Mechanical Response of a Wide-Temperature-Range Gas-Mixing System for a High-Altitude Test Chamber

The front-end wide-temperature-range gas-mixing flow system of a high-altitude test chamber is a key component for simulating inlet conditions over a wide temperature range, and the temperature uniformity of the mixed gas as well as the positional stability of the downstream interface directly affect the accuracy of the test boundary conditions. To address the nonuniform temperature field, thermal deformation of the main pipeline, and local thermal stress concentration arising from the combined effects of large temperature differences, long-distance piping, and high-pressure operating conditions, this study establishes a multiphysics coupling analysis framework based on ANSYS Fluent 2021 R1, CAESAR II, and ANSYS Mechanical, and systematically investigates the temperature distribution in the mixing section, thermal boundary transfer, the global thermal response of the piping system, locally refined models, and the compensating effect of expansion joints. The fluid–thermal results show that the temperature distribution of the mixed gas in the right-side section of the pipeline varies among the representative operating conditions, under which the branch mass-flow allocation, total flow rate, inlet total pressure, and inlet total temperature change simultaneously. To enable quantitative comparison, a temperature nonuniformity coefficient γ (the standard deviation of cross-sectional temperature divided by mean temperature) is introduced. For each temperature combination, Cases 1 to 3 represent operating conditions dominated by the 40 °C branch, whereas Cases 4 and 5 represent conditions dominated by the −70 °C branch in the 40 °C/−70 °C group and by the 550 °C branch in the 40 °C/550 °C group. Under the selected 40 °C/−70 °C cases, the γ values at the downstream monitoring section decrease from 8.72% in Case 1 to 3.86% in Case 3, confirming progressively smoother temperature distributions, whereas γ increases from 6.31% in Case 4 to 9.87% in Case 5, indicating stronger thermal stratification. Under the selected 40 °C/550 °C cases, γ reaches its minimum of 2.87% in Case 2, while the comparison between Case 4 (γ = 9.54%) and Case 5 (γ = 5.78%) shows that the increase in flow-rate difference in the dominant branch is associated with improved temperature uniformity. Thermo-structural coupling analysis further indicates that the main pipeline is the key component governing the overall thermal deformation of the system, that axial thermal expansion dominates under high-temperature conditions, and that local high stresses are mainly concentrated at fixed supports and in adjacent regions with abrupt geometric constraint changes. Further analysis demonstrates that expansion joints modeled using equivalent stiffness can reduce the Z-direction displacement at key locations of the main pipeline by 10.94, 16.46, 9.58, and 10.50 mm under four typical operating conditions, respectively, while shifting the critical region from the main pipeline support area to the vicinity of the compensating components. These results can provide a reference for controlling interface thermal displacement, designing the main pipeline structure, and arranging expansion joints in front-end gas-mixing systems.