Study on temperature evolution and associated phase behavior inside an industrial-scale large-diameter test pipeline during liquid CO₂ discharge

With the large-scale application of carbon capture, utilization, and storage (CCUS) technologies, safe operation of CO₂ pipelines and clarification of leakage disaster mechanisms are critical to its reliable deployment. In this work, we performed dense/liquid phase CO₂ release comparison tests in a self-built 203.2 m × 323 mm industrial-scale annular pipeline, measured leakage-induced dynamic pressure and temperature responses, and analyzed related phase transition and heat transfer behaviors. The results show that the pressure during CO₂ release presents staged evolutionary characteristics, and the initial pressure imposes a more pronounced effect on the pressure decay rate than the leakage orifice diameter; the most prominent finding is that the internal pipeline temperature exhibits distinct spatiotemporal distributions and vertical stratification differences, where the minimum temperature zone dynamically migrates from the near-leakage region to the pipeline far end as the release proceeds, and the temperature drop rate is negatively correlated with the initial pressure. The cooling mechanism varies across leakage stages: the sharp temperature drop near the leakage point in the early stage is dominated by the Joule–Thomson effect, the continuous temperature reduction in the middle stage is driven by the combined effects of expansion cooling and latent heat absorption accompanying CO₂ phase transition, and the temperature in the later stage recovers gradually under the dominant heat exchange between the pipeline and the ambient environment. Meanwhile, the phase evolution displays sectional stratification in the temporal domain and axial hysteresis along the pipeline, with non-equilibrium phase transition behavior becoming more prominent with increasing initial pressure. The industrial-scale data, evolution laws and core finding of minimum temperature zone migration from this study offer reliable support for risk assessment, emergency monitoring and model validation of CCUS pipeline leakage, and a key reference for its safe industrial operation.