Experimental study on the phase transition evolution and dynamic characteristics of high-pressure CO₂ pipeline leakage

Brittle fracture of pipelines is one of the common consequences of high-pressure CO₂ leakage accidents. Understanding the phase transition mechanism and flashing flow characteristics during high-pressure liquefied CO₂ leakage is crucial for the safety of pipeline systems. In this study, a high-pressure liquefied CO₂ horizontal visualization pipeline was established to investigate pressure responses, temperature variation patterns, and temperature reduction characteristics along the pipeline during flashing flow. The phase transition evolution was recorded using a high-speed camera. Results show that the phase transition process can be divided into six stages. Pressure and temperature responses are more sensitive to the initial temperature: an increase in the initial temperature slows down the depressurization at a higher pressure level, but results in a smaller temperature reduction. A temperature gradient forms along the flow direction, with greater and faster temperature reductions near the release orifice (lowest temperature: 50.85∼54.82 °C) and smaller drops further away (24.87∼30.39 °C). Moreover, higher initial temperatures cause greater fluctuations in average temperature reduction rates under the same pressure change. At 16 °C, the rate was 0.93∼0.96 °C/s for 8∼11 MPa, while at 27 °C, it increased to 1.12∼1.25 °C/s, indicating that operating at lower temperatures can effectively expand the safety margin for pressure regulation.