Study on phase evolution modes and transient mass flow rate characteristics during SCO2 pipeline leakage

Supercritical carbon dioxide (SCO2) Brayton cycle is a key development direction for Generation IV advanced reactors and holds promising prospects in deep-sea and deep-space exploration. However, pipeline leakage under extreme operating conditions threatens system safety. The mass flow rate at the leakage orifice is a critical parameter for assessing accident consequences, yet the predictive accuracy of existing models for leakage processes involving complex phase transitions remains unclear. In this study, a full-bore, high-pressure visualized experimental system was established, enabling simultaneous measurements of phase distribution, temperature, pressure, and mass flow rate during SCO2 leakage. The results indicate that SCO2 transcritical leakage can be categorized into two modes based on the presence of liquid phase formation. In Mode I (two-phase leakage mode), liquid phase vaporization compensates for pressure drop, resulting in an approximately linear pressure decline and a slower decay in mass flow rate. Image grayscale analysis maps the evolution of void fraction, with grayscale values stabilizing around 50 (on a 0–255 scale). In Mode II (gas-phase leakage mode), the fluid transitions directly from the supercritical phase to the gas phase, leading to a faster decay in mass flow rate. The homogeneous equilibrium model (HEM) demonstrates high predictive accuracy for mass flow rate in Mode II (error <10%) but significantly underestimates it in Mode I (error >30%). In the early stage of leakage, subcooling drives homogeneous condensation to form a liquid layer, whereas in the later stage, superheating leads to boiling nucleation dominance and a sharp increase in void fraction. The transition from subcooling to superheating corresponds to a shift in the dominant phase change mechanism from “vapor-to-liquid” to “liquid-to-vapor”. A phase transition criterion was established based on initial specific enthalpy (critical value: approximately 389.0 kJ/kg). Below this value, leakage follows Mode I; above it, Mode II applies. Based on this criterion, the SCO2 Brayton cycle is partitioned: the precooler outlet and compressor inlet/outlet are identified as high-risk regions for Mode I, while high-temperature and high-pressure sections (heater, turbine, etc.) correspond to Mode II. This study provides an important basis for consequence assessment and safety design of SCO2 leakage accidents.