Numerical simulation of the flow flashing of carbon dioxide in a circular pipe

Abstract

The study of Loss of Coolant Accidents (LOCA) in supercritical carbon dioxide (S-CO₂) reactors constitutes a critical and challenging domain in nuclear safety analysis, with the investigation of flow flashing phenomena in pipelines being particularly pivotal. In this work, reactor pipelines were simplified, and a transient numerical simulation code for channel flow flashing was developed based on the homogeneous flow model, integrated with a suite of relevant correlations. Computational results from this code were validated against experimental data for water flow flashing, showing strong consistency with the measured values. Specifically, the maximum error in predicting flash onset (the point where vapor first forms) was approximately 5 %, thereby confirming the model accuracy. Utilizing the validated code, an in-depth analysis of the flow flashing process with CO₂ as the working fluid was conducted. For vertical pipes, higher inlet fluid temperature, lower outlet pressure, and lower inlet velocity were found to extend the length of the flashing two-phase region and increase the outlet void fraction. Furthermore, transient depressurization processes at the outlets of vertical pipes were numerically simulated to verify the code's transient analysis capabilities. For horizontal pipes, lower inlet pressure triggers adiabatic flashing, resulting in higher steam quality. Faster outlet depressurization initiates earlier flashing, enhances volume expansion, accelerates flow velocity, and elevates the peak mass flow rate. Overall, the computational results provide a solid theoretical foundation for LOCA analysis in S-CO₂ reactors.