Experimental Study on High-parameter Critical Flow in Supercritical Carbon Dioxide Brayton Cycle System

The supercritical carbon dioxide (CO₂) power cycle nuclear energy system offers advantages such as simplified system configuration, high thermal efficiency, and modular construction capabilities, representing an emerging advanced technology in next-generation nuclear power systems. The challenge of its application is the seal design for turbomachinery and the prevention of loss of coolant accident. The generic scientific issue behind the challenge is that the mechanisms of critical flow for supercritical carbon dioxide is still not well established. The operating parameters of the supercritical CO₂ power cycle system cover a wide range (8.0-15.0 MPa, approximately 31-500 ℃), and vary across different locations. A system breach introduces novel multiphase critical flow phenomena: 1) Near the core and regenerator, high-temperature single-phase critical flow occurs with gradually varying compressibility; 2) Near the compressor, two-phase critical flow is dominated by condensation phase transition arises. In this paper, a novel critical flow experimental apparatus was constructed to investigate the operating parameter range relevant to supercritical CO₂ power cycle systems. The effects of upstream stagnation temperature, upstream stagnation pressure, nozzle dimensions, and length-to-diameter ratio on critical flow rate were systematically analyzed, and the published critical flow experimental data were expanded. The range of critical flow test parameters: pressure 8.0-15.0 MPa, temperature 35.0-501.0 ℃, length-to-diameter 0.99-150.0 (L/D), inner diameter 1.0-1.5 mm, with a total of 290 sets of experiments. The research shows that the critical flow mass flow rate increases with the decrease of the upstream stagnation temperature, the increase of the stagnation pressure, and the reduction of L/D. Under a small L/D, the mass flow rate decreases more significantly as the L/D increases. In different temperature ranges, the extent to which the mass flow rate decreases with the increase of temperature varies. Near the pseudo-critical temperature, the mass flow rate decreases most significantly with the increase of temperature. Based on the experimental data of critical flow, an empirical correlation for critical flow applicable over a wide range of parameters was obtained through fitting. The applicable ranges are as follows: pressure from 8.0 to 15.0 MPa, temperature from 35.0 to 500.0 ℃, and L/D from 1.0 to 40.0. When compared with the existing critical flow experimental data and the authors’ own experimental data, 97.4% of the data have errors within 10%, and the maximum error is 12.09%. This correlation can be directly applied in engineering research. The experimental data and empirical correlation presented in this study can be applied to safety analysis of supercritical CO₂ power cycle systems and validation of critical flow theoretical models.