Experimental investigation of discharge coefficient of a converging nozzle for supercritical kerosene

In advanced aeroengines, hydrocarbon fuel is used as coolant in the thermal management system, which may turn to supercritical before injected into the combustor. Inside the injecting nozzle, supercritical hydrocarbon fuel may experience phase transition because of the sharp expansion, which triggers violent variations in fuel thermal properties. Thus, conventional nozzle design is faced with serious challenges. In this work, the discharge coefficient (C_d) of a converging nozzle for the supercritical RP-3 aviation kerosene is experimentally investigated, focusing specifically on the effects of violent phase transition on the flow characteristics in advanced aeroengine fuel systems. A high-temperature, high-pressure experimental platform was developed, enabling precise control of the injection pressure (1.0–6.6 MPa), temperature (600–740 K), and mass flux (0.40–1.59 kg·cm⁻²·s⁻¹). Experimental results demonstrate that C_d firstly increases then decreases in the liquid-two phase-supercritical transition, instead of keeping constant. It is noticed part of C_d values with two-phase outflow even exceeds 1, which proves the limitations of conventional isentropic assumptions. Further thermodynamic analysis reveals that fluid density and isentropic compressibility at the nozzle exit are two critical parameters influencing the discharge coefficient. Particularly, isentropic compressibility shows the strongest correlation and is identified as the dominant factor. Finally, a predictive model of C_d is proposed based on the real-fluid thermophysical data and a modified ideal gas framework, the precision of which has been validated in the following conditions: The reduced injection temperature ranges from 1.02 to 1.22, and the reduced injection pressure ranges from 1.0 to 2.92. It reveals deeper understanding of the flow behavior in the liquid-two phase-supercritical transition and provides enhanced guidance for the nozzle design with supercritical kerosene.