Experimental and comparative analysis of choked flow models in CO₂ expansion

Abstract Predicting the speed of sound in two-phase flows is critical for optimizing systems involving refrigerants or high-pressure working fluids, such as transcritical cycles and ejector-based technologies. Carbon dioxide has emerged as a promising working fluid due to its favorable thermodynamic properties; however, its non-ideal behavior in two-phase states complicates the prediction of properties like the speed of sound. This study experimentally investigates the choked flow parameters and compares them with two Homogeneous Equilibrium Models (HEM) and the classic thermodynamic approach, focusing on their application in transcritical CO₂ expansion systems. A high-pressure experimental setup was designed to analyze CO₂ flow dynamics, incorporating precision instrumentation for stagnation pressure, temperature, and mass flow measurements. Experimental results were validated against theoretical predictions using the HEM, alongside the classic thermodynamics’ model, to identify their accuracy in predicting flux flow and choked pressures. Results reveal that Wallis’ model better aligns with pressure predictions at lower operating pressures. Conversely, Katto’s model proves more accurate for mass flow rate estimations. The findings emphasize the need for model refinement to address high-pressure regimes and highlight the critical role of accurate experimental data in advancing the design of CO₂-based energy recovery systems.