A physics-based hybrid model for supercritical CO2 ejector in critical flow regime

Abstract

Supercritical CO2 (s-CO2) is a natural eco-friendly refrigerant finding global acceptance in energy systems. Supersonic ejectors are passive gasdynamic devices where a motive flow energizes and compresses a secondary stream in a varying area duct having energy applications. Mathematically modeling ejectors, including real gas effects present in CO2, is challenging. We develop a comprehensive physics-based hybrid model of the ejector operating in the critical flow regime where both the primary and secondary mass flow rates are choked. The method of characteristics is used to model the primary supersonic flow and is concurrently solved with a discrete quasi-1D model for the secondary flow with appropriate pressure-matching interface conditions and boundary conditions. The compressible turbulent mixing layer growth between the primary and secondary flow is modeled, and the location of choking is evaluated without any prior assumptions. We introduce empirical fits of the non-mixed length in the ejector to ascertain the length of the mixing duct, and the shock location in the mixed flow is estimated using an entropy minimization principle. Real gas thermodynamic properties are fetched from thermophysical database at each discrete point. The overall model exhibits remarkable fidelity and robustness in the prediction of previous experimental results of air ejectors. Comparisons between numerical results and the physics-based model with s-CO2 as working fluid confirm the accuracy of prediction of the current model (<5% difference in entrainment ratio) compared with conventional modeling approaches (10%–15% difference in entrainment ratio). The computationally effective model developed in this study is invaluable for optimization of ejectors.