An improved thermodynamic model for supersonic ejectors

The use of supersonic ejectors in heat pump and refrigeration cycles is especially compelling, as those devices could allow the use of natural refrigerants whilst maintaining satisfying performance of the system. Thermodynamic models are simple models that allow to swiftly predict ejector performance. Almost all of the state-of-the-art models that can be found in the literature are based on the so-called Fabri-choking criterion, which states that double-choking of the ejector occurs when the secondary stream reaches the speed of sound. However, it has recently been demonstrated that an alternative choking criterion, the so-called compound-choking criterion, provides more accurate predictions of the ejector performance and a sound physical explanation of the choking phenomenon. This criterion states that double-choking occurs when the secondary stream remains subsonic. In the present work, this compound-choking theory has been extended to real-gas and lead to a new thermodynamic model. Subsequent analyses were then made to link the Fabri- and compound-choking criteria. Analytical developments were finally performed to demonstrate that the compound-choking criterion maximises the mass flow through the ejector, which is the admitted physical feature of a choked flow in general