Multi-scale evaluation of ejector performances: The influence of refrigerants and ejector design

Ejector refrigeration systems pose a promising alternative to vapour compressor technologies. Still, they have not been penetrating the market because of the strong dependency of the ejector behaviour ("component-scale") on the underlying fluid-dynamics ("local-scale"), which directly affects the system performances ("system-scale"). Furthermore, within the broader discussion concerning refrigerant phase-out, the refrigerant selection is a cutting-edge and challenging task, especially because the ejector performance is influenced by the coupling taking place between the working fluid and component design. This paper contributes to the broader framework of ejector research investigating the complex relationships between the factors mentioned above that arise during the operation of a standard ejector refrigeration system. An integrated model (CFD-LPM) has been applied to compare fourth generation and natural refrigerants (R1233zd(E), R1234yf, R1234ze(E), R290, R1270, and R600a) to commonly used refrigerants (R134a, R245fa, R152a) on five ejector geometries, obtained changing the nozzle exit position and the mixing chamber throat dimension from a reference case. Based on the results in terms of performance curves, tested refrigerants could be divided into three groups: (i) R1270 and R290, having higher COP (0.5-1.03) but lower critical temperature Tcrit (20.7-25.0 degrees C), (ii) R134a, R152a, R1234yf, R1234ze R600a, with an intermediate value of COP (0.27-0.83) and critical temperature Tcrit (22.8-28.0 degrees C), (iii) R245fa and R1233zd, with lower COP (0.25-0.58) but higher critical temperature Tcrit (26.0-31.0 degrees C). Increasing nozzle exit position has been found to slightly affect the performances (+5% COP) while the ejector entrainment process has been found to highly depend on the mixing chamber diameter (up to + 35% COP). One of the tested geometries has been then selected, with R290 as operative fluid, to perform a boundary conditions sensitivity analysis. This led to the definition of a system performance map for different system temperature levels at the generator and evaporator.