Ejectors on the cutting edge: The past, the present and the perspective

Highlights: • "the multi-scale ejector challenge" should guide the future research. • Medium-term and long-term research activities are discussed. • Ongoing research activities and achievements are discussed. • Ejector technology and applications are discussed. • Ejector modeling approaches are presented and discussed. -- Abstract: The applications of ejectors are many and encompass the refrigeration, the power generation and the chemical sectors. On one hand, ejector technology needs limited maintenance, has low operational costs and has no restrictions concerning the working fluids; on the other hand, the complex single- and multi-phase fluid dynamics make ejector design and performance prediction a real challenge. This perspective explores the main advancements in ejector technology and proposes a critical discussion with an outlook for the future research; the proposed discussion is grounded on the multi-scale relationship between the "local-scale" phenomena and the "component-scale" performances. After a look at the past, this perspective examines the ongoing research activities and achievements regarding four state-of-the-art research areas, namely refrigeration systems, power conversion plants, chemical process and technology and computational methods. For the different research areas discussed, directions and opportunities for the future research activities are appraised. Finally, this perspective defines a fundamental challenge that needs be addressed in the forthcoming long-term activities: "the multi-scale ejector challenge".