Experimental and numerical characterization of an axisymmetric supersonic ejector

Supersonic ejectors are passive devices that use the high stagnation pressure of a driver fluid stream to induce the entrainment of a secondary stream (which is at a lower stagnation pressure), owing to the viscous entrainment effect. In this process, both streams are mixed, compressed and eventually yield a uniform flow at an intermediate stagnation pressure. Used for more than a century in numerous applications (from aerospace engineering to water desalination cycles and vacuum generators), supersonic ejectors are chiefly envisioned as an alternative to traditional mechanical compressors in air-conditioning systems. The need for renewable and diversified energy sources makes supersonic ejectors a promising technology. Indeed, when used in refrigeration cycles, they offer the advantage of using heat as main energy input, allowing the use of low-grade thermal energy such as waste heat (i.e., heat resulting from industrial processes that would be wasted otherwise), geothermal or solar energy. In addition, as the mixing and compression of the fluid do not require any moving parts, maintenance and operation costs are low. However, shortcomings linked to their inherent nature have prevented ejectors from becoming an economically attractive technology in the air-conditioning market. Their low coefficient of performance and low versatility regarding their operating conditions are limiting factors to their industrial progress. To tackle this issue, scientific research has been conducted for several decades, but some ejector phenomena still suffer a lack of comprehension. The principal stakes are the blockage (choking) of the entrained mass flow rate and the mixing phenomena. Following the research conducted in previous Ph.D. and Master's theses at UCLouvain (both experimental and numerical studies), this Master's thesis is devoted to the study of supersonic air ejectors with an axisymmetric geometry. In contrast, previous works used mainly a rectangular geometry. The purpose of this work is twofold: First, it aims at designing a novel axisymmetric ejector, with a geometry optimizing its performance, and to embed it in a high-quality experimental setup. The mechanical design has to overcome the challenges that arise from the axisymmetric geometry. Second, it pursues the objective of performing a full numerical characterization of the various phenomena at stake within the designed ejector. Using 2D steady-state RANS numerical methods and a state-of-the-art computational domain, a numerical database composed of simulations spanning a broad range of operating conditions is obtained, and exploited using adequate post-processing tools. An in-depth investigation of the choking phenomenon and other outstanding flow features is then performed, aiming at shedding light on their origin and characteristics. Finally, a quantitative assessment of the mixing phenomenon and associated transfers is carried out.