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Choking and mixing phenomena in supersonic ejectors

By Olivier Lamberts

Abstract

The purpose of a supersonic ejector consists in the mixing of two fluids with different stagnation pressures in order to obtain a fluid at an intermediate stagnation pressure at the discharge. Devoid of any moving parts, they are fully passive devices that have been exploited for a long time in several applications, notably to serve as vacuum generators in power stations, in the food industry or in aerospace engineering. In the current context of growing energy concerns, the use of supersonic ejectors is considered in the areas of refrigeration and air-conditioning to perform the compression of a fluid by using low-grade heat release which would be wasted otherwise, or renewable energy sources such as solar energy. So far, the low coefficient of performance of ejector refrigeration cycles has prevented ejector refrigerators to penetrate the market. Due to its importance on the whole system, the ejector in itself is actually the main culprit for this shortcoming. The limitation on the performance of supersonic ejectors can be principally traced to the choking (i.e. the blockage of the flow rate) and the mixing mechanisms at play in such devices. Hence, a better understanding of these intricate flow phenomena is necessary in order to identify potential improvements of the performances of ejector systems. This thesis aims at gaining insights into the complex flow physics in supersonic ejectors through numerical and experimental investigations. The main approach consists in exploiting the vast potential of 2D RANS numerical simulations – validated with experimental data both at the global and at the local scales – to investigate the mixing and the choking mechanisms through novel post-processing approaches. Throughout these investigations, theoretical considerations are used to support the different findings. To the author’s knowledge, such a detailed analysis of mixing and choking phenomena within supersonic ejectors has never been proposed before.(FSA - Sciences de l'ingénieur) -- UCL, 201

Topics: Supersonic, Ejector, Choking, Fabri, Exergy, Compound-choking
Year: 2018
OAI identifier: oai:dial.uclouvain.be:boreal:203675
Provided by: DIAL UCLouvain
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