Numerical simulation of a supersonic ejector : influence of the geometry and study of a transient scenario

The purpose of supersonic ejectors consists in the mixing of two fluids with different stagnation pressures in order to obtain a fluid at an intermediate pressure. These passive devices have been widely used at the beginning of the last century for steam locomotives and in refrigeration cycles. Nowadays, even if some domains are less promising than others, the future of the supersonic ejector is not compromised due to the large range of applications, notably in power stations, in the food industry or in aerospace engineering. For more than twenty years, many efforts have been made to characterize the mixing mechanisms at play in such devices. Hence, a better understanding of these flow phenomena is necessary in order to identify the limitations of the ejector and the potential improvements. In 2018, the research of Lamberts et al. [29] greatly improved the scientific understanding of mixing and choking phenomena within supersonic ejectors. In particular, they developed an original exergy based approach to investigate the exchange between both streams, and a compound-choking theory to predict the performance of the ejector. This master thesis is a continuation of their work and aims to adapt their analysis to the axisymmetric case. The objective is to compare it to the rectangular case of Lamberts et al. [29] and investigate the influence of the geometry using 2D RANS numerical simulations. To the author’s knowledge, such analysis has never been proposed before. Based on the results, an improved axisymmetric ejector was presented with better performance. Moreover, few studies on transient phenomena have been carried out, and this thesis constitutes a first approach in the field of unsteady flows. Many transient scenarios are possible, but this paper focuses on the valve closure at the secondary inlet. This study case is directly related to the vacuum generator, in the high-altitude test facilities. In this work, two operating modes have been identified: the un-started mode and the started mode, as well as an optimal vacuum performance. Depending on the mode, the transient scenarios differ in time response, flow structure and evacuation performance.