A numerical investigation was carried out in a constant-pressure mixing (CPM) ejector with an under-expanded supersonic primary jet. CPM ejectors are widely used because of their ability to operate against larger backpressures. The subsonic, secondary stream upon contact with the primary jet accelerates and may reach sonic condition, known as Fabri choke or second choke, at a downstream location from the primary nozzle exit. Occurrence of Fabri choke is an indication that the ejector is operating in a double choking mode (first choking occurs at the throat of the converging-diverging primary nozzle) with the maximum possible entrainment ratio. Moreover, as the ejector becomes doubly choked it becomes immune from the back-pressure variations, as choked flows are independent of the back-pressure variations. Numerous 1D models developed earlier extensively uses the assumption of Fabri-choke and can give reasonable accuracy while designing a double-choked ejector. Even though widely used, only recently the experimental and numerical evidence of the Fabri-choking is demonstrated conclusively for a supersonic ejector (Olivier Lamberts et al., International Journal of Heat and Fluid Flow, 2018). A serious limitation with their study is that they considered only a steady state operation with a fixed low value of back-pressure. Single-choked ejector is highly sensitive to the back pressure, as a high back pressure can reduce the entrainment ratio or can even cause back-flow, causing malfunction of the ejector.&nbsp;Hence, the main objectives of the present effort are to understand the evolution of Fabri-choking numerically, mixing layer development and unsteady oblique shock/mixing layer interactions as the backpressure changes continuously. Steady state simulations were conducted for a 2D ejector with dry air as the working substance and a relatively high backpressure, which gives single choking, is utilized. Steady state solution is then taken as the initial value for the unsteady simulations with an assumed unsteady linear reduction in backpressure. Such a reduction will change the operational mode of the ejector form single-choked to double-choked state. Results show that, at sufficiently low values of backpressure, Fabri-choke appears, indicating that secondary flow has accelerated to sonic conditions. The Fabri-choke was made visible by the use of sonic-line. Furthermore, under certain conditions, it was found that secondary flow has accelerated to supersonic conditions and terminated by an unsteady shock wave. This unsteady shock wave interacts with the starting shock from the primary nozzle and the pseudo-shock waves present inside the ejector, thus making the&nbsp;flow and shock structures complex. The unsteady shock wave significantly increases the turbulent intensity which further enhances the vortex structures and hence mixing inside the ejector.

A Computational Study of the Evolution of Fabri-Choke in a Two-Dimensional Supersonic Ejector

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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