Ejector/Engine/Nacelle Integration for Increased Thrust Minus Drag

Abstract

Numerical investigations of a cylindrical shroud ejector and its integration with a subsonic mixed flow turbofan installation were conducted. The objectives included obtaining an insight into the mechanism responsible for secondary flow entrainment and thrust augmentation, the trends in ejector performance, and the feasibility of harnessing ejector pumping to extract installation boundary layer with an estimation of the resulting drag reduction. The presented results are to be considered indicative of trends only. The 2D axi-symmetric viscous CFD analysis indicated that the static pressure inside the ejector was lowered to 65% to 70% of ambient pressure. This was attributed to the pumping action created by the momentum transfer from the primary stream to the secondary stream, effectively pushing aft the secondary flow. This low static pressure induced the secondary flow, imparting kinetic energy to it which resulted in enhanced mixed flow momentum. A numerical experiment demonstrated the feasibility of using ejector suction to remove installation boundary layer with an estimated drag reduction of 6.5% of net thrust at high altitude (over 60000 ft, cruise Mach No.). Since 2-D axi-symmetric analysis was performed, a forced mixer at the exit of the primary nozzle was not modeled. Instead, the effect was emulated by a long constant diameter shroud. The results showed that complete mixing did not result in the best ejector performance, attributed to the inherently higher losses with complete mixing. A configuration with partial mixing indicated a significant gross thrust augmentation: 24% at sea level, static and 18% at altitude, Mach 0.6 to 0.8. However, net thrust augmentation was small at high subsonic Mach numbers, with only 2.3% at 35000 ft, Mach 0.8 due to secondary flow ram drag. Over 60000 ft, cruise Mach No., where the nacelle is prone to have high drag, ejector net thrust augmentation combined with nacelle drag reduction through ejector suction of the boundary layer, amounted to a net thrust minus drag increase of more than 10%. A higher fidelity assessment of performance improvements can best be obtained from subscale ejector model tests and a calibrated 3-D viscous CFD software. Hence future work should include a subscale ejector model test to calibrate a 3-D CFD tool, and then an exhaustive numerical experimentation can be performed to determine an optimum ejector configuration incorporating an optimum forced mixer.