Conceptual development of quiet turbofan engines for supersonic aircraft

This is a joint thermodynamic and acoustic study of engines for next-generation supersonic aircraft. It explores fixed-cycle concepts with potential for quiet takeoff and efficient cruise. The flowpath is simple, without mechanical suppressors. The strategy is to take a representative state-of-the-art,military turbofan engine and increase its bypass ratio to a moderate value. The engine core stays the same. Three exhaust configurations are considered: mixed flow, coaxial separate flow, and eccentric separate flow. Engine-cycle analysis predicts thermodynamic performance and nozzle exhaust conditions at takeoff and Mach 1.6 cruise. Subscale experiments duplicate the static exhaust conditions at takeoff power and measure the far-field sound. Flyover perceived noise levels are estimated for a twin-engine aircraft in the 120,000-lb class. In terms of effective perceived noise level (EPNL), the eccentric arrangement is 6.5 dB quieter than the mixed-flow arrangement and 5 dB quieter than the coaxial configuration. Spectral- and time-domain analyses indicate that the eccentric exhaust is free of strong Mach wave radiation. The acoustic benefit of the eccentric arrangement, combined with faster climb afforded by the modified engine, leads to a reduction of 14 dB in EPNL. Compared to the baseline engine, the specific fuel consumption of the modified engine is about 13% less at subsonic climb and 3% less at supersonic cruise.