A Tip-Turbine Driven Propulsion Fan Concept

Abstract

The current drive for low specific fuel consumption (SFC) has resulted in increasing the bypass ratio (BPR) to improve the propulsive efficiency. Conventional high BPR engines face several practical limits when augmenting BPR i.e. an increase in the number of low-pressure turbine stages and in weight of the engine. This paper is an account of an investigation of the tip-turbine driven propulsion fan (TTDPF) as one potential solution. In particular, the TTDPF is considered as the main propulsion source for the Blended Wing Body Aircraft (BWB) developed by the College of Aeronautics at Cranfield University. The concept combines a gas generator(s) in the fuselage in combination with three or four over-wing mounted propulsion fans driven by tip-turbines. A double pass single stage configuration for the tip-turbine is assessed. This comprises two partial admission segments sharing the same circular annulus. The paper considers some special features of this novel engine concept. For example, the complex ducting (volutes) between the two passes and the aerodynamic arrangement of a two-pass turbine with a single rotor are investigated. Mechanical integrity issues and nacelle sizing are also considered. Finally, estimates are made of the noise generated by the fan and the exhaust configuration and a comparison is made with a traditional high by-pass ratio turbofan. The study concludes that the potential benefits in terms of SFC are somewhat overshadowed by the less favourable outcomes in terms of weight, nacelle size, noise and tip-turbine efficiency.