Analysis of Ultra-High Bypass Ratio Turbofan Nacelle Geometries With Conventional and Short Intakes at Take-Off and Cruise

Abstract

Turbofan engines with ultra-high bypass ratio require more aggressive nacelle designs to limit the drag and weight associated to the increased fan diameter. Reducing the inlet length-to-diameter ratio appears a possibility for further weight saving and mitigation of the total pressure losses. However, these benefits are counteracted by the higher fan exposure to nonuniform in-flow conditions. In this paper, we compare three nacelle geometries for an ultra-high bypass ratio turbofan with a progressively shorter intake having a length-to-diameter ratio of 0.475, 0.35, and 0.22 and two different compact cowl designs. The complete isolated nacelle model, including the exhaust system and a pylon, is simulated numerically with the fan rotor and stator cascades represented through a body force method, allowing to capture the engine/nacelle interaction. The three geometries are examined at take-off and cruise, measuring the variation of integral metrics related to the inlet performance and the propulsive forces. Although with shorter intakes the total pressure losses and the separation onset are positively impacted at high incidence, a careful shaping of the lip is needed to control the diffusion in the limited axial extension available. The effect on the propulsive forces is found to be limited at take-off for a fully attached flow, whereas at cruise the gross thrust improvement of the more compact nacelle is counterbalanced by a larger drag.