Theoretical acoustic benefit of high bypass ratio and variable-area nozzle in turbofan engines

Authors

Antoine Moreau - DLR

Abstract

This paper presents a procedure to evaluate the noise emitted by modern aero-engines at an early design stage where only a few global parameters about flight mission and engine design are available. The procedure is then applied for a parameter study considering several engines of increasing bypass ratio varying from 10 to 16. The work presented here was performed within the DLR-funded project KonTeKst, whose goal was to assess the main characteristics of a future single-aisle mid-range aircraft, specifically designed for low noise and with an entry in service around 2030. The reduction of engine noise at the source is one the possibilities to achieve that.Several projections of the expected noise decrease due to high-bypass ratio engines were provided in the past, for example in the frame of a joint NASA / General Electric study in the late 90s. These estimations mostly rely on empirical correlations derived from static tests on past engines. Here, we propose to re-evaluate the impact of increasing bypass ratio with a modern more physics-based analytical prediction model for fan noise, where tonal and broadband acoustic contributions coming from the rotor-stator interaction and from the rotor alone (buzz-saw noise) are separated. Moreover, the question of bypass ratio is addressed in the light of the recent developments in engine architecture, such as a variable-area nozzle.The noise estimations are the final output of a procedure including aerodynamic calculations with a simplified meanline approach for fan design, aerodynamic fan map calculation, and search inside the map for the noise certification points Approach, Cutback, and Sideline, which are specified by a given thrust that the engine must provide. Hence the acoustic comparisons between different designs are thermodynamically equivalent and fair.As part of a parametric study, engines with fan pressure ratio varying from 1.5 to 1.3 (corresponding roughly to bypass ratio between 10 and 16) are appraised based on the analytical noise estimation procedure mentionned above. The trends obtained are compared with the results from previous studies from NASA in order to validate the method. Also, an evaluation of the impact of a variable-area nozzle system is presented and it is shown that this technology becomes aerodynamically mandatory for very-high-bypass-ratio engines but may provide additionally some acoustic benefit in terms of jet noise and fan noise.