Session: 36-03 Fan and Engine Noise

Paper Number: 101882

101882 - Experimental Investigation of Mode-Frequency Scattering at Fan Stages 

The fans of modern aero engines are equipped with less and less blades to the advantage of reduced aerodynamic losses and overall weight. At the same time, larger bypass ratios lead to lower rotational speeds of the fans. Hence, the larger fans result in lower frequencies at which the blade passing frequency (BPF) occurs, but also generate a richer spectrum of higher harmonics of the BPF that have to be considered regarding noise emissions. Both developments facilitate the occurrence of mode-frequency scattering at the rotor. Scattering at the rotor alters the azimuthal mode order of a reflected resp. transmitted acoustic mode and the frequency, at which it oscillates, simultaneously. The scattered modes superpose with the modes, which are directly generated by the fan stage at the respective frequency. This may cause significant variations of the total radiated sound field due to constructive or destructive interference and should be considered already in the noise-related fan predesign phase.

For this purpose, several models have been developed that are capable of predicting scattering effects at blade rows. The development began in the early 1970s and continues until now, with more and more sophisticated models being published during the last 10 years. However, there is a lack of specific experimental data for validation of these models. Most available data comprise acoustic measurements of the total radiated sound field. This poses the problem that sound generation of the fan stage and propagation effects through the rotor resp. stator are combined and cannot be separated easily.

To this end, an extensive experimental study has been carried out on the DLR fan test rig CRAFT. The CRAFT reference fan is representative for the ducted fan stage of small airplanes and features all relevant sub-sonic fan noise sources that occur in large-scale aero engines. For the investigation of the mode-frequency scattering effects individual dominant modes were generated by a loudspeaker array and emitted onto the fan from the downstream side. The reflected, transmitted and scattered modes were recorded by microphone arrays installed at the inlet and outlet section of the test rig and were analyzed by use of radial mode detection techniques. A strong dominance of the generated target modes was achieved by application of an adaptive technique. It allows to suppress distorting modes that may stem from reflections and scattering at test bench installations downstream of the loudspeaker array. Special care has been taken to compensate for random rotor speed fluctuations using rotor-coherent signal synthesis for the loudspeakers. The rotor-coherent signal synthesis ensures complete coherence between the directly generated and the scattered frequency components. Overall an extensive data base was obtained that comprises the variation of the most relevant acoustic parameters.

In this study, differences between the impinging target mode and its transmitted portion, its reflected portion and the scattered portion are presented. Results are given in terms of sound power level differences for a variation of mode order, frequency and operating conditions. The data base is used for the systematic identification of the physical dependencies of mode-frequency scattering. As a result, the importance of scattering effects is discussed in comparison to the directly generated transmitted and reflected modes.

Presenting Author: Maximilian Behn German Aerospace Center (DLR)

Presenting Author Biography: - Studies in Engineering Science at Technische Universität Berlin, Graduation with Master of Science
- Research Engineer at DLR, Institute of Propulsion Technology, Department of Engine Acoustics in the research group "Turbomachinery Acoustics"

Authors:

Maximilian Behn German Aerospace Center (DLR)
Ulf Tapken German Aerospace Center (DLR)