DGEN AEROPROPULSION RESEARCH TURBOFAN CORE/COMBUSTOR-NOISE MEASUREMENTS — PART 1: EXPERIMENT AND MODAL STRUCTURE AT CORE-NOZZLE EXIT

Devin K. Boyle, Brenda S. Henderson and Lennart S. Hultgren, NASA Glenn Research Center, Cleveland, OH 44135, USA

Contributions from the combustor to the overall propulsion noise of civilian transport aircraft are expected to become important due to turbofan design trends and advances in mitigation of other noise sources. Future propulsion systems for ultra-efficient commercial air vehicles are projected to be of increasingly higher bypass ratios from larger fans combined with much smaller cores, with ultra-clean burning fuel-flexible combustors. Unless effective noise-reduction strategies are developed, combustor noise is likely to become a prominent contributor to overall airport community noise. The noise/environmental impact is an issue of great importance not only to future gas-turbine propulsors, but also to proposed far-term hybrid-electric aircraft-propulsion systems.  

The NASA DGEN Aeropropulsion Research Turbofan (DART) is a cost-efficient testbed for the study of core-noise physics and mitigation. The kernel of this resource is an AKIRA MecaTurbines DGEN 380 turbofan engine. It is a two-spool 500 lbf (2.2 kN) thrust-class geared turbofan engine with a bypass ratio of approximately 7.6, a 3.32 fan gear ratio, a single centrifugal compressor on the high pressure spool, a reverse flow annular combustor, and single stage uncooled axial high-pressure and low-pressure turbines. Its modular design allows the replacement of major components with parts modified for invasive instrumentation with comparative ease.

This paper describes the recently completed DART source-diagnostic test (SDT) in the NASA Glenn Research Center Aero-Acoustic Propulsion Laboratory (AAPL). The SDT campaign continued the exploration and documentation of the DART core/combustor noise begun in an earlier baseline test \[1\], but with more extensive instrumentation. Both tests were aimed at developing a better understanding of propulsion-noise sources and their impact on the farfield noise signature, in order to enable improved turbofan noise-prediction methods and noise-mitigation techniques.

Simultaneous high-data-rate acoustic measurements were obtained using a circumferential infinite-tube-probe (ITP) array at the core-nozzle exit in conjunction with sideline and farfield microphone arrays. The circumferential array consisted of 8 equally spaced ITP sensors; the linear sideline array was made up of 61 microphones placed parallel to the engine center axis at two alternate offset distances in the acoustic farfield; and a 24-microphone array in the geometric farfield completed the acoustic instrumentation. Data were acquired, utilizing 24-bit analog-to-digital conversion, at a sampling rate of 100 kHz for 60 s at each test point. The test matrix consisted of seven relevant engine operational points, ranging from engine idle to max power, and was repeated for different circumferential and sideline array configurations, as well as for redundancy. The extensive data set (up to 93 channels of data and various configurations) will allow the application of advanced source-separation and phased-array methods to elucidate not only the core-noise structure, but also the propagation characteristics of other propulsion noise sources.

A detailed documentation of the unsteady pressure field at the core-nozzle exit will be presented. Previous work \[1\] suggested that the +/-1 azimuthal duct mode could be cut-on at this location, which would have implications for combustor-noise modeling and prediction. The data analysis is in progress and the final version of the paper is expected to conclusively establish the modal structure of the combustor noise emanating from the engine. Relevant farfield sound-pressure-level (SPL) spectra will also be presented. A proposed companion paper will discuss combustor-noise results obtained by the application of advanced source-separation methods that combine correlation methods with mode-decomposition techniques, eg. \[2\], to the time-series data.

The NASA Advanced Air Transport Technology Project supported this work.

\[1\] Boyle et al., AIAA Paper 2018-3281    \[2\] Davis and Bennet, AIAA Paper 2011-2825


DGEN Aeropropulsion Research Turbofan Core/Combustor-Noise Measurements: Experiment and Modal Structure at Core-Nozzle Exit

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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