Effect of Trailing Edge Flow Injection on Fan Noise and Aerodynamic Performance

Abstract

An experimental investigation using trailing edge blowing for reducing fan rotor/guide vane wake interaction noise was completed in the NASA Glenn 9-by 15-foot Low Speed Wind Tunnel. Data were acquired to measure noise, aerodynamic performance, and flow features for a 22" tip diameter fan representative of modern turbofan technology. The fan was designed to use trailing edge blowing to reduce the fan blade wake momentum deficit. The test objective was to quantify noise reductions, measure impacts on fan aerodynamic performance, and document the flow field using hot-film anemometry. Measurements concentrated on approach, cutback, and takeoff rotational speeds as those are the primary conditions of acoustic interest. Data are presented for a 2% (relative to overall fan flow) trailing edge injection rate and show a 2 dB reduction in Overall Sound Power Level (OAPWL) at all fan test speeds. The reduction in broadband noise is nearly constant and is approximately 1.5 dB up to 20 kHz at all fan speeds. Measurements of tone noise show significant variation, as evidenced by reductions of up to 6 dB in the 2 BPF tone at 6700 rpm.: and increases of nearly 2 dB for the 4 BPF tone at approach speed. Aerodynamic performance measurements show the fan with 2 % injection has an overall efficiency that is comparable to the baseline fan and operates, as intended, with nearly the same pressure ratio and mass flow parameters. Hot-film measurements obtained at the approach operating condition indicate that mean blade wake filling in the tip region was not as significant as expected. This suggests that additional acoustic benefits could be realized if the trailing edge blowing could be modified to provide better filling of the wake momentum deficit. Nevertheless, the hot-film measurements indicate that the trailing edge blowing provided significant reductions in blade wake turbulence. Overall, these results indicate that further work may be required to fully understand the proper implementation of injecting flow at/near the trailing edge as a wake filling strategy. However, data do support the notion that noise reductions can be realized not only for tones but perhaps more importantly, also for broadband. Furthermore, the technique can be implemented without adversely effecting overall fan aerodynamic performance.Nomenclature

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Counter-Rotating Open Rotor

“…[15] Methods for accounting for the background noise of the facility have not been included in the tone/broadband results presented in this report. The spectra were computed using a sample size of 3 million and a Fourier transform of length 2 14 A table of the reading numbers and a few relevant parameters are shown in Table 2.…”

Active Control of Fan Tones by means of Trailing Edge Blowing

“…The successful outcome of the aforementioned studies led to a comprehensive test to fully investigate the performance of TEB in a relevant environment. Fite et al and Woodward et al [22], [23] from NASA Glenn designed a new fan model with TEB, fig. 7.…”

Control of Tonal Fan Noise Using Flow Induced Secondary Sound Sources Generated by Air Jet Actuation

“…The resulting aerodynamic sound sources can be adjusted in both amplitude and phase by controlling the injected mass flow and the circumferential nozzle position. Note that this active flow control technique to reduce the blade tone noise is fundamentally different from the rotor wake filling concept presented by, e.g., Fitel et al [7]. Results for closed loop control of the blade passing frequency component were presented by Lemke et al [8].…”