FIGHTING NOISE IN GAS TURBINES Noise reduction techniques in gas turbines

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1. Introduction According to a study on noise pollution, children can be physically effected by aircraft noise exposure: ‘Chronic aircraft noise exposure to children impairs reading

comprehension and long-term memory and may be associated with raised blood pressure’ (Stansfeld & Matheson, 2003). Residents close to airports are gaining

increasing influence in the western world, and airports are now subject to noise pollution requirements that often stop them from growing. However, passenger

and cargo traffic is growing at an average rate of 5% worldwide each year (Boeing, 2015). This makes noise restriction a hot topic. One cause of flight noise

pollution is gas turbines. Especially during take-off and landing most noise comes from the engines. But what exactly are engine manufacturers doing to reduce

this noise?

The aim of this factsheet is to describe noise reduction techniques in gas turbines. It starts with a description of the current generation of gas turbines, along

with a short explanation of how they work and which ones are currently used. Noise-producing components are then illustrated, followed by an explanation of

techniques used to minimize their noise. Finally, noise-reduction techniques of the future will be explained.

2. Current generation gas turbines The high bypass ratio turbofan (turbofan, for short), is currently the most widely-used gas turbine in aviation. The GE90 (Boeing 777), the PW1000 (Airbus 320 NEO), and the TRENT1000 (Boeing 787) are examples of ordinary turbofans. All turbofans consist of a core engine and a bypass, as depicted in Figure 1.

Figure 1: Turbofan buildup (Huff, 2007)

The core engine consists of an inlet, a fan, a compressor, a combustor, a turbine and an exhaust. The bypass consists of an inlet, a fan, a stator and an exhaust. These turbofans are characterized by their bypass ratio, which is equal to the airflow through the fan divided by the airflow through the core engine. The bypass ratios of the current generation gas turbines are between eight and twelve. In some flight phases, approximately 80% of the total thrust is generated by the bypass airflow. Figure 2 contains a graph that shows the decrease in aircraft engine noise pollution over the years – the result of air traffic growth and the growing importance of noise impact on the community (Huff, 2007).

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Figure 2: Progress in aircraft noise reduction (Huff, 2007)

3. Noise sources Engine noise is generated by all engine sections: the fan, the compressor, the combustor, the turbine and the exhaust – also referred to as the jet (Huff, 2007). The main focus of this factsheet is on noise generated by the fan and the exhaust. Most noise reduction techniques are applied on these two modules, since they generate most of the noise. Figure 3: Turbofan engine with major noise components and their directions shows a graphical representation of noise emission.

Figure 3: Turbofan engine with major noise components and their directions (Allen, 2012)

3.1. Fan Noise Sound is produced by distortion – such as pressure fluctuation – in an elastic medium. A fan system will produce sounds with tonal characteristics and broadband characteristics (Acoustic Glossary, n.d.):

Tonal sound: a sound or noise recognizable by its regularity. A simple or pure tone – a musical note, for example – has one frequency.

Broadband sound: noise whose energy is distributed over a wide section

of the audible range, such as noise coming from wind. Because these two sound types are generated differently, a separate discussion will be provided for each of them. 3.1.1. Tonal Noise Tonal noise is generated when there is an interactive effect between airflow distortions in the path of a rotating blade. Most frequently, these interactions involve pressure fields or turbulent-wake disturbances. A discrete tone, propagated to the far-field, is generated by an almost identical pressure field. This pressure field is associated with each blade on a single rotating stage. The propagation to the far-field occurs when a signal is recognized as a sound-wave. Usually this sound-wave is a wavelength to where the sound pressure and the particle velocity are in phase. At subsonic conditions, when no shock pattern exists, the cyclic pressure field and wake interactions between rotating and stationary stages are the source of discrete tones. If the stages are close together, there is generally an intense pressure-field interaction and a very strong tone generation (see Figure 4).

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Figure 4: Mechanisms of tonal sound generation in a compressor stage (Smith, 1989).

3.1.2. Broadband Noise Broadband noise results from the propagation of sound produced near the surface of the blade as a result of pressure fluctuations associated with turbulent flow in the vicinity. In the first stage of the compressor – or the fan in a turbofan engine – broadband noise is generated by the movement of the tip of the rotating blade within the turbulent boundary layer close to the wall of the inlet duct. Turbulence in the wakes shed by fan blades (see Figure 5) is also an important source of broadband noise, particularly for fan blades with large surface areas. It plays a significant role in the generation of broadband noise in the downstream stages of the turbomachine. In a modern single-stage fan, these are the outlet stators in the bypass duct and the multistage core compressor. (Smith, 1989)

Figure 5: Generation of broadband noise

3.2. Jet Noise (only broadband noise) Jet noise covers the sources associated with the mixing process of the exhaust flow into the atmosphere. For aircraft flying below the speed of sound, the mixing noise (also referred to as turbulent mixing noise) is the only noise source coming from the exhaust (Smith, 1989). Turbulent mixing noise is generated by high velocity exhaust flow that pushes against a stable and static atmosphere. Since this high-speed flow is moving through a stationary medium, friction – and therefore sound – will occur. Turbulent mixing noise is basically composed of two contributions associated with large turbulence (large-scale eddies) and with small scale turbulence (small-scale eddies) developing in the shear layers of the jet flow (see Figure 6). The small-scale eddies and the large-eddies produce high and low-frequency sounds respectively (Bailly, Bogey, Marsden, & Castelain, 2014).

Figure 6: Origin of shock and mixing noise components of jet noise spectrum (Kala, 2012)

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4. Noise reduction techniques The techniques used for noise reduction in gas turbines reduce fan noise and jet noise. The new generation gas turbines mainly cope with broadband noise rather than tones. 4.1 Fan Noise Fan noise is lowered by reducing the rotational tip speed of the rotor blades or reducing the fan pressure ratio. This causes the engine diameter to increase in order to achieve the same thrust production. The best fan tip speed is just below the speed of sound. Once this has been achieved in an engine design, the fan pressure ratio becomes the controlling factor for reducing (broadband) noise. Techniques for reducing fan noise are: the UHBR engine, scarf inlets, active noise control, forward swept fans, swept and leaned stators, variable area fan nozzles, fan trailing edge blowing, soft stator vanes, and acoustic treatment (Huff, 2007). 4.1.1 Ultra-high bypass ratio (UHBR) The ultra-high bypass ratio engine is an engine with an ultra-high bypass, as depicted in Figure 7. Compared with a bypass ratio engine, the nacelles of an UHBR engine are considerably reduced in length and volume. By decreasing the bypass nacelles of the turbofan, noise is lowered through a reduction in the rotational tip speed of the fan blades and a reduction in the fan pressure ratio. By decreasing the diameter, the UHBR solution is the most effective and easiest way to reduce noise. The other main advantage of the UHBR engine is that it increases sustainability in an efficient way (Huff, 2007). 4.1.2 Scarf inlets Scarf inlets reduce fan noise by redirecting the forward-radiated sound – both tones and broadband noise – away from the community (upwards), as depicted in Figure 7. Scarf inlets also have aerodynamic advantages (Huff, 2007).

Figure 7: Ultra-high bypass ratio engine including scarf inlet (Enoval, 2014)

4.1.3. Active Noise Control Active noise control (see Figure 8) is used in aviation gas turbine fans, called the Active Noise Control Fan (ANCF). An actuator in the fan duct walls and inside the stator vanes captures the tones of a fan and directly send a 180 degree shifted tone back (anti-noise) to cancel out the noise source (Huff, 2007).

Figure 8: Active noise control (Wikipedia, 2016)

4.1.4. Forward swept fans Forward swept fans, also called forward swept fan tip blades (see Figure 9) mainly reduce aerodynamic losses associated with shocks and improve stall margin. By reducing the opportunity of shocks, noises associated with these shocks are eliminated by delaying the onset of multiple pure tones (Huff, 2007).

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Figure 9: Forward swept fans (Huff, 2007)

4.1.5. Swept and leaned stators Swept and leaned stators are depicted in Figure 10. They reduce fan noise by increasing the phase changes from root to tip – and the effective distance from the fan to the stator vanes (both producing the sound). Increasing the effective distance and phase changes creates less turbulence. Swept and leaned stators reduce broadband noise as well as tones. Unfortunately swept and leaned stator blades have additional aerodynamic performance losses, resulting in a trade-off between noise reduction and performance (Huff, 2007).

Figure 10: Swept and leaned stators (Huff, 2007)

4.1.6. Variable area fan nozzles A variable area fan nozzle is a moving mechanism that can increase and decrease the area of the fan nozzle. Controlling the incidence angle of the

flow (as depicted in Figure 11) reduces noise coming from the fan. If the fan nozzle area increases, then the fan noise coming from the back of the engine decreases. Variable area fan nozzles can also increase and decrease thrust (Mabe, 2008).

Figure 11: Variable area fan nozzles (Mabe, 2008)

4.1.7. Fan trailing edge blowing Fan noise can also be reduced by ejecting air at the trailing edge of the fan blades. Specially-designed passages pump air from the hub of the fan to the trailing edge of the fan, as depicted in Figure 12. Air is pumped into the engine to smoothen the air flow. By smoothening the air flow, fan trailing edge blowing reduces the wake defect and thereby tone noises. Fan trailing edge blowing also reduces the turbulence intensity of the fan, which reduces broadband noise (Huff, 2007).

Figure 12: Fan trailing edge blowing (Huff, 2007)

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4.1.8. Soft stator vanes Soft stator vanes reduce tone noise coming from the fan. They are vanes on the trailing edge of a porous surface (also referred to as soft), with hollow chambers, as depicted in Figure 13. Each chamber is tuned in a specific way to damp out a particular frequency. Soft stator vanes reduce the unsteady pressure response on the stator surface and absorb energy that would otherwise become sound radiating from the stator. Soft stator vane design can be tailored for specific frequency ranges (Fite, 2015).

Figure 13: Soft stator vanes (Fite, 2015)

4.1.9. Acoustic treatment Acoustic treatment is accomplished by putting acoustic liners in the inlet (see Figure 14), aft fan ducts and the tip of the rotor. These liners are mostly honeycomb structures with porous or felt metal face sheets. These sheets reduce tone noise around a desired target frequency. Honeycomb structures reduce fan noise by wearing out the tones (Huff, 2007).

Figure 14: Acoustic liners (Wikipedia, 2015 & Kempton, 2011)

4.2. Jet Noise Jet noise is reduced by lowering jet exhaust velocity. The biggest challenge is to reduce jet noise without changing the engine cycle. Currently, the only common noise reducing technique is the use of chevron nozzles. 4.2.1. Chevron Nozzles Chevrons are the saw-toothed patterns on the trailing edge of an engine, as depicted in Figure 15. Chevron nozzles use this saw-toothed pattern to reduce jet noise by mixing the core and bypass air flow in a smoother manner. Chevrons reduce the low frequency noise from highly turbulent flows without changing the engine cycle.

Figure 15: Chevron nozzles (R. Kanmaniraja, 2014)

5. Expectations for the future The Federal Aviation Administration (FAA) has created a program called Continuous Lower Energy, Emissions and Noise (CLEEN). This program is a NextGen effort to accelerate the development and commercial deployment of environmentally-promising aircraft technologies and sustainable alternative fuels. These technologies focus on reducing aircraft noise, emissions and fuel burn (Federal Aviation Administration, 2016). The Federal Aviation Administration has awarded five-year agreements to Boeing, General Electric, Honeywell, Pratt & Whitney, and Rolls-Royce (Federal Aviation Administration, 2016). One of the program’s goals is ‘developing and demonstrating certifiable aircraft technologies that reduce noise levels by 32dB cumulative’. The following two innovations will be a part of

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achieving the goal of CLEEN (Federal Aviation Administration, 2016). Current engine noise pollution is at an average of 140 dB (Asha, 2016). General Electric completed scaled Open Rotor wind tunnel tests in 2012. These noise reduction tests indicated a 15 dB cumulative noise reduction relative to FAA’s Stage 4 noise standards. (Federal Aviation Administration, 2014). Research conducted by teams led by Airbus and Snecma has concluded that a 2030-timeframe short/medium-range airliner with counter-rotating open-rotor engines is technically feasible (Warwick, 2014). Several tools will be used to reduce noise coming from the engine, including the trend of disk loading, design changes, and clipping (General Electric, 2013). Pratt & Whitney is also developing and demonstrating an ultra-high bypass ratio geared turbofan (GTF) engine with associated advanced technologies. This GTF engine will contribute to noise reduction of 25 dB relative to the Stage 4 noise standards (Federal Aviation Administration, 2014). Rolls Royce also has a research program running, called DREAM (valiDation of Radical Engine Architecture systeMs). One of DREAM’s objectives is to reduce noise by 9 dB cumulative compared with the current Y2000 turbofan engines (Rolls-Royce, 2016).

Glossary Acoustic liners: protective covering designed to control sounds.

Anti-noise: noise designed to reduce or ban other noise.

Broadband noise: noise whose energy is distributed over a wide section of the audible range (such as noise coming from wind).

Bypass ratio: airflow through the fan divided by airflow through the core engine.

Cumulative: increasing or increased in quantity, degree or force by successive additions.

Eddies: currents at variance with the main current in a stream of liquid or gas, especially one having a rotary or whirling motion.

Engine cycle: any series of thermodynamic phases constituting a cycle for the conversion of heat into work.

Fan pressure ratio: the pressure at the beginning of the fan divided by the pressure at the end of the fan.

Honeycomb structure: a multiple layer hexagonal structure.

Pressure fluctuation: continual change from one pressure to another pressure.

Shear layer: a region of airflow with a significant velocity gradient.

Shocks: sudden acceleration of air velocity.

Stage: a rotor and stator combined (from the turbine or compressor).

Stall margin: margin that delineates the differences between the RPM of rotor blades and the RPM at which the blades will stall.

Tones: sound or noise recognizable by regularity. A simple or pure tone has one frequency (for example, a musical note).

Turbulence: air turbulence caused by the disturbance of an airflow.

Turbulent boundary layer: a current mixing all airflow layers each other, creating intense agitation.

Wake defect: turbulence coming from the collision of a streamlined air flow with a high pressure area (also blade tip vortices).

Wake: distortion in airflow coming from the rotor (like a moving ship leaving a distortion in the water – also called ‘Kielwater’ in Dutch).

References Acoustic Glossary. (n.d.). Retrieved from Gracey & Associates:

http://www.acoustic-glossary.co.uk/ Allen, M. P. (2012, April). Analysis and synthesis of aircraft engine fan noise

for use in psychoacoustic studies. Retrieved from Virginia polytechnic institute and state university: https://theses.lib.vt.edu/theses/available/etd-05022012-155708/unrestricted/Allen_MP_T_2012.pdf

Asha. (2016). Noise. Retrieved from American Speech-Language-Hearing Association: http://www.asha.org/public/hearing/Noise/

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Bailly, C., Bogey, C., Marsden, O., & Castelain, T. (2014, December). Subsonic and supersonic jet mixing noise. Retrieved from laboratoire de Mécanique des Fluides et d'acoustique: http://acoustique.ec-lyon.fr/cours/vki_JetNoise_notes_bailly.pdf

Boeing. (2015). Current market outlook. Boeing. Retrieved from Boeing: http://www.boeing.com/resources/boeingdotcom/commercial/about-our-market/assets/downloads/Boeing_Current_Market_Outlook_2015.pdf

Enoval. (2014). About Enoval - Objectives. Retrieved from Enoval: http://www.enoval.eu/page/about-enoval/objectives.php

Federal Aviation Administration. (2014, 08 01). Fact Sheet – Continuous Lower Energy, Emissions, and Noise (CLEEN) Program. Retrieved from Federal aviation administration: https://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=16814

Federal aviation administration. (2016, 04 04). Continuous Lower Energy, Emissions, and Noise (CLEEN) Program. Retrieved from Federal aviation administration: https://www.faa.gov/about/office_org/headquarters_offices/apl/research/aircraft_technology/cleen/

Fite, E. B. (2015, March). A softer vane. Retrieved from National Aeronautics and Space Administration: http://www.grc.nasa.gov/WWW/Acoustics/testing/facilities/softvane.htm

General electric. (2013). Open Rotor Engine Aeroacoustic Technology Final Report. Evendale: Federal Aviation Administration.

Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf

Kala, B. (2012, May). Noise suppression engine noise. Retrieved from Model aircraft: http://aeromodelbasic.blogspot.nl/2012/05/noise-suppression-engine-noise.html

Mabe, J. (2008). variable area jet nozzle for noise reduction using shape memory alloy actuators. Retrieved from Acoustics Paris: http://webistem.com/acoustics2008/acoustics2008/cd1/data/articles/003595.pdf

R. Kanmaniraja, R. F. (2014). 3D Numerical studies on jets acoustic characteristics of chevron nozzles for aerospace application. Retrieved from World Academy of Science, Engineering and Technology: http://waset.org/publications/9999247/3d-numerical-studies-on-jets-acoustic-characteristics-of-chevron-nozzles-for-aerospace-applications

Rolls-Royce. (2016, 09 22). DREAM. Retrieved from Rolls-royce: http://www.rolls-royce.com/about/our-technology/research/research-programmes/dream.aspx

Smith, M. J. (1989). Aircraft noise. New York: Press Syndicate of the University of Cambridge.

Stansfeld, S. A., & Matheson, M. P. (2003). Noise pollution: non-auditory effects on health. oxford journals.

Warwick, G. (2014, 03 31). Airbus, Snecma Tackle Open-Rotor Integration. Retrieved from Aviationweek: http://aviationweek.com/equipment-technology/airbus-snecma-tackle-open-rotor-integration

Image references (top to bottom, left to right) Front page: Zoezoquinha. (2016). Grote motor met een klein hartje… Retrieved from KLM Royal Dutch Airlines: https://www.facebook.com/KLM/photos/a.378259560772.168963.273795515772/10153917422105773/?type=3&theater 1 Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf

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2 Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf 3 Allen, M. P. (2012, April). Analysis and synthesis of aircraft engine fan noise for use in psychoacoustic studies. Retrieved from Virginia polytechnic institute and state university: https://theses.lib.vt.edu/theses/available/etd-05022012-155708/unrestricted/Allen_MP_T_2012.pdf 4 Smith, M. J. (1989). Aircraft noise. New York: Press Syndicate of the University of Cambridge. 5 Smith, M. J. (1989). Aircraft noise. New York: Press Syndicate of the University of Cambridge. 6 Kala, B. (2012, May). Noise suppression engine noise. Retrieved from Model aircraft: http://aeromodelbasic.blogspot.nl/2012/05/noise-suppression-engine-noise.html 7 Enoval. (2014). About Enoval - Objectives. Retrieved from Enoval: http://www.enoval.eu/page/about-enoval/objectives.php 8 Wikipedia. (2016). Active noise control. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Active_noise_control

9 Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf 10 Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf 11 Mabe, J. (2008). variable area jet nozzle for noise reduction using shape memory alloy actuators. Retrieved from Acoustics Paris: http://webistem.com/acoustics2008/acoustics2008/cd1/data/articles/003595.pdf 12 Huff, D. L. (2007, September). Noise Reduction Technologies for Turbofan Engines. Retrieved from NASA: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080001448.pdf 13 Fite, E. B. (2015, March). A softer vane. Retrieved from National Aeronautics and Space Administration: http://www.grc.nasa.gov/WWW/Acoustics/testing/facilities/softvane.htm 14 Wikipedia. (2015). Acoustic liner. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Acoustic_liner & Kempton, A. (2011) Acoustic liners for modern aero-engines. Retrieved from: http://www.win.tue.nl/ceas-asc/Workshop15/CEAS-ASC_XNoise-EV_K1_Kempton.pdf 15

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R. Kanmaniraja, R. F. (2014). 3D Numerical studies on jets acoustic characteristics of chevron nozzles for aerospace application. Retrieved van World Academy of Science, Engineering and Technology: http://waset.org/publications/9999247/3d-numerical-studies-on-jets-acoustic-characteristics-of-chevron-nozzles-for-aerospace-applications

Dutch Summary Geluidsuitstoot is momenteel een hot topic. Geluidsuitstoot van vliegtuigen ontstaat door de vorm van het vliegtuig en de gasturbines. Buiten dat gasturbine producenten rekening moeten houden met de duurzaamheid van het product, moeten zij dus ook rekening houden met geluidsuitstoot. Deze fact sheet behandeld de vraag: ‘Welke geluidsvermindering technieken worden gebruikt in de huidige generatie gasturbines en wat kan worden verwacht in de toekomst?’ Deze vraag zal beantwoord worden door eerst een korte introductie te gegeven over de opbouw van een gasturbine en waar het geluid ontstaat. Daarna worden de technieken voor het reduceren van geluid uitgelegd. Als laatste zal worden omschreven wat er in de toekomst verwacht kan worden van de gasturbine producenten op het gebied van geluidsuitstoot.

This is a Luchtvaartfeiten.nl / AviationFacts.eu publication.

Authors: Dost, M; Jansen, R; Meer van der, D.

Editorial staff: R.J. de Boer, PhD, MSc; & G.J.S. Vlaming, MSc.

Copying texts is allowed. Please cite:

‘AviationFacts.eu (2016), Title Fact sheet,

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13 October 2016