1、NASACONTRACTOR REPORT STUDY AND DEVELOPMENT OF TURBOFAN NACELLE MODIFICATIONS TO MINIMIZE FAN-COMPRESSOR NOISE RADIATION Volume I1 - Acoustic Lining Development Prepared by THE BOEING COMPANY Seattle, Wash. 98124 joy Lagley Resend Cefzter NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 0 WASHINGTON, D
2、. C. JANUARY 1971 I Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TECH LIBRARY KAFB, NM 1. Report No. 2. Government Accession No. NASA CR-1712 L 4. Title and Subtitle STUDY AND DEVELOPMENT OF TURBOFAN NACELLE MODIFI- ZATIONS TO “IZE FAN-COMPRESSOR
3、NOISE RADIATION. VOLUME II -ACOUSTIC LINING DEVELOPMENT. 7. Authorls) .“ _- I 3. Recipients Catalog No. I 5 1971 I 6. Performing Organization Code 8. Performing Organization Report No. “ . 10. Work Unit No. 9. Performing Organization Name and Address The Boeing Company Seattle, Wash. 98124 I. 11. Co
4、ntract or Grant No. NAS 1-7129 13. Type of Re rt and Rriod Covered 12. Sponsoring Agency Name and Address “_ - Contractor %port May 1, 1967 to Nov. 1, 1969 National Aeronautics and Space Administration Washington, D.C. 20546 14. Sponsoring Agency Code - “ I 15. Supplementary Notes 16. Abstract An ac
5、oustic lining development program is described, which covers selection of facing naterials and lining concepts suitable for attenuating turbofan engine noise generated by a fan :ompressor and development of acoustic iining design technology. A theoretical method for predict- ng lining attenuation in
6、 a duct without airflow is presented and is combined with an empirical Irediction method developed from parametric results of the flow-duct tests to establish a lining tesign procedure. Concurrent and interrelated investigations, which comprised the acoustic lining levelopment program, are described
7、 as is the facilities equipment used. Evaluation of more than LO types of acoustic facing materials resulted in the selection of woven metallic fiber, metallic felt, *einforced metallic felt, and resin impregnated fiberglass laminate facing materials as most suitable or acoustic linings in a turbofa
8、n nacelle environment. A study of lining concepts resulted in ;electing a broadband resistive-resonator type lining as best suiting engine-fan-duct and inlet-. nstallation requirements. Results are presented of analytical and experimental investigations mdertaken to develop an acoustic lining design
9、 procedure and to select suitable lining concepts and naterials for reducing fan-compressor-generated noise in turbofan engines. A comprehensive Lcoustic-lining flow-duct test program established the parameters controlling the amount and peak requency of attenuation provided by a lining. Test result
10、s also showed metallic and nonmetallic inings, having the same flow resistance and geometry, provide similar attenuation characteristics. kod correlation was obtained between the results of the flow-duct and full-scale-engine tests. Zvaluation of acoustic and mechanical properties of polyimide-fiber
11、glass sandwich panels as acoustic inings resulted in the selection of this material for follow-on boilerplate fan-duct development. 17. KeyWords (Suggested by Authorls) Noise, aircraft Material, acoustic Acoustically absorptive duct linings 10. Distribution Statement Unclassified - Unlimited - 19. S
12、ecurity Classif. (of this report) 20. Security Classif. (of this page) 22. Rice* 21. NO. of Pages Unclassified Unclassified $3 .OO 184 For sale by the National Technical Information Service, Springfield, Virginia 22151 Provided by IHSNot for ResaleNo reproduction or networking permitted without lice
13、nse from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-STUDY AND DEVELOPMENT OF TURBOFAN NACELLE MODIFICATIONS TO MINIMIZE FAN-COMPRESSOR NOISE RADIATION OVERALL REPORT ORGANIZATION VOLUME II -ACOUSTIC LINING DEVELOPMENT VOLUME 111 -CONCEPT
14、 STUDIES AND GROUND TESTS VOLUME IV - FLIGHTWORTHY NACELLE DEVELOPMENT VOLUME V -SONIC INLET DEVELOPMENT VOLUME VI - ECONOMIC STUDIES VOLUME Vll -SUBJECTIVE EVALUATION TESTS iii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for R
15、esaleNo reproduction or networking permitted without license from IHS-,-,-CONTENTS Page SUMMARY 1 INTRODUCTION 2 SYMBOLS . 3 BASIC CONCEPTS FOR ATTENUATING SOUND IN DUCTS . 6 Lining Concepts . 6 Facing Materials . 7 Development Procedure 7 ACOUSTIC EVALUATION METHODS 8 Flow Resistance . 8 Acoustic I
16、mpedance . 10 Acoustic Attenuation . 10 Absorption Coefficient 10 MATERIALS SEARCH 14 Manufacturer/Vendor Survey . . . 14 Evaluation Criteria 14 Pratt namely, a resistive Rs(f) and a reactive Xs(f) component that are frequency f dependent. Acoustic impedance is expressed in the complex form Zs = Rs(
17、f) + j Xs(f). The reactive component Xs(f) is due to the inertia of the air in the facing of the lining and the stiffness or inertia of the air in the cavity behind the lining. Absorption Coefficient The normal incidence absorption coefficient of the porous materials and acoustic lining panels was d
18、erived from the measurements made in an acoustic impedance tube. The coefficient CXN is the ratio of the acoustic energy absorbed by the sample in the tube to the acoustic energy impinging on it. Acoustic Attenuation Three types of tests were conducted to determine acoustic attenuation properties of
19、 the facing materials and lining concepts. First, tests were conducted using the flow duct facility to determine the attenuation properties of various facing materials. Second, using the same flow duct facility, acoustic lining concepts were evaluated by means of acoustic lining panels. Third, the m
20、ost promising lining concepts and facing materials were tested in the fan exhaust duct of a full-scale engine. 10 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Flow duct tests.-Tests were conducted in the flow duct facility to measure the acoustic
21、attenuation or power insertion loss of various panels faced with a number of selected facing materials. Power insertion loss is equal to the difference in the power spectra measured with a lined duct and an unlined baseline duct with the same noise and airflow input conditions. Since the sound power
22、 level in a reverberant chamber is equal to the logarithmic sum of the sound pressure level and a constant (characteristic of the chamber), the difference between sound power spectra is equal to the difference between corresponding sound pressure level spectra. In all the following work, therefore,
23、the terms “attenuation” and “power insertion loss7 are interchangeable. To reduce the number of tests made with a given lining configuration, advantage was taken of the fact that the attenuation achieved in a duct 6 in. wide and lined on one wall was equal to that provided in a duct 12 in. wide and
24、lined on two facing walls, thus reducing these tests by half. The facility used is described in the following section. The results obtained permitted parametric evaluation of geometric and acoustic characteristics of the linings. Flow duct facility.-The flow duct facility was developed for evaluatin
25、g acoustic characteristics of duct lining panels and facing materials. Test provisions included a range of high-velocity airflow through the duct test section with airflow and noise propagating in the same direction to simulate the fan exhaust ducts of a turbofan engine. A schematic diagram of the f
26、acility is shown in figure 7. A reverberant chamber provided a diffuse acoustic source for the test duct and also acted as a plenum chamber to ensure uniform airflow into the duct test section. The plenum chamber volume was approximately 45 ft3. The large diffuser in the air supply line reduced the
27、entry air velocity into the plenum chamber and hence improved the flow duct inlet aerodynamics. A bellmouth at the duct inlet ensured minimum aerodynamic distortion in the test section and minimized the generation of aerodynamic noise. Three noise sources were available in the reverberation/plenum c
28、hamber. 0 Aerodynamic noise generated by the air turbulence in the supply line 0 A jet noise source to augment the above aerodynamic noise source 0 An air-driven siren generating discrete tones over the frequency range from 2000 to 8000 Hz with a fundamental peak amplitude of 150 dB in a 1 /I 0-octa
29、ve band- width measured in the reverberation/plenum chamber The test duct was connected to a calibrated semireverberant receiving chamber in which the total acoustic power transmitted through the duct was measured. The chamber volume was approximately 1750 ft3 and was vented by an acoustically lined
30、 exhaust stack. Three sizes of test section-6 by 10 in., 4 by 10 in., and 6 by 7 in. in cross section-were used in the test program. Wall positioning was adjustable to maintain constant working sec- tion dimensions for various thicknesses of lining treatments. The length of the lining test 11 Provid
31、ed by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-panels could be varied from 0 to 44 in., the latter being the full length of one test section element. Longer ducts could be tested by bolting two or more test sections together. An S-bend duct, 24 in. long w
32、ith a 6-in. offset, also was used for one test series. Lining panel test specimens could be mounted on any one of the four walls in the test section. Figure 8 shows the range of test section configurations used in the study. The maximum airflow velocity throua the test section was Mach 0.45 in the 6
33、- by IO-in. duct and Mach 0.65 in the 6- by 7-in. duct. The majority of lining panel tests was conducted in the Boeing-Seattle flow duct facility; however, some tests in which the duct airflow direction was reversed were conducted in the Boeing-Wichita flow duct facility. The latter facility can be
34、operated with air flowing the same direction as the sound propagation to simulate an engine fan duct as well as with air flowing in the opposite direction to simulate an engine inlet. The two facilities have the same basic acoustic and flow characteristics. Test procedure: The acoustic evaluation of
35、 an acoustic lining panel configuration was obtained by the measurement of acoustic power in the receiving chamber with and without test panels installed in the flow duct. Noise generated by the fan and compressor of a jet engine has both broadband and discrete frequency characteristics. Early tests
36、, therefore, were conducted to determine whether linings responded differently to the two types of noise. Results showed no measurable difference in attenuation at siren frequencies of 1800 and 2700 Hz. It was concluded that duct linings absorbed equal amounts of acoustic energy for either broadband
37、 or discrete fre- quency noise in a given bandwidth. All subsequent tests were conducted using broadband noise inputs only. Flow duct experiments were conducted at ambient laboratory temperature and pressure conditions; hence, for engine operating environments of other ambient levels, the design cal
38、culations must take into account changes in temperature and dmsity, which are reflected in the effective flow resistance of the linings. Duct wall pressure: Theoretical work indicated that the effective flow resistance of a lining increases with the magnitude of the rms particle velocity normal to t
39、he panel sur- face. The magnitude of this velocity is dependent upon both the acoustic pressure and the airflow turbulence pressure occurring at the lining surface. Wall pressure measurements were made to compute the rms particle velocity so that effective flow resistance (equivalently the resistive
40、 impedance) of the linings tested could be determined at the various test airflow Mach numbers. To measure the aeroacoustic spectra at the surface of porous laminates, microphones were wall-mounted close to each end of the test panels (40-in. separation). The microphone diaphragms were mounted flush
41、 with the laminate surface and without a dia- phragm protective grid. Aerodynamic measurements: To measure the horizontal and vertical velocity profiles in the duct test section, a traversing pitot tube apparatus was installed close to each end of 12 Provided by IHSNot for ResaleNo reproduction or n
42、etworking permitted without license from IHS-,-,-the test panels with a 40-in. separation along the duct. Tests to calibrate the facility showed low turbulence in the test section because of care taken with the aerodynamic design of the bellmouth inlet into the test section. Vertical- and horizontal
43、-velocity profiles in the test section were uniform with a normal boundary layer growth along the test section walls pro- ducing a downstream boundary layer thickness of approximately 0.5 to 1 in. in the 6-in. duct. Turbulence and boundary layer effects on the acoustic attenuation in ducts were not
44、studied separately. Full-scale engine evaluation of acoustic linings.-Subsequent to the development of duct-acoustic-lining concepts using the flow duct facility, a number of duct-lining configura- tions were evaluated in a full-scale engine fan duct. This study provided data for selecting the most
45、promising configuration for evaluation in a boilerplate version of the final flight nacelle. The study was conducted using the engine test facility described below. The acoustic attenuation properties of fan duct linings evolved from the earlier flow duct tests were determined. Test configurations:
46、Acoustic linings to be evaluated were installed in the constant- area section of an extended three-quarter-long fan duct. A constant treatment length of 50 in. was used for all configurations except two, which were 25 in. long and were tested to investigate the influence of lining length. The surfac
47、es treated, in various combinations, were: Outer wall Inner wall 0 Radial splitters 0 Circumferential splitter (with a porous septum) Nonporous blariking panels were fitted in the test ducts to maintain a constant flow area when some of the acoustically treated panels were not used (fig. 9). Two vie
48、ws of the fan discharge duct linings are shown in figures 10 and 1 1. Full-scale engine facility: Tests to investigate the acoustic performance of duct linings installed in a full-scale engine fan duct were conducted using a Pratt & Whitney JT3D-3B turbofan engine. The basic engine with an existing
49、short fan duct (fig. 12) was used as an acoustics baseline. The lining studies were conducted with an experimental three-quarter- long fan duct installation (fig. 13). Tests were conducted with a bellmouth inlet and also with an existing large blow-in door inlet. Test site: A plan view of the test site (fig. 14) indicate
