REG NASA-TM-X-71627-1974 Interim prediction method for low frequency core engine noise.pdf

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1、!iNAIA fIIGHN ICAL NASATMX-71627 MIMORANOUMo IIZ x:3i(NASA- _-l-7 1697) ; _, 2 L I, ; _-“;P5 _DCgiO_: _75-12950 “METHCD FCR gO_ _,_!15L_.! _O_L -N31tJ_ ?LiO_.aE(NAS7 3t,65 “oINTERIMPREDICTIONMETHODFORLOWFREQUENCYCOREENGIP_NOISEby K.nald G. Huff. B:dce J. Clark, :and Robe: _ c. _._rschLewis Researcl,

2、 Center:.,._ Cleveland, Ohio 44135, j Nove_,ber 1974 t /,00000002Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-+ ABSTRACTA literature survey on low-frequenCy core engine noise is presented.Possible sources of low frequency internally generated nois

3、e in coreengines are discussed with emphasis on combustion and componentscrub-bing noise. Core noise generated by the turbine is high frequeucy, and +:is not included. An interim method, taken from the literature, is recom-mended for predicting low frequency core engine noise that is dominant jwhen

4、Jet velocities are low. This prediction procedure is developed insupport of the NASA Aircraft Noise Prediction Program. Recommendationsare made for future research on low frequency core engine noise that willi aid in improving the prediction method and help define possible additionalinternal noise-s

5、ources._t_4COr_ NASA,dlstributlon cateSorles: 23, Physlcs, General (Acoustlcs)Key words: ANOPP; Acoustics, Aircraft Noise, Combustion, Predictions,F_tglne Noise, Cores, Core, Noise!iProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-INTERIM PREDICTION M

6、ETHOD FOR LOW FREQUENCY CORE ENGINE NOISEby Ronald G. Huff, Bruce J. Clark, and Robert G. DorschLewis Research CenterSUMMARYA literature survey on low-frequency core engine noise is presented.Possible sources of low frequency internally generated noise in coreengines are discussed with emphasis on c

7、ombustion and component scrub-blng noise. Core noise generated by the turbine is high frequency, andis not included. An interim method, taken from the literature, is recom-mended for predicting low frequency core engine noise that is dominantwhen Jet velocities are low. This prediction procedure is

8、developed insupport of the NASA Aircraft Noise Prediction Program. Recommendationsare made for future t_search on low frequency core engine noise that willaid in improving the prediction method and help define possible additionalinternal noise sources.INTRODUCTION_ The noise reduction potential of e

9、ngine cycles and various noiset suppression technique may be limited by the presence of internal noisesources in the core engine. Further engine noise suppression, in suchcasea, will require reducing core noise generation or the use of corenoise suppression. For evaluation of the noise reduction ben

10、efits ofnew engines or modifications to existing engines, a prediction techniquefor core e _ine noise is required.Measurements of the acoustic power generated by low velocity Jetsoften show a deviation from the eighth-power velocity relation found athigher velocities. This discrepancy has been shown

11、 to be at least par-Lially caused by additional noise sources upstream of the nozzle. Inthe case of noise from an engine, the internal noise from the core engineadds to the Jet exhaust noise.At the present stage of definition of core engine noise sources, itis not possible to identify all the source

12、s with certainty. Some of theprobable sources are:(1) The combustion process(2) Flow around internal obstructions(3) Scrubbing of the duct walls(4) Local temperature fluctuations or hot spots flowing through theturbine and nozzlek00000002-TSA04Provided by IHSNot for ResaleNo reproduction or networki

13、ng permitted without license from IHS-,-,-_“ 2(5) Flow through the turbine(6) Turbine blade and stator interactionsOf these, the noises associated with the turbine appear at relativelyhigh i_equencies and therefore are not considered in this report. Com-bustion noise and internal flow noises appear

14、in the low- to mid-frequency range and are difficult to separate from Jet noise. In a cur-_ rent study (DOT-FAA Contract No. DOT-FA72WA-3023), an attempt is beingmade to isolate and study these effects separately, but the results are not yet available.Core engine noise data are sparse and of questio

15、nable accuracy. Fannoise and jet noise from the engine must be adequately suppressed so thatthe core noise remains as the major noise contribution in the Jow- andmid-frequency range. In most cases Jet noise is deducted from the datafor the total engine noise by means of existing jet noise prediction

16、:_ techniques. Unfortunately, much of the resulting core noise data is con-sdered proprietary by the engine manufacturers and cannot be shown inthis report.Prediction of core noise levels has followed two main lines of ap-proach. The first approach was based on treatment of the core noise asdeviatio

17、n from pure jet noise (refs. 1 and 2), and hence related thecore noise levels to jet exhaust velocity. The second approach usedengine operating parameters to predict the low frequency core noise. Theprediction method chosen herein is based on engine operating parameters,that Is, engine mass flow rat

18、e, compressor pressure and temperature ratio, _Iand the combustor temperature rise.This prediction method is presented in response to the need for pre-dicting low-frequency core noise as a component of total aircraft noisefor the NASA Aircraft Noise Prediction Program (ANOPP). The responsi-billty of

19、 this program :Isassigned to the Langley Research Center; how-ever, it is being developed Jointly by various NASA centers with help._ from industry representatives. In the Program, the various contributorsto and modifiers of aircraft noise are summed at various ground locationsin order to predict a

20、noise footprint for single- or multiple-event air-craft flights. The need for the ANOPP requires that this predictioni._i method be based on the present state-of-the-art. Refined techniques and_ _ better data may be available in the near future to permit up-dating thisii i prediction method.This rep

21、ort selects an interim prediction method for estimation ofi the low-frequency component of core engine noise. Combustion noise, in-terr.al flow noise, and perhaps entropy fluctuations contribute to thelow-frequency core noise. A review of recent published work in thisarea is included. Various recomm

22、endations are made for further work in_ the ar_a of low-frequency core noise, which might contribute to betterunderstanding of the controlling mechanisms and lead to firmer prediction“ techniques.:00000002-TSA05Provided by IHSNot for ResaleNo reproduction or networking permitted without license from

23、 IHS-,-,-3All symbols used in this report are defined in the appendix. U.S.Customary Units were found t_ be used in the literature, hence the equa-tions given in the literature survey section retain these units Theequations given for the recommended prediction method are also given inU.S. Customary

24、Units bvt factors for converting to S. I. Units are given.In the llst of symbols, U.S. Customary Units are given in parenthesesfollowing the S. I. Units.LITERATURE SURVEYFor convenience of presentation, the literature dealing with low-frequency core engine noise is divided into applied and fundament

25、al re-search.Applied research is considered thatwork which makes use of enginedata, and fundamental research is considered that which uses componenttesting for its primary source of data. In general, the fundamental re-search work has been performed by the universities; however, several air-craft en

26、gine companies are conducting basic acoustic tests with individualcombustion chambers. The applied research is generally being done by theaircraft engine companies.This llterature survey is not comprehensive. However, it should besufficient to give the reader an appreciation of the state of the art

27、oflow frequency core engine noise prediction techniques. Table I llstsmost of the references and identifieb th_ type of information containedin each reference for power level, sound pressure level, spectra, anddrectivity. The equations given herein were modified from those found Iin the references i

28、n order to use the standard symbols list given in theappendix. Reference power and reference sound pressure used in this re-port are 10-13 watts and 20 _N/m2, respectlvely.Applied ResearchBushell (refo I) and Marshall (ref. 2) are among the many investi-gators to note that typical Jet noise measurem

29、ents at low Jet velocities ifrom both jet noise rigs and engines show a deviation from the classical i_eighth power dependence on Jet velocity derived by Lighthill (ref. 3).Bushell cites internal noise sources associated with turbulence, struts,and flow through combustors as possible reasons for the

30、 deviation Hegives curves of overall sound pressure level as a function of jet velocitythat can be used to calculate the noise at low Jet velocities. Marshall(ref. 2) makes use of the curves presented by Bushell and also directsattention to the genera, dlrectlvlty of the various sound sources inengi

31、nes, i.e.p compressor, fan, Jet, turbinej and tailplpe (internalnoise). In general both Bushel1 and Marshall agree that the source ofnoise causing the deviation from the Jet noise is oz can bc internallygenerated.00000002-TSA06Provided by IHSNot for ResaleNo reproduction or networking permitted with

32、out license from IHS-,-,-: 4Ho and Tedrick (ref. 4) investigated combustion noise in small gasturbine engines. They state that “one of the most significant sources ofnoise from small turboshaft engines and auxiliary power units is the com-bustion pro_ess.“ The power spectra of individual combustors

33、and of tur-bine engines using the same combustor were obtained experimentally.These measurements showed that the combustion noise peaked at low fre-quencies, on the order of 125 hertz. Several types of combustors andengines were tested_ Empizical methods and dimensional analysis wereused to obtain e

34、quations for the acoustic power generated for both thecomponent combustor tests and the engine tests. No directivity and onlyone spectrum was reported in reference 4. The equations given for theacoustic power due to the combustion process are:OAPWL= 81 + 10 lOgl0 T4 P4V4(1 + F) 2 (1)based on data ta

35、ken in the combustion test r_g; and(T4 - T3 )4 _2,2 tOAPNLffi 23 + 10 lo810 _-2- _4v4(1 + F) 4 D (2)T4based on data derived from engine tests.Motsinger (ref. 5) used the work of Knott (ref. 6) ia conjunctionwith TF39 combustor pressure fluctuation data to predict the noise of theT64 turboshaft engin

36、e. Knotts data for nonpremixed (diffusion) flamesat atmospheric pressure showed that the “acoustic combustion efficiencyis a simple function of the fuel mass flow rate.“ Motsinger added cor-zections for pressure and temperature effects based on TF39 engine com-bustor data and found good correlation

37、_th the noise data from a T-64turboshaft engine. The final prediction equation given by Motsinger foracoustic power due to combustion noise is:2 (3), OAeNL = 56.5 + ZO lOgl 0 - -2 TO P0/T3The power spectrum for the T-64 engine is given in tabular form in refer-ence 5. The peak frequenc_ and spectrum

38、 for the T-64 engine may not bethe same as for other engines, as pointed out by Motsinger. Directivitydata for the T-64 engine shows a maximumsound pressure level at 120 de-grees from the inlet axis.: In a contractor report for NASA, Neitzel etal. (ref. 7) reportedthat the maximum overall sound pres

39、sure level at a 200-foot sideline dueto combustion noise is:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- “ 5OASPL- -2.5 + 10 loS10 4 - 3)2 i (4)No spectral or directivity da_a are 8iven other than a statement that thepeak OASPL occurred at an ans

40、le of 110 degrees from the inlet axis.Grands (ref. 8) concludes that the primary contribution to coreengine noise is combustion. Be presents an equation for combustion sen-stated noise based on Arnolds analysis of open flames (ref. 9). Theequation is modified for the transncLssion of this noise thro

41、ugh the tur-bine and Jet exhaust nozzle. The equations are:OAPWL - 205.5 + 10 .Ogl0BNEfWc (5)where the nozzle transmission coefficient BN is. TOBN = _SS (5.1)the turbine transmission coefficient _ isthe combustion noise WC vst2 Vb2 _ “4=,-,i (.T4T_ T3 1 Y4 a= - v. (1 + F) DL (5.3)where :DL L- - - fo

42、-,annular combustors (5.3.i)_“ D_ ADann_, ADann -22 tlme_ the annulus heightfor can type combustors (5.3.2)i , D_ D:an00000002-TSA08Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-6Grande states “Ho and Tedztcks empirical correlation,“ for engines,“a

43、grees fully with“ the above equation for WC, “except for a factor ofT_4.“ He compares data from low and higL bypass ratio engines and findsagreement with his correlation to within 1 dB_ However, he points out:“, .similar comparisons involving other engines are desirable to permitassessment of the fr

44、edictlon scheme accuracy,“ No spectra or dlrec-t_ity predictions are given by Gran_,Gerend et ale (refo 10) investigated core engine noise using bothmodel and engine tests. The model test consisted of an inte_al noisesource in a pipe s_plying air to _e inner nozzle of a co_ial nozzle ina configurati

45、on _proximating bypass type engines. _ese tests show thatinte_al noise c_ trigger system resonance so as to increase the reso-nant peak SPL 0vez that of the inte_al noise generator alone.F_I scale engine tests of a JT9D high bypass turbofan engine(ref. 10) showed that there was _tern_ly generated lo

46、w-frequency noise,and that it could be atten_ted by acoustic treatment of the plug used inthe core engine. _e des_ frequency of the acoustic absorber was500 Hz,The coze engine noise predicti_ parameters used by Oerend (ref. 10)assume that “co_ustor monopole noise is si_ificant“ and “dngine gasgenera

47、tor size will influence the ma_itude of the core noise.“ Frommonopole source theory and the fact that _e turbine nozzle is choked inmost engines, the combustion noise ia found to be proportional to thesquare of the co_ustor outlet temperature. _e corrected engine m_sflow rate is _sumed to provide a

48、size correction. The frequency rangeconsidered is 50 to 1000 hertz. Gerends equation for the overall soundpressure level at a 200 foot sideline and 110 degrees from the inlet a_sis:The recommended constant, C, was -15 dB for _nular combustors and -6 dBfor c_ or can-_nular combustorso _r high bypass

49、engines the accuracyof the prediction was quoted as 5 dB on OASPL.The reconnr_ended spectral shoe used by Oerend (zef. 10) was the SAgflight Strouhal Jet spect_m given in reference 11 with the pe_ fre-quency dependent on the corrected mass flow rate. The equation for thepeak frequency is:740fp ffi . (6.1)00000002-TSA09Provided by IHSNot for ResaleNo repr