Exh Loss Paper

7/27/2019 Exh Loss Paper

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NASA Tsctinlcil, Uorary

3 1176 01425 7738NATIONALADVISORY COMMITTEEFOE AERONAUTICS

TECHNICALNOTE NO.1149

PERFORMANCEOE HOODSFOBAIRCRAFTEXHAUST-GAS TURBINESByL.RichardTurner,Warren H.LowdermilkandAlbert M.Lord

SUMMARYTheperformanceofaturbosupercharger turbine was measuredwiththreetypesofexhausthood.heeffectivenessofthe turbine-hood combinationwas determined " b y measuringtheturbine powerandthethrustofthejetdischargedfrom thehoodatpressureratiosacrossthe turbine andhoodof1.35,1.7*smd. 2.0 for a rangeofblade-to-jetspeedratiosfrom0.1 to0.9.ompressedair wasusedasthedriving fluid.'Theresultsof thesetestsindicatethatan exhaust hood should

have aninletareaequal to thebucket-annulus area of theturbineand shouldbeequippedwithstraighteningvanes.system of vanes,the total chord of whichwasequal to theturbinepitch-linecircum-ference,achievedcompletestraighteningof theflow.ossesdueto


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NACA TWNo.1148

exhaust hoodsthatthe bucket-leavinglossIsrecovered(forexample,forjetpropulsion)Isohvlous.venwithsometypesofinefficientexhausthood,anappreciablejetpowercanheobtainedbyan increaseIn exhaustbackpressure but this method ofoperationcausesalossin enginepowerandalsoIn enginefueleconomy.urthermore,theincreasedbackpressure hasahadverseeffectonengine-operationlimitsand on thepowerfor turbineoperation in cruising.Theimportanceofprovidingefficientexhausthoodsfor turbine-compressorjet-propulsionenginesisevenmoreobvious as all thepoweroftheseunitsis providedbythedischargeoftheexhaust gasfromtheturbine.Anideal exhausthoodwouldrearwardlydirectthe axialmomentumofthe gasleaving the turbinewithoutlossandwouldalso rearwardlyredirect anytangentialmomentum.nInefficientexhaust hood might,however,destroy theexistingmomentum andperhapsmight require anadditionalpressuredrop to dischargethegases;or the hood mightIncreasethetangential'momentumofthejet attheexpenseof an

Increasedbackpressure.ngularmomentum remainingInthejetafterItisdischargedfrom the hood Isnotuseful forj o tpropulsionanditsexistencemayreduce thethrustobtainablewitha givenhood-exitarea anda given mass flow.


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NACATNNo.114S

showedthatthepressures near theturbine wheel varied markedlyfromplaceto place]an evaluation ofthe hoodlosses basedonthesemeasurements wouldtherefore beinsufficientlyaccurate.Asecondattempttoevaluatethedata usedan empirical relationof turbine power,speed,an d inletpressuretoestimatethe bucket-dischargepressure.egativelosses for the hoodsweresometimesobtainedfromthis method,however,when noangularmomentumcouldpossiblyhavebeenrecovered;therefore,the methodwas abandonedbecausetheturbineefficiencywas beinginfluenced by the hooddesign,Finallyallattempts atseparation of hoodlosses and effectsofthehoodson theturbineperformance were abandoned andthe anal-ysisofthetestdatawasbaaed ona se tofover-allturbine and hoodperformanceparameters.Performance parameters.-In thisreporttheperformanceofthehoodsisdescribedbycomparingtheturbinepower andmass flowpro-

ducedat variouswheelspeeds for a rangeof valuesofthe ratioofthestaticpressure attheinletof theturbineto thetotalpressureatthe hoodexit.hestaticpressure atthe turbineinlet waschosenastheupstreampressure because theeffectofexhaustbackpressureon engine power isusually based onstaticexhaust back


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NACA T I T No.1149

The following equation,basedonanassumedieentropicconver-sionof velocity energytopressureenergyand on the neglectofheatlossesor gainsin the hood,wasused tocalculate the bood-dxschargetotalpressure27-1 2L g E T - L P

2 r gRTl( 4 )

?P -u2

The neglectof heatlossesor gainsin the hoodin equation( 4 )isshown in appendixA tohavea negligibleeffecton the valueoftheover-all pressure ratio requiredforagiventurbinepower andwheels ' D o e d .

Turbine calibration.T h e "heperfonnanoe ofturbineexhausted directlyto the atmosphere

As aconvenientstandardofcomparison,turbinewithoutahoodwasmeasured.heTheperformance wasplottedintermsofthe factors/(* (xV3


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NACATN No.1149

where2sthe average hucket-dischargestaticpressure,as theturbinehadnohood,and2sthe "bucket-annulusarea.valueof41.5squareinches wasusedfor the hucket-annulusarea.

Effectof humidityondensityandavailableenergy.-Intheseteststheworking fluidwasai r atatemperatureofabout80 3 ? .thigh pressure ratiosthe work abstractionwassufficienttocool theairbelow itsdewpointandoccasionallybelowthe freezingpoint.Visual observation ofthewakeofthe turbinewiththe hoods removedshowedthat novisible condensationoccurred inthe airstream untilafter theflowleftthe buckets,although in afewcases aslightamountof frost wasseen onthe turbinebuckets.heseobservationsindicate thatlittleor no energywasliberatedby condensation whilethe ai rwasintheturbineandthat the turbinepowerwasthereforenot affectedby condensation.

Condensation did occur in thehoods.hemethod ofanalysisofthetestdata isshown inappendixAto permit neglectofcondensa-tion exceptin thecalculation ofthe densityof thegasatthe hoodexit.hiscalculation was basedon the assumption that,between theturbine and the hood exit,the mixture attained the equilibriumcon-dition at whichthe airissaturatedwithmoisture.he method usedto calculate thedensity ofthe wetairandthecorresponding correc-tiontothe velocityisdescribed inappendix C .


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I J A C A TN Ko.1149

similar to thatdescribed in reference1 .hethrust,orreactionof turning the air through,anangleof90,was measuredbya beamscale.hetargetconsists,of asleeveclosedaton eendand mountedtopivotabout an axis at right anglestothedirection.ofdischargeofthe airfromthesleeve.hesleevewasmounted inside atank.The airentered thesleevethrougha holeneartheclosed end andwasturnedthroughanangleof90.henthe axisofsupportofthesleeveis normal tothedirection ofthe flow,ofthe air asitentersthetarget,the momentofmomentumproducedisequaltothe productofthe momentumofthe airstreamtimesitsmeandistancefrom thesupportingaxis.hedistance from thecenter oftheentering airstreamtotheaxisofsupportofthesleeve and thelengthofthereactionarm ar eequal;thescaletherefore readsthe. J e tthrust.

Calibration of thrust'target.-The thrusttarget wascalibratedby measuringthe reactionof anairjetflowing axially from a noz-zle.hethrustof the airjet wasdeterminedbymakinga wakesur-vey ofthe nozzle.comparison ofthej e tthrustas measuredbythetargetandbythe nozzlewakesurvey iashown in figure2 .hedif-ferences aresmall.Turbinecalibration.-Th eturbinefirst wascalibratedas areferenceusing a fairing posttostablizetheair flow fromthewheel. (Seefig.3.)hestaticpressure atthewheelexitwas


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10 ACATN No.1149

areshownin figure4 .drawingofthe hoodisshown in figure4(a).Hiepowerandmass-flowfactorsareshown In figure4(h).Atapressureratioof1.35,theturbinepower withthehoodisequaltotheturbinepowermeasured duringtheturbinecalibrationhutthe mass flowisslightlyreduced(fig.3(b)).scomparedwiththeturbinecalibrationtheover-allefficiency computedfromequa-tion( 1 )isincreasedabout1.4points(from68.4to69.8 percent).Atapressure ratio,of1.7, theturbinepower andmass-flowfactorsareequaltothosemeaavred during theturbinecalibration exceptat

veryhighwheelspeeds.hen the mass flowand power are reduced,apparentlybecauseoflarge hood losses.heover-all turbine-hoodefficiency islower than that for theturbinecalibration in thishigh-wheel-speedregion.tapressure ratioof2.0,alossin poweroccursascomparedwith the turbinecalibration(fig.3(b)),withthe hood corresponding to about2pointsofturbineefficiency.The ratiooftheeffective hood-discbargevelocityJ J ,asmeasured bythethrusttarget)tothe hood-discharge velocityc

calculatedfrom the continuityequation isshownin figure4(c).Thevalueofthis ratioissubstantially constantovertheturbine-speedrangean dnearlyequaltounity in agreement withtheobser-vation,madewiththetarget removed,thatthe flow fromthe hoodwassubstantiallyaxial an duniform atall times.


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NACATN No.114S 1

Corticalhoodwithlong-chord straightening vanes,wheelfairing,a r - d hoodentrance fairing " b a n d .-Thepreviouslymentioned conicalhood( f i t .4(a))did notprovide asmooth and close-fitting entranceforthepassageof airfrom the buckets to the hood.n order todetermine thepossibility of reducing thelossesinthis region,tests were run withafairingband installedinthe hood entrance.(Seefig.5(a).)hi s fairingbandmade the hood-entrance area equalto the bucket-annuluBarea.

Thepowerandthe mass flowobtainedwiththis hood ar eshownin figure5(b).heprincipal effectofthe fairingwastoincreasethepoweratevery point.heimprovementattained isequivalent toabout1pointin turbineefficiency.he velocityratio/ u , ,Sshown in figure5(c).he variation issimilar to thatofthe vanedconicalhoodwithout the fairingband(fig.4 . ( c ) ) .

Conicalhoodwithwheelfairing; supportedby 5/8-inchdiametertubes.-Inorder todetermine thevalueofstraighteningvanesinconical hoods,testswere runwith aconicalhood(fig.6(a))sim-ilar to thevaned conicalhood,figure4(a),exceptthat nostraight-eningvanoswere used.he wheelfairingwassupportedby six3/e-inch-diameter tubes.

The power and mass-flow factorsfor this hoodareshown in


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12 CATB No.1149

would producetheeffectiveaxialvelocity.he pover oftur-bines withvanelesshoods foraconstant ratio oftheinletstaticpressuretototal hood-discharge pressurei s . thereforeunusually lowwhenanappreciable '. partoftheavailable pressuredropisexpendedin the generation ofincreased swirl.helong-chordstraighteningvanesin theconicalhood(figs.4(a)and5(a))prevented the growthofswirl.Lossesdu etoswirlin the vaneleBSconical hood are moreserious forwheelspeedsgreater than thosefor axialflowthanforwheelspeedslessthan thosefor axialflow.hepressurelossisprincipally determinedbytheamountofswirl.he hoodlossesincrease the bucket-discharge pz^essureand thus reducethespeed oftheJetapproaching theturbinebuckets,increasethe actualblade-to- Jetspeedratio,anddecreasethe availableturbinepower.henthe wheelspeedisincreased abovethe wheel "speedforaxial flow,theactual blade-to-Jetspeed ratio and thereforetheswirl and thelosses rapidly increase becausetheJetspeed decreasesasthewheelspeedincreases.henthe wheelspeed i s " decreasedbelowthe wheel

speedfor axialflow,theswirl an d thelossesincreaselessrapidlythan in the previouscasebecause thetrueJetspeedisdecreased asthe wheelspeedisdecreased and thetrue blade-to-Jetspeedratiothereforechangeslessrapidly.


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14 ACATNNo.1149

threeabort-chordstreamlinestruts(fig.8(a)).hechord ofeachstrutIs2.0i n c h e s " .he turbinepover forthisarrangementisshownin figure8(h).scomparedwith the vaneless hoodwith nocoolingcap(fig.7 ( 8 - ) ) ,thepowerwasmuchgreater especiallyathighwheelspeeds. Thepowerwasalwayslessthanthepowerohtainedwiththelong-chord straightening-vanedhood(fig.5(h)).partofthislosswaaundoubtedlythrottlinglossdu etothechangein areaimmediatelybehind thecoolingcap.

Variationofthe-ratiooftheeffective hood-discharge velocitytothecalculatedhood-discharge velocityAUit hhlade-to-Jetspeedratio/vsshown infigure8(c).hisfigureshows evi-denceofexistenceofaconsiderableswirlintheJ e t ,when comparedwiththesame ratiosfor the hoodwiththelong-chord straighteningvanes(fig.4(c)).he rateof reduction ofthe velocityratioU j f l / u , - . ,sthe wheelspeed ischanged from that for axial flow,ismuchloss thanthatohtainedwith the vanelessconical hoodwithorwithout awheelfairing(fig3.6(c)and7(c))indicatingthat anappreciablestraighteningof the flowhadbeenachieved t> ythethreeshort-chord vanes.

Conioal hoodwithflatcoolingcap s'upportedby 3/8-inch tubes.-In order-todistinguish between thestraighteningeffectofthesmallstreamlinedstruts an d anyeffectsdirectly attributabletothe pres-


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NACA TNNo.1149 5

the flatcoolingcapsupportedbythreeshort-chord vanes(fig.8 ) underallconditions.hemaximumover-allefficiency is decreasedabout3to4 points ascompared w i t h ,theincreasesinefficiencygiven bythe "hoodsof figures4(b)and 5(b).lthough somestraight-ening ofthe flow vas attained withtheelliptical vanes,the vaneswereeithertoolargein diameter,or too poorlystreamlinedto pro-videaflowpath withlowaverage resistance.

The variationofthevelocityratioJ J J / U , ,ith blade-to-jetspeedratio/vhownin figure10(c) issimilar tothat for theconicalhood with theshort-chordstruts(fig.8(c)).he formationof a pronouncedhollow-coredjetfrom the hooddidnotoccurwitheithertheshort-chord streamlinedvanes orwith theconventionalcooling capasevidencedbythe factthatthe velocity ratiodidnottendto riseatthe high andlow blade-to-Jetspeed ratios.hisbehaviorindicatesthattheflow remainedmore nearly uniformthaninthecaseofthe vaneless hood(fig.7(c)).

Flat-Nozzle HoodsThe flat-nozzlehood(fi^s,11(a)and11(b))wasso designedthattheentrance areawasequal to the bucket-annulus area.twasalso designedto bring the gas from the roundannulussection


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16 ACATNNo.1149

Plat-nozzlehood -with 9 0 - " b e n d extension.-A SO0 bend exten-sionw i t h . theradiusofcurvatureofthebend equal tothe width ofthehood extension wastestedwiththeflat-nozzlehood. (Seefig.12(a).)he powerand mass-flowfactorsforthishoodandextension areshown in figure12(b).tthe pressure ratiosof1.35and1.7* e power factorfor the 90 bend extension is nearly equaltothatfor thestraightextension.tthe pressureratioof2.0,aseriouslossoccurs.heTelocityJU/UQor the flat-nozzle hoodwith90 bendisshownin figure12(c).hehighpeakofthisratioan d thepoor over-allperformanceofthis hoodatapressureratioof2.0are probablytheresultofthe formationofastablesystemofshockwavesatthe dcwistreamend of the bend.

Short-Turning-HadiusHoodAsketchoftheconventionalshort-turning-radius hoodisshownin figure13(a).he power andmass-flowfactors for thishood areshown in figure13(b).hemaximum power factor ofthis

turbine-hood combinationis about11.5 percentlower than the powerfactorfor theconical hoodwithlong-chordstraightening vanesandhood-entrance fairingband(fig.5) .


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HACATNNo.1 3 . 4 9 7againstthe wheel-speed parameter/ygRTir theblade-to-.jetspeedratio/v.ta fixedvalue of/v,rof/y^gRTi, thechangesin the torquecoefficient are proportional tochangesintheefficiency.ecause theefficiencyisdividedby/ v ,thetorquecoefficient varies morerapidlywith lossesthandoestheefficiency.he usualmethodofplotting efficiencyorpower againstwheel speed ragainst/vends tomasktheeffectoflossesin the regionoflowwheel speedsand toenlargethem athigh wheelspeeds.

Therelation between torquecoefficient andthe wheel-speedparameter atapressure ratioof2.0isshownin figure14 for theturbine withoutahoodandfor the hoodsdescribedin figures5 ,6 ,8 ,11,and 13.hedifferencein the valuesoftorquecoefficientfor theseveralexhaustconfigurationsisanindicationofthe dif-ferencesin efficiency.

Thethreeshort-chord streamlined struts(having asolidity of0.175,wheresolidity isdefined asthetotalchordlengthdividedby theturbinepitch-linecircutcference)effected aboutone-half asgreatan improvementin performance asthelong-chord strut,whichhadasolidityof1.0.his resultsuggeststhananintermediatestraightening-vanesoliditymay " b eadequate formostapplications.


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18 TACATNNo. 1149

pitch-linecircumference.ossesdu etoswirl, were reduced,however,toaboutone-halftheir, maximum by straightening vaneswitha totaichord 0.175 timesthepitch-linecircumferenceofthe turbine.

4 .The flat-nozzlehood,whichwasdesigned to guide the gasfromthe roundbucket^annulussection i n t o , aflatduct,imposed nogreaterloss than the bestconical hood.thighMachnumbers,thelossesin a90bend,high-aspect-ratio,flatductw i t h , the axisofbending parallel to thelongersidewere very large.5 .Theconventionalshort-turning-radius hood often usedfor aflighthood in turbosuperchargerinstallationsgave11.5 percentlesspowerthantheconical hoodwithlong-cherdstraighteningvanesand hood-entrance fairingband atablade-to-Jetspeedratioof0.4and apressure ratio'of2.0.

Aircraft Engine Research Laboratory,National AdvisoryCommittee for Aeronautics,Cleveland,Ohio,May 27 ,1946.


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NACATNNo.1149 9APEENDIX A

EFFECTOF HEATADDITION ONTOTAL PRESSUREThemain effectofmoisturecondensation onthedetailsoffluidflow,inparticular theeffecton thepressuredrop,iscausedbythe releaseof the heatofvaporization ofthecondensedwaterin the fluid.nthe following discussion,theeffectsofcondensationarethereforeconsidered as theeffectofthe additionofheattoamovinggasstream." W h e n heatis added toastream of gasmoving atasubsonicvelocity,apressuredrop occursin thedirection of flow "becausethe velocityof the gasisincreased.n ductsofconstantcross-sectional area,thechangeinstaticpressureisequalbutoppositein signto thechangein momentumperunitof area.henthe amountof heataddedissmall and theinitial Mach numberissmallenoughtLattheeffectofchangein static pressure ondensitymaybeneglected,thedropinstaticpressrarebetween thepoints1and 2

is givenbytheequationPl-p2= PXU-L2*2-tl = plUl2At6)

t


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20 ACATNNo.11491 2The truetotalpressure?+ Pgu2 ^s - ' w'er ' t h a n " t h etotal

pressurebefore the addition of heat.n equation(1),however,thetemperatureused,in effect,to calculatethedensity2snot2ut.he apparenttotalpressure2ctsection 2istherefore

1 P2U22At iP2U22 . .E2 c = p 2 + 2&H~ =Pl ' Pl Ui :H + 2 T (8)Ifthechancein pressureisassumed tohave a negligibleeffectinchanging thedensity,

1 P2U22SH2c~p2+2gRt2LI P2U22 1P2U22AtP2+ " 5 gRt2 2 gRt2x

= s2-l 2li


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HACATN No.1149 1

APPENDIXBCALCULATION OF TEETOTAL PRESSUREOF AGASFLOWING- INATUBEWien the assumption,ismadethatthe velocity,the temperature,and thestatic pressureof a gasflowing through auniform t u b e , athigb Reynolds number ar e uniform,thetotalpressure may becalcu-latedfromaconsideration ofthecontinuityand energyequationsandthetemperature-pressure relationforanisentropicexpansion.Theserelations ar eexpressedby the following equations:

and

M = P 2 u2A2

4 - u,2*2 * - 27

p2 P 2S3t2

(11)

(12)

(13)


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KACATW No.1149

(17)

Thecombination ofequations(12),(14),an d(17)givesthe relation

2=1 H2\7

2 7 gET227 _ 27~r\s 2 u2

2?~l 27 P2A27-1 M Po ? \ 27.7-1 M + FI^2 ,7-1 M/

(18)


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24 ACANNo, 1149APPENDIX

CORRECTIONFHEALCULATEDOOD-D ISCHARGEVELOCITY FORHUMIDITYOFTHEAIR

Becauseofthe high apparentspecificheatofthe mixtureofair and - w a t e rJustbelowthedew-pointtemperature,the temperaturedropin thehood wasless whencondensation occurred than thatcal-culatedfordry air. Calculation shows,however,thatthediffer-ence betweenthe mean density of airwith asmallamountofcondensedwater andthatofdryair is negligible.;Figure15was therefox-eprepared showingtheenthalpyanddensity ofdryair and theenthalpyof air withvariousabsolute humiditiesasa function ofthetempera-ture fora pressureof1 atmosphere.

Thecorrection ofthecalculatedhood-discharge velocityQforeffectofcondensation was appliedby'calculating the dry-airdownstreamtemperature and findingthecorrespondingdensityandenthalpyon thechart.hetrue dischargetemperatureIs foundatthesameenthalpy on thelinecorresponding tothe absolutehumidity.hecorrected,orwet,density is foundatthistrue


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I 7 A C A T U " Eo.1149 5

APPENDIXDEVALUATION OFANOZZLE SURVEY WITHSWIRLABOUT THEAXISOFSYMMETRY

Thethrustproducedbytheflowfroma nozzlemayheobtainedby evaluation oftheintegralF = /V p U a 2d A + JJ ( p - p 0)dA (21)

wherep localstaticpressurep atmospheric pressureU Q , xialcomponentof velocityof flowfrom nozzle

Whenasurvey ismadeof the flow fromanozzle,thetotalpressureand thedirection ofeach radialelementismeasured.fthe nozzleissmalland the velocityhigh,measurementofthestaticpressureisverydifficult.henaxialsymmetry existsin theflow,theradialvariation ofthestaticpressuremay beestimated,


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26 NACA TNNo.1149

where.= Usin 9 andI I absolute velocity of fluid0 angleof velocityvectormeasuredfrom planethroughaxisofnozzle andpointofsurvey

Thedifferential equation(22)thenbecomes

1 dp 1 U2sin2ePor gRT U2 =

0 (24)

Thevelocityr is given " b y theequation

U 2 7 gRT7-1

(25)When thevalueof fromequation (25)issubstitutedin


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l i A C A TN I ' o .1149 2 7pressure.tis necessarytoassumethattheflow becomes a"solid-body" flownear thecenter;thatis,thetangentialTelocityispro-portionaltothe radius.newformoftheintegral equationismoreconvenientforthiscentral core,namely

logeP = *b2*r

2712 2 U | jsc (28)

Thevalueof inthiscentral core may beobtainedwith sufficientaccuracy eitherbj extrapolationofvaluesof easured,outsidethiscoreor byassumingthatan9 equalsc l/ua,here isassumed constant.his assumptionievalidonlyinthe region verynear= 0 .he axialvelocitygillbe negativenear the axiswhen a flowreversaloccurs.

Themass flow may beobtainedfrom thestatic-pressuredistri-bution calculatedwiththe aidofequations(27)and(28)bytheuseofthe followingequation:

..//puadA = 27 2=1 7-17 pcos82 j o * d r2G)


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NATIONAL ADVISORYCOMMITTEE FOR AERONAUTICS Tl 1 1 1 1 IIII

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66/66

Lord,A.M .,andthersAUTHOXS)

C R O S SE F E R E N C E S : Nozzles,xhaust67001);urbouper-chargers (95550) TN-11RAMER.I T L E :PerformancefoodsoraircraftxhaustasurbinesFORG'N.TTIE:ORIGINATINGGENCY:NationalAdvisoryCommitteeorAeronautics,Washington,D.C.TRANSLATION: CTg[lF]OUNTRYU.S. LANGUAGE Big. 'ORG'NOASS U.S.CLASS.Unclass. DAT INov Idiagrs,raphs,rw

ABSTRACTConical,lat-nozzle,hort-turning-radiusoodswereperformanceestednair-drivenurbosupercharger. Resultshowedhathrottlingndeakageossereducedbyuseofaoodhavingntrancelowareaequalobucketannulusreacausedywirlwereeduced,ndtraighteningfhelowwaschievedytrvanes. Flat-nozzleoodmposedogreaterossha nhebestconicalhood. Tturning-radiusoodgave1.556essowerhananedonicalhood.

NOTBi RequestsorcopiesWashington,.. of this report must e addressed to: N.A C.A. T-2 .HQ.AATERIEL COMMAND AIR TECHNICAL INDEX WR I GHTIELD OHIO USWf-

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