冲压式喷气和超燃冲压发动机原理Word格式文档下载.docx
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冲压式喷气和超燃冲压发动机原理Word格式文档下载.docx
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3,675km/h).ThistypeofenginecanoperateuptospeedsofMach6(2,041.7m/s;
7,350km/h).
Ramjetscanbeparticularlyusefulinapplicationsrequiringasmallandsimplemechanismforhigh-speeduse,suchasmissilesorartilleryshells.Weapondesignersarelookingtouseramjettechnologyinartilleryshellstogiveaddedrange;
a120mmmortarshell,ifassistedbyaramjet,isthoughttobeabletoattainarangeof35km(22mi).Theyhavealsobeenusedsuccessfully,thoughnotefficiently,astipjetsontheendofhelicopterrotors.
Ramjetsdifferfrompulsejets,whichuseanintermittentcombustion;
ramjetsemployacontinuouscombustionprocess.Theyaresimilartoscramjets,asystemdesignedforhigherspeedsthatusesasupersonicairflowinitscombustionchamber.Whileascramjetworkswiththesametechnology,thecombustionprocessdiffersslightly,resultinginahighercruisespeed.
Enginecycle
TheBraytoncycleisathermodynamiccyclethatdescribestheworkingsofthegasturbineengine,thebasisoftheairbreathingjetengineandothers.ItisnamedafterGeorgeBrayton(1830–1892),theAmericanengineerwhodevelopedit,althoughitwasoriginallyproposedandpatentedbyEnglishmanJohnBarberin1791.ItisalsosometimesknownastheJoulecycle
Design
Aramjetisdesignedarounditsinlet.Anobjectmovingathighspeedthroughairgeneratesahighpressureregionupstream.Aramjetusesthishighpressureinfrontoftheenginetoforceairthroughthetube,whereitisheatedbycombustingsomeofitwithfuel.Itisthenpassedthroughanozzletoaccelerateittosupersonicspeeds.Thisaccelerationgivestheramjetforwardthrust.
Aramjetissometimesreferredtoasa'
flyingstovepipe'
averysimpledevicecomprisinganairintake,acombustor,andanozzle.Normally,theonlymovingpartsarethosewithintheturbopump,whichpumpsthefueltothecombustorinaliquid-fuelramjet.Solid-fuelramjetsareevensimpler.
Bywayofcomparison,aturbojetusesagasturbine-drivenfantocompresstheairfurther.Thisgivesgreatercompressionandefficiencyandfarmorepoweratlowspeeds,wheretherameffectisweak,butisalsomorecomplex,heavierandexpensive,andthetemperaturelimitsoftheturbinesectionlimitthetopspeedandthrustathighspeed.
Diffuser
Ramjetstrytoexploittheveryhighdynamicpressurewithintheairapproachingtheintakelip.Anefficientintakewillrecovermuchofthefreestreamstagnationpressure,whichisusedtosupportthecombustionandexpansionprocessinthenozzle.
Mostramjetsoperateatsupersonicflightspeedsanduseoneormoreconical(oroblique)shockwaves,terminatedbyastrongnormalshock,toslowdowntheairflowtoasubsonicvelocityattheexitoftheintake.Furtherdiffusionisthenrequiredtogettheairvelocitydowntoasuitablelevelforthecombustor.
Subsonicintakesonramjetsarerelativelysimple.
Subsonicramjetsdonotneedsuchasophisticatedinletsincetheairflowisalreadysubsonicandasimpleholeisusuallyused.Thiswouldalsoworkatslightlysupersonicspeeds,butastheairwillchokeattheinlet,thisisinefficient.
Theinletisdivergent,toprovideaconstantinletspeedofMach0.5(170.15m/s;
612.5km/h).
Combustor
Aswithotherjetengines,thecombustor'
sjobistocreatehotair,byburningafuelwiththeairatessentiallyconstantpressure.Theairflowthroughthejetengineisusuallyquitehigh,soshelteredcombustionzonesareproducedbyusing'
flameholders'
tostoptheflamesfromblowingout.
Sincethereisnodownstreamturbine,aramjetcombustorcansafelyoperateatstoichiometricfuel:
airratios,whichimpliesacombustorexitstagnationtemperatureoftheorderof2,400K(2,130°
C;
3,860°
F)forkerosene.Normally,thecombustormustbecapableofoperatingoverawiderangeofthrottlesettings,forarangeofflightspeeds/altitudes.Usually,ashelteredpilotregionenablescombustiontocontinuewhenthevehicleintakeundergoeshighyaw/pitchduringturns.Otherflamestabilizationtechniquesmakeuseofflameholders,whichvaryindesignfromcombustorcanstosimpleflatplates,tosheltertheflameandimprovefuelmixing.Overfuellingthecombustorcancausethenormalshockwithinasupersonicintakesystemtobepushedforwardbeyondtheintakelip,resultinginasubstantialdropinengineairflowandnetthrust.
Nozzles
Thepropellingnozzleisacriticalpartofaramjetdesign,sinceitacceleratesexhaustflowtoproducethrust.
ForaramjetoperatingatasubsonicflightMachnumber,exhaustflowisacceleratedthroughaconvergingnozzle.ForasupersonicflightMachnumber,accelerationistypicallyachievedviaaconvergent-divergentnozzle.
OneofthetwoBristolThorramjetenginesonaBristolBloodhoundmissile
Performanceandcontrol
Althoughramjetshavebeenrunasslowas45metrespersecond(160km/h),belowaboutMach0.5(170.15m/s;
612.5km/h)theygivelittlethrustandarehighlyinefficientduetotheirlowpressureratios.
Abovethisspeed,givensufficientinitialflightvelocity,aramjetwillbeself-sustaining.Indeed,unlessthevehicledragisextremelyhigh,theengine/airframecombinationwilltendtoacceleratetohigherandhigherflightspeeds,substantiallyincreasingtheairintaketemperature.Asthiscouldhaveadetrimentaleffectontheintegrityoftheengineand/orairframe,thefuelcontrolsystemmustreduceenginefuelflowtostabilizetheflightMachnumberand,thereby,airintaketemperaturetoreasonablelevels.
Duetothestoichiometriccombustiontemperature,efficiencyisusuallygoodathighspeeds(aroundMach2–Mach3[680.6–1,020.9m/s;
2,450–3,675km/h]),whereasatlowspeedstherelativelypoorpressureratiomeanstheramjetsareoutperformedbyturbojets,orevenrockets.
Types
Ramjetscanbeclassifiedaccordingtothetypeoffuel,liquidorsolid;
andthebooster.
Inaliquidfuelramjet(LFRJ),hydrocarbonfuel(typically)isinjectedintothecombustoraheadofaflameholderwhichstabilisestheflameresultingfromthecombustionofthefuelwiththecompressedairfromtheintake(s).Ameansofpressurizingandsupplyingthefueltotheramcombustorisrequired,whichcanbecomplicatedandexpensive.Aé
rospatiale-CelergdesignedanLFRJwherethefuelisforcedintotheinjectorsbyanelastomerbladderwhichinflatesprogressivelyalongthelengthofthefueltank.Initially,thebladderformsaclose-fittingsheatharoundthecompressedairbottlefromwhichitisinflated,whichismountedlengthwiseinthetank.Thisoffersalower-costapproachthanaregulatedLFRJrequiringaturbopumpandassociatedhardwaretosupplythefuel.
Aramjetgeneratesnostaticthrustandneedsaboostertoachieveaforwardvelocityhighenoughforefficientoperationoftheintakesystem.Thefirstramjet-poweredmissilesusedexternalboosters,usuallysolid-propellantrockets,eitherintandem,wheretheboosterismountedimmediatelyaftoftheramjet,e.g.SeaDart,orwraparoundwheremultipleboostersareattachedalongsidetheoutsideoftheramjet,e.g.SA-4Ganef.Thechoiceofboosterarrangementisusuallydrivenbythesizeofthelaunchplatform.Atandemboosterincreasestheoveralllengthofthesystem,whereaswraparoundboostersincreasetheoveralldiameter.Wraparoundboosterswillusuallygeneratehigherdragthanatandemarrangement.
Integratedboostersprovideamoreefficientpackagingoption,sincetheboosterpropellantiscastinsidetheotherwiseemptycombustor.Thisapproachhasbeenusedonsolid,forexampleSA-6Gainful,liquid,forexampleASMP,andductedrocket,forexampleMeteor,designs.Integrateddesignsarecomplicatedbythedifferentnozzlerequirementsoftheboostandramjetphasesofflight.Duetothehigherthrustlevelsofthebooster,adifferentlyshapednozzleisrequiredforoptimumthrustcomparedtothatrequiredforthelowerthrustramjetsustainer.Thisisusuallyachievedviaaseparatenozzle,whichisejectedafterboosterburnout.However,designssuchasMeteorfeaturenozzlelessboosters.Thisofferstheadvantagesofeliminationofthehazardtolaunchaircraftfromtheejectedboostnozzledebris,simplicity,reliability,andreducedmassandcost,althoughthismustbetradedagainstthereductioninperformancecomparedwiththatprovidedbyadedicatedboosternozzle.
Integralrocketramjet/ductedrocket
Aslightvariationontheramjetusesthesupersonicexhaustfromarocketcombustionprocesstocompressandreactwiththeincomingairinthemaincombustionchamber.Thishastheadvantageofgivingthrustevenatzerospeed.
Inasolidfuelintegratedrocketramjet(SFIRR),thesolidfueliscastalongtheouterwalloftheramcombustor.Inthiscase,fuelinjectionisthroughablationofthepropellantbythehotcompressedairfromtheintake(s).Anaftmixermaybeusedtoimprovecombustionefficiency.SFIRRsarepreferredoverLFRJsforsomeapplicationsbecauseofthesimplicityofthefuelsupply,butonlywhenthethrottlingrequirementsareminimal,i.e.whenvariationsinaltitudeorMachnumberarelimited.
Inaductedrocket,asolidfuelgasgeneratorproduces
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- 冲压 喷气 发动机 原理