冲压式喷气和超燃冲压发动机原理.docx

冲压式喷气和超燃冲压发动机原理.docx

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冲压式喷气和超燃冲压发动机原理

Ramjet

Aramjet,sometimesreferredtoasaflyingstovepipeoranathodyd（anabbreviationofaerothermodynamicduct）,isaformofairbreathingjetenginethatusestheengine'sforwardmotiontocompressincomingairwithoutanaxialcompressor.Ramjetscannotproducethrustatzeroairspeed;theycannotmoveanaircraftfromastandstill.Aramjetpoweredvehicle,therefore,requiresanassistedtake-offlikearocketassisttoaccelerateittoaspeedwhereitbeginstoproducethrust.RamjetsworkmostefficientlyatsupersonicspeedsaroundMach3（2,284mph;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.Aérospatiale-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