MILLIMETER-SCALE, MEMS GAS TURBINE ENGINES

Copyright©2003byASME1ProceedingsofASMETurboExpo2003PowerforLand,Sea,andAirJune16-19,2003,Atlanta,Georgia,USAGT-2003-38866MILLIMETER-SCALE,MEMSGASTURBINEENGINESAlanH.EpsteinGasTurbineLaboratoryMassachusettsInstituteofTechnologyCambridge,[e-mail]ofmarketdemandforgreatlyimprovedcompactpowersourcesforportableelectronicswiththerapidlyexpandingcapabilityofmicromachiningtechnologyhasmadefeasiblethedevelopmentofgasturbinesinthemillimeter-sizerange.Withairfoilspansmeasuredin100’sofmicronsratherthanmeters,these“microengines”haveabout1millionththeairflowoflargegasturbinesandthusshouldproduceabout1millionththepower,10-100W.Basedonsemiconductorindus-try-derivedprocessingofmaterialssuchassiliconandsiliconcarbidetosubmicronaccuracy,suchdevicesareknownasmicro-electro-mechanicalsystems(MEMS).Currentmillime-ter-scaledesignsusecentrifugalturbomachinerywithpressureratiosintherangeof2:1to4:1andturbineinlettemperaturesof1200-1600K.Theprojectedperformanceoftheseenginesareonaparwithgasturbinesofthe1940’s.ThethermodynamicsofMEMSgasturbinesarethesameasthoseforlargeenginesbutthemechanicsdifferduetoscalingconsiderationsandmanufac-turingconstraints.Theprincipalchallengeistoarriveatadesignwhichmeetsthethermodynamicandcomponentfunctionalrequirementswhilestayingwithintherealmofrealizablemicro-machiningtechnology.Thispaperreviewsthestate-of-the-artofmillimeter-sizegasturbineengines,includingsystemdesignandintegration,manufacturing,materials,componentdesign,acces-sories,applications,andeconomics.Itdiscussestheunderlyingtechnicalissues,reviewscurrentdesignapproaches,anddis-cussesfuturedevelopmentandapplications.INTRODUCTIONFormostofthe60-year-plushistoryofthegasturbine,economicforceshavedirectedtheindustrytowardeverlargerengines,currentlyexceeding100,000lbsofthrustforaircraftpropulsionand400MWforelectricpowerproductionapplica-tions.Inthe1990’s,interestinsmaller-sizeenginesincreased,inthefewhundredpoundthrustrangeforsmallaircraftandmissilesandinthe20-250kWsizefordistributedpowerpro-duction(popularlyknownas“microturbines”).Morerecently,interesthasdevelopedinevensmallersizemachines,1-10kW,severalofwhicharemarketedcommercially[1,2].Gasturbinesbelowafewhundredkilowattsinsizegenerallyusecentrifugalturbomachinery(oftenderivativeofautomotiveturbochargertechnologyinthesmallersizes),butareotherwiseverysimilartotheirlargerbrethreninthattheyarefabricatedinmuchthesameway(cast,forged,machined,andassembled)fromthesamematerials(steel,titanium,nickelsuperalloys).Recently,manufacturingtechnologiesdevelopedbythesemiconductorindustryhaveopenedanewandverydifferentdesignspaceforgasturbineengines–onethatenablesgasturbineswithdiam-etersofmillimetersratherthanmeters,withairfoildimensionsinmicronsratherthanmillimeters.Theseshirt-button-sizedgasturbineenginesarethefocusofthisreview.Interestinmillimeter-scalegasturbinesisfueledbybothatechnologypushandauserpull.Thetechnologypushisthedevelopmentofmicromachiningcapabilitybasedonsemicon-ductormanufacturingtechniques.Thisenablesthefabricationofcomplexsmallpartsandassemblies–deviceswithdimensionsinthe1-10,000µmsizerangewithsubmicronprecision.Suchpartsareproducedwithphotolithographically-definedfeaturesandmanycanbemadesimultaneously,offeringthepromiseoflowproductioncostinlarge-scaleproduction.SuchassembliesareknownintheUSasmicro-electrical-mechanicalsystems(MEMS)andhavebeenthesubjectofthousandsofpublica-tionsoverthelasttwodecades[3].InJapanandEurope,devicesofthistypeareknownas“microsystems”,atermwhichmayencompassawidervarietyoffabricationapproaches.EarlyworkinMEMSfocusedonsensorsandsimpleactuators,andmanydevicesbasedonthistechnologyareinlarge-scaleproduction,suchaspressuretransducersandairbagaccelerometersforauto-mobiles.Morerecently,fluidhandlingisreceivingattention.Forexample,MEMSvalvesarecommerciallyavailable,andtherearemanyemergingbiomedicaldiagnosticapplications.Also,chemicalengineersarepursingMEMSchemicalreactors(chemicalplants)onachip[4].Userpullispredominantlyoneofelectricpower.Theprolif-erationofsmall,portableelectronics–computers,digitalassis-tants,cellphones,GPSreceivers,etc.–requirecompactenergyCopyright©2003byASME2supplies.Increasingly,theseelectronicsdemandenergysupplieswhoseenergyandpowerdensityexceedthatofthebestbatteriesavailabletoday.Also,thecontinuingadvanceinmicroelectron-icspermitstheshrinkingofelectronicsubsystemsofmobiledevicessuchasgroundrobotsandairvehicles.Thesesmall,andinsomecasesverysmall,mobilesystemsrequireincreasinglycompactpowerandpropulsion.Hydrocarbonfuelsburnedinairhave20-30timestheenergydensityofthebestcurrentlithiumchemistry-basedbatteries,sothatfuelledsystemsneedonlybemodestlyefficienttocompetewellwithbatteries.Giventheneedforhighpowerdensityenergyconversioninverysmallpackages,amillimeter-scalegasturbineisanobvi-ouscandidate.Theairflowthroughgasturbinesofthissizeisaboutsixordersofmagnitudesmallerthanthatofthelargestenginesandthustheyshouldproduceaboutamilliontimeslesspower,10-100wattswithequivalentcycles.WorkfirststartedonMEMSapproachesinthemid1990’s[