英文原文4.1IntroductionThischapterdemonstratesthescopeofthemethoddevelopedforthethree-dimensionalanalysisofascrewcompressor.TheCFDpackageusedinthiscasewasCOMETdevelopedbyICCMGmbHHamburg,todayapartofCD-Adapco.Theanalysisoftheflowandperformancecharacteristicsofanumberoftypesofscrewmachinesisperformedtodemonstrateavarietyofparametersusedforgridgenerationandcalculation.Thefirstexampleisconcernedwithadryairscrewcompressor.Acommoncompressorcasingisusedwithtwoalternativepairsofrotors.Therotorshaveidenticaloverallgeometricpropertiesbutdifferentlobeprofiles.Theapplicationoftheadaptationtechniqueenablesconvenientgridgenerationforgeometricallydifferentrotors.Theresultsobtainedbythreedimensionalmodellingarecomparedwiththosederivedfromaone-dimensionalmodel,previouslyverifiedbycomparisonwithexperimentaldata.Therelativeadvantagesofeachrotorprofilearedemonstrated.Thesecondexampleshowstheapplicationofthreedimensionalflowanalysistothesimulationofanoilinjectedaircompressor.Theresults,thusobtained,arecomparedwithtestresultsobtainedbytheauthorsfromacompressorandtestrig,designedandbuiltatCityUniversity.Theyarepresentedintheformofbothintegralparametersandap-indicatordiagram.Calculationsbasedontheassumptionsofthelaminarflowarecomparedtothoseofturbulentflow.Theeffectofgridsizeontheresultsisalsoconsideredandshownhere.Thethirdexamplegivestheanalysisofanoilinjectedcompressorinanammoniarefrigerationplant.Thisutilisestherealfluidpropertysubroutinesintheprocesscalculationsanddemonstratestheblowholeareaandtheleakageflowthroughthecompressorclearances.Thefourthexamplepresentstwocases,oneofadryscrewcompressortoshowtheinfluenceofthermalexpansionoftherotoronscrewcompressorperformanceandoneofahighpressureoil-floodedscrewcompressortoshowtheinfluenceofhighpressureloadsuponthecompressorperformance.4.2FlowinaDryScrewCompressorDryscrewcompressorsarecommonlyusedtoproducepressurisedair,freeofanyoil.Atypicalexampleofsuchamachine,similarinconfigurationtothecompressormodelled,isshowninFigure4-1.Thisisasinglestagemachinewith4maleand6femalerotorlobes.Themaleandfemalerotorouterdiametersare142.380mmand135.820mmrespectively,whiletheircentrelinesare108.4mmapart.Therotorlengthtomaindiameterratiol/d=1.77.Thus,therotorlengthis252.0mm.Themalerotorwithwrapangle=248.40isdrivenataspeedof6000rpmbyanelectricmotorthroughagearbox.Themaleandfemalerotorsaresynchronisedthroughtiminggearswiththesameratioasthatofthecompressorrotorlobesi.e.1.5.Thefemalerotorspeedistherefore4000rpm.Themalerotortipspeedisthen44.7m/s,whichisarelativelylowvalueforadryaircompressor.Theworkingchamberissealedfromitsbearingsbyacombinationoflipandlabyrinthseals.Eachrotorissupportedbyoneradialandoneaxialbearing,onthedischargeend,andoneradialbearingonthesuctionendofthecompressor.Thebearingsareloadedbyahighfrequencyforce,whichvariesduetothepressurechangewithintheworkingchamber.Bothradialandaxialforces,aswellasthetorquechangewithafrequencyof4timestherotationalspeed.Thiscorrespondsto400Hzandcoincideswiththenumberofworkingcyclesthatoccurwithinthecompressorperunittime.Figure4-1CrosssectionofadryscrewcompressorThecompressortakesinairfromtheatmosphereanddischargesittoareceiverataconstantoutputpressureof3bar.Althoughthepressureriseismoderate,leakagethroughradialgapsof150missubstantial.Inmanystudiesandmodelling,procedures,volumetriclossesareassumedtobealinearfunctionofthecrosssectionalareaandthesquarerootofpressuredifference,assumingthattheinterlobeclearanceiskeptmoreorlessconstantbythesynchronisinggears.Theleakagethroughtheclearancesisthenproportionaltotheclearancegapandthelengthoftheleakageline.However,alargeclearancegapisneededtopreventcontactwiththehousingcausedbyrotordeformationduetothepressureandtemperaturechangeswithintheworkingchamber.Hence,theonlywaytoreduceleakageistominimisethelengthofthesealingline.Thiscanbeachievedbycarefuldesignofthescrewrotorprofile.Althoughminimising,leakageisanimportantmeansofimprovingascrewcompressorefficiency,itisnottheonlyone.Anotheristoincreasetheflowareabetweenthelobesandtherebyincreasethecompressorflowcapacity,therebyreducingtherelativeeffectofleakage.Modernprofilegenerationmethodstakethesevariouseffectsintoaccountbymeansofoptimisationprocedureswhichleadtoenlargementofthemalerotorinterlobesandreductioninthefemalerotorlobes.Thefemalerotorlobesaretherebystrengthenedandtheirdeformationthusreduced.Todemonstratetheimprovementspossiblefromrotorprofileoptimisation,athreedimensionalflowanalysishasbeencarriedoutfortwodifferentrotorprofileswithinthesamecompressorcasing,asshowninFigure4-2.Bothrotorsareofthe“N”typeandrackgenerated.Figure4-2NRotors,Case-1upper,Case-2lowerCase1isanolderdesign,similarinshapetoSRM“D”rotors.Itsfeaturesimplythatthereisalargetorqueonthefemalerotor,thesealinglineisrelativelylongandthefemalelobesarerelativelyweak.Case2,shownonthebottomofFigure4-2,hasrotorsoptimisedforoperatingondryair.Thefemalerotorisstrongerandthemalerotorisweaker.Thisresultsinhigherdelivery,arelativelyshortersealinglineandlesstorqueonthefemalerotor.Allthesefeatureshelptoimprovescrewcompressorperformance.Theresultsofthesetwoanalysesarepresentedintheformofvelocitydistributionsintheplanesdefinedbycross-sectionsA-AandB-B,showninFigure4-1.Inthecaseofthisstudy,theeffectofrotorprofilechangesoncompressorintegralperformanceparameterscanbepredictedfairlyaccuratelywithone-dimensionalmodels,evenifsomeofthedetailedassumptionsmadeinsuchanalyticalmodelsareinaccurate.Hencetheintegralresultsobtainedfromthethree-dimensionalanalysisarecomparedwiththosefromaone-dimensionalmodel.4.2.1GridGenerationforaDryScrewCompressorInCase-1,therotorsaremappedwith52numericalcellsalongtheinterlobeonthemalerotorand36cellsalongeachinterlobeonthefemalerotorinthecircumferentialdirection.Thisgives208and216numericalcellsrespectivelyinthecircumferentialdirectionforthemaleandfemalerotors.Atotalof6cellsintheradialdirectionand97cellsintheaxialdirectionisspecifiedforbothrotors.Thisarrangementresultsinanumericalmeshwith327090cellsfortheentiremachine.ThecrosssectionfortheCase-1rotorsisshowninFigure4-3.Thefemalerotorisrelativelythinandhasalargeradiusonthelobetip.Therefore,itismoreeasilymappedthaninCase-2wherethetipradiusissmaller,asshowninFigure4-4.Figure4-3CrosssectionthroughthenumericalmeshforCase-1rotorsTherotorsinCase2aremappedwith60cellsalongthemalerotorlobeand40cellsalongthefemalelobe,whichgives240cellsalongbothrotorsinthecircumferentialdirection.Intheradialdirection,therotorsaremappedwith6cellswhile111cellsareselectedformappingalongtherotoraxis.Thus,theentireworkingchamberforthiscompressorhas406570cells.Inthiscase,differentmeshsizesareappliedanddifferentcriteriaarechosenfortheboundaryadaptationofthesero