英文原文1IntroductionThescrewcompressorisoneofthemostcommontypesofmachineusedtocompressgases.Itsconstructionissimpleinthatitessentiallycomprisesonlyapairofmeshingrotors,withhelicalgroovesmachinedinthem,containedinacasing,whichfitscloselyroundthem.Therotorsandcasingareseparatedbyverysmallclearances.Therotorsaredrivenbyanexternalmotorandmeshlikegearsinsuchamannerthat,astheyrotate,thespaceformedbetweenthemandthecasingisreducedprogressively.Thus,anygastrappedinthiscaseiscompressed.Thegeometryofsuchmachinesiscomplexandtheflowofthegasbeingcompressedwithinthemoccursinthreestages.Firstly,gasentersbetweenthelobes,throughaninletportatoneendofthecasingduringthestartofrotation.Asrotationcontinues,thespacebetweentherotorsnolongerlinesupwiththeinletportandthegasistrappedandthuscompressed.Finally,afterfurtherrotation,theoppositeendsoftherotorspassasecondportattheotherendofthecasing,throughwhichthegasisdischarged.Thewholeprocessisrepeatedbetweensuccessivepairsoflobestocreateacontinuousbutpulsatingflowofgasfromlowtohighpressure.Thesemachinesaremainlyusedforthesupplyofcompressedairinthebuildingindustry,thefood,processandpharmaceuticalindustriesand,whererequired,inthemetallurgicalindustryandforpneumatictransport.Theyarealsousedextensivelyforcompressionofrefrigerantsinrefrigerationandairconditioningsystemsandofhydrocarbongasesinthechemicalindustry.Theirrelativelyrapidacceptanceoverthepastthirtyyearsisduetotheirrelativelyhighrotationalspeedscomparedtoothertypesofpositivedisplacementmachine,whichmakesthemcompact,theirabilitytomaintainhighefficienciesoverawiderangeofoperatingpressuresandflowratesandtheirlongservicelifeandhighreliability.Consequently,theyconstituteasubstantialpercentageofallpositivedisplacementcompressorsnowsoldandcurrentlyinoperation.Themainreasonsforthissuccessarethedevelopmentofnovelrotorprofiles,whichhavedrasticallyreducedinternalleakage,andadvancedmachinetools,whichcanmanufacturethemostcomplexshapestotolerancesoftheorderof3micrometersatanacceptablecost.Rotorprofileenhancementisstillthemostpromisingmeansoffurtherimprovingscrewcompressorsandrationalproceduresarenowbeingdevelopedbothtoreplaceearlierempiricallyderivedshapesandalsotovarytheproportionsoftheselectedprofiletoobtainthebestresultfortheapplicationforwhichthecompressorisrequired.Despitetheirwideusage,duetothecomplexityoftheirinternalgeometryandthenon-steadynatureoftheprocesseswithinthem,uptillrecently,onlyapproximateanalyticalmethodshavebeenavailabletopredicttheirperformance.Thus,althoughitisknownthattheirelementsaredistortedbothbytheheavyloadsimposedbypressureinducedforcesandthroughtemperaturechangeswithinthem,nomethodswereavailabletopredictthemagnitudeofthesedistortionsaccurately,norhowtheyaffecttheoverallperformanceofthemachine.Inaddition,improvedmodellingofflowpatternswithinthemachinecanleadtobetterportingdesign.Also,moreaccuratedeterminationofbearingloadsandhowtheyfluctuateenablebetterchoicesofbearingstobemade.Finally,ifrotorandcasingdistortion,asaresultoftemperatureandpressurechangeswithinthecompressor,canbeestimatedreliably,machiningprocedurescanbedevisedtominimisetheiradverseeffects.Screwmachinesoperateonavarietyofworkingfluids,whichmaybegases,dryvapourormulti-phasemixtureswithphasechangestakingplacewithinthemachine.Theymayinvolveoilflooding,orotherfluidsinjectedduringthecompressionorexpansionprocess,orbewithoutanyformofinternallubrication.Theirgeometrymayvarydependingonthenumberoflobesineachrotor,thebasicrotorprofileandtherelativeproportionsofeachrotorlobesegment.Itfollowsthatthereisnouniversalconfigurationwhichwouldbethebestforallapplications.Hence,detailedthermodynamicanalysisofthecompressionprocessandevaluationoftheinfluenceofthevariousdesignparametersonperformanceismoreimportanttoobtainthebestresultsfromthesemachinesthanfromothertypeswhichcouldbeusedforthesameapplication.Asetofwelldefinedcriteriagovernedbyanoptimisationprocedureisthereforeaprerequisiteforachievingthebestdesignforeachapplication.Suchguidelinesarealsoessentialforthefurtherimprovementofexistingscrewmachinedesignsandbroadeningtheirrangeofuses.Flemingetal.,1998givesagoodcontemporaryreviewofscrewcompressormodelling,designandapplication.Amathematicalmodelofthethermodynamicandfluidflowprocesseswithinpositivedisplacementmachines,whichisvalidforboththescrewcompressorandexpandermodesofoperation,ispresentedinthisMonograph.Itincludestheuseoftheequationsofconservationofmass,momentumandenergyappliedtoaninstantaneouscontrolvolumeoftrappedfluidwithinthemachinewithallowanceforfluidleakage,oilorotherfluidinjection,heattransferandtheassumptionofrealfluidproperties.Bysimultaneoussolutionoftheseequations,pressure-volumediagramsmaybederivedoftheentireadmission,dischargeandcompressionorexpansionprocesswithinthemachine.Ascrewmachineisdefinedbytherotorprofilewhichisheregeneratedbyuseofageneralgearingalgorithmandtheportshapeandsize.Thisalgorithmdemonstratesthemeshingconditionwhich,whensolvedexplicitly,enablesavarietyofrotorprimaryarcstobedefinedeitheranalyticallyorbydiscretepointcurves.Itsusegreatlysimplifiesthedesignsinceonlyprimaryarcsneedtobespecifiedandthesecanbelocatedoneitherthemainorgaterotororevenonanyotherrotorincludingarack,whichisarotorofinfiniteradius.Themostefficientprofileshavebeenobtainedfromacombinedrotor-rackgenerationprocedure.Therotorprofilegenerationprocessor,thermofluidsolverandoptimizer,togetherwithpre-processingfacilitiesfortheinputdataandgraphicalpostprocessingandCADinterface,havebeenincorporatedintoadesigntoolintheformofageneralcomputercodewhichprovidesasuitabletoolforanalysisandoptimizationofthelobeprofilesandothergeometricalandphysicalparameters.TheMonographoutlinestheadoptedrationaleandmethodofmodelling,comparestheshapesofthenewandconventionalprofilesandillustratespotentialimprovementsachievedwiththenewdesignwhenappliedtodryandoil-floodedaircompressorsaswellastorefrigerationscrewcompressors.ThefirstpartoftheMonographgivesareviewofrecentdevelopmentsinscrewcompressors.Thesecondpartpresentsthemethodofmathematicaldefinitionofthegeneralcaseofscrewmachinerotorsanddescribesthedetailsoflobeshapespecification.Itfocusesonanewlobeprofileofaslendershapewiththinnerlobesinthemainrotor,whichyieldsalargercross-sectionalareaandshortersealinglinesresultinginhigherdeliveryratesforthesametipspeed.Thethirdpartdescribesamodelofthethermodynamicsofthecompression-expansionprocesses,discussessomemodellingissuesandcomparestheshapesofnewandconventionalprofiles.Itillustratesthepotentialimprovementsachievablewiththenewdesignappliedtodryandoil-floodedaircompressorsaswellastorefrigerationscrewcompressors.Theselectionofthebestgaterotortipradiusisgivenasanexampleofhowmathematicalmodellingmaybeusedtooptimisethedesignandthemachinesoperatingconditions.Thefourthpartdescribesthedesignofahighefficiencyscrewcompressorwithnewrotorprofiles.Awellprovenmathematicalmodelofthecompressionprocesswithinpositivedisplacementmachineswasusedtodeterminetheoptimumrotorsizeandspeed,thevolumeratioandtheoilinjectionpositionandjetdiameter.Inaddition,moderndesignconceptssuchasanopensuctionportandearlyexposureofthedischargeportwereincluded,togetherwithimprovedbearingandsealspecification,tomaximisethecompressorefficiency.Theprototypesweretestedandcomparedwiththebestcompressorscurrentlyonthemarket.Themeasuredspecificpowerinputappearedtobelowerthananypublishedvaluesforotherequivalentcompressorscurrentlymanufactured.Boththepredictedadvantagesofthenewrotorprofileandthesuperiorityofthedesignprocedureweretherebyconfirmed.1.1BasicConceptsThermodynamicmachinesforthecompressionandexpansionofgasesandvapoursarethekeycomponentsofthevastmajorityofpowergenerationandrefrigerationsystemsandessentialfortheproductionofcompressedairandgasesneededbyindustry.Suchmachinescanbebroadlyclassifiedbytheirmodeofoperationaseitherturbomachinesorthoseofthepositivedisplacementtype.Turbomachineseffectpressurechangesmainlybydynamiceffects,relatedtothechangeofmomentumimpartedtothefluidspassingthroughthem.Theseareassociatedwiththesteadyflowoffluidsathighvelocitiesandhencethesemachinesarecompactandbestsuitedforrelativelylargemassflowrates.Thuscompressorsandturbinesofthistypearemainlyusedinthepowergenerationindustry,where,asaresultofhugeinvestmentinresearchanddevelopmentprogrammes,theyaredesignedandbuilttoattainthermodynamicefficienciesofmorethan90%inlargescalepowerproductionplant.However,theproductionrateofmachinesofthistypeisrelativelysmallandworldwide,isonlyoftheorderofsometensofthousandsofunitsperannum.Positivedisplacementmachineseffectpressurechangesbyadmittingafixedmassoffluidintoaworkingchamberwhereitisconfinedandthencompressedorexpandedand,fromwhichitisfinallydischarged.Suchmachinesmustoperatemoreorlessintermittently.Suchintermittentoperationisrelativelyslowandhencethesemachinesarecomparativelylarge.Theyarethereforebettersuitedforsmallermassflowratesandpowerinputsandoutputs.Anumberoftypesofmachineoperateonthisprinciplesuchasreciprocating,vane,scrollandrotarypistonmachines.Ingeneral,positivedisplacementmachineshaveawiderangeofapplication,particularlyinthefieldsofrefrigerationandcompressedairproductionandtheirtotalworldproductionrateisinexcessof200millionunitsperannum.Paradoxically,butpossiblybecausethesemachinesareproducedbycomparativelysmallcompanieswithlimitedresources,relativelylittleisspentonresearchanddevelopmentprogrammesonthemandthereareveryfewacademicinstitutionsintheworldwhichareactivelypromotingtheirimprovement.Oneofthemostsuccessfulpositivedisplacementmachinescurrentlyinuseisthescrewortwinscrewcompressor.Itsprincipleofoperation,asindicatedinFig.1.1,isbasedonvolumetricchangesinthreedimensionsratherthantwo.Asshown,itconsists,essentially,ofapairofmeshinghelicallobedrotors,containedinacasing.Thespacesformedbetweenthelobesoneachrotorformaseriesofworkingchambersinwhichgasorvapouriscontained.Beginningatthetopandinfrontoftherotors,showninthelightshadedportionofFig.1.1a,thereisastartingpointforeachchamberwherethetrappedvolumeisinitiallyzero.Asrotationproceedsinthedirectionofthearrows,thevolumeofthatchamberthenincreasesasthelineofcontactbetweentherotorwithconvexlobes,knownasthemainrotor,andtheadjacentlobeofthegaterotorFig.1.1.ScrewCompressorRotorsadvancesalongtheaxisoftherotorstowardstherear.Oncompletionofonerevolutioni.e.360bythemainrotor,thevolumeofthechamberisthenamaximumandextendsinhelicalformalongvirtuallytheentirelengthoftherotor.Furtherrotationthenleadstoreengagementofthemainlobewiththesucceedinggatelobebyalineofcontactstartingatthebottomandfrontoftherotorsandadvancingtotherear,asshowninthedarkshadedportionsinFig.1.1b.Thus,thetrappedvolumestartstodecrease.Oncompletionofafurther360ofrotationbythemainrotor,thetrappedvolumereturnstozero.ThedarkshadedportionsinFig.1.1showtheenclosedregionwheretherotorsaresurroundedbythecasing,whichfi