涡轮增压器的压力测量PPT格式课件下载.pptx
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ConceptsNREC,PressureMeasurement,NickBainesConceptsNREC,Probesareusedformeasurementsoftotalpressure,staticpressure,andflowanglewithintheflowitself.Awiderangeofprobesisavailable,andprobesareoftenmanufacturedforspecialpurposes.Dynamic(time-varying)pressuremeasurementrequiresspecialconsideration.Conventionalpneumatictapsandprobesdonothaveanadequateresponsetimetocaptureanybuttheslowestoftime-varyingevents,butmanyspecialtransducersandmeasuringsystemsareavailableforthispurpose.,ConceptsNREC2003,Pressuremeasurement,PrinciplesWallstatictapsTotalpressureprobesTotal-staticprobesDirectionalpressureprobesPressuretransducersDynamicpressuremeasurementPressurescanners,Pressuremeasurement,ThedifferencebetweentotalandstaticpressurecanbeseenbysolvingtheEulerequationalongastreamline.Thisissimplytheforce-momentumequation,inwhichwebalancetheforces(pressureforcesandgravitationalforce)againstthechangeofmomentumthatoccursasthefluidmovesalongastreamline.Thisgivesadifferentialequationthatcanbesolvediftheflowisincompressibletogiveanequationwithpressure,kineticenergy,andpotentialenergyterms.Formostpracticalpurposeswecanneglectthepotentialenergy.Wethendefinethetotalpressurep0asthesumofthestaticpressurepandthedynamicpressurerC2.Iftheflowiscompressible(densityisnotconstant)thenwehavetotakeaslightlydifferentapproach,butwecanstillobtainanequation,thistimeincludingtheMachnumberM,thatexpressestherelationbetweenthetotalandstaticpressures.,ConceptsNREC2003,Totalandstaticpressures,Compressibleflow:
@#@,Pressuremeasurement,Asinglesensorlocatedinthe“wrong”positionmaygivemisleadingmeasurements.Ideallyseveralsensorscircumferentiallyspacedshouldbeusedateachlocation.Ifmeasurementsarebeingmadeadjacenttoarowoffixedbladesorvanes,thesensorlocationsshouldbephasedrelativetothebladespacinginordertocoveracompletepassage.Sidewallstatictapsshouldbelocatedonboththehubandshroudsurfacesifatallpossible.Oneshouldneverassume(withoutgoodevidence)thatthestaticpressureisconstantacrossapassage,particularlyifthereisanycurvature.,ConceptsNREC2003,Singleandmultiplepointmeasurements,SinglepointmeasurementsWherefinespatialdefinitionisrequired,orWhereweareverysuretheflowisaxisymmetric,orWhereaccessisverydifficultMulti-pointmeasurementsarealmostalwayspreferredwherepossible,Pressuremeasurement,Pressureand/ortemperaturerakesmeasureacross-sectionoftotalpressure,staticpressureand/ortotaltemperatureofamovingfluid.Rakesoffertheadvantageofprovidingmanyseparatereadingssimultaneouslyorasimpleaverageofmanyreadings.Rakesarecustomdesignedandconstructedtomatchtheflowconditionsofaparticularapplication.Rakesmayincorporatefromtwotoseveraldozenindividualmeasuringelements.Thedesignofagivenrakeisdeterminedbymanyfactors,includingfluidvelocityandtemperature,flowanglevariation,thetypeofmeasurementrequiredandthephysicaldimensionsoftheflowpassage.Manyflowortemperatureprobesmaybeincorporatedintoarake.,ConceptsNREC2003,Traversingversusproberake,TraverseprobeGoodspatialresolutionLessblockageRequiresactuation(usuallyremote)andcontrolProberakeSimpleMoreblockage,Pressuremeasurement,Todeterminetheerrorduetotapdiameter,testscanbedonecomparingtheresponseofdifferentsizetapstothesamepressure,andtheresultingcurveofpressureagainstdiameterextrapolatedtozerodiameter.Thiscanthenformacorrectiontothepressuresmeasuredwithfinitediametertaps.,ConceptsNREC2003,Wallstatictaps1,Maketapsassmallaspossible,consistentwithtimeresponseandmanufacturinglimits,Pressuremeasurement,ConceptsNREC2003,Wallstatictaps2,Manufactureandfinishareimportanttoobtainingaccuratemeasurements,Pressuremeasurement,Wallstatictaps3,Whereholesizeisalimitingfactor,itisbestdeterminedbyexperiment:
@#@carefullymeasurethepressureatseveralholesizesandextrapolatetoConceptsNREC2003zero,Pressuremeasurement,Foraccuratemeasurementitisessentialthatprobesarealignedwiththeflowdirection.Staticpressureprobescanbeusedintransonicandsupersonicflows,butmustbeindividuallycalibratedforthepurpose.,ConceptsNREC20,03,Staticpressureprobes,AccuracydependsonthelocationofstaticholesrelativetonoseNotusefulinrestrictedspaces,Pressuremeasurement,Totalpressureprobes,PitotprobeConceptsNREC2003,Stagnationprobes,Pressuremeasurement,ConceptsNREC20,03,Effectofflowangle,Pressuremeasurement,MachnumberrangeTruetotalpressureisindicateduptoaMachnumberof1.0.ThereisaslightdropinyawinsensingrangeaboveM=0.3.Thisdecreasesaveragesabout4%foralltypesataMachnumberof1.0.Reynolds