flow measurements(Orifice and Venturi).pdf

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1、FLOWMEASUREMENT(Venturimeter,OrificePlateandRotameter)OBJECTIVESTostudythecharacteristicsandapplicationsofvariousflowmeasuringdevice(venturimeter&orificeplate).Tocalculatethevolumeflowrateofwaterfromthepressuredifferenceofbothventuriandorificedevices.Tocomparebetweentheoreti

2、calandactualvolumetricflowratethroughthedischargecoefficientconcept.Toknowhowrotameterworks.INTRODUCTIONThemeasurementoffluidflowisimportantinapplicationsrangingfrommeasurementsofblood-flowratesinhumanarterytothemeasurementofliquidoxygeninarocket.Theselectionoftheproperinstr

3、umentforaparticularapplicationisgovernedbymanyvariables,includingcost.Flow-rate-measurementdevicesfrequentlyrequireaccuratepressureandtemperaturemeasurementsinordertocalculatetheoutputoftheinstrument.Themostwidelyusedflowmeteringprincipleinvolvesplacingafixedareaflowrestrict

4、ionofsometypeinthepipeorductcarryingthefluid.Thisflowrestrictioncausesapressuredropthatvarieswiththeflowrate.Thus,measurementofthepressuredropbymeansofasuitabledifferential-pressurepickupallowsflowratemeasurement.Eachoftheflowmeasurementdevicesinherentlyhasitsownadvantagesan

5、ddisadvantages.Someofthoseinstrumentsare:•TheVenturiMeterIntheventurimeter(showninfigure(1)below)thefluidisacceleratedthroughaconvergingconeofangle15-20°andthepressuredifferencebetweentheupstreamsideoftheconeandthethroatismeasuredandprovidesthesignalfortherateofflow.Upstream

6、pressuretapDownstreampressuretapVenturiThroatUpstreamDownstreamFlowFlowFigure(1):TheVenturimeteroperation.Thefluidslowsdowninaconewithsmallerangle(5-7°)wheremostofthekineticenergyisconvertedbacktopressureenergy.Becauseoftheconeandthegradualreductionintheareathereisno"venacon

7、tracta".Theflowareaisatminimumatthethroat.Highpressureandenergyrecoverymakestheventurimetersuitablewhereonlysmallpressureheadsareavailable.AdischargecoefficientCv-of0.975maybetakenasstandard,butthevaluevariesnoticeablyatlowvaluesoftheReynolds'number.WThepressurerecoveryismuc

8、hbetterfortheventurimeterthanfortheorificeplate.WTheventuritubeissuitablefo