Heat transfer performance comparison of steam and air flow in gas turbine cooling Channels.docx
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Heat transfer performance comparison of steam and air flow in gas turbine cooling Channels.docx
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HeattransferperformancecomparisonofsteamandairflowingasturbinecoolingChannels
Heattransferperformancecomparisonofsteamandairingasturbinecoolingchannelswithdifferentribangles
XiaojunShi*,JianminGao,LiangXu,FajinLi,WeiWang
StateKeyLaboratoryforManufacturingSystemsEngineering,SchoolofMechanicalEngineering,Xi’anJiaotongUniversity,Xi’an710049,China
*Correspondingauthor.Tel.:
+862983395089;E-mailaddress:
shixiaojun@.
Abstract
Usingsteamasworkingfluidtoreplacethecompressedairisapromisingcoolingtechnologyforinternalcoolingpassagesofbladesandvanes.Thelocalheattransfercharacteristicsandthethermalperformanceofsteaminwideaspectratiochannelswithdifferentangledribsontwooppositewallshavebeenexperimentallyinvestigatedinthispaper.TheaveragedNusseltnumberratiosandthefrictionfactorratiosofsteamandairinfourribbedchannelswerealsomeasuredunderthesametestconditionsforcomparison.TheReynoldsnumberrangeis6,000-70,000.Theribanglesare90°,60°,45°,and30°,respectively.Theribheighttohydraulicdiameterratiorangesfrom0.047to0.078.Thepitch-to-ribheightratiois10.TheresultsshowthattheNusseltnumberratiosofsteamare1.19–1.32timesgreaterthanthoseofairovertherangeofReynoldsnumbersstudied.Forwideaspectratiochannelsusingsteamasthecoolant,the60°angledribshasthebestheattransferperformanceandisrecommendedforcoolingdesign.
Keywords:
Ribbedrectangularchannel;Steamcooling;Ribangle;Heattransferenhancement.
Nomenclature
A
heattransfersurfacearea(notincludetheribsurfacearea)(m2)
cp
specificheatatconstantpressure(kJkg-1K-1)
D
hydraulicdiameteroftestchannel(m)
e
ribheight(m)
F
heattransferperformancefactor
f
frictionfactor
f0
frictionfactorforfullydevelopedturbulentflowinsmoothcirculartubes
G
coolantmeanmassvelocityinthechannel(kgm-2s-1)
H
flowchannelheight(mm)
hx
localheattransfercoefficient(Wm-2K-1)
L
channellength(m)
m
coolantmassflowrate(kgs-1)
Nu
averageNusseltnumberalongcenterlineoftheribbedwall
Nux
localNusseltnumbers
Nu0
Nusseltnumberforfullydevelopedturbulentflowinsmoothtubes
P
pressure(Pa)
p
ribpitch(mm)
ΔP
pressuredropacrossthetestsection(Pa)
Prandtlnumber
Q
heattransferrate(W)
Qel
inputelectricpowertothetestchannel(W)
Qnc
naturalconvectiveheattransferratetothesurroundings(W)
Qrad
radiationheattransferratetothesurroundings(W)
Qdis
heattransferratedissipatedthroughtwoflangesofthetestchannel(W)
Qloss
heatlosstotheenvironment(W)
qconv
convectiveheatflux(Wm-1)
qc
coolantsideheatflux(Wm-1)
qm
meanheattransferflux(Wm-1)
Re
Reynoldsnumber
Tb
bulkmeantemperature(K)
Tw
localwalltemperature(K)
Tin,c
coolanttemperatureattheentranceofthetestchannel(K)
Tout,c
coolanttemperatureattheoutletofthetestchannel(K)
Tw,x
localribbedwallsurfacetemperature(K)
Tb,x
localbulkcoolanttemperatureatdistancex(K)
V
velocity(ms-1)
W
flowchannelwidth(mm)
x
axialdistancefromchannelentrance(mm)
Greeksymbols
α
ribangleofattack(oC)
λ
thermalconductivity(Wm-1K-1)
μ
dynamicviscosityofcoolant(Pas)
ρ
density(kgm-3)
1.Introduction
Theefficiencyandpoweroutputofagasturbinecanbesignificantlyimprovedthroughincreasingtheturbineinlettemperature.Theinlettemperatureofmoderngasturbineiswellabovethemateriallimitationofthefirstturbinestages.Therefore,efficientcoolingtechnologies(filmcooling,impingementcooling,rib/pinaugmentedcoolingandinternalcooling)usingcompressedairasworkingfluidareemployedforvanesandbladesofmoderngasturbines.However,theconventionalcoolingtechnologiesusingairascoolantisdifficulttomeetthecoolingrequirementwiththedevelopmentofnextgenerationadvancedgasturbineoperatingathighertemperatures.
ItcanbeseenfromFig.1thatsteamcantransportmoreenergyperunitvolumethanairalthoughthedensityofsteamislowerthanthedensityofair.Therefore,steamascoolingworkingfluidisapromisingoptiontocoolvanesandbladesofhightemperaturegasturbines.Steamcanbeconvenientlyprovidedundertheproperconditionstogasturbineinacombinedcyclepowerplant.Ifsteamisusedascoolantininternalcoolingpassagesofvanesandbladestoreplacecompressedair,thecoolingefficiencywillbesignificantlyimprovedandthencompressedcoolingaircanbesavedtoexpansioninthegasturbine.Sohighercycleefficiencywillbeachieved.Previousinvestigationsonthevanesteamcoolingtechnologyhavemainlyconcentratedoncoolingefficiencyofsteamcooledvanesandparametricanalysisofcombinedcyclepowerplantwithclosedcircuitsteamcooledgasturbine(Nomotoetal.,1997;Krügeretal.,2001;Jordaletal.,2000;Facchinietal.,2001).Theresultsofabovestudiesshowthatthecoolingefficiencyofclosedcircuitsteamcooledvaneishigherandtheefficiencyofthecombinedcyclepowerplantisincreasedby1.5%comparedwithaircooledgasturbineatthesamefiringtemperature.Eventhoughitisprovedthatsteamascoolantforbladesandvanesofmoderngasturbinesisapromisingcoolingtechnology,heattransferandfrictionexperimentaldataofsteamininternalcoolingpassagesofvanesandbladesisnotavailable.Inparticularitisunclearthattheeffectofinternalcoolingpassagegeometryontheheattransferandpressuredropoflaminarandturbulentsteamflow.Alsothereislittleinformationonheattransferandfrictioncharacteristicsofinternalcoolingpassagewallforentrancedevelopinganddevelopedturbulentsteamflow.
Intheinternalcoolingpassagesofgasturbinevanesandblades,repeatedribsaretypicallycastoninnerwallsofthepressureandsuctionsurfacetoenhanceheattransfertothecoolant.Theinternalcoolingpassagesofgasturbinecanbeapproximatelymodeledasrectangularchannelswithapairofoppositeribbedwallsinexperimentalinvestigations(Hanetal.,2000).TheeffectsofribconfigurationparametersandchannelaspectratioonheattransfercoefficientsandfrictionfactorsoverawiderangeofReynoldsnumbershavebeensystematicallystudiedinstationarystraightrectangularchannelsforaircooling.HanandPark(Hanetal.,1978;Hanetal.,1985;Hanetal.,1988;Hanetal.,1989;Parketal.,1992)reportedtheeffectsofvaryingthechannelaspectratio,theribangleofattack,theribpitch-to-heightratio,theribheight-to-channelhydraulicdiameterratioandflowReynoldsnumbersontheheattransferandfrictioncharacteristicsforrectangularchannelswithcontinuousribsontwooppositewalls.Theseresultsshowthatangledribsprovidebetterheattransferperformancebecauseofthesecondaryflowinducedbytheribs.Thesesecondaryflowsaffectthemainflowreattachmentandrecirculationbetweenribsandinterruptboundarylayergrowthofthereattachmentregions.Comparedwiththesmoothchannel,theaverageheattransfercoefficientofribbedsideisgenerallyenhancedtwotothreetimesandpressuredroppenaltyincreaseabout3to18fold.Semi-empiricalfrictionandheattransfercorrelationshavebeendevelopedforheattransferdesigners.SystematicstudiesontheeffectoftheribspacingonheattransferforaspectratiolargerthanonewerereportedbyGiovanniTandaetal(2011).Thelocalheattransferandpressuredropcharacteristicsofdevelopingturbulentflowsofairinribbeddivergentsquarechannelandribbedconvergentsquarechannelwerereportedalso[Wangetal.,2001].TheeffectoftheheatingofalimitednumberofsidewallshasbeenstudiedbyHanetal.(1992)andAhnetal.(2008).Furtherexperimentalinvestigationshaveperformedontheeffectofribshape(straightorv-shape;continuousordiscrete),ribbed-groovedwalls,thenumberofribbedwallsonheattransferandfrictioncharacteristicsofafullydevelopedturbulentairflowinchannels(Lauetal.,1991;Zhangetal.,1994;Taslimetal.,1996;Chandraetal.,1997;Tandaetal.,2004).Itcanbeseenfromliteraturereviewthatalmostallstudiesofinternalcoolingpassagesofgasturbinevanesandbladeswerebasedonusingairasworkingfluid.However,therearenotonlynosystematicinvestigationsontheeffectofthesteamflowcharacteristicsonheattransferofrectangularchannelswithapairofoppositeribbedwallsbutalsonoexperimentaldatainliterature.
Theobjectiveofthisinvestigationistostudytheturbulentheattransferandfrictionfordevelopingandfullydevelopedsteamflowinwideaspectratiochannelswithdifferentangledribsontwooppositewalls.Thisinvestigationmainlydiffersfrompreviousstudiesandrelativeliteratureasfollows:
1)Theheattransferandfrictioncharacteristicsofsteamandairintheuniformlyheatedrectangularchannelswithtwooppositeribbedwallsareexperimentallycompared.
2)Withsteamastheworkingfluid,thecombinedeffectsofReynoldsnumberandribanglesonlocalheattransferofgasturbineinternalcoolingpassageindevelopingandfullydevelopedregionarestudiedunderconstantheatfluxconditions.
3)Mostribsusedinpreviousexperimentsweremanufacturedbynonmetalmaterials.Buttheribsstudiedinthisresearchweremilledfrom6mmthickstainlesssteelplates.Theyexchangeheatwithconvectivesteamflow.Sotheeffectoftheribsthermalconductivityisconsidered.
4)Previousexperimentswereconductedbasedonthethin-foilheatermethodandtypicallyrunatandaroundroomtemperature.Testchannelsinthisworkareresistanceheatedbyan
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