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Fin Applications in Motor Body: Different Fin Configurations for Heat Exchangers

This text provides an overview of various fin applications in motor bodies, focusing on different fin configurations used in heat exchangers. It also discusses concepts related to convection boundary layers, thermal boundary layers, and the development of velocity profiles in circular pipes. Additionally, it covers boiling and condensation processes, including the classification of boiling and the different regimes on the boiling curve.

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Fin Applications in Motor Body: Different Fin Configurations for Heat Exchangers

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  1. UNIT I FINS (EXTENDEDSURFACES)

  2. FinApplications

  3. FinsApplications

  4. Finsin MotorBody

  5. Finconfigurations straightfinofuniform cross-sectiononplanewall straightfinofuniformcross-sectiononcirculartube annularfin d) straightpinfin

  6. UNIT III HEAT EXCHANGERS

  7. ConcentricTubeParallel&Counter

  8. ConcentricTube:parallel,Counter

  9. CrossFlow:Mixed& Unmixed

  10. CrossFlow:Mixed& Unmixed 10

  11. Shell&TubeHX. 11

  12. UNIT II CONVECTIONBOUNDARY LAYER CONCEPT HeatandMass Transfer:Fundamentals &Applications FourthEdition YunusA.Cengel,AfshinJ.GhajarMcGraw-Hill,2011

  13. VELOCITYBOUNDARYLAYER Velocityboundarylayer:Theregionoftheflowabovetheplate bounded byinwhichtheeffects oftheviscousshearing forcescaused byfluidviscosityare felt. Theboundary layerthickness,,is typicallydefinedas thedistance yfromthesurfaceatwhichu=0.99V. The hypotheticallineofu=0.99Vdivides theflowover a plateinto two regions: Boundarylayer region:Theviscous effects andthe velocitychanges aresignificant. Irrotationalflowregion:Thefrictionaleffects arenegligible and thevelocityremains essentially constant. 13

  14. THERMALBOUNDARYLAYER Athermal boundary layerdevelopswhena fluidataspecified temperatureflowsoverasurface thatisatadifferenttemperature. Thermalboundarylayer:The flowregion overthesurface inwhichthetemperaturevariationinthedirection normaltothesurface issignificant. The thickness ofthethermalboundarylayertatanylocation alongthesurface isdefined as the distance fromthesurfaceat which thetemperature differenceT−Tsequals 0.99(T−Ts). The thicknessofthe thermalboundarylayerincreases in the flowdirection,since theeffects ofheattransfer are feltatgreater distancesfromthesurfacefurther downstream. Theshapeofthe temperatureprofile in the thermal boundary layerdictatestheconvection heattransferbetweenasolid surface and the fluid flowing overit. Thermal boundarylayer ona flatplate(thefluidishotterthantheplatesurface). 14

  15. Developmentof thevelocityprofilein a circular pipe • Aflowfield is bestcharacterized byits velocitydistribution. • Aflowis saidtobe one-,two-,orthree-dimensionaliftheflowvelocity variesinone,two,orthreedimensions,respectively. • However,thevariationofvelocityincertaindirections can besmall relativetothevariationin otherdirectionsandcan be ignored. The developmentofthevelocityprofileinacircular pipe.V=V(r,z) andthustheflowistwo-dimensionalin theentranceregion, and becomes one-dimensionaldownstreamwhen the velocity profilefullydevelopsandremainsunchanged in theflowdirection,V=V(r). 15

  16. UNIT III BOILINGANDCONDENSATION

  17. BOILINGHEATTRANSFER • Evaporationoccurs attheliquid–vaporinterfacewhenthevaporpressureis less thanthesaturationpressureoftheliquidatagiventemperature. • Boilingoccurs atthesolid–liquidinterfacewhenaliquidis broughtintocontactwithasurfacemaintainedatatemperaturesufficientlyabovethesaturationtemperatureoftheliquid.

  18. Boilingheatfluxfromasolidsurfaceto thefluid excesstemperature Classificationofboiling • Boilingiscalled poolboilingin the absenceofbulkfluid flow. Anymotionofthe fluid is duetonaturalconvectioncurrentsand the motionofthe bubbles under the influenceofbuoyancy. Boilingiscalled flowboilinginthe presenceofbulkfluid flow. Inflowboiling,thefluid isforcedtomove ina heatedpipeorovera surface byexternalmeanssuchasapump. • • •

  19. SubcooledBoiling Whenthetemperatureofthemainbodyoftheliquid is belowthesaturationtemperature. • Saturated Boiling Whenthetemperatureoftheliquid is equaltothesaturationtemperature. •

  20. POOLBOILING In poolboiling,the fluid is notforced to flow byamoversuch asa pump. Anymotionofthefluidisdueto naturalconvectioncurrents andthemotionofthebubblesunderthe influenceofbuoyancy. BoilingRegimes andtheBoilingCurve Boilingtakesdifferentforms, Texcess=TsTsat dependingonthe 20

  21. NaturalConvectionBoiling (toPointAontheBoilingCurve) • Bubbles donotform onthe heatingsurface untilthe liquid is heated afewdegrees abovethesaturationtemperature(about2to6°Cforwater) Theliquid is slightlysuperheatedinthis case(metastablestate). Thefluidmotioninthis modeof boilingis governedbynaturalconvectioncurrents. • • • Heattransferfromthe heatingsurfacetothefluidis bynaturalconvection. Fortheconditions of Fig.10–6,naturalconvectionboilingends at anexcesstemperatureofabout5°C. •

  22. Nucleate Boiling(between Points A andC) • Thebubblesformatanincreasingrateatan increasingnumberofnucleationsites aswe move alongtheboilingcurvetoward point C. • RegionA–B─ isolated bubbles. • RegionB–C─ numerouscontinuouscolumnsofvaporin the liquid. PointAisreferredtoasthe onset ofnucleate boiling(ONB).

  23. • InregionA–Bthe stirringand agitationcausedbytheentrainmentoftheliquidto theheatersurface isprimarily responsiblefortheincreasedheat transfercoefficient. InregionA–Bthe large heatfluxesobtainableinthisregionarecaused bythe combined effect ofliquid entrainment and evaporation. For theentirenucleateboiling range,theheattransfercoefficientrangesfromabout2000 to 30,000W/m2·K. • • • AfterpointBthe heatfluxincreases at alowerratewithincreasingTexcess,and reaches amaximumat pointC. • The heatflux atthis pointis calledthe critical(or maximum) heatflux,andis ofprime engineering importance.

  24. TransitionBoiling (betweenPointsCandD) WhenTexcessis increased pastpointC, the heatflux decreases. Thisisbecausea large fractionoftheheatersurface is covered byavaporfilm,whichacts as an insulation. • • • Inthetransition boiling regime, both nucleateandfilm boiling partiallyoccur. Operation in the transitionboilingregime,which isalsocalledthe unstablefilmboilingregime,is avoidedin practice. For water,transition boiling occursovertheexcesstemperaturerangefromabout30°Ctoabout • • 25 120°C.

  25. Film Boiling(beyondPointD • BeyondpointDtheheatersurfaceiscompletely coveredby acontinuous stable vaporfilm. PointD,where the heatfluxreaches aminimumiscalled theLeidenfrostpoint. Thepresenceofa vaporfilmbetweenthe heatersurfaceand the liquidisresponsibleforthe lowheattransferrates in thefilmboilingregion. Theheattransferrateincreases with increasingexcess temperature due • • • 26 to radiationtotheliquid.

  26. BurnoutPhenomenon • Atypical boiling processdoesnotfollowtheboilingcurve beyondpointC. Whenthe power appliedtotheheatedsurface exceededthevalueatpointCevenslightly,thesurfacetemperatureincreasedsuddenlytopointE. Whenthe power is reducedgraduallystartingfrompointEthecoolingcurvefollowsFig. 10–8withasuddendropinexcess temperaturewhenpointDis reached. • •

  27. Anyattemptto increasetheheat fluxbeyondqmax willcausetheoperation pointontheboilingcurveto jumpsuddenlyfrom pointCto pointE. However,surfacetemperaturethatcorresponds to pointEis beyondthemelting point ofmostheatermaterials,andburnout occurs. Therefore,pointContheboilingcurveis alsocalledtheburnoutpoint,andthe heatfluxatthispointtheburnoutheat flux. Most boilingheattransferequipmentinpractice operateslightlybelowqmax toavoidany disastrousburnout.

  28. Heat Transfer CorrelationsinPoolBoiling • • • Boilingregimesdifferconsiderablyin theircharacter. Differentheattransferrelations needtobe usedfordifferent boilingregimes. Inthe naturalconvection boilingregimeheattransferratescan beaccuratelydeterminedusing naturalconvectionrelations. NucleateBoiling • No generaltheoreticalrelationsfor heattransfer in thenucleateboilingregimeis available. • Experimental basedcorrelations are used. • Therateof heattransferstrongly depends on thenatureofnucleation and thetype andtheconditionofthe heatedsurface.

  29. CONDENSATIONHEATTRANSFER Condensation occurs whenthetemperature ofavapor isreducedbelow Filmcondensation its saturationtemperature. • Thecondensatewetsthesurfaceand formsa liquid film. Thesurfaceisblanketedbya liquid filmwhichserves asaresistance toheattransfer. • Dropwisecondensation • Thecondensedvaporformsdropletsonthesurface. The dropletsslidedownwhen they reachacertainsize. No liquidfilmtoresistheattransfer. As aresult,heattransferratesthataremore than 10times larger thanwith filmcondensationcanbeachieved. • • • 30

  30. FILMCONDENSATION • Liquidfilmstarts forming atthetopoftheplate andflows downwardunder the influenceofgravity. increasesintheflowdirectionx Heatintheamounthfgis releasedduringcondensationandistransferredthroughthefilmtotheplatesurface. Tsmust bebelowthesaturationtemperatureforcondensation. Thetemperatureofthecondensateis Tsatatthe interfaceanddecreasesgraduallyto Tsatthewall. • • • •

  31. DROPWISE Dropwisecondensation,CchaOracNteriDzedEbyN SATION countless droplets ofvarying diametersonthe condensing surfaceinstead ofa continuousliquidfilmand extremelylarge heattransfercoefficients canbe achievedwiththis mechanism. Thesmalldroplets thatformatthenucleationsites onthesurface growas aresultofcontinued condensation,coalesceintolarge droplets,andslide downwhentheyreach acertainsize,clearingthesurfaceand exposingittovapor.Thereis noliquidfilminthiscaseto resist heattransfer. As aresult,with dropwisecondensation,heattransfercoefficients canbe achievedthat aremorethan 10times largerthanthoseassociated withfilmcondensation. Thechallengein dropwisecondensationis nottoachieveit,but rather,tosustainitforprolonged periods oftime. Dropwisecondensationof steamoncoppersurfaces:

  32. UNIT V MASSTRANSFER

  33. INTRODUCTION Wheneverthereisanimbalanceofacommodityinamedium,naturetendstoredistributeituntila“balance”or“equality”is established.Thistendencyisoftenreferredtoasthedrivingforce,whichisthemechanismbehindmanynaturallyoccurringtransport phenomena. Thecommoditysimplycreepsawayduringredistribution,andthustheflowisadiffusionprocess.The rateofflowofthecommodityisproportionaltotheconcentrationgradientdC/dx,whichisthechangeintheconcentrationCperunitlengthintheflowdirection x,andtheareaAnormaltoflowdirection. kdiffisthe diffusioncoefficientofthe medium,whichisameasureofhowfasta commoditydiffuses in the medium,andthe negativesign istomakethe flowin thepositivedirectiona positive quantity(notethatdC/dxisanegativequantitysince 34 concentration decreases in the flowdirection).

  34. Thediffusioncoefficientsand thusdiffusionratesofgasesdependstronglyontemperature. Thediffusionratesare higherathighertemperatures. Thelargerthemolecularspacing,the higherthediffusionrate. Diffusionrate:gases>liquids>solids 35

  35. ANALOGY(SIMILARITY) BETWEEN HEATANDMASSTRANSFER Wecandevelopanunderstandingofmass transfer inashorttimewithlittleeffortby simplydrawingparallels between heat andmass transfer. Temperature The drivingforceformasstransfer isthe concentration difference. Bothheatandmass aretransferredfromthemoreconcentratedregionsto the less concentrated ones. Ifthere is no differencebetweentheconcentrations ofaspecies atdifferent parts ofamedium,therewillbenomass transfer. 36

  36. Conduction Mass is transferredbyconduction(calleddiffusion)andconvectiononly. Rateofmassdiffusion Fick’slawofdiffusion 37 DABis thediffusioncoefficient(ormassdiffusivity)ofthespeciesinthemixture CAisthe concentrationofthespeciesinthe mixture

  37. HeatGeneration Heat generationreferstotheconversionofsomeformofenergysuchas electrical,chemical,ornuclearenergyintosensiblethermalenergyinthemedium. Somemasstransfer problems involvechemicalreactions that occurwithinthemedium andresult inthegeneration ofaspeciesthroughout. Therefore,species generationis avolumetric phenomenon,andthe rate of generationmayvaryfrompointtopointinthemedium. Suchreactions thatoccurwithinthemediumarecalledhomogeneousreactionsand areanalogous to internal heat generation. In contrast,somechemicalreactions resultinthegenerationofaspecies atthe surfaceas aresult ofchemicalreactions occurring atthesurfaceduetocontact betweenthemediumandthesurroundings. This isasurfacephenomenon,andas suchitneeds to betreatedas aboundarycondition. In mass transferstudies,suchreactions arecalledheterogeneous reactionsand areanalogous tospecifiedsurface heatflux. 38

  38. Convection Massconvection(orconvectivemasstransfer) is the masstransfermechanismbetweenasurface andamovingfluidthatinvolves bothmassdiffusion andbulkfluidmotion. Fluidmotion alsoenhancesmasstransferconsiderably. Inmassconvection,we defineaconcentration boundary layer inananalogousmannertothethermalboundarylayerand definenewdimensionlessnumbersthatarecounterpartsofthe Nusseltand Prandtlnumbers. Newton’slawofcooling Rateofmass convection hmass the masstransfercoefficient Asthesurface area Cs−Casuitableconcentrationdifference 39 acrosstheconcentrationboundarylayer.

  39. MASSDIFFUSION Fick’slawofdiffusionstatesthatthe rateofdiffusionofachemicalspeciesata location ina gasmixture(orliquidorsolidsolution)isproportionaltotheconcentrationgradientofthatspeciesatthatlocation. 1MassBasis Ona massbasis,concentrationisexpressed in termsofdensity(ormassconcentration). Thedensityofamixtureata location isequaltothe sumofthedensitiesofitsconstituentsatthatlocation. Themassfractionofaspeciesrangesbetween0 and1, and thesumofthemassfractionsof 40 the constituents ofamixture beequalto1.

  40. TYPEISTEADYMASSDIFFUSIONTHROUGHAWALL Manypracticalmasstransferproblemsinvolvethediffusionofaspeciesthrougha plane-parallelmediumthatdoesnotinvolveanyhomogeneouschemicalreactionsunder one-dimensionalsteadyconditions. diffusionresistance 41 ofthewall

  41. TYPEI:STEADY ONE-DIMENSIONALMASSTRANSFERTHROUGH NONREACTINGCYLINDRICALAND SPHERICALLAYERS Onamolar basis

  42. TYPEII:Equimolar Counterdiffusion equimolarcounterdiffusion

  43. TYPEIII:DIFFUSIONOFVAPORTHROUGHASTATIONARYGAS:STEFANFLOW Manyengineeringapplicationssuchasheat pipes,coolingponds,andthefamiliar perspirationinvolvecondensation,evaporation,andtranspirationinthepresenceofa noncondensablegas,andthusthediffusionofavaporthroughastationary(orstagnant) gas. Tounderstandandanalyzesuchprocesses,consideraliquidlayerofspeciesAinatank surroundedbyagasofspecies B,suchasalayerofliquidwaterinatankopentothe atmosphericairatconstant pressurePandtemperatureT.

  44. Thisrelation isknownasStefan’slaw,and theinducedconvective flowdescribedthatenhancesmassdiffusioniscalled theStefanflow.

  45. CONVECTIVEMASSTRANSFER Nowweconsidermassconvection (orconvectivemasstransfer),whichisthe transferofmass betweenasurfaceandamovingfluid duetoboth massdiffusionandbulkfluidmotion. The analogybetweenheatandmassconvectionholdsforbothforced andnatural convection,laminarandturbulentflow,andinternalandexternalflow. Massconvection isalsocomplicated becauseofthecomplicationsassociatedwith fluidflowsuch asthesurface geometry,flowregime,flowvelocity,andthe variationofthefluid propertiesandcomposition. Therefore,wehavetorelyon experimentalrelationsto determinemasstransfer.Massconvection isusuallyanalyzedonamassbasis rather thanona molar basis. Concentrationboundarylayer:Inmass convection,the regionofthe fluidinwhichconcentration gradientsexist.

  46. Ininternalflow,we havea concentrationentrance regionwhere the concentrationprofiledevelops,inadditionto thehydrodynamicand thermal entryregions. Theconcentration boundarylayercontinuesto developintheflowdirectionuntil itsthicknessreachesthetubecenterandthe boundarylayersmerge. The distance fromthetube inlettothe location where thismergingoccursis calledthe concentrationentrylengthLc, andtheregionbeyondthatpoint iscalled the fullydevelopedregion.

  47. SIMULTANEOUSHEATANDMASS TRANSFER Manymasstransferprocesses encounteredinpracticeoccur isothermally,and thustheydo notinvolve anyheattransfer. Butsomeengineering applications involve thevaporizationofaliquidand the diffusionofthisvapor intothesurroundinggas. Such processesrequirethe transferofthe latentheatofvaporizationhfgtothe liquid in ordertovaporizeit,and thussuchproblemsinvolvesimultaneous heatandmasstransfer. To generalize,anymass transfer probleminvolvingphasechange(evaporation,sublimation,condensation,melting,etc.)mustalsoinvolveheattransfer,and thesolutionofsuchproblems needsto beanalyzed byconsideringsimultaneous heatandmass transfer. 48

  48. UNIT IV FUNDAMENTALSOFTHERMAL RADIATION

  49. INTRODUCTION Radiation differsfromconductionandconvectioninthatitdoes notrequirethepresenceofamaterialmediumtotakeplace. Radiationtransfer occurs insolidsaswellas liquidsand gases. The hotobject invacuumchamberwilleventuallycooldown andreachthermalequilibriumwith itssurroundings byaheattransfer mechanism:radiation.

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