Boiling and its various Modes.pdf

Boiling and Evaporation, Modes of boiling, Pool Boiling Regimes

1. Unit-5: Boiling Modes
Prepared by:
Ankur Sachdeva
Assistant Professor, ME

2. Boiling and Evaporation
•Thephasechangefromliquidtovapourat
liquidvapourinterfaceiscalledtheboiling.
•Boilingisaconvectiveheattransferprocess
thatinvolvesphasechangeataconstant
temperaturefromliquidtovapour.
•Duringtheboiling,theheataddedtoliquid
foritsphasechangeisthelatentheat.
•Evaporationoccursattheliquid–vapor
interfacewhenthevaporpressureisless
thanthesaturationpressureoftheliquidat
agiventemperature.
•Boiling,ontheotherhand,occursatthe
solid–liquidinterfacewhenaliquidis
broughtintocontactwithasurface
maintainedatatemperatureT
s,sufficiently
abovethesaturationtemperature,T
sat,of
theliquid.

3. Modes of Boiling
•Thegrowthofvapouranditsdynamicsdepend
ontemperatureexcess,natureofsurfaceand
thermo-physicalpropertiesofliquid,suchasits
surfacetension,latentheatetc.
•Variousmodesofboiling
–PoolBoiling
–FlowBoiling
–SaturatedBoiling
–Sub-cooledBoiling

4. Modes of Boiling: Pool & Flow
•Poolboiling.Itreferstoasituationinwhichtheheated
surfaceissubmergedbelowthefreesurfaceofa
stagnantliquidanditsmotionnearthesurfaceisdue
tofreeconvectiononlyandthemixingisinducedby
bubblegrowthanditsdetachment.
•Forcedconvectionboiling.Itreferstoasituationof
boiling,inwhichthefluidmotionisinducedartificially
byanexternalmeansaswellasbynaturalconvection
andthebubbleinducedmixing.
–Thistypeofboilingoccursinhighpressurewatertube
boilersinvolvingforcedconvection.

5. Modes of Boiling:
Sub-cooled and Saturated
•Sub-cooledorlocalboiling.Itreferstoasituationofboiling,in
whichthetemperatureoftheliquidisbelowitssaturation
temperatureandthebubblesformedattheheatedsurfaceare
condensedintheliquidastheyleavethesurface.
•Saturatedboiling.Itreferstoasituationofboiling,inwhichthe
temperatureoftheliquidisequaltoitssaturationtemperature.
–Thebubblesformedattheheatedsurfacearepushedthroughthe
liquidbybuoyancyeffectstoescapefromfreesurfaceoftheliquid

6. Pool Boiling Regimes (For water at 1 atm)

7. Pool Boiling Regimes: Region I
•Freeconvectionboiling:
IntheregionI,the
temperatureexcessΔT
e≤
ΔT
e,
A≅5°C,thefree
convectioncurrents,are
responsibleforfluid
motionandheattransfer.
•Theliquidnearthe
heatedsurfaceisslightly
superheated and
evaporateswhenitrises
thesurface.

8. Pool Boiling Regimes: Region II & III
•NucleateBoiling:Thenucleate
boilingexistswhenΔT
e,A≤ΔT
e≤
ΔT
e,C≅30°C.
•Inthisrange,thetwodifferent
flowregimesmaybe
distinguished:Region-IIand
Region-III.
•IntheregionII,thebubbles
begintoformatsome
nucleationsitesonthesurface
ofthewireandarecollapsedin
theliquidafterdetachingfrom
thesurface.
•ThisregionIIisreferredas
unstableorisolatednucleate
boiling(A–B).

9. Pool Boiling Regimes: Region II & III
•NucleateBoiling:Asthe
temperatureexcessis
increasedfurtherintheregion
III,morenucleationsites
becomeactiveandthebubbles
formmorerapidlyandriseto
freesurfaceoftheliquidand
formslugsofvapour.
•Itisstablenucleateboiling(B
–C).
•Theinterferencebetweenthe
denselypopulatedbubbles
retardthemotionoftheliquid
nearthesurface.

10. Some Important Points on
Boiling Curve
•Inflectionpoint:ThepointPintheboilingcurve
atwhichtheheattransfercoefficienthreaches
maximumvalue.
•CriticalHeatFlux:ThepeakofcurveatpointCis
thelocusofmaximumheatflux,wherethe
excesstemperatureisraisedtoabout35°C.
–Atthispoint,thetemperatureofthesurfaceexceeds
themeltingpointofsolidandthusthemeltingofwire
mayoccur
–Itisalsoknownasburn-outpointorboilingcrisis.

11. Pool Boiling Regimes: Region IV
•UnstableFilmBoiling(C–D):In
thisregionIV,∆T
e,C≤∆T
e≤∆T
e,D
≅120°C.
•Bubbleformationissohighthat
thevapourfilmformonthe
fractionofsurface.
•Atanypointonthesurface,the
filmandnucleateboilingmay
appearalternativelybutthe
growthoffilmincreaseswith
increasein∆Te.
•Thisfilmpreventstheinflowof
freshliquidtakingtheirplace

12. Pool Boiling Regimes: Region V
•FilmBoiling:Itappearsin
regionV,when∆Te≥120°C,
thesurfaceiscompletely
coveredbyvapourblanket.
•Theheatfluxbecomes
minimumatpointD.
•Theheattransferfromthe
surfacetotheliquidoccursby
conductionthroughthestable
vapourfilm.
•Asthesurfacetemperature
increases,theheatfluxslowly
increasesduetoincreaseinh.