Download - Boiling Condensation
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BOILING AND CONDENSATION
BOILING AND CONDENSATION
Mihir SenUniversity of Notre Dame
September 29, 2010
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BOILING AND CONDENSATION
Outline
1 Outline
2 Overview
3 BoilingPool boilingFlow boiling
4 CondensationDropwise condensationFilm condensation
5 Boilers
6 Condensers
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BOILING AND CONDENSATION
Overview
Overview
Boiling: heat is transferred to the liquid to be vaporized
Condensation: heat is transferred from the vapor to becondensed
In thermodynamics, phase change at constant pressureoccurs without temperature change. The difference inenthalpy is the latent heat of transformation hfg
In reality, heat transfer is due to temperature differences
Boiling and condensation can achieve very high heattransfer rates qs for small differences in temperature(Ts T)
Convective heat transfer coefficient is high (2500100,000W/m2K) according to Newtons law of coolingq = h(Ts T)
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BOILING AND CONDENSATION
Boiling
Boiling
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BOILING AND CONDENSATION
Boiling
Boiling at a solid surface
The thermodynamical saturation temperature isdetermined by the liquid pressure.
Boiling is possible when the surface temperature Ts exceedsthe saturation temperature Tsat.
Excess temperature Te = Ts Tsat
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BOILING AND CONDENSATION
Boiling
Types of boiling
Pool boiling
Flow boiling
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BOILING AND CONDENSATION
Boiling
Pool boiling
Pool boiling
Nukiyama (1934) identified different regimes in pool boiling
Measured Te vs. qs in a submerged wire
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BOILING AND CONDENSATION
Boiling
Pool boiling
Boiling curve
Below A: free convectionONB: onset of nucleateboilingAC: nucleate boiling (AB:isolated bubbles, BC: jetsand columnsC: critical heat flux qmaxCD: transition boilingD: Leidenfrost pointAbove D: film boiling
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BOILING AND CONDENSATION
Boiling
Pool boiling
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BOILING AND CONDENSATION
Boiling
Pool boiling
http://www.youtube.com/watch?v=jcXVLMjWvsc
Nucleate boiling
Isolated bubbles: bubbles form at nucleation sites andseparate from the surface; fluid mixing induces increasingconvective heat transfer
Jets or columns: More nucleation sites are activated;densely populated bubble jets at the surface inhibit liquidmotion; convective heat transfer coefficient begins todecrease.
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BOILING AND CONDENSATION
Boiling
Pool boiling
Nucleation sites
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BOILING AND CONDENSATION
Boiling
Pool boiling
Nucleate boiling
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BOILING AND CONDENSATION
Boiling
Pool boiling
Nucleate boiling correlations
Nu = CRemPrn
D
g(l v)
q = lhfg
[g(l v)
]1/2( clTeCshfgPr
nl
)3
Nu = Nusselt numberl = liquidv = vaporD = diameter of bubblec = specific heat atconstant pressureC = empirical constant = surface tensioncoefficient
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BOILING AND CONDENSATION
Boiling
Pool boiling
Critical heat flux (CHF)
This is the highest heat flux that is safe to operate at
qmax = Chfgv
[g(l v)
2v
]1/4
C is an empirical constant that depends on geometry
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BOILING AND CONDENSATION
Boiling
Pool boiling
Transition boiling
Bubble formation is so rapid that a vapor film forms on thesurface
The state oscillates between film and nucleate boiling
The heat flux decreases during this mode, because thethermal conductivity of vapor is much lower than liquid
At Leidenfrost point
qmin = 0.09hfgv
[g(l v)
(l + v)2
]1/4
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BOILING AND CONDENSATION
Boiling
Pool boiling
Film boiling
The surface is completely covered by a vapor blanket
Heat transfer is only by conduction and radiation throughthe vapor
As the surface increases in temperature radiation heattransfer ( T 4) dominates and heat flux increases
Surface temperature becomes very high and damage orsoftening may occur
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BOILING AND CONDENSATION
Boiling
Pool boiling
Nucleate boilinghttp://www.youtube.com/watch?v=ALfwp6D3lmU&feature=related
Leidenfrost effecthttp://www.youtube.com/watch?v=gjsMV1MglA4
http://www.youtube.com/watch?v=6NiZlFNXPlw&feature=related
Critical heat fluxhttp://www.youtube.com/watch?v=NO3I5MGErOE
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BOILING AND CONDENSATION
Boiling
Pool boiling
Other factors
Gravitational field; CHF 0 as g 0
Rotation in machinery; artificial g
Surface roughness: nucleation site density increases withroughness.
Enhanced boiling surfaces can be used
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BOILING AND CONDENSATION
Boiling
Pool boiling
Enhanced boiling surface
Koratkar (RPI), 2008 A scanning electron microscopeshows copper nanorods depositedon a copper substrate. Air trappedin the forest of nanorods helps todramatically boost the creation ofbubbles and the efficiency ofboiling, which in turn could lead tonew ways of cooling computer chipsas well as cost savings for anynumber of industrial boilingapplication.
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BOILING AND CONDENSATION
Boiling
Flow boiling
Flow boiling
Depends greatly on geometry and orientation
External flow: over heated plates or cylinders
Internal (duct) flow: in piping; sometimes calledtwo-phase flow
heat in
flowvertical, internal
heat in
flow
horizontal, internal
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BOILING AND CONDENSATION
Boiling
Flow boiling
External flow boiling correlations
Low velocity
qmaxvhfgV
=1
pi
[1 +
(4
We
)1/3]
High velocity
qmaxvhfgV
=(l/v)
3/4
169pi+
(l/v)1/2
19.2piWe1/3
Weber number
We =vV
2D
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BOILING AND CONDENSATION
Boiling
Flow boiling
Two phase flow in ducts
Flow regimes
Single-phase liquid
Bubbly flow
Slug flow
Annular flow
Annular flow withentrainment
Drop flow
Single-phase vapor
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BOILING AND CONDENSATION
Boiling
Flow boiling
Patterns in two phase flow
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BOILING AND CONDENSATION
Boiling
Flow boiling
Two phase flow correlations
X = average mass fraction of vapor in fluid (quality)
h
hsp= 0.6683
(lv
)0.1X0.16(1X)0.64f(Fr)
+ 1058
(q
mhfg
)0.7(1X)0.8Gs,f
Froude number
Fr =m
l
2 1
gD
Stratification parameter f(Fr); for horizontal tubes
f(Fr) = 2.63Fr 0.3
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BOILING AND CONDENSATION
Condensation
Condensation
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BOILING AND CONDENSATION
Condensation
Types of condensation
Surface condensation
Dropwise condensation: higher heat transfer rateFilm condensation: lower heat transfer rate
Homogeneous condensation, e.g. like in a fog
Direct contact condensation: vapor condenses at avapor-liquid interface.
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BOILING AND CONDENSATION
Condensation
left dropwise right film
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BOILING AND CONDENSATION
Condensation
Dropwise condensation
Dropwise condensation correlation
Steam on copper (SI units)
h =
{51, 104 + 2044Tsat[
C] for 22C < Tsat < 100C
255, 510 for 100C < Tsat
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BOILING AND CONDENSATION
Condensation
Film condensation
Film condensation correlations
Laminar film on vertical plate
NuL =hLL
kl
= 0.943
[lg(l v)h
fgL3
lkl(Tsat Ts)
]1/4
hfg = hfg + 0.68cp,l(Tsat Ts)
L
Vgravity
L = liquidV = vapor
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BOILING AND CONDENSATION
Condensation
Film condensation
Turbulent film on vertical plate
Re =
3.78
[klL(Tsat Ts)
lh
fg(2/g)1/3
]3/4for Re < 30
[3.70klL(Tsat Ts)
lh
fg(2/g)1/3
+ 4.8
]0.82for 30 < Re < 1800
[0.069klL(Tsat Ts)
lh
fg(2/g)1/3
Pr0.5 151Pr 0.5l + 253
]4/3for Re > 1800
hL(2
l /g)1/3
kl=
1.47Re1/3 for Re < 30
Re
1.08Re1.22 5.2for 30 < Re < 1800
Re
8750 + 58Pr0.5l (Re0.75 253)
for Re > 1800
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BOILING AND CONDENSATION
Boilers
Boilers
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BOILING AND CONDENSATION
Boilers
Types
Fossil fuel boilers
Nuclear boilers
Solar boilers
Waste heat recovery boilers
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BOILING AND CONDENSATION
Boilers
Types of boilers
Water-tube or fire-tube
Natural or forced circulation
Subcritical or supercritical pressure
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BOILING AND CONDENSATION
Boilers
Design considerations
Heat flux is controlled not the temperature, so there is adanger of film boiling
CHF must be accurately pre-determined
Boiling crisis
The pipe temperature needs to be monitored to preventthis
Occurs due to two mechanisms
Local metal temperature rises to levels where the creep lifeis rapidly exceededRapid corrosion occurs due high concentrations of dissolvedsolids at the steam-water interface.
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BOILING AND CONDENSATION
Condensers
Condensers
Types
Direct contact
SprayBarometric and jet
Surface condensers
Single passMultipass
barometric
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BOILING AND CONDENSATION
Condensers
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OutlineOverviewBoilingPool boilingFlow boiling
CondensationDropwise condensationFilm condensation
BoilersCondensers