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BOILING AND CONDENSATION BOILING AND CONDENSATION Mihir Sen University of Notre Dame September 29, 2010 1/ 36

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Boiling Condensation

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  • BOILING AND CONDENSATION

    BOILING AND CONDENSATION

    Mihir SenUniversity of Notre Dame

    September 29, 2010

    1/ 36

  • BOILING AND CONDENSATION

    Outline

    1 Outline

    2 Overview

    3 BoilingPool boilingFlow boiling

    4 CondensationDropwise condensationFilm condensation

    5 Boilers

    6 Condensers

    2/ 36

  • 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

    9/ 36

  • 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

    11/ 36

  • BOILING AND CONDENSATION

    Boiling

    Pool boiling

    Nucleate boiling

    12/ 36

  • 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

    13/ 36

  • 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

    14/ 36

  • 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

    17/ 36

  • 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

    20/ 36

  • 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

    28/ 36

  • 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

    36/ 36

    OutlineOverviewBoilingPool boilingFlow boiling

    CondensationDropwise condensationFilm condensation

    BoilersCondensers