THE FUNDAMENTALS OF WATER MIST .THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST

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Transcript of THE FUNDAMENTALS OF WATER MIST .THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST

  • THE FUNDAMENTALS OF WATER MIST TECHNOLOGY

    Dr. Jukka Vaari, VTT Fire Technology

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 2

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    THE SPECTRUM OF DROP SIZES

    DROP SIZE, m

    0.1 1.0 10 100 1000

    SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

    SPRINKLERSSPRAYERSAEROSOLS

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 3

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    WHAT IS WATER MIST - FORMAL DEFINITIONS

    NFPA 750:1996 Class 1, Dv0.1=100 m, Dv0.9=200 m Class 2, Dv0.1=200 m, Dv0.9=400 m Class 3, Dv0.9>400 m

    0.1 1.0 10 100 1000

    SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

    SPRINKLERSSPRAYERSAEROSOLS

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 4

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    WHAT IS WATER MIST - FORMAL DEFINITIONS

    NFPA 750:2003 Dv0.99 < 1000 m

    prCEN/TS 14972 Dv0.9 < 1000 m

    0.1 1.0 10 100 1000

    SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

    SPRINKLERSSPRAYERSAEROSOLS

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 5

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    HOW CAN WE MAKE WATER MIST

    Usually to atomize water, all that is needed is a high relative velocity between water and the surrounding air

    A few alternative techniques exist to produce low velocity and very fine (< 10 m) water mist

    ultrasonic

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 6

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    THE PLAIN ORIFICE PRESSURE ATOMIZER

    High-velocity water jet atomized due to friction at the air-liquid interface (drag force)

    W = work needed to create an amount of A of new surface area = surface tension (0.073 N/m) FD = drag force CD = drag coefficient air = density of air v = velocity of water jet

    = AW2

    airDD v21ACF =

    SECONDARYATOMIZATION

    PRIMARYATOMIZATION

    DRAGFORCE

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 7

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    Taylor & Hoyt: Exp. Fluids, vol.1, p. 113-120

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 8

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    THE SIMPLEX ATOMIZER

    Lefebvre: Atomization and Sprays, pp. 105-153

    SOLID-CONE HOLLOW-CONE

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 9

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    WHAT KIND OF DROPLETS DO WE GET

    spray = a system of drops immersed in a gaseous continuous phase

    for practical nozzles, the atomization process yields a spectrum of drop sizes

    mean drop size spectrum shape spectrum width

    20

    15

    10

    5

    0

    Pro

    babi

    lity

    dist

    ribut

    ion

    250200150100500Drop diameter

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 10

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    THE DROPLET SIZE SPECTRUM

    Several analytical functions exist to represent actual droplet size spectra

    20

    15

    10

    5

    0

    Pro

    babi

    lity

    dist

    ribut

    ion

    250200150100500Drop diameter

    Rosin-Rammler Log-normal

    ( )( )( )

    =

    2

    lnln

    21exp

    ln21)(

    DD

    DDf

    =

    qq

    q DDD

    DqDf exp)( 1

    LOG-NORMAL DISTRIBUTION

    ROSIN-RAMMLER DISTRIBUTION

    D=100 m, q=2.5, =1.545

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 11

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    THE MEAN DROP SIZE

    20

    15

    10

    5

    0P

    roba

    bilit

    y di

    strib

    utio

    n250200150100500

    Drop diameter

    ( )ba

    bii

    aii

    ab DNDN

    D

    =

    1

    SAUTER MEAN DIAMETER (SMD): a=3, b=2

    D0.1 D0.9

    VOLUME MEAN DIAMETER (D30): a=3, b=0

    SMD

    Dpeak

    D0.5

    D0.99

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 12

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    10

    100

    1000

    Sur

    face

    are

    a (m

    2 / li

    tre)

    1 10 100 1000

    Drop size (m)

    1014

    1013

    1012

    1011

    1010

    109

    108

    107

    106

    Drops per litre

    THERMAL MANAGEMENT

    Q hA T=

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 13

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    AEROSOL CHARACTERISTICS: WATER MIST

    70x10-3

    60

    50

    40

    30

    20

    10

    0

    Pro

    babi

    lity

    dist

    ribut

    ion

    10080604020Drop size (m)

    10-4

    10-3

    10-2

    10-1

    Terminal velocity (m

    /s)

    Plain orifice atomizer, d=0.5 mm, p=15 MPa

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 14

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    30x10-3

    25

    20

    15

    10

    5

    0

    Pro

    babi

    lity

    dist

    ribut

    ion

    54321Drop size (mm)

    10

    8

    6

    4

    2

    Terminal velocity (m

    /s)

    AEROSOL CHARACTERISTICS: SPRINKLERS

    13.5 mm orifice, p=206 kPaH.-Z. Yu, Proc. 1st Int. Symp. On Fire Safety Science, pp. 1165-1176

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 15

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    DROP-PLUME INTERACTION: DIRECT SUPPRESSION

    Surface wetting Flame cooling

    D = 1 mm D = 100 m

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 16

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 17

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    DROP-ROOM INTERACTION: VAPOUR INERTING

    P,Flow

    PUMPVentilation

    Vapor formationoutside flame

    Gas sample,thermocouple

    Obstructionplates

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 18

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 19

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    DROP-ROOM INTERACTION: VAPOUR INERTING

    Fire only

    Dry oxygen

    True oxygen

    Extinction limit

    text=12 min0.20

    0.18

    0.16

    0.14

    0.12

    O2

    mol

    e fra

    ctio

    n

    151050

    Time (min)

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 20

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    PARTIAL PRESSURE OF WATER VAPOUR

    1.0

    0.8

    0.6

    0.4

    0.2

    0.0

    Vap

    our p

    ress

    ure

    (bar

    )

    100806040200

    Temperature (C)

    Oxygen concentration (%

    )

    0

    4.2

    8.4

    12.6

    16.8

    21

    14

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 21

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    DROP-RADIATION INTERACTION: STOPPING FIRE SPREAD

    I

    D

    P()

    Dx =

    Scattering cross section highest for x = 1

    106

    105

    104

    103

    102

    101

    100

    10-1A

    ttenu

    atio

    n co

    effic

    ient

    (1/m

    )

    2 4 6 8

    10-62 4 6 8

    10-52 4 6 8

    10-42

    Wavelenght (m)

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 22

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    MIST-RADIATION INTERACTION

    dx

    2

    1I

    3

    I

    4

    I

    I+I-

    Radiation solvers

    Two-fluxapproximation

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 23

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    MIST-RADIATION INTERACTION

    SMD Dv0.1 Dv0.9Mist 1 30 17.3 80.7Mist 2 100 57.5 269Mist 3 300 173 807

    0.30

    0.25

    0.20

    0.15

    0.10

    0.05

    0.00

    Tran

    smitt

    ed fr

    actio

    n

    2.01.51.00.50.0Water film thickness (mm)

    TF=1200C TF=800C

    1.0

    0.8

    0.6

    0.4

    0.2

    0.0Tr

    ansm

    itted

    frac

    tion

    0.50.40.30.20.10.0Water mist concentration (kg/m3)

    Mist 1

    Mist 2

    Mist 3

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 24

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 25

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    FACTORS AFFECTING THE SUPPRESSION EFFICIENCY OF A WATER MIST SYSTEM

    Water droplets diameter velocity with respect to the surrounding gas

    Water spray momentum distribution

    penetration of fire fighting water ability of the spray to capture, transport and circulate the

    surrounding gases

    Water mist system positioning of the water sprays with respect to the fire, the

    room, and each other interaction with fire detection

    SC

    ALE

    SM

    ALL

    LAR

    GE

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 26

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    IN OTHER WORDS ...

    Interaction of a droplet with a flame; droplet vaporization

    Atomization, spray dynamics

    Large-scale flows with particle tracking; fire dynamics, fire spread

    SC

    ALE

    SM

    ALL

    LAR

    GE

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 27

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    PREDICTING ACCURATELYWATER MIST SYSTEM

    PERFORMANCE IS BEYOND CURRENT MODELING

    CAPABILITIES

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 28

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    WHAT IS A WATER MIST SYSTEM

    NFPA 750:2006 A system... discharging water mist and demonstrated

    to meet the performance requirements... (3.3.20) Test protocols shall be conducted to verify the working

    limits and installation parameters (8.2.5)

    PERFORMANCE-BASED TECHNOLOGY

  • AIIA-SFPE Milan 12.2.2008 Jukka Vaari 29

    VTT TECHNICAL RESEARCH CENTRE OF FINLAND

    FEATURES OF A FIRE TEST PROTOCOL

    Represents a simplification of rea