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Page 1: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

THE FUNDAMENTALS OF WATER MIST TECHNOLOGY

Dr. Jukka Vaari, VTT Fire Technology

Page 2: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 2

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

THE SPECTRUM OF DROP SIZES

DROP SIZE, μm

0.1 1.0 10 100 1000

SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

SPRINKLERSSPRAYERSAEROSOLS

Page 3: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 μm, Dv0.9=200 μm • Class 2, Dv0.1=200 μm, Dv0.9=400 μm• Class 3, Dv0.9>400 μm

0.1 1.0 10 100 1000

SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

SPRINKLERSSPRAYERSAEROSOLS

Page 4: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 μm

• prCEN/TS 14972• Dv0.9 < 1000 μm

0.1 1.0 10 100 1000

SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

SPRINKLERSSPRAYERSAEROSOLS

Page 5: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 μm) water mist

• ultrasonic

Page 6: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 ΔA of new surface area• σ = surface tension (0.073 N/m)• FD = drag force• CD = drag coefficient• ρair = density of air• v = velocity of water jet

σ⋅Δ= AW2

airDD v21ACF ρ=

SECONDARYATOMIZATION

PRIMARYATOMIZATION

DRAGFORCE

Page 7: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 7

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

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

Page 8: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 9: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 10: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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)(

σσπDD

DDf

⎥⎥⎦

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−= −

qq

q DDD

DqDf exp)( 1

LOG-NORMAL DISTRIBUTION

ROSIN-RAMMLER DISTRIBUTION

D=100 μm, q=2.5, σ=1.545

Page 11: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 12: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 12

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

10

100

1000

Sur

face

are

a (m

2 / litr

e)

1 10 100 1000

Drop size (μm)

1014

1013

1012

1011

1010

109

108

107

106

Drops per litre

THERMAL MANAGEMENT

Q hA T= Δ

Page 13: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 (μm)

10-4

10-3

10-2

10-1

Terminal velocity (m

/s)

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

Page 14: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 15: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 μm

Page 16: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 16

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

Page 17: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 18: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 18

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

Page 19: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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)

Page 20: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 21: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 21

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

DROP-RADIATION INTERACTION: STOPPING FIRE SPREAD

θλ

D

P(θ)

λπDx =

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)

Page 22: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 23: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 24: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 24

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

Page 25: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 26: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 27: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 28: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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

Page 29: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

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 reality• Contains essential features with respect to the

application, such as• fire loads, ignition scenarios, fire scenarios• physical dimensions, ventilation, obstructions

• Consists of several individual fire tests, in order to• find performance limits of a system• avoid using a single favourable fire scenario as the

measure for system performance• Quantitatively measures the system performance against

a set of performance requirements• Yields system design and installation parameters

Page 30: THE FUNDAMENTALS OF WATER MIST TECHNOLOGY · THE FUNDAMENTALS OF WATER MIST TECHNOLOGY ... WHAT IS A WATER MIST SYSTEM • NFPA 750:2006 ... • Yields system design and installation

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 30

VTT TECHNICAL RESEARCH CENTRE OF FINLAND