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