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

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, μm

0.1 1.0 10 100 1000

SMOKE SEA FOG CLOUDS MIST DRIZZLE RAIN

SPRINKLERSSPRAYERSAEROSOLS



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





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





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



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



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



THE SIMPLEX ATOMIZER

Lefebvre: Atomization and Sprays, pp. 105-153

SOLID-CONE HOLLOW-CONE



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



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



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



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



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



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



DROP-PLUME INTERACTION: DIRECT SUPPRESSION

Surface wetting Flame cooling

D = 1 mm D = 100 μm



DROP-ROOM INTERACTION: VAPOUR INERTING

P,Flow

PUMPVentilation

Vapor formationoutside flame

Gas sample,thermocouple

Obstructionplates



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)



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



DROP-RADIATION INTERACTION: STOPPING FIRE SPREAD

θλ

Iλ

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)



MIST-RADIATION INTERACTION

dx

2

1I

3

I’

4

I

I+I-

Radiation solvers

Two-fluxapproximation



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



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



… 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



PREDICTING ACCURATELYWATER MIST SYSTEM

PERFORMANCE IS BEYOND CURRENT MODELING

CAPABILITIES



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



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

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

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