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

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

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

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

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

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)(
DD
DDf
=
qq
q DDD
DqDf exp)( 1
LOG-NORMAL DISTRIBUTION
ROSIN-RAMMLER DISTRIBUTION
D=100 m, q=2.5, =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 / 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=

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

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 m

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
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)

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 rea

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

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