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

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

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

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

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

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)(
σσπDD
DDf
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠⎞
⎜⎝⎛−= −
q DDD
DqDf exp)( 1
LOG-NORMAL DISTRIBUTION
ROSIN-RAMMLER DISTRIBUTION
D=100 μm, q=2.5, σ=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 / 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

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

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 μm

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
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)

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 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

AIIA-SFPE Milan 12.2.2008 Jukka Vaari 30
VTT TECHNICAL RESEARCH CENTRE OF FINLAND