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Fire modelling of sprinklers and water mist
Jukka Vaari, VTT
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Owned by
Ministry of
Economic
Affairs and
Employment
VTT – beyond the obvious
Established in
1942
258M€Net turnover and
other operating
income (VTT
Group 2017) 2,368
Total of personnel
(VTT Group
31.12.2017)
36%from abroad
(VTT Group
2017)
27%Doctorates and
Licentiates
(VTT Group
2017)
VTT is one of the leading research, development and
innovation organizations in Europe. We help our customers
and society to grow and renew through applied research. The
business sector and the entire society get the best benefit
from VTT when we solve challenges that require world-class
know-how together and translate them into business
opportunities.
Our vision
A brighter future is created through science-based
innovations.
Our mission
Customers and society grow and renew through
applied research.
Strategy
Impact through scientific and technological excellence.
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Outline
• Modelling and simulation• Sources of uncertainty• Spatial resolution
• Water sprays• Spray dynamics• Water distribution
• Interaction between fire and water sprays• Gas-phase cooling• Solid-phase cooling• Absorption of radiation• Gas-phase suppression• Solid-phase suppression
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Modelling and simulation
• Simulation is the imitation of the operation of a real-world process or system over time.
• The act of simulating something first requires that a model be developed; this model represents the key characteristics or behaviors/functions of the selected physical or abstract system or process.
• The model represents the system itself, whereas the simulation represents the operation of the system over time.
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Modelling and simulation
“Essentially, all models are wrong, but some are useful.”- George E.P. Box (1919-2013)
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Sources of uncertainty in a simulation
Model
Input
User
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CFD: spatial resolution
• All quantities of interest could be accurately calculated provided a very fine spatial resolution was used (below 1 mm)
• In practial simulations, the spatial resolution is 10-50 cm due to computational resources
• The significance of the error due to coarse resolution can and should be estimated performing a grid sensitivity test
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Spray dynamics
• Accurate modelling of spray dynamics requires high-quality experimental data
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Spray dynamics
• Complex spray patterns constructed by superposition of micronozzle sprays• Collective spray dynamics OK provided micronozzle
spray dynamics is OK
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Spray dynamics
• Spatial resolution is an important consideration for correct description of water mist spray dynamics
Dx=2.5 cm Dx=20 cm
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Cooling performance
• Protection of ceiling steel structures• Direct cooling by water impinging on surface
• Indirect cooling by cooling the fire plume
1000
800
600
400
200
0
Tem
pera
ture
(°C
)
108642
Height (m)
10MW Freeburn
SSP 12.5 mm/min
ESFR K363
1000
800
600
400
200
0
Tem
pe
ratu
re (
°C)
108642
Height (m)
Freeburn
1400 m
1200 m
1000 m
800 m
600 m
https://www.vtt.fi/inf/pdf/technology/2012/T54.pdf
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Radiation attenuation
• High-pressure water mist micronozzles
https://www.vtt.fi/inf/pdf/technology/2012/T54.pdf
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Gas-phase suppression
• Cup burner: apparatus to measure Minimum Extinguishing Concentration (MEC) for combinations of fuels and suppression agents
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Gas-phase suppression
• Results from FDS Validation Guide, inert gas agents
Water vapour ?
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Gas-phase suppression
• USCG/HAI machinery space tests (100 m3)
Diesel spray fires 250, 500, 1000 kW’Navy’ WM system, natural ventilation
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Gas-phase suppression
• Results from FDS Validation Guide, water mist total flooding (machinery space) systems
Fire extinguishment due to oxygen depletion by fire, oxygen displacement by water vapour, and water vapour acting as an inert gas suppression agent in the flame.
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Solid-phase suppression
• The ability of FDS to model solid-phase suppression depends on how the user chooses to describe fire
• The options are• Pre-defined (time dependent) Heat Release Rate Per
Unit Area (HRRPUA)
• Pyrolysis model
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Solid-phase suppression
• The usual case in performance-based fire protection engineering: no sprinklers explicitly in the simulation
Time
Hea
t R
elea
se R
ate
Freeburn
Sprinklers
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Solid-phase suppression
• Advanced HRRPUA: E_COEFFICIENT
5000
4000
3000
2000
1000
0
HR
R (
kW)
150010005000
Time (s)
5 mm/min Experiment FDS
7.5 mm/min Experiment FDS
10 mm/min Experiment FDS
Arvidson & Lönnermark, SP Report 2002:03
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Solid-phase suppression
• Road tunnel protection with water mist
9 m24 m
1.8 m/s
80
60
40
20
0
HR
R (
MW
)
2000150010005000
Time (s)
Freeburn (FDS) Mist (FDS) Mist (experiment)
1400
1200
1000
800
600
400
200
0
Te
mp
era
ture
(°C
)
420410400390380370
Position (m)
Freeburn (FDS) Water mist (FDS) Water mist
(experiment)
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Solid-phase suppression
• Pyrolysis: residential sprinkler test
Wall panels: birchFuel (75%)
Char (25 %)
Foam pads: polyether Fuel (100 %)
Wood crib & cotton wicks: HRRPUA
REFERENCE_TEMPERATURE=300.
PYROLYSIS_RANGE=160.
REFERENCE_TEMPERATURE=200.
PYROLYSIS_RANGE=180.
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Solid-phase suppression
• Efficiency of a residential sprinkler: wall wetting
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Summary
• Verification and validation excercises (open simulations) give information on the capabilities and accuracy of the model
• Effects related to user skills and quality of input data are less well understood
• Ultimate decision on whether a model is suitable for an application remains solely with the user
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Acknowledgements
• Financial support has been provided by the Finnish funding Agency for Technology and Innovation
• Marioff Corporation Oy has supported this research financially and by providing hardware