Fire modelling of sprinklers and water mist

Jukka Vaari, VTT

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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

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.

Modelling and simulation

“Essentially, all models are wrong, but some are useful.”- George E.P. Box (1919-2013)

Sources of uncertainty in a simulation

Model

Input

User

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

Spray dynamics

• Accurate modelling of spray dynamics requires high-quality experimental data

Spray dynamics

• Complex spray patterns constructed by superposition of micronozzle sprays• Collective spray dynamics OK provided micronozzle

spray dynamics is OK

Spray dynamics

• Spatial resolution is an important consideration for correct description of water mist spray dynamics

Dx=2.5 cm Dx=20 cm

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

Radiation attenuation

• High-pressure water mist micronozzles

https://www.vtt.fi/inf/pdf/technology/2012/T54.pdf

Gas-phase suppression

• Cup burner: apparatus to measure Minimum Extinguishing Concentration (MEC) for combinations of fuels and suppression agents

Gas-phase suppression

• Results from FDS Validation Guide, inert gas agents

Water vapour ?

Gas-phase suppression

• USCG/HAI machinery space tests (100 m3)

Diesel spray fires 250, 500, 1000 kW’Navy’ WM system, natural ventilation

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.

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

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

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

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)

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.

Solid-phase suppression

• Efficiency of a residential sprinkler: wall wetting

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

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