Predicting fire suppression by water spray with numerical codes: model development

and validation

Alexandre JENFT1, Armelle MULLER1, Grégoire PIANET1,

Arnaud BRETON1 Pascal BOULET2, Anthony COLLIN2,

1 CNPP, Route de la Chapelle Réanville, BP 2265, F-27950 Saint Marcel - FRANCE

2 LEMTA, 2 Avenue de la Forêt de Haye - TSA 60604 - 54518 Vandoeuvre-lès-Nancy cedex – FRANCE

Contents

• Suppression mechanisms by water spray

• Experimental study

• Numerical results

• New suppression model writing

• New suppression model results

• Conclusions / current work

2

Suppression mechanisms

• Gas phase cooling;

• Oxygen displacement and fuel vapor dilution;

• Fuel surface cooling and wetting;

• Radiative transfer attenuation;

• Kinetic effects.

3

Experimental setup

Metrology:• 18 thermocouples• Gas analyser for O2, CO2

and CO• Load cell • Video camera

Water mist characteristics:• Pressure = 10 bars• Flow rate = 6.3

l/min/nozzle• Injection angle = 130° • D32 = 112 µm 4

Suppression observationTest 14tapp = 1 min

a) t0+10 s b) tapp-1 s

c) tapp+5 s d) tapp+10 s e) tapp+20 s

5f) tapp+30 s g) tapp+60 s h) tapp+65 s

Experimental results: fuel oil

6

N° Dpool

[cm]T0

[°C]tapp

[s]HRRapp

[kW]TA2,290,app

[°C]dtvid

[s]dtgas

[s]dtpool

[s]

13 35 12 38 28 21 40 35 35

14 35 11 64 48 32 65 61 65

20 35 7 94 52 42 65 58 78

21 35 6 126 56 51 70 57 78

31 25 10 38 7 17 31 29 35

32 25 10 64 12 23 30 30 62

33 25 10 99 19 29 69 71 104

34 25 10 128 24 33 99 108 126

35 25 10 186 32 39 106 95 126

36 25 10 305 40 47 105 103 125

37 25 10 544 46 50 126 131 156

Time - [s]0 50 100 150 200 250 300 350 4000

50

100

150

200

250

300

350

400

450

500

TA1,50 - [°C]TA1,150 - [°C]TA1,250 - [°C]TA1,290 - [°C]

tgas

tapp

Time - [s]0 50 100 150 200 250 300 350 400

100

200

300

400

500

600

700

Tpool,1 - [°C]Tpool,2 - [°C]Tpool,3 - [°C]Tpool,4 - [°C]Tpool,5 - [°C]

tpool

tapp

Numerical model

7

Main parameters:

• Cell size : 5 cm x 5 cm x 5 cm;

• Power increase defined as a ramp following the actual measured curve until stationary regime;

• After mist activation, HRR guided toward a reduction through suppression model.

Suppression model:

with

dttk

ff emtm )(''

0,'' )(

)()( '' tmatk w

8

N° 13 14 20 21

HRRapp – [kW] 28 48 52 56

a – [m²/kg/s] 0.2 0.15 0.4 0.2

N° 31 32 33 34 35 36 37

HRRapp – [kW] 7 12 19 24 32 40 46

a – [m²/kg/s] 1 1 0.24 0.26 0.42 0.74 0.24

It is impossible to predict the value of ‘a’ for a test which has not been carried out prior to the simulation.

This model does not allow predictive simulations.

Fire suppression model

9

Pyrolysis rate reduction during water application is linked to fuel surface temperature. The model is written:

B and E are empirical coefficient which can be easily determined, even with no preliminary real test.

The model is based on Arrhenius law:

ignfuelpyro

ignfuelfuel

ignfuelpyro

TTtm

TTtRT

ETtTBtm

if 0)(

if ))(

exp()()(

''

''

))(

exp()(''

tRT

EAtm

fuelpyro

Tfuel - [°C]

mpyro-[kg/s]

100 150 200 250 3000

0.2

0.4

0.6

0.8

1

1.2

1.4

ExperimentsModel

x 10-3

.

10

How does it work ?

1. Simulate the part before water application with a specific interest in HRR (or pyrolysis rate) evolution and fuel surface temperature;

2. Identify B et E on this part;

3. Put optimal values for B and E in simulation input file;

4. Simulate the whole test.B = 0.00122 kg/m²/K0.5/s

E = 3751 J/mol

Results on suppression time

11

N° 13 14 20 21 31 32 33 34 35 36 37

tsup,exp

(s)40 65 65 70 31 30 69 99 106 105 129

tsup,num

(s)22 39 50 66 8 17 32 43 57 74 86

Gap(s)

18 26 15 4 23 13 37 56 49 31 43

Gap(%)

45 40 23 6 74 43 54 57 46 30 33

• The new model predicts suppression by fuel cooling in every tests, just like in real tests;

• Suppression time prediction still needs improvement.

12

Conclusions

1. An experimental study has been carried out to understand suppression mechanisms;

2. For the “fuel cooling” cases, a new suppression model has been developed and integrated to FDS;

3. This model allows predictive simulations for fire suppression by water spray.

13

Current work

1. Improve model results by improving fuel temperature

calculation through particles / fuel exchanges modeling;

2. Validate the model on other configurations;

3. Determine FDS capability to determine extinction by flame cooling and inerting effects in FDS 6.