Computational modelling as an alternative to full-scale testing for tunnel fixed fire fighting systems

Kenneth J. Harris & Bobby J. Melvin

Parsons Brinckerhoff

Sacramento, CA USA

E-mail: harris@pbworld.com

Presented By Aaron McDaid

Key modeling bases

Fundamental energy analysis can be used to estimate water application rates.

Subroutines that model the key elements of solid and liquid vaporization have been written.

Subroutines that model the key elements of combustion energy have been written.

Dynamics of Fire and Extinguishment

Water Application Rate Equation

Comparison of two identical fire test set-ups

Flaming Radiative Heat Flux & Pyrolysis Model

Common Heat Flux Levels

Source kW/m2

Irradiance of sun on the earth’s surface ≤1 Minimum for pain to skin (relatively short exposure) ~1Minimum for burn injury (relatively short exposure) ~4Usually necessary to ignite thin items ≥10 Usually necessary to ignite common furnishings ≥20Surface heating by a small laminar flame 50-70Surface heating by a turbulent wall flame 20-40 ISO 9705 room-corner test burner to wall 100 kw 40-60 ISO 9705 room-corner test burner to wall 300 kw 60-80Within a fully-involved room fire (800-1000 C) 75-150 Within a large pool fire (800-1200 C) 75-267

Description of LTA Fire Tests

LTA Test No.

Water Application Rate (mm/min)

Activation Time after 60 C

Peak FHRR (MW)

Target Ignited?

Max Target Heat Flux (kw/m2)

1 12 4 min 37.7 No 2

2 8 4 min 44.1 Unknown Unknown

7 0 none 150 Yes 225

Tabulation and comparison of fuel quantities

Model Values Wood Plastic Total Test Values

Volume (m3)/% 7.6/82 1.7/18 9.3 80/20

Mass (kg)/% 3,410/67 1,711/33 5,121 5,000

Energy (GJ)/% 58.0/61 37.6/39 95.6 99.2

Total inc. Target (GJ)

    117  

Fuel Properties

Property (11) (12) (12)D F

(13) (14) (15) (16) Value Used

Wood  

Specific Heat ~1.5-2.0 2.5-7.4 2.2-4.0 1.2-2.0       2.2

Thermal Conductivity

0.12 .19-2.08 .23-.80         0.23

Density 600 354-753 455-502 300-550       450

Heating Rate               5

Heat of Reaction   1600-3500

1600-2900

        1600

Heat of Combustion

  17000           17000

Plastic  

Specific Heat         1.4-1.5 .92-2.3   1.4

Thermal Conductivity

        .17-.19     0.17

Density         1150-1190

  570-3900 1000

Heating Rate               5

Heat of Reaction           800-6400   1500

Heat of Combustion

          14000-47000

  22000

Comparison of model and test results for unsuppressed fire

Comparison of model and test results for 12 mm/min. suppressed fire

Peak heat flux and FHRR for various leakage rates

0 2 4 6 8 10 120

5

10

15

20

25

30

35

40

45

0

20

40

60

80

100

120

Water Application Rate (mm/min)

Pea

k N

et H

eat

Flu

x kW

/m2

Pea

k F

HR

R (

MW

)

Heat flux

FHRR

Comparison of model and test results for unsuppressed and 12 mm/min. suppressed

fire

Dynamics of Fire and Extinguishment

Water application rate for external heat flux only

Vaporized water heat flux

Water Application Rate 2 mm/min

Water Application Rate 4 mm/min

Main/Target Rate 4/0 mm/min

Conclusion

o Computer modelling provides a more cost-effective means of demonstrating proposed system performance.

o The fuel vaporization process is well-defined in fire science and the computer models can be set up to utilize this approach. • Some significant differences in modelling are required for this approach. • The fuel properties and structure must be explicitly defined.

o Comparison with a test is beneficial to calibrate the model.  • Modelling of the unsuppressed fire in particular can produce results very close to

that shown in testing. • Modelling of fire suppression can provide results that give a reasonable degree of

confidence of what can be expected of the system.o Computer modelling can be used to model the interaction of water and fire for

design purposes, making individual full-scale testing unnecessary and making FFFS more likely to be implemented in road tunnels.

o Pyrolysis-based input rather than fire heat release rate input should be used to more accurately model the effects of water and fire interaction.

Fire Sprinkler International

FSI 201422

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