computational modelling as an alternative to full-scale testing for tunnel fixed fire fighting...
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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: [email protected]
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
Thank you