Presentation delivered by Matthew Bilson and Katie McQuade from WSP | Parsons Brinckerhoff in the USA, on March 16, 2016 at the 7th International Symposium on Tunnel Safety and Security (ISTSS) held in Montreal, Canada.

1. Matthew Bilson and Katie McQuade
Energy Budget in
Tunnel Fires – FFFS
Considerations
March 2016
© WSP | Parsons Brinckerhoff

2. MAIN POINTS
2
 In a tunnel fire with a fixed fire fighting
system (FFFS) operating:
 How much water is used for cooling
versus drained away?
 Can the amount of water used be
correlated to ventilation design?
 Energy balances can help to quantify
these questions.
 Outline:
 Energy budget concept.
 Cases with no FFFS:
- Energy budget (convection, radiation,
condition), critical velocity.
 Cases with FFFS:
- Energy budget redistribution.
 Impact on design approaches.

3. ENERGY DISTRIBUTION CONCEPT
3
 Energy distribution:
 To walls (about 30% of the heat):
- Radiation.
- Conduction.
 Convection to the airflow.
 Evaporation of water.
FIRE HEAT
RELEASE
CONVECTION
CONDUCTION
RADIATION
EVAPORATION
(FFFS)

4. ENERGY DISTRIBUTION – VENTILATION DESIGN
4
 Typical practice to deduct 30% of the fire heat release rate (FHRR) in
calculations:
 Can we take out more when the FFFS is involved?
 Is it even valid to take out 30%?
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0.0 20.0 40.0 60.0 80.0 100.0
V(m/s)
Q (MW)
Critical Velocity
NFPA 502: 100% FHRR
NFPA 502: FHRR - 30%

5. ENERGY BUDGET – PREVIOUS CONSIDERATIONS
5
Critical velocity reduction: 3.35 m/s to 2.75 m/s
(Connell and Melvin, using CFD)
0.67
0.33
CFD (Blanchard et. al.), backlayering
RAD+COND
CONV
0.500.50
CFD (Blanchard et. al.), no backlayer
RAD+COND
CONV
0.30
0.70
RULE OF THUMB
RAD+COND
CONV
0.500.50
CFD, (Blanchard et. al.)
RAD+COND
FFFS

6. ENERGY BUDGET – FURTHER QUESTIONS
6
 What is the impact of different
fire heat release rates (FHRR),
tunnel geometry, FFFS (water
application, droplet diameter)?
 How do results compare to critical
velocity equations?
 Approach:
 CFD models (using Fire
Dynamics Simulator Version
6).
 Suppression is not modeled –
only cooling.
 CFD models verified by
comparison with work by
Blanchard et. al. (scale tests).
0
50
100
150
200
250
0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0
Temperature(degC)
Time (s)
CFD, z=1.8 m
Exp't, z=1.8 m
CFD, z=1.5 m
Exp't, z=1.5 m
Memorial Tunnel Fire Ventilation and Test Program (left)
Typical road tunnel, flat ceiling (right)

7. 0.65
0.05
0.30
8.5 MW MTVFTP
CONV
COND
RAD
Vc=2.75 m/s 0.70
0.04
0.26
25.5 MW MTFVTP
CONV
COND
RAD
Vc=3.5 m/s
0.67
0.02
0.31
100 MW MTFVTP
CONV
COND
RAD
Vc=3.5 m/s0.70
0.03
0.27
50 MW MTFVTP
CONV
COND
RAD
Vc=3.5 m/s
CASES WITH NO FFFS OPERATING – MTFVTP
7
Confirms about 30% of the heat release is radiation,
over a range of FHRRs

8. 0.52
0.10
0.38
10 MW FLAT CEILING
CONV
COND
RAD
Vc=2 m/s
0.43
0.10
0.48
30 MW FLAT CEILING
CONV
COND
RAD
Vc=2.25 m/s
0.680.04
0.29
MTFVTP AVERAGE
CONV
COND
RAD
Vc=2.75 m/s
at 8.5 MW
0.41
0.04
0.55
100 MW FLAT CEILING
CONV
COND
RAD
Vc=2.5 m/s
CASES WITH NO FFFS OPERATING – FLAT CEILING
8
Geometry sensitivity is apparent, average 47%
radiation versus 29%

9. 0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0 20 40 60 80 100 120 140
V(m/s)
Q (MW)
Flat Ceiling
NFPA 502 (0% grade)
Wu and Bakar
Li and Ingason
CFD (0% grade, 6 m high, 8.8 m wide)
CRITICAL VELOCITY 9
 Flat ceiling:
 30% radiative
 0% grade
 MTFVTP:
 30% radiative
 -3.2% grade
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0 20 40 60 80 100 120 140
V(m/s)
Q (MW)
MTFVTP
Memorial Tunnel (-3.2%) - no backlayering
Memorial Tunnel (-3.2%) - backlayering
CFD (-3.2%, Memorial Tunnel)
NFPA 502 (-3.2% grade)
Wu and Bakar (-3.2% grade)
Li and Ingason

10. CRITICAL VELOCITY 10
 Flat ceiling:
 47% radiative
 0% grade
 MTFVTP:
 29% radiative
 -3.2% grade
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0 20 40 60 80 100 120 140
V(m/s)
Q (MW)
Flat Ceiling
NFPA 502 (0% grade)
Wu and Bakar
Li and Ingason
CFD (0% grade, 6 m high, 8.8 m wide)
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0 20 40 60 80 100 120 140
V(m/s)
Q (MW)
MTFVTP
Memorial Tunnel (-3.2%) - no backlayering
Memorial Tunnel (-3.2%) - backlayering
CFD (-3.2%, Memorial Tunnel)
NFPA 502 (-3.2% grade)
Wu and Bakar (-3.2% grade)
Li and Ingason

11. RECAP
11
 Energy budget:
 Between 30% and 50% radiative.
 Sensitive to geometry.
 Critical velocity:
 Sensitive to geometry.
 No one equation for critical velocity is universal.
 For design – use a combination of equations and CFD.

12. CASES WITH FFFS OPERATING – FLAT CEILING
12
FFFS droplet size is 1000 μm
About 40% of the FHRR is absorbed by the water
Split between reducing convection and radiation
Critical velocity reduction 0.25 m/s to 0.5 m/s – not so much!
0.52
0.10
0.38
10 MW, NO FFFS
CONV
COND
RAD
Vc=2 m/s
0.12
0.11
0.28
0.51
10 MW, FFFS 6 mm/min
CONV
COND
RAD
FFFS
Vc=1.5 m/s
0.14
0.07
0.34
0.46
30 MW, FFFS 6 mm/min
CONV
COND
RAD
FFFS
Vc=2 m/s
0.43
0.10
0.48
30 MW, NO FFFS
CONV
COND
RAD
Vc=2.25 m/s

13. CASES WITH FFFS OPERATING – IMPACT OF
WATER APPLICATION RATE
13
Increase in FFFS portion of energy budget for water increase
More water affects convection
Some impact on critical velocity
0.43
0.10
0.48
30 MW, NO FFFS
CONV
COND
RAD
Vc=2.25 m/s
0.20
0.08
0.37
0.36
30 MW, FFFS 3 mm/min
CONV
COND
RAD
FFFS
Vc=2 m/s
0.06
0.06
0.31
0.58
30 MW, FFFS 12 mm/min
CONV
COND
RAD
FFFS
Vc=1.75 m/s
0.14
0.07
0.34
0.46
30 MW, FFFS 6 mm/min
CONV
COND
RAD
FFFS
Vc=2 m/s

14. CASES WITH FFFS OPERATING – IMPACT OF
DROPLET SIZE
14
Reduction in the critical velocity
Substantial decrease in convection
0.43
0.10
0.48
30 MW, NO FFFS
CONV
COND
RAD
Vc=2.5 m/s
0.14
0.07
0.34
0.46
30 MW, FFFS 6 mm/min, 1000 μm dia
CONV
COND
RAD
FFFS
Vc=2 m/s
0.06
0.06
0.31
0.58
30 MW, FFFS 6 mm/min, 250 μm dia
CONV
COND
RAD
FFFS
Vc=1.5 m/s
0.14
0.07
0.34
0.46
30 MW, FFFS 6 mm/min, 500 μm dia
CONV
COND
RAD
FFFS
Vc=1.5 m/s, 16 m
backlayer

15. FINDINGS AND DISCUSSION
15
 Water application evaporation
 % of water energy potential evaporated:
- 10 MW = 4%, 30 MW = 11%, 100 MW = 33%.
- These values would increase with smaller droplets.
 Exit temperature:
 100MW: 200 deg C down to 100 deg C.
 FFFS and energy budget:
 Typically, around 45% of the FHRR is absorbed by water.
 Consistent with previous work (Blanchard et. al. ~50%).
 Order of magnitude balance of energy (flat ceiling):
No FFFS With FFFS
Radiation 45% Radiation 35%
Conduction 10% Conduction 5%
Convection 45% Convection 15%
FFFS 45%

16. FINDINGS AND DISCUSSION
16
 Critical velocity:
 Reduction by 35% for radiation, 45% for FFFS
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0 20 40 60 80 100 120 140
V(m/s)
Q (MW)
NFPA 502
Wu and Bakar
Li and Ingason
35% radiation + 45% FFFS deduction
CFD (cases with no FFFS)
CFD (cases with FFFS)

17. SUMMARY AND CONCLUSION
17
 Energy balances and critical velocity (no FFFS):
 35% radiation (curved), 45% (flat).
 Energy balances and critical velocity (with FFFS):
 Up to 45% energy is taken away by the FFFS.
 Impact on design approaches:
 Reduced heat load on fans.
 Reduced velocity that fans must achieve.
 CFD can be used to quantify these factors.
FIRE HEAT
RELEASE
CONVECTION
CONDUCTION
RADIATION
EVAPORATION
(FFFS)