Matthew Bilson and Katie McQuade

Energy Budget in

Tunnel Fires – FFFS

Considerations

March 2016

© WSP | Parsons Brinckerhoff

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.

ENERGY DISTRIBUTION CONCEPT

3

Energy distribution:

To walls (about 30% of the heat):

- Radiation.

- Conduction.

Convection to the airflow.

Evaporation of water.

FIREHEATRELEASE

CONVECTION

CONDUCTION

RADIATION

EVAPORATION(FFFS)

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%

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(Blanchardet.al.),backlayering

RAD+COND

CONV

0.500.50

CFD(Blanchardet.al.),nobacklayer

RAD+COND

CONV

0.30

0.70

RULEOFTHUMB

RAD+COND

CONV

0.500.50

CFD,(Blanchardet.al.)

RAD+COND

FFFS

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

Tem

per

atu

re (

deg

C)

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)

0.65

0.05

0.30

8.5MWMTVFTP

CONV

COND

RAD

Vc=2.75m/s 0.70

0.04

0.26

25.5MWMTFVTP

CONV

COND

RAD

Vc=3.5m/s

0.670.02

0.31

100MWMTFVTP

CONV

COND

RAD

Vc=3.5m/s0.700.03

0.27

50MWMTFVTP

CONV

COND

RAD

Vc=3.5m/s

CASES WITH NO FFFS OPERATING – MTFVTP7

Confirms about 30% of the heat release is radiation,

over a range of FHRRs

0.52

0.10

0.38

10MWFLATCEILING

CONV

COND

RAD

Vc=2m/s

0.43

0.10

0.48

30MWFLATCEILING

CONV

COND

RAD

Vc=2.25m/s

0.680.04

0.29

MTFVTPAVERAGE

CONV

COND

RAD

Vc=2.75m/sat8.5MW

0.41

0.04

0.55

100MWFLATCEILING

CONV

COND

RAD

Vc=2.5m/s

CASES WITH NO FFFS OPERATING – FLAT CEILING8

Geometry sensitivity is apparent, average 47%

radiation versus 29%

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

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

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.

CASES WITH FFFS OPERATING – FLAT CEILING12

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

10MW,NOFFFS

CONV

COND

RAD

Vc=2m/s

0.12

0.11

0.28

0.51

10MW,FFFS6mm/min

CONV

COND

RAD

FFFS

Vc=1.5m/s

0.140.07

0.34

0.46

30MW,FFFS6mm/min

CONV

COND

RAD

FFFS

Vc=2m/s

0.43

0.10

0.48

30MW,NOFFFS

CONV

COND

RAD

Vc=2.25m/s

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

30MW,NOFFFS

CONV

COND

RAD

Vc=2.25m/s

0.20

0.08

0.37

0.36

30MW,FFFS3mm/min

CONV

COND

RAD

FFFS

Vc=2m/s

0.060.06

0.310.58

30MW,FFFS12mm/min

CONV

COND

RAD

FFFS

Vc=1.75m/s

0.140.07

0.34

0.46

30MW,FFFS6mm/min

CONV

COND

RAD

FFFS

Vc=2m/s

CASES WITH FFFS OPERATING – IMPACT OF

DROPLET SIZE

14

Reduction in the critical velocity

Substantial decrease in convection

0.43

0.10

0.48

30MW,NOFFFS

CONV

COND

RAD

Vc=2.5m/s

0.140.07

0.34

0.46

30MW,FFFS6mm/min,1000μmdia

CONV

COND

RAD

FFFS

Vc=2m/s

0.060.06

0.310.58

30MW,FFFS6mm/min,250μmdia

CONV

COND

RAD

FFFS

Vc=1.5m/s

0.140.07

0.34

0.46

30MW,FFFS6mm/min,500μmdia

CONV

COND

RAD

FFFS

Vc=1.5m/s,16mbacklayer

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%

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)

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.

FIREHEATRELEASE

CONVECTION

CONDUCTION

RADIATION

EVAPORATION(FFFS)