Control of smoke propagation in a wide road tunnel with...

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Control of smoke propagation in a wide road tunnel with side wall jet fans through a dedicated ventilation strategy Application to Hatfield tunnel (UK) Frederic WAYMEL, Len SMALL, Mélanie LORENZ 6th International Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

6th International Conference ‘Tunnel Safety and Ventilation’ 2012, Graz

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Contents of the presentation

Introduction Issue of longitudinal ventilation in wide cut & cover tunnel

Case of Hatfield tunnel

Design assessment of the proposed ventilation strategy Design methodology : 3 steps design

Design results

Test verification Test protocol

Test results

Conclusion


3

INTRODUCTION


4

General

Most of the tunnels built during the 20th century need to be refurbished to achieve the new fire safety requirements

Longitudinal ventilation strategy in a twin bore unidirectional tunnel is generally the best cost / effective solution

Jet fans are generally used to achieve the critical longitudinal velocity

3 m/s


5

Presentation of the main purpose

Issue of cut and cover tunnels Height of the tunnel generally optimized as per the dynamic gauge of the trucks ⇒

Installation of jet fans under the ceiling generally not possible

Other solutions such as Saccardo can be proposed when possible

Most of the time, installation of side walls jet fans is the only possible and the most cost / effective solution


6

Presentation of the main purpose

Issue raised with side wall jet fans in wide cut and cover tunnels Jet fans induce local aerodynamic effects leading to heterogeneity of the velocity field

Effects increased in wide tunnels ⇒ low velocity and backflow in the middle of the tunnel ⇒ Risk of significant backlayering

Main reasons Difficulty to transfer the momentum from the jet into the middle of the tunnel

Significant airflow absorbed and dragged by the jet fans which tends to reduce the velocity in the remaining part of the cross-section

Length


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Twin bore cut and cover tunnel of 1.1 km long / 3 lanes per bore in the North of London (A1-M25 Motorway) Wide cross-section (19.2 m wide and 5.7 m high)

Existing ventilation system was based on 28 small jet fans per bore in the corner of the cross-section unable to achieve the new design requirements (based on risk analysis) :

50 MW with 15 Pa adverse wind pressure 100 MW with 0 Pa adverse wind pressure


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Initial design based on “Banana” Jet fans 3D simulation shown that significant backlayreing still occurs due to low velocity in the middle

Saccardo : extension of the tunnel for a new building over the carriageway Increase of the cost

Obtaining planning permission: Lengthy process (tunnel to be refurbished one year before the London Olympic games)

Architectural aspect ?


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Proposed ventilation system for Hatfield tunnel

7 banks of fans with 160 m apart / 4 jet fans per bank

Reversible JF of 1000 N nominal thrust – 1000 mm inner diameter (ZITRON JZR

10-30/4)

80 m 160 m 160 m 160 m 160 m 160 m 160 m 100 m

Entry portal

Exit portal


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DESIGN ASSESSMENT OF THE PROPOSED

VENTILATION STRATEGY


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3 steps design methodology

1st step : 3D CFD simulation in a limited part of the tunnel without any vehicles and without any fire Assessment of local effects

Find the most relevant design solution for the fans arrangement and the ventilation control philosophy to minimize the effects of low velocity in the middle of the cross-section

2nd step : 1D simulation based on the jet fans configuration and control ventilation strategy determined during the first step Accurate calculation of volume flow rates and average longitudinal velocities for various situation (fire

location, adverse wind pressure, traffic conditions, fire size…)

Comparison with the critical velocity

3rd step : 3D simulation including vehicles and fire for the worst ventilation situation Based on 2d step 1D simulations for the boundary conditions

Assessment of smoke control for the worst cases


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1st step design results : 3D assessment of the ventilation strategy

Control philosophy based on the non-activation of the nearest bank of fans upstream of the fire

Need of accurate detection of the fire location to switch off the appropriate bank of fans Linear detection system with 10 m accuracy provided in Hatfield tunnel

Inoperative jet fans

Region of fire

Length

3 m/s

3 m/s


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2nd step design results : 1D assessment of the longitudinal velocity

Main 1D design assumptions No barriers at entry portal (tunnel full of vehicles upstream the fire)

Linear friction coefficient : 0.02

Jet fan efficiency : 0.64 estimated during the 3D-CFD simulation of the 1st design step

10% of the fans unavailable prior the fire / Nearest bank of fans upstream the fire not activated /

Nearest bank of fans downstream the fire burnt

Results V > Critical velocity

Lowest velocity : Fire at 100 m from exit portal

3.3 m/s for a 50 MW fire-15 Pa

3.6 m/s for a 100 MW fire-0 Pa

Longitudinal velocity upstream the fire

2.50

3.00

3.50

4.00

4.50

5.00

5.50

0 100 200 300 400 500 600 700 800 900 1000 1100

Distance of fire from entry portal (m)

V (m

/s)

50 MW - 15 Pa100 MW - 0 Pa

Design criterion 50 MW

Design criterion 100 MW


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3rd step design results : 1D assessment of the longitudinal velocity

Isotherm 50°C – 100 MW fire – 3.6 m/s longitudinal velocity


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ON SITE TEST VERIFICATION


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Test protocol

Test the performance for the aerodynamic conditions corresponding to the worst ventilation flow rate observed during the design 3.3 m/s longitudinal velocity

Empty tunnel ⇒ limited number of fans activated to avoid non-realistic high longitudinal velocity

Entry portal

Exit portal

Measurement sectionsLongitudinal velocity measurement (control at 3.3 m/s)

Activated fans

Shut down fans

Bank n°1 Bank n°2 Bank n°3 Bank n°4 Bank n°5 Bank n°6 Bank n°7


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Test protocol

Measurements Velocity measurements with anemometers

Measurement grid

3.5

m

1.02 m

3.90 m

7.03 m

9.60 m

12.2 m

15.3 m

18.2 m

1.5

m


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Test results

Entry portal

Exit portal

Measurement sectionsLongitudinal velocity measurement (control at 3.3 m/s)

Activated fans

Shut down fans

Bank n°1 Bank n°2 Bank n°3 Bank n°4 Bank n°5 Bank n°6 Bank n°7

Transverse velocity profile (midway banks 4 & 5)

0.00.51.01.52.02.53.03.54.04.55.05.5

0 2 4 6 8 10 12 14 16 18

Distance from the side wall (m)

Vel

ocity

(m/s

)

H = 3.5 mH = 1.5 m

Transverse velocity profile (bank 5)

0.00.51.01.52.02.53.03.54.04.55.05.5

0 2 4 6 8 10 12 14 16 18

Distance from the side wall (m)

Vel

ocity

(m/s

)

H = 3.5 mH = 1.5 m


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Conclusion

Installation of side wall jet fans in a wide cut and cover tunnel has to be considered carefully Efficiency of the fans can be reduced significantly

Low velocity and even backflow may occur in the middle of the cross-section (Risk of significant backlayering in case of fire)

A specific 3 steps design methodology was developed to find and assess the most appropriate ventilation strategy

A solution based on the inactivation of the nearest bank of fans upstream the fire can be applied Design and on site tests have provided successful results for the Hatfield tunnel

Additional needs Fire detection is required to determine accurately the location of the fire

Additional or more powerful jet fans are required to compensate the thrust losses of the inactivated fans and achieve the critical velocity

More sophisticated tunnel ventilation SCADA software


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THANK YOU FOR YOUR ATTENTION

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