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
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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
6th International Conference ‘Tunnel Safety and Ventilation’ 2012, Graz
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INTRODUCTION
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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
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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
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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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Case of Hatfield tunnel
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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Case of Hatfield tunnel
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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