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1 Research Findings on Ventilation in Tunnels, Car Parks and Atria By Ghent University – UGent, Dept. Flow, Heat and Combustion Mechanics, Belgium Dr Tarek Beji
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Research Findings on Ventilation in Tunnels, Car Parks and Atria
By
Ghent University – UGent, Dept. Flow, Heat and Combustion Mechanics, Belgium
Dr Tarek Beji
2
Outline
1. Ventilation in tunnels
2. Ventilation in car parks
3. Ventilation in atria
Based on work conducted at the Institute of Mechanics of Marseille with
Dr Olivier Vauquelin
Based on: (i) Results presented in an International Workshop on “Fire and
Explosion Safety in Large Car Parks” Organized by Pr. Bart Merci (Ghent
University) + (ii) work conducted by Dr Nele Tilley (Ghent Uni.)
Based on work conducted by Dr Nele Tilley (Ghent Uni.)
Introduction
Conclusion
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Introduction Importance of ventilation:
•Provide tenable conditions for: (i) evacuation of people and (ii) intervention of fire fighters
•Major cause of death in fires: Smoke inhalation . CO “Chief killer”
The design of a ventilation system depends on:
• Type of building (tunnels, car parks, atria…etc),
• Occupancy and use of the building…etc
Example:
2-directional long tunnels + congested one-directional tunnels smoke transverse control.
1-directional low traffic tunnel smoke longitudinal control
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1. Ventilation in Tunnels
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There are several ventilation strategies:
1. Longitudinal ventilation: Smoke is pushed by jet fans in one
direction. The opposite direction remains smoke free.
2. Transverse ventilation: Smoke is removed by extraction ducts
1.1 Ventilation strategies
3. Other options (e.g. semi-transverse ventilation system: confine and
then extract the fire induced smoke)
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1.2 Design calculations
Fire characteristics:
1. Heat Release Rate (e.g. Van-Bus => 15 MW, Tank => 100 MW)
2. Smoke flow rate (e.g. Van-Bus => 50 m3/s , Tank=> 200-300 m3/s)
Mechanical ventilation system:
1. Ventilation System Efficiency,
2. Extraction capabilities,
3. Vent shape and locations…etc.
Ventilation system efficiency:
e.g. preventing backlayering
Flow patterns of smoke longitudinal control [1]
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1.3 Research Tools
1. Experimental testing
2. Computational Fluid Dynamics (CFD)
1.1. Large-scale testing (expensive)
1.2. Reduced-scale testing (scaling laws)
-Visualization of temperature iso-contours in a tunnel [2]-
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1.4 Reduced-Scale Helium Experiments (principle: 1/2)
Principle of two-step splitting [3]
From a full-scale fire plume to a small-scale Helium plume
1. Fire source – non-thermal buoyant release: fire HRR and smoke
density and flow rate used as release conditions for a buoyant fluid.
2. Non-thermal buoyant release – Scale reduction: conservation of
reduced density difference and Richardson number
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1.4 Reduced-Scale Helium Experiments (principle: 2/2)
)( airsspsc TTqcQ −= ρ• Convective HRR of a fire source
• Buoyant flux of a buoyant jetair
sairsqgB
ρρρ −
=
helhelairairs ρχρχρ +=• Density of air-Helium mixture
air
helairhelqgB
ρρρ −
=(2) + (3)
(1)
(2)
(3)
• The buoyant flux depends only on the Helium volume flow rate [1].
• Air has to be added to increase the mixing flow rate, decrease the density deficit and reach coherent fire plume characteristics [1].
(4)
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1.4 Reduced-Scale Helium Experiments [1, 4]) (set-up)• Rectangular plexiglass channel (10m × 0.5m × 0.25m)
• Circular opening located at the floor level (at the centre)
• Injection of a mixture air-Helium
• Mixture seeded by ammonium salt particles
• Visualization of smoke flow patterns: vertical laser sheet in the medium plan
- Picture of the set-up- - Sketch of the set-up- - LASER set-up -
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“Aeraulic transparencies”
1.4 Reduced-Scale Helium Experiments: Natural Ventilation
Smoke is extracted through openings placed at the ceiling
- Smoke extraction through openings- - Set-up -
Objective:
Critical opening width to have full extraction of smoke ?
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Total extraction
Partial extraction
Total/Partial extraction
(Intermittent extraction)
Partial extraction
(low efficiency)
1.4 Reduced-Scale Helium Experiments: Natural Ventilation
+ He
+ Air
+ Air/He
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5
*
gH
qq m
m =Non-dimensional mixing flow rate
Critical opening width (minimum width for total extraction), Lcrit
- Correspondence between mixing relative density difference and
mixing flow rate at critical condition for several opening lengths -
- General correspondence between mixture density and flow rate for a given opening length at critical condition -
1.4 Reduced-Scale Helium Experiments: Natural Ventilation
2/1*
/Δ036.0
8.1 ⎟⎟⎠
⎞⎜⎜⎝
⎛ −≈
mm
mcrit
ρρq
HL
Correlation [4]
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The required opening length Lcrit
as a function of Qc [4]
1.4 Reduced-Scale Helium Experiments: Natural Ventilation
• Design experiments
• Derive results
• Apply results derived from reduced-scale experiments to real-scale fires
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1.5 Numerical simulations (1/3)
- Geometry and meshing in FDS for the simulation of the reduced-scale Helium model-
CFD code: The Fire Dynamics Simulator (FDS), designed specifically for
fire calculations
- Visualization of the flow at the opening -
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1.5 Numerical simulations (2/3)
Calculation of the extraction efficiency from CFD results
LW H
Slice plane downstream
• Simulated convective HRR (FDS input) mmpinletc ρqTcQ Δ0, =
• Non-extracted convective HRR
WdzzρzuTcQH
xpextractednonc ⎟⎟⎠
⎞⎜⎜⎝
⎛= ∫−
00, )(Δ)( Wz
uρuTcQ
K
k
kkkk
pextractednonc ⎟⎟⎠
⎞⎜⎜⎝
⎛ += ∑
−
=
++
− Δ2
Δ1
1
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0,
Δρ
• Extraction efficiencyinletc
extractednoncextraction Q
Qe
,
,1 −−=
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1.5 Numerical simulations (3/3)
Simulated convective HRR (MW)
0 2 4 6 8 10Ope
ning
leng
th (L
) in
the r
educ
ed-s
cale
mod
el (i
n cm
)
0
2
4
6
8
10
Exp. correlationFDS (full extraction)FDS (partial extraction)
Comparison: Experimental correlation Vs FDS results
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2. Car parks(References [5-8])
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• Set-up:
– Car park 30 m x 28.6 m x 2.7 m
– Inlet opening modular
– Pool fire: tray of 3 m x 1.5 m
– Fuel height: 45 cm above floor level
– 4 extraction fans (4 x 50000m3/h)
– 2 induction type jet fans (50 N each)
2.1 Full-Scale Experiments: Hexane Pool Fire
Conducted at Warrington Fire - GENT
Modular inlet opening
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2.1 Full-Scale Experiments: Hexane Pool Fire
- Picture of the car park - - Sketch of the exp. set-up -
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2.1 Full-Scale Experiments: Hexane Pool Fire
Results (1/3)
Different flow patterns depending on the inlet opening configuration
- Flow patterns near the ceiling -
Once smoke is trapped in a recirculation region, it is hard to get it out!
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Smoke backlayering distance determined from mean temperature
profiles along centerline under ceiling
2.1 Full-Scale Experiments: Hexane Pool Fire
Results (2/3)
Smoke direction in backlayering
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2.1 Full-Scale Experiments: Hexane Pool Fire
Results (3/3)
• Activation of jet fans for the case of full open inlet
(OOOO) hardly affects results on the centreline.
• For the case XXOXX the exact position of
extraction fans is not crucial.
•Transversal beam: blocking of smoke backlayering
(momentum is broken).
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2.2 Full-Scale Experiments: Real-car fires
Collaboration with Brandweer Gent
Research purposes:
• Effect of SHC system on fire spread to next car.
• Effect of fire service intervention on temperatures.
• Use of video data.
• Use of thermal camera.
Practical purpose:
Development of SOPs for Fire Service interventions in
case of car park fire.
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2.2 Full-Scale Experiments: Real-car fires
Experimental set-up
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2.2 Full-Scale Experiments: Real-car fires
Results (3 cars, configuration XXOX)
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Objective: design a system with a necessary extraction velocity
to guarantee smoke clearance in the car parks
Similarity with tunnels for the ‘critical velocity’ to keep all the smoke
in the downstream direction But Different ratio height/width
- Principle of critical horizontal velocity in a car park -
2.3 Numerical study
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‘Numerical experiments’ in order to depict relationships between:
1. Convective HRR per unit area,
2. Fire source area.
Domain of validation:
1. Large closed car parks,
2. Flat ceiling,
3. Uni-directional smoke and ventilation flow pattern
2.3 Numerical study
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Correlations between the critical velocity, the convective HRR per unit
area and the fuel surface
4/1''., convincr qv ∝
5/1, Fincr Av ∝Dependence on
5/14/1''., 26.0 Fconvincr Aqv =
Dependence on
General correlation
''.convq
FA
Required ventilation velocity when a backlayering distance d is allowed
)(40 vvd cr −=
2.3 Numerical study
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3. Atria(References [7-8])
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3.1Objective
Design a mechanical ventilation system which allows a given
smoke-free height in case of a fire
- Small-scale atrium configuration and spill
plume-
• M(z): mechanical mass flow rate (kg/s)
• Mb: emerging smoke flow rate
• Qconv.: convective HRR (kW)
• z: height above spill edge (m)
• z0: virtual origin (m)
• W: width of atrium (m)
• Db: smoke depth (m)
3/1.0 )()( convQzzCzM +=
3/203.0 WρCC m= 3/1
.0
conv
bb QC
MDz +=
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3.2 Performance of CFD
- Experimental and CFD results of smoke mass flow extraction rate as a function or rise height-
• Validation of CFD results for small-scale atria
• Can we perform “numerical experiments” to capture trends?
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3.3 Scaling issues
• Nele Tilley: “Scaling based on only the Froude number is allowed as long as all
configurations are sufficiently turbulent”.
• Olivier Vauquelin: “The most important aspect is to represent the duality between
inertial and buoyant forces. This imposes a strict conservation of the Froude and
Richardson numbers. The Reynolds number cannot be preserved, but it has to be high
enough in order to ensure turbulent flows”.
Froude number velocitynalgravitatiovelocityticcharcteris
hgu
Fr ==
energykineticenergypotential
uhg
Ri == 2Richardson number
Densimetric Froude numberρρρ
gg 21' −=
hg
uFr
'= ;
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3.4 Multi-dimensional smoke layer effect (1/2)
zs,min ≈ zs,ave
zs,ave
zs,min
Temp (°C)
20
50
Hs
skgm /37.0
1-D smoke layer Multi-D smoke layer
Reduced-scale calculations
= skgm /69.0=
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3.4 Multi-dimensional smoke layer effect (2/2)
When multi-dimensional smoke layer is present, a new slope is found
New expression
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Conclusion
• The design of efficient ventilation systems is challenging.
• Research tools: Experiments and CFD modelling
• Experiments must be carefully designed (e.g. scaling issues)
• CFD modelling could be used for “numerical experiments”
==> capture trends
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References
[7] N. Tilley, P. Rauwoens, B. Merci, Verification of the accuracy of CFD simulations in small-scale tunnel and atrium fire configurations, Fire Safety Journal 46 (2011) 186-193
[8] N. Tilley Numerical study on Fire Smoke Extraction in Large Complex Buildings. PhD Dissertation, Ghent University (2011).
[5] N. Tilley, X. Deckers, B. Merci, CFD study of relation between ventilation velocity and smoke backlayering distance in large closed car parks, Fire Safety Journal 48 (2012) 11-20
[6] X. Deckers, S. Haga, N. Tilley, B. Merci Smoke control in case of fire in a large car park: CFD simulations of full-scale configurations, Fire Safety Journal (2012)
[4] O. Vauquelin, , T. Beji, G. Michaux, Laboratory experiments on smoke natural extraction in case of tunnel fire. Thriteenth International Symposium on Aerodynamics and Ventilation of Vehicle Tunnel, BHRGroup, New Brunswick, NJ, USA, May 2009
[1] O. Vauquelin, Experimental simulations of fire-induced smoke control in tunnelsusing an ‘‘air–helium reduced scale model’’: Principle,limitations, results and future, Tunnel and Underground Space Technology 23 (2008) 171-178
[2] H.Y. Wang, Numerical and theoretical evaluations of the propagation of smoke and fire in a full-scale tunnel, Fire Safety Journal 49 (2012) 10-21
[3] O. Vauquelin, G. Michaux, C. Lucchesi, Scaling laws for a buoyant release used to simulate fire-induced smoke in laboratory experiments, Fire Safety Journal 44 (2009) 665-667
