1. Researchers at Ghent University are studying smoke movement and backlayering in car parks using computational fluid dynamics (CFD) simulations. 2. The simulations vary key parameters like car park height, width, fire source area, and heat release rate to determine the required inlet velocity for a given backlayering distance. 3. Preliminary results from the CFD simulations will help establish a formula to define the critical inlet velocity in car parks based on the allowed backlayering distance.

1. FACULTY OF ENGINEERING
ONGOING RESEARCH and PRELIMINARY CFD‐
CALCULATIONS ON CAR PARK
Nele Tilley and Bart Merci
Department of Flow, Heat and Combustion Mechanics
Ghent University – UGent
SBO‐gebruikersgroep bijeenkomst 06/02/2009 – WFR Gent
pag. 1

2. ongoing research SBO car park
outline
1. Relation between horizontal ventilation velocity and
backlayering distance in large closed car parks
Research 2008
Development of formula for velocity in car park corresponding to certain
backlayering distance
Presented at IAFSS – september 2008 – Karlsruhe
2. Application of developed formula to SBO car park
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3. ongoing research SBO car park
Relation between horizontal ventilation velocity and backlayering
distance in large closed car parks
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4. introduction numerical setup results conclusion
fire in a car park
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5. introduction numerical setup results conclusion
fire in a car park
smoke movement
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6. introduction numerical setup results conclusion
fire in a car park
smoke movement
• vertical rise of smoke plume
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7. introduction numerical setup results conclusion
fire in a car park
smoke movement
• vertical rise of smoke plume
• horizontal movement beneath ceiling
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8. introduction numerical setup results conclusion
fire in a car park
smoke movement
• vertical rise of smoke plume
• horizontal movement beneath ceiling
most car parks: smoke control smoke extraction at back end of car park
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9. introduction numerical setup results conclusion
fire in a car park
vin
smoke movement
• vertical rise of smoke plume
• horizontal movement beneath ceiling
most car parks: smoke control smoke extraction at back end of car park
no backlayering allowed
critical ventilation velocity = smallest
inlet velocity (vin) without backlayering
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10. introduction numerical setup results conclusion
fire in a car park
d vin
smoke movement
• vertical rise of smoke plume
• horizontal movement beneath ceiling
most car parks: smoke control smoke extraction at back end of car park
no backlayering allowed backlayering allowed
critical ventilation velocity = smallest • backlayering distance (d) of 10–15 m
inlet velocity (vin) without backlayering • firemen able to extinguish fire
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11. introduction numerical setup results conclusion
critical ventilation velocity defined for fire in tunnels
1st question
Is the formula for vcr in tunnels also valid in car parks?
• tunnel: w ≈ h < l
• car park: h < w ≈ l
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12. introduction numerical setup results conclusion
critical ventilation velocity defined for fire in tunnels
1st question
Is the formula for vcr in tunnels also valid in car parks?
• tunnel: w ≈ h < l
• car park: h < w ≈ l
l
h
w
l
w
h
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13. introduction numerical setup results conclusion
critical ventilation velocity defined for fire in tunnels
1st question
Is the formula for vcr in tunnels also valid in car parks?
• tunnel: w ≈ h < l
• car park: h < w ≈ l
most regulations in car parks: backlayering is allowed
2nd question
What is the required inlet velocity, when a certain backlayering
distance is allowed?
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14. introduction numerical setup results conclusion
critical ventilation velocity defined for fire in tunnels
1st question
Is the formula for vcr in tunnels also valid in car parks?
• tunnel: w ≈ h < l
• car park: h < w ≈ l
most regulations in car parks: backlayering is allowed
2nd question
What is the required inlet velocity, when a certain backlayering
distance is allowed?
critical inlet velocity car parks: smoke control based on smoke extraction
3rd question
How to account for the difference between inlet and outlet velocity?
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15. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
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16. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
variation of four key parameters
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17. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
variation of four key parameters
• car park height h
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18. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
variation of four key parameters w
• car park height h
• car park width
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19. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
AF
variation of four key parameters w
• car park height h
• car park width
• fire source area
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20. introduction numerical setup results conclusion
use CFD‐simulations as numerical experiments
• advantage: relatively easy to vary parameters
• simulations carried out in FDS (version 4.0.7)
′′
qc
AF
variation of four key parameters w
• car park height h
• car park width
• fire source area
• convective heat release rate per unit area
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21. introduction numerical setup results conclusion
outline
• introduction
• numerical setup
• results
• conclusion
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22. introduction numerical setup results conclusion
configuration
basic configuration is kept constant – variation of one parameter at a time
• AF between 1 m2 and 26 m2 basic: 26 m2
′′
• qc between 50 kW/m2 and 1500 kW/m2 basic: 192 kW/m2
• h between 1.8 m and 3 m basic: 2.4 m
• w between 12 m and 32 m basic: 16 m
• # cells between 115200 and 307200 basic: 153600
cell size: 20cm x 20cm x 20cm
′′
qc
AF
w
h
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23. introduction numerical setup results conclusion
configuration
basic configuration is kept constant – variation of one parameter at a time
• AF between 1 m2 and 26 m2 basic: 26 m2 2 cars
′′
• qc between 50 kW/m2 and 1500 kW/m2 basic: 192 kW/m2 on fire
• h between 1.8 m and 3 m basic: 2.4 m
• w between 12 m and 32 m basic: 16 m
• # cells between 115200 and 307200 basic: 153600
cell size: 20cm x 20cm x 20cm
′′
qc
AF
w
h
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24. introduction numerical setup results conclusion
configuration
basic configuration is kept constant – variation of one parameter at a time
• AF between 1 m2 and 26 m2 basic: 26 m2 2 cars
′′
• qc between 50 kW/m2 and 1500 kW/m2 basic: 192 kW/m2 on fire
• h between 1.8 m and 3 m basic: 2.4 m
• w between 12 m and 32 m basic: 16 m
• # cells between 115200 and 307200 basic: 153600
cell size: 20cm x 20cm x 20cm
′′
qc
modeling AF
w
outlet: constant extraction velocity over entire surface
h
length is 32m ‐ view as significant part of larger car park
‐ no local effects of outlet fans on flow field
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25. introduction numerical setup results conclusion
modeling (ctd.)
• source: convective heat release rate
• radiation off exclude insecurity of
• walls: adiabatic radiation modeling
• turbulence model: standard Smagorinsky LES (Cs = 0.2)
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26. introduction numerical setup results conclusion
modeling (ctd.)
• source: convective heat release rate
• radiation off exclude insecurity of
• walls: adiabatic radiation modeling
• turbulence model: standard Smagorinsky LES (Cs = 0.2)
measure in FDS
in each simulation:
• backlayering distance temperature profile underneath ceiling
• inlet velocity corresponding to imposed outlet velocity
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27. introduction numerical setup results conclusion
outline
• introduction
• numerical setup
• results
qc′′
• variation of four parameters: AF, , h, w
• difference between inlet and outlet velocity
• conclusion
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28. introduction numerical setup results conclusion
variation of one parameter at a time
qc′′
1) find a relation vcr = f(AF, , h, w)
and compare to the formula of Wu and Bakar1 for fire in tunnels
′′ ′′
qc ⋅ AF
vcr ∝ qc 1/3 AF1/3 (1/h + 1/w)1/3 Q* < 0.2 Q* =
2 ⋅w ⋅ h ⎞
5
vcr ∝ (1/h + 1/w)‐1/2 Q* > 0.2 ρ0cpT0 g ⎛
⎜ ⎟
⎝ w+h ⎠
1 Wu, Y., Bakar, M.Z.A., (2000) Fire Safety Journal 35, 363‐390
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29. introduction numerical setup results conclusion
variation of one parameter at a time
qc′′
1) find a relation vcr = f(AF, , h, w)
and compare to the formula of Wu and Bakar1 for fire in tunnels
′′ ′′
qc ⋅ AF
vcr ∝ qc 1/3 AF1/3 (1/h + 1/w)1/3 Q* < 0.2 Q* =
2 ⋅w ⋅ h ⎞
5
vcr ∝ (1/h + 1/w)‐1/2 Q* > 0.2 ρ0cpT0 g ⎛
⎜ ⎟
⎝ w+h ⎠
qc′′
basic configuration < 241 kW/m2 AF < 33 m2
h > 2.2 m w > 9.2 m
1 Wu, Y., Bakar, M.Z.A., (2000) Fire Safety Journal 35, 363‐390
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30. introduction numerical setup results conclusion
variation of one parameter at a time
qc′′
1) find a relation vcr = f(AF, , h, w)
and compare to the formula of Wu and Bakar1 for fire in tunnels
′′ ′′
qc ⋅ AF
vcr ∝ qc 1/3 AF1/3 (1/h + 1/w)1/3 Q* < 0.2 Q* =
2 ⋅w ⋅ h ⎞
5
vcr ∝ (1/h + 1/w)‐1/2 Q* > 0.2 ρ0cpT0 g ⎛
⎜ ⎟
⎝ w+h ⎠
qc′′
basic configuration < 241 kW/m2 AF < 33 m2
h > 2.2 m w > 9.2 m
a linear relation is found between the difference of critical
and inlet velocity and the backlayering distance
d = a (vcr – vin)
qc ′′
2) find a relation a = f(AF, , h, w)
1 Wu, Y., Bakar, M.Z.A., (2000) Fire Safety Journal 35, 363‐390
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31. introduction numerical setup results conclusion
AF = 26 m2 h = 2.4 m w = 16 m
variation of convective heat release rate per unit area
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
0 400 800 1200 1600
′′
qc (kW/m2)
′′
vcr ∝ qc 0.28
′′
Wu and Bakar: vcr ∝ qc 1/3 ′′
qc < 241 kW/m2
′′
vcr = c > 241 kW/m2
qc
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32. introduction numerical setup results conclusion
AF = 26 m2 h = 2.4 m w = 16 m
variation of convective heat release rate per unit area
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
0 400 800 1200 1600
′′
qc (kW/m2)
′′
vcr ∝ qc 0.28
′′
Wu and Bakar: vcr ∝ qc 1/3 ′′
qc < 241 kW/m2
′′
vcr = c > 241 kW/m2
qc
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33. introduction numerical setup results conclusion
AF = 26 m2 h = 2.4 m w = 16 m
variation of convective heat release rate per unit area
critical ventilation velocity backlayering distance
20
2.6
vcr (m/s) d (m)
2.2 15
1.8
10 ′′
qc (kW/m2)
1500
1.4 1000
500
5 320
1 192
100
50
0.6 0
0 400 800 1200 1600 0 0.2 0.4 0.6
′′
qc (kW/m2)
vcr – vin (m/s)
′′
vcr ∝ qc 0.28 d = a (vcr – vin)
′′
a ∝ qc ‐0.2
′′
Wu and Bakar: vcr ∝ qc 1/3 ′′
qc < 241 kW/m2
′′
vcr = c > 241 kW/m2
qc
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34. introduction numerical setup results conclusion
intermediate results
based on the variation of parameters in FDS
result n°1
′′
vcr ∝ AF0.2 qc 0.28 h0.27 w‐0.1
result n°2
d = a (vcr – vin)
′′
a ∝ qc ‐0.2
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35. introduction numerical setup results conclusion
difference between inlet and outlet velocity
v (m/s)
inlet velocity
2 outlet velocity
basic configuration outlet velocity theory
• outlet velocity imposed
1.6
• inlet velocity measured
in FDS
1.2
0.8
0 5 10 d (m) 15
simplified theory:
• conservation of mass vin ρin = vout ρout
• ideal gas law Tin ρin = Tout ρout
• energy balance Qc = ΔT ρin vin w h cp,in ′′
qc ⋅ AF
vout = vin +
• assumption of homogeneous m ρin ⋅ Tin ⋅ w ⋅ h
temperature in outlet plane
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36. introduction numerical setup results conclusion
difference between inlet and outlet velocity
v (m/s)
inlet velocity
2 outlet velocity
basic configuration outlet velocity theory
• outlet velocity imposed
1.6
• inlet velocity measured
in FDS
1.2
0.8
0 5 10 d (m) 15
simplified theory:
• conservation of mass vin ρin = vout ρout
• ideal gas law Tin ρin = Tout ρout
• energy balance Qc = ΔT ρin vin w h cp,in ′′
qc ⋅ AF
vout = vin +
• assumption of homogeneous m ρin ⋅ Tin ⋅ w ⋅ h
temperature in outlet plane
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37. introduction numerical setup results conclusion
difference between inlet and outlet velocity
v (m/s)
inlet velocity
2 outlet velocity
basic configuration outlet velocity theory
• outlet velocity imposed
1.6
• inlet velocity measured
in FDS
1.2
0.8
0 5 10 d (m) 15
result n°3
• calculate vin with formulae
• apply simplified theory to obtain vout ′′
qc ⋅ AF
vout = vin +
conservative design for smoke extraction ρin ⋅ Tin ⋅ w ⋅ h
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38. introduction numerical setup results conclusion
outline
• introduction
• numerical setup
• results
• conclusion
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39. introduction numerical setup results conclusion
1st question
Is the formula for vcr in tunnels also valid in car parks?
l
result n°1
′′
vcr ∝ AF0.2 qc 0.28 h0.27 w‐0.1 h
w
Wu and Bakar for fire in tunnels
′′
vcr ∝ qc 1/3 AF1/3 (1/h + 1/w)1/3 Q* < 0.2 l
w
vcr ∝ (1/h + 1/w)‐1/2 Q* > 0.2
h
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40. introduction numerical setup results conclusion
1st question
Is the formula for vcr in tunnels also valid in car parks?
l
result n°1
′′
vcr ∝ AF0.2 qc 0.28 h0.27 w‐0.1 h
w
Wu and Bakar for fire in tunnels
′′
vcr ∝ qc 1/3 AF1/3 (1/h + 1/w)1/3 Q* < 0.2 l
w
vcr ∝ (1/h + 1/w)‐1/2 Q* > 0.2
h
answer Formula for tunnels is not valid in car parks!
′′
new formula: vcr = 0.2 AF0.2 qc 0.28 h0.27 w‐0.1
within the range of configurations studied
Main differences: ‐ no threshold value observed
‐ inverse influence of height
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41. introduction numerical setup results conclusion
2nd question
What is the required inlet velocity, when a certain backlayering
distance is allowed?
result n°2
d = a (vcr – vin)
′′
a ∝ qc ‐0.2
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42. introduction numerical setup results conclusion
2nd question
What is the required inlet velocity, when a certain backlayering
distance is allowed?
result n°2
d = a (vcr – vin)
′′
a ∝ qc ‐0.2
answer A linear relation was found between the difference of critical
and inlet velocity, and the backlayering distance. Of the four studied
parameters, the coefficient in this linear relation only depends on the
convective heat release rate per unit area.
′′
The relation is: d = 111 ‐0.2 (vcr – vin)
qc
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43. introduction numerical setup results conclusion
3rd question
How to account for the difference between inlet and outlet velocity?
result n°3
• calculate vin with formulae
• apply simplified theory to obtain vout
conservative design for smoke extraction
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44. introduction numerical setup results conclusion
3rd question
How to account for the difference between inlet and outlet velocity?
result n°3
• calculate vin with formulae
• apply simplified theory to obtain vout
conservative design for smoke extraction
answer The difference between inlet and outlet velocity can be substantial.
It is therefore extremely important to recall that a smoke extraction system
design in car parks is based on outlet velocity!
Calculating the inlet velocity with the suggested formulae, and applying the
simplified theoretical relation to obtain the outlet velocity is a conservative
way of designing the smoke extraction system.
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45. ongoing research SBO car park
Application of developed formula to SBO car park
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46. ongoing research SBO car park
SBO car park
• 28 m x 28 m x 2.4 m
• maximum convective HRR: 4 MW – 3 m x 3 m
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47. ongoing research SBO car park
SBO car park
• 28 m x 28 m x 2.4 m
• maximum convective HRR: 4 MW – 3 m x 3 m
h = 2.4 m w = 28 m ′′
qc = 444.44 kW/m2 AF = 9 m2
′′
vcr = 0.2 AF0.2 qc 0.28 h0.27 w‐0.1 = 1.55 m/s
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48. ongoing research SBO car park
SBO car park
• 28 m x 28 m x 2.4 m
• maximum convective HRR: 4 MW – 3 m x 3 m
h = 2.4 m w = 28 m ′′
qc = 444.44 kW/m2 AF = 9 m2
′′
vcr = 0.2 AF0.2 qc 0.28 h0.27 w‐0.1 = 1.55 m/s
′′
qc ⋅ AF
vout ≈ vin + 0.6 = 1.65 m/s
ρin ⋅ Tin ⋅ w ⋅ h
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49. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
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50. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
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51. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
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52. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
closed walls and ceiling
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53. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
closed walls and ceiling
open wall (air inlet)
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54. ongoing research SBO car park
FDS‐simulations of car park with vout = 1.65 m/s
V = 399168 m3/h 2 extraction ventilators, each ± 200000 m3/h
closed walls and ceiling
open wall (air inlet)
car fire: 4 MW conv. – 3 m x 3 m
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55. ongoing research SBO car park
some simulation pictures ...
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56. ongoing research SBO car park
some simulation pictures ...
smoke layer
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57. ongoing research SBO car park
some simulation pictures ...
temperature slice
at height 2 m
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58. ongoing research SBO car park
from similar temperature slices
• derive maximum temperatures at ceiling
• find out the position of these temperatures
• decide where to put thermocouples
• ...
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59. ongoing research SBO car park
from similar temperature slices
• derive maximum temperatures at ceiling
• find out the position of these temperatures
• decide where to put thermocouples
• ...
Useful to know where to look and what to look for, in the experiments!
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60. ongoing research SBO car park
Thank you for your attention!
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61. introduction numerical setup results conclusion
′′
qc = 192 kW/m2 h = 2.4 m w = 16 m
variation of fire source area
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
0 5 10 15 20 25 30
AF (m2)
vcr ∝ AF0.2
Wu and Bakar: vcr ∝ AF1/3
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 9
Ghent University – UGent

62. introduction numerical setup results conclusion
′′
qc = 192 kW/m2 h = 2.4 m w = 16 m
variation of fire source area
critical ventilation velocity backlayering distance
2.6 20
vcr (m/s)
d (m)
2.2
15
1.8
10
AF (m2)
1.4
26
15.2
5
1 10.2
4.8
1
0.6 0
0 5 10 15 20 25 30 0 0.2 0.4 0.6
AF (m2)
vcr – vin (m/s)
vcr ∝ AF0.2 d = a (vcr – vin)
a independent of AF
Wu and Bakar: vcr ∝ AF 1/3
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 9
Ghent University – UGent

63. introduction numerical setup results conclusion
AF = 26 m2 ′′
qc = 192 kW/m2 w = 16 m
variation of car park height
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
1.6 2 2.4 2.8 3.2
h (m)
vcr ∝ h0.27
Wu and Bakar: vcr ∝ (1+ w/h)‐1/2 h < 2.2 m
vcr ∝ (1+ w/h) 1/3 h > 2.2 m
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 11
Ghent University – UGent

64. introduction numerical setup results conclusion
AF = 26 m2 ′′
qc = 192 kW/m2 w = 16 m
variation of car park height
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
1.6 2 2.4 2.8 3.2
h (m)
vcr ∝ h0.27
Wu and Bakar: vcr ∝ (1+ w/h)‐1/2 h < 2.2 m
vcr ∝ (1+ w/h) 1/3 h > 2.2 m
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 11
Ghent University – UGent

65. introduction numerical setup results conclusion
AF = 26 m2 ′′
qc = 192 kW/m2 w = 16 m
variation of car park height
critical ventilation velocity backlayering distance
2.6 20
vcr (m/s)
d (m)
2.2
15
1.8
10 h (m)
1.4 3.0
2.6
5 2.4
1 2.2
2.0
1.8
0.6 0
1.6 2 2.4 2.8 3.2 0 0.2 0.4 0.6
h (m)
vcr – vin (m/s)
vcr ∝ h0.27 d = a (vcr – vin)
a independent of h
Wu and Bakar: vcr ∝ (1+ w/h)‐1/2 h < 2.2 m
vcr ∝ (1+ w/h) 1/3 h > 2.2 m
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 11
Ghent University – UGent

66. introduction numerical setup results conclusion
AF = 26 m2 ′′
qc = 192 kW/m2 h = 2.4 m
variation of car park width
critical ventilation velocity
2.6
vcr (m/s)
2.2
1.8
1.4
1
0.6
10 14 18 22 26 30 34
w (m)
vcr ∝ w‐0.1
Wu and Bakar: vcr ∝ (1+ h/w)1/3
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 12
Ghent University – UGent

67. introduction numerical setup results conclusion
AF = 26 m2 ′′
qc = 192 kW/m2 h = 2.4 m
variation of car park width
critical ventilation velocity backlayering distance
2.6 20
vcr (m/s)
d (m)
2.2
15
1.8
w (m)
10
32
1.4 28
24
5 20
1
16
12
0.6 0
10 14 18 22 26 30 34 0 0.2 0.4 0.6
w (m)
vcr – vin (m/s)
vcr ∝ w‐0.1 d = a (vcr – vin)
a independent of w
Wu and Bakar: vcr ∝ (1+ h/w)1/3
Department of Flow, Heat and Combustion Mechanics – www.FloHeaCom.UGent.be 12
Ghent University – UGent