I. Basics of Fire Science
A. Definition:- Fire: It is a rapid oxidation process accompanied by evolution of heat, light, flames and emission of sound.
B. Ignition:
- Pilot ignition: Fire ignited by an external heating source;
- Spontaneous ignition: Fire is ignited by itself under elevated temperature.
Materials
|
Pilot
Ignition Temperature (°C)
|
Spontaneous
Ignition Temperature (°C)
|
Cotton
|
230 –
266
|
254
|
Paper
|
230
|
230
|
White
pine
|
228 –
264
|
260
|
Polyethylene
|
341
|
349
|
PVC
|
391
|
454
|
Perspex
|
280 -
300
|
450 - 462
|
Polystryrene
foam
|
346
|
491
|
Polyurethane
|
310
|
416
|
C. Combustion:
- a series of very rapid chemical reactions. (i.e. fuel, heat, oxygen).
i. Smouldering Combustion - Burning process without flame due to limited supply of oxygen.
ii. Flaming Combustion - Visible manifestation of combustion between gaseous fuel and oxygen.
iii. Heat of Combustion (ΔHc):
- Heat of combustion is the energy released as heat when a material undergoes complete combustion with oxygen under standard conditions.
- Different fuels have different heat of combustion. Usually, fuels with carbon-rich molecules have higher heat of combustion but also require higher ignition energy.
iiii. Combustion Reaction:
- Propane (C3H8)
H. Hot and cold layers
- When the ceiling jet reaches the wall boundaries, it reflects back and accumulates at the upper part of the compartment.
- A thermal interface exists in the compartment to demarcate the upper hot gas layer and the lower air layer. The interface is quite stable.
- It is defined as the level which the largest change in temperature.
- When the thermal interface reaches the door soffit, the hot gas emerges out of the compartment.
I. Neutral Plane
- At the upper part of the door opening, the hot gas is emerging out of the compartment.
- At the lower part of the door opening, the ambient air entering into the compartment.
- There exist a level at the door opening in which the air velocity is zero. It is defined as Neutral Plane.
J. Modelling Hot Gas Temperature
ii. Foote et al.
ΔTg /T∞ = 0.63 [ Q/(mg cp T∞]0.72 [hk AT / (mg cp ) ]-0.36
iiii. Combustion Reaction:
- Propane (C3H8)
- C3H8
+ O2 ―> 3CO2
+ 4H2O
-The reaction produces 2044kJ/mole of C3H8 Or 46.45kJ/g of C3H8.
- The energy produced by the combustion reaction will be presented in form of light and heat. (e.g. a flame)
D. Flammability & Flame Structure
E. Flames
i. Premixed flame - Fuel gas and oxygen are mixed before combustion.
ii. Diffusion flame - Fuel gas and oxygen are separated before combustion (e.g. bunsen burner)
F. Buoyant plume
- The heat produced by the combustion reaction will heat up the surrounding air.
- When the air temperature increase the density is reduced.
- The density different between the hot gas and the surrounding ambient air increases.
- The buoyancy force of the hot air increases and pushes the hot air to higher level.
- The hot gases created from the fire forms a hot gas column extending to the ceiling of the compartment.
- The upward movement of the hot gas column induces the entrainment of the surrounding ambient air by turbulent mixing and molecular diffusion.
- Due to the air entrainment, the temperature and velocity of the plume decreases along the upward direction. Therefore, a plume shape can be approximated by an inverted cone.
G. Ceiling Jet
- When the hot gas reaches the ceiling, it can not penetrate through the slab. It spreads not penetrate through the slab. It spreads radially under the slab soffit.
-The air entrainment along the horizontal spread of the hot gas is not efficient.
- The speed of the ceiling jet is fast due to the thin layer of the hot smoke under the ceiling.
- The ceiling jet in contact with sprinkers and detectors.
Modelling of Ceiling Jet Temperature:
- Alpert's equation (for temperature)
Modelling of Ceiling Jet Temperature:
- Alpert's equation (for temperature)
H. Hot and cold layers
- When the ceiling jet reaches the wall boundaries, it reflects back and accumulates at the upper part of the compartment.
- A thermal interface exists in the compartment to demarcate the upper hot gas layer and the lower air layer. The interface is quite stable.
- It is defined as the level which the largest change in temperature.
- When the thermal interface reaches the door soffit, the hot gas emerges out of the compartment.
I. Neutral Plane
- At the upper part of the door opening, the hot gas is emerging out of the compartment.
- At the lower part of the door opening, the ambient air entering into the compartment.
- There exist a level at the door opening in which the air velocity is zero. It is defined as Neutral Plane.
J. Modelling Hot Gas Temperature
i. McCaffrey
et al. equation
ΔTg = 480 [Q / (g1/2 cpρ∞ T∞ Ao Ho
1/2)] 2/3 [ hk AT / (g1/2
cpρ∞ Ao Ho 1/2)] -1/3
- hk is the heat transfer coefficient which is time dependent
- If t ≦ tp,
hk = ( kρc /t)1/2,
otherwise hk = k /δ
- tp = (ρc /k) (δ/2) 2 is the penetration time
ΔTg /T∞ = 0.63 [ Q/(mg cp T∞]0.72 [hk AT / (mg cp ) ]-0.36
- mg is the mass ventilation rate in (kg/s)
K. Flashover
- The fire plume and the hot gas layer emit radiation to all unburnt combustible materials inside the compartment
- When the radiation is sufficiently high, it will ignite all combustible materials
- All fuels inside the compartment are involved in the fire
- The heat release rate / temperature are rapidly increased
L. Determination of Flashover
- Hot gas temperature at 10mm below ceiling soffit ≧ 600°C
- Radiation at the floor of the compartment ≧ 20kW/m²
- Minimum heat release rate can be estimated by use of the McCaffrey equation by setting the
ΔTg = 600- ambient temperature in °C.
- Method of Babrauskas
Q = 750 Ao Ho1/2
- Method of McCaffrey et al.
Q = 610( hk ATAo Ho1/2 )1/2
- Method of Thomas
Q = 7.8 AT + 378 Ao Ho1/2
M. Growth stages of enclosure fire
- Ignition
-Grown stage (pre-flashover stage)
- Flashover
- Fully developement stage (post-flashover stage)
-Decay stage
N. Typical Fire Growth Curve
O. Design Fire Growth Rate
- t² fire is commonly adopted in fire engineering.
- It owns a parabolic increasing profile
* Extra fast growth (75s to reach 1MW)
* Fast growth (150s to 1MW)
* Medium growth (300s to 1MW)
* Slow growth (600s to 1MW)
- The fire size increases parabolically until it reaches the maximum heat rekease rate.
P. Smoke / Hot Gas Production Rate
Smoke Production Rate is approximated by the air entrainment rate
m smoke = m fuel + m air
m fuel << m air => m smoke ≒ m air
R. Smoke Production Rate
Reference:
1. NIST Engineering Laboratory
2. Fire standards and flammability standards developed by ASTM
3. A New Curve for Temperature - Time Relationship Compartment Fire, Thermal Science 2011.
4. International Journal on Engineering Performance-Based Fire Codes - PolyU
5. International Journal on Architectural Science - PolyU
B. Combustion Theory
- Heat: conduction, convection & radiation
- Oxygen: does not support combustion at >15%
- Fuel: Liquid, gas & solid (Pyrolysis); Only 25% of gaseous fuel will be burnt and the rest will be accumulated in the atmosphere
a. Pyrolysis
- Decomposition of a substance by heat
- Does not involve catalyst and oxygen
- Pyrolysis can start to be product at about 80°C
- At 150°C - 200°C pyrolysis will occur in wood
b. Fire Gases (Smoke)
- Non flammable gases - mainly CO2 and water vapour
- Flammable gases - due to pyrolysis and incomplete combustion, includes Carbon Monoxide
- Air - entrained in by rising temperature
- Soot - small solid particles of carbon
Reference:
1. Resources CFB-US
3. Publication and Code of Practices- HKFSD
2. Fire safety engineering of structures (Blog)
K. Flashover
- The fire plume and the hot gas layer emit radiation to all unburnt combustible materials inside the compartment
- When the radiation is sufficiently high, it will ignite all combustible materials
- All fuels inside the compartment are involved in the fire
- The heat release rate / temperature are rapidly increased
L. Determination of Flashover
- Hot gas temperature at 10mm below ceiling soffit ≧ 600°C
- Radiation at the floor of the compartment ≧ 20kW/m²
- Minimum heat release rate can be estimated by use of the McCaffrey equation by setting the
ΔTg = 600- ambient temperature in °C.
- Method of Babrauskas
Q = 750 Ao Ho1/2
- Method of McCaffrey et al.
Q = 610( hk ATAo Ho1/2 )1/2
- Method of Thomas
Q = 7.8 AT + 378 Ao Ho1/2
M. Growth stages of enclosure fire
- Ignition
-Grown stage (pre-flashover stage)
- Flashover
- Fully developement stage (post-flashover stage)
-Decay stage
N. Typical Fire Growth Curve
O. Design Fire Growth Rate
- t² fire is commonly adopted in fire engineering.
- It owns a parabolic increasing profile
* Extra fast growth (75s to reach 1MW)
* Fast growth (150s to 1MW)
* Medium growth (300s to 1MW)
* Slow growth (600s to 1MW)
- The fire size increases parabolically until it reaches the maximum heat rekease rate.
P. Smoke / Hot Gas Production Rate
Smoke Production Rate is approximated by the air entrainment rate
m smoke = m fuel + m air
m fuel << m air => m smoke ≒ m air
R. Smoke Production Rate
If (z –z
fire) / L flame ≧ 1 (smoke height > flame length
mg = 0.071 Qc 1/3 (z – z fire – L fame + 0.166Qc 2/5)
5/3 x [1 +0.026 Qc2/3 (z – z fire – L flame + 0.166 Qc 2/5)-5/3]
If 0
< (z –z fire) / L flame < 1
Mg =
0.0054Qc (z – z fire)
Where
Qc ≈
0.65 Qtotal
L flame
= -1.02 D + 0.235Qc 2/5
D id the
diameter of fire bed
S. Mechanical Smoke Extraction
- Smoke is removed by extraction fan
- The mass extraction rate should be higher than the mass of smoke generation rate
- Volumetric flow rate = mass flow rate / density
- By ideal gas law with constant pressure, we have ρT =
constant.
ρT =ρ273T273
= 1.2922 x 273 =352.77
T.
Static Smoke Vent
- Smoke
is drive by its buoyancy to the atmosphere naturally (natural vent)
Mg = Cd,v
Av ρo [2 g zlay (T – To)]1/2 / [ T/To + (Cd,v
Av / Cd,i Ai)² (To / T] 1/2
where Cd,v
and Cd,I are the discharge coefficients of vent outlet and inlet respectively.
U. Zone Modelling
- Zone model approximate the smoke layer can be defined as one zone; the clear air underneath is another
- Heat and mass are transferred from lower zone to upper zone is treated as the third zone
- Heat release rate should be specified as a function of time (e.g. t² fire)
- Generally responds quick and useful in sample geometry
V. Field models
- Computational fluid dynamics (CFD) techniques
- Computational domain is divided into many small volumes
- A large set of partial differential equations to describe the chemistry of combustion, heat soot production of one volume and its neighboring volumes
- The problem is solved iteratively by numerical approach until the solution converge
W. Meshing
(finite element approach)
- Some
CFD models provide equation to determine the grid size. Others may require grid
sensitivity study.
X.
Fire Size and Growth
- Some
of the CFD model can solve the fire chemistry and determine the fire size by
its own
- However,
pre-determining a maximum fire size can cope with the worst scenario but it
should be determined reasonably with supports from literature
- T-square
fire is usually adopted
Y.
Smoke Generation
- With
only heat release rate specified, the fire source is only a heating element.
The soot yield rate should also be pre-determined since it will affect the
visibility
Z.
Boundary Condition Setting
-
Boundary materials
-
Extended regions should be provided at openings
-
Patching of fire location
- The
fire bed area
e.g.
When door is opening, the pressure will reduce. Fire/smoke will be extract suddenly.
AA. Convergence
- Some
of the CFD packages can terminate the simulation of the preset convergence
criterion
- Some
of the CFD packages (e.g. LES model) require user to determine the simulation
time. To confirm the convergence, the time averages of two consecutive time
frames should be compared
Reference:
1. NIST Engineering Laboratory
2. Fire standards and flammability standards developed by ASTM
3. A New Curve for Temperature - Time Relationship Compartment Fire, Thermal Science 2011.
4. International Journal on Engineering Performance-Based Fire Codes - PolyU
5. International Journal on Architectural Science - PolyU
II. Compartment Fire Behaviour & Fire Fighting
A. Compartment Fire Behavior TrainingB. Combustion Theory
- Heat: conduction, convection & radiation
- Oxygen: does not support combustion at >15%
- Fuel: Liquid, gas & solid (Pyrolysis); Only 25% of gaseous fuel will be burnt and the rest will be accumulated in the atmosphere
a. Pyrolysis
- Decomposition of a substance by heat
- Does not involve catalyst and oxygen
- Pyrolysis can start to be product at about 80°C
- At 150°C - 200°C pyrolysis will occur in wood
b. Fire Gases (Smoke)
- Non flammable gases - mainly CO2 and water vapour
- Flammable gases - due to pyrolysis and incomplete combustion, includes Carbon Monoxide
- Air - entrained in by rising temperature
- Soot - small solid particles of carbon
c. Limits of Flammibility
|
Gas
|
Limits
of Flammability (%)
|
Auto
Ignition Temperature (°C)
|
|
Acrolein
|
3 – 31
|
278
|
|
Ammonia
|
16 -
25
|
651
|
|
1,3
Butadiene
|
2 –
11.5
|
429
|
|
Carbon
Monoxide
|
12.5 –
74
|
609
|
|
Formaldehyde
|
7 - 73
|
430
|
|
Hydrogen
cyanide
|
6 – 41
|
538
|
|
Hydrogen
sulphide
|
4.3 -
46
|
260
|
Reference:
1. Resources CFB-US
3. Publication and Code of Practices- HKFSD
