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Annual Fire Safety Engineering Conference 2013
Björn Karlsson
Arnheim, 12-13 November 2013
1
Workshop 2: IGNITION AND FLAME SPREADBased on textbook “An Introduction to Fire Dynamics” by Dougal Drysdale
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of origin2
Premixed and Diffusion Flames
Two modes of combustion:
• Flaming Combustion
• Smouldering Combustion
Premixed and Diffusion Flames
Flaming Combustion
Gaseous fuel + Air FLAME Products +Heat
Premixed Flame
Gaseous fuel and air mixed before ignition
Diffusion flame
Gaseous fuel and air burn as they mix
Premixed and Diffusion Flames
Premixed Diffusion
Premixed and Diffusion Flames
Premixed
Premixed flames
• Fuel vapour and air intimately mixed before ignition
• “Flammability limits” apply
↑
Gas/air mixture
Premixed and Diffusion Flames
Diffusion
Diffusion flames
• Fuel vapour and air initially separate: combustion occurs where and as they mix
• “Flammability limits” do notapply
Gases: gaseous fuel released directly (e.g. from burner)
Premixed and Diffusion Flames
m
FQ
m
Liquids: fuel vapour from rapid evaporation (boiling)
Solids: fuel vapour from chemical decomposition (pyrolysis)
Air entrainment
LQ
m
Change of state
q
SOLID LIQUID VAPOURMelting Evaporation
Sublimation
Chemical decomposition and vapourisation
Chemical decomposition and vapourisation
CHAR
Premixed and Diffusion Flames
Premixed flames
• Fuel vapour and air intimately mixed before ignition
• “Flammability limits” apply
Flammability limits
Lower
Flammability Limit (%)
Stoichiometric Concentration
(%)
Upper Flammability
Limit (%)
Methane (CH4)
5.0 9.5 15.0
Propane (C3H8)
2.2 4.02 9.5
Ethylene (C2H4)
3.1 6.54 36.0
Hydrogen (H2)
4.0 29.6 75.0
Flammability limits
Stoichiometric Concentration
(%)
Minimum Ignition
Energy (mJ)
Autoignition Temperature
(oC)
Methane (CH4)
9.5 0.26 601
Propane (C3H8)
4.02 0.25 450
Ethylene (C2H4)
6.54 0.12 490
Hydrogen (H2)
29.6 0.01 400
Gas Explosions
The remains of a house after a gas explosion
Gas Explosions
Ronan Point 16th May 1968 – the result of a gas explosion in SE corner flat on 18th floor
18th floor
Explosion following the ignition of a suspension of 50g polyethylene powder (demonstration at FM Global Research Labs, RI, USA)
Dust Explosions
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of originDr. Björn Karlsson
16
Premixed and Diffusion Flames
What is the relevance of premixed burning to “fire”?
Ignition of liquids and solids
Flashpoint and Firepoint
“Closed Cup Flashpoint”
Lowest temperature at which a flammable
mixture exists above the surface of the liquid.
Determined in a “closed cup” apparatus.
Flashpoint and Firepoint
“Closed Cup Flashpoint”
Lowest temperature at which a flammable
mixture exists above the surface of the liquid.
Determined in a “closed cup” apparatus.
“Firepoint”
Lowest temperature at which ignition of the vapours
leads to sustained burning of the liquid.
Determined in an “open cup” apparatus.
Flashpoint and Firepoint
q
Closed Cup Apparatus Open Cup Apparatus
n-decane CC Flashpoint = 46oC
(C10H22) OC Flashpoint = 52oC
OC Firepoint = 62oC
* *●
Flashpoint and Firepoint
q
“Closed Cup Flashpoint” Lowest temperature at which a flammable mixture exists above the surface of the liquid.
*
CC Flashpt (oC)
OC Flashpt (oC)
Firepoint (oC)
Gasoline -38 - -
n-Decane 46 52 61.5
n-dodecane 74 - 103
p-Xylene 27 31 44
Kerosene >37.5 - -
Corn Oil 255 320 -
●
Flashpoints and firepoints of liquid fuels
Closed cup flashpoint
(oC)
Open cup flashpoint
(oC)
Firepoint
(oC)
Gasoline
-38
-
-
iso-octane -12 - -
n-decane 46 52 61.5
n-dodecane 74 - 103
Methanol 11 1(13.5) 1(13.5)
p-xylene 27 31 33
More data are quoted by Babrauskas, Ignition Handbook
Flashpoint and firepoint
Flashpoint measurement. (a) Closed Cup:
(b) Open Cup: (c) showing the vapour
pressure gradient above the liquid surface in
the open cup.
Uniform vapour concentration
Vapour concentration decreases with height above surface
Height above liquid surface
Vapour concentration
(a) (b) (c)
Firepoint
At or above the firepoint, ignition of vapours (premixed flame) is followed by continuous burning of the liquid (diffusion flame)
Firepoint > OC Flashpoint > CC Flashpoint
Note: the OC Flashpoint increases if the height of the ignition source above the surface is increased
Flashpoint and firepoint
Height of the ignition source above liquid surface (mm)
Measurements of Open Cup flashpoint and firepoint for n-decane with an elevated ignition source
Flashpoint
ll Firepointl
Flashing
Continuous flaming
No OC ignition
Open cup flashpoint
Firepoint
Flashpoint Classification
Closed cup flashpoint
0oC
50oC
100oC
Temperature
Combustible Liquids
60oC
32oC
Flammable Liquids
Highly Flammable Liquids
UK USA
100oC 212oF
Temperature 60oC
200oF
Closed cup flashpoint
Class IIIB
Class IIIA
50oC
140oF
37.8oCClass II
Class IA, IB
Class IC
32oF0oC
100oF
22.8oC
93.4oC
73oF
A comparison between the 1972 UK Regulations and the USA classifications of flammable and combustible liquids (Table 3)
Flashpoint Classification
Closed cup flashpoint
0oC
50oC
100oC
Temperature
Combustible Liquids
60oC
32oC
Flammable Liquids
Highly Flammable Liquids
UK USA
100oC 212oF
Temperature 60oC
200oF
Closed cup flashpoint
Class IIIB
Class IIIA
50oC
140oF
37.8oCClass II
Class IA, IB
Class IC
32oF0oC
100oF
22.8oC
93.4oC
73oF
A comparison between the old UK and the EU classifications of highly flammable, flammable and combustible liquids
Highly Flammable Liquids
21oC
55oC
Flammable Liquids
(Combustible Liquids??)
EU
(under COMAH Regulations)100oC
50oC
0oC
Extremely flammable liquids –
CC Flashpoint < 0oC Boiling point < 35oC
Fire in the Hotel International Zurich (16th February 1988). It started in the restaurant when a junior waiter tried to top up a flambé lamp before the flame had extinguished
Flashpoint and firepoint
Ignition of liquids
Flammable and combustible liquids must be heated above their firepoints
Kerosene > 40oC
Diesel Oil > 70oC
Corn Oil > 280oC
Bulk heating, or surface heating?
Ignition of a high flashpoint liquid
(a) Bulk heating to the firepoint
e.g. Chip pan fires
(b) Local surface heating
Surface-tension driven flows
Distance from wick (mm) Velocity away from wick
(a) Surface tension driven flows and convective motion in a liquid subjected to a localised ignition source;
(b) Velocity profile 10 mm away from the wick.
(a)
(b)
Wick Flame
Distance from wick (mm) Velocity away from wick
Wick Flame
Heat transfer from flame to the surface of the liquid
Increasing surface tension Liquid surface
Figure 2.
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of originDr. Björn Karlsson
32
Ignition of solids
Combustible solids also exhibit flashpoints and firepoints
They are not easily measured:
(a) surface temperatures
(b) transient conditions
(c) method of heating affects result
Ignition of solids
“Fire properties of combustible solids”
Ease of ignition
Surface spread of flame
Rate of heat release
Smoke and toxic gases
These are not material properties - they all depend on the “fire scenario” and the physical form of the “fuel” (cf Al dust)
Ignition of solids
Ignition / Ignitability
Distinguish between:
• Piloted ignition
• Spontaneous ignition
Ignition of solids
Piloted ignition
* Ignition source
Combustible material
(1) Critical surface temperature (cf. “firepoint” for
flammable liquids)
(2) Critical flux of flammable vapours
These have been confirmed experimentally, but they are configuration-dependent
critm
Ignition of solids
Piloted ignition
Thermal decomposition at surface at elevated temperatures
Spark, flame, hot surface
Surface temperature must be greater than the firepoint
Firepoint temperatures of solids
QMaterial* Heat Flux
Range
Average Tig
(kW/m2) (
oC)
PX
17-37.5 310 + 3
FINN 18.5-38 309 + 6
POM 21-34 281 + 5
PE 19-34 363 + 3
PP 21-42.5 334 + 5
PS 19-34 366 + 4
PX and FINN are trade names for PMMA (polymethylmethacrylate); POM is polyoxymethylene, PE, PP and PS are polyethylene, polypropylene and polystyrene respectively
Firepoint temperatures of solids
Q
Time
Tem
pera
ture
T1 Chemical decomposition begins
T2 “Flashpoint”
T3 “Firepoint” (Piloted Ignition)
T4 Spontaneous ignition
Ignition of solids
Spontaneous ignition
Spontaneous ignition
occurs in the gas phase
Ignition of solids
Distinguish between “Thick fuels” and “Thin fuels”
k
hBi
Ignition of solids
Distinguish between:
“Thin fuels” (Bi small)
“Thick fuels” (Bi large)
THIN FUELS:
HEAT FLUX
Convective heat loss
Time to ignition Thickness ()
Ignition of a thin combustible solid
oigR
R
igTThQa
Qa
h
ct
2ln.
2
tig = time to ignition h = heat transfer cfft = thickness QR = radiant heat flux = density a = absorptivityc = heat capacity Tig = firepoint temperature
To = ambient temperature
.″
igt
Ignition of solids
Distinguish between:
“Thin fuels” (Bi small)
“Thick fuels” (Bi large)
THIN FUELS: THICK FUELS:
HEAT FLUX
Convective heat loss
Conductive heat transfer
HEAT FLUX
Time to ignition = f(Thermal Inertia (kc))
Ignition of a thermally thick solid
The response of the surface to an imposed heat flux (convective or radiative) is strongly dependent on the thermal inertia (kc)
Ts
To
Surface of
semi-infinite
solid
Temperature profile beneath the heated surface of a semi-infinite solid
Distance from surface (depth)
Ignition of a thermally thick solid
The response of the surface to an imposed heat flux (convective or radiative) is strongly dependent on the thermal inertia (kc)
Ts
To
Surface of
semi-infinite
solid
Distance from surface (depth)
ck
hterfc
ck
th
TT
TT
o
os
5.02
.exp1
k = thermal conductivity = densityc = heat capacity
Derived from the General Heat Conduction Equation:
Ignition of a thermally thick solid
The effect of thermal inertia on the rate of temperature rise at the
surface of a semi-infinite solid (semi-infinite behaviour if t > 2(at)0.5)
kc (Table 5)
Steel 1.6x108
Oak 3.2x105
Asbestos 9.2x104
FIB 2.0x104
PUF 9.5x102
(units - W2.s/m4.K2)
PUFFIB
Asbestos
Oak
Steel
o
oss
TT
TT
Time (mins)
Fig. 9
(p. 12)
Ignition of a thermally thick solid
The effect of thermal inertia on the rate of temperature rise at the
surface of a semi-infinite solid (semi-infinite behaviour if t > 2(at)0.5)
kc (Table 5)
Steel 1.6x108
Oak 3.2x105
Asbestos 9.2x104
FIB 2.0x104
PUF 9.5x102
(units - W2.s/m4.K2)
PUFFIB
Asbestos
Oak
Steel
o
oss
TT
TT
Time (mins)
Fig. 9
(p. 12)
Ignition of solids
Identification of ignition
What heat flux would have been required for ignition to have occurred?
Could this have been provided by any heat source present?
How long would ignition have taken?
Discussion here is based on Fundamental laws of Heat transfer
Rate of surface spread of flame
Factors which influence “ignitability” also affect the rate of flame spread:
Thermal inertia of a “thick fuel”
Presence of edges
Presence of an imposed heat flux
Thickness of a “thin fuel”
In addition:
Orientation of the surface
Direction of spread
Rate of surface spread of flame
Interaction of a spreading flame and the surface of a thick combustible solid at different angles of orientation
Curtains
Wall linings
High rack storage
Rate of surface spread of flame
Rate of upward spread of flame on a thin fuel (computer card) as a function of angle of orientation
Angle (degrees)
Rate
of
flam
e s
pre
ad (
mm
/s)
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of originDr. Björn Karlsson
53
Smouldering combustion
Smouldering - does not involve flame
- occurs with char-forming materials only
- char produced at temperatures > 250oC
- char undergoes surface oxidation
- heat released produces more char
Smouldering combustion
Model of the smouldering process
Virgin cellulose Discoloration of cellulose
Black charMaximum temperature
Propagation
Residual ash/char
Smoke
Glowing char
Smouldering combustion
Typical materials which smoulder:
Wood-based products
Cellulose
Viscose rayon
Dusts and fibres of vegetable origin
Rubber latex foam
Some leathers
Some polyurethane foams
Smouldering combustion
Sequence showing the smouldering of a corrugated carton packed with expanded polystyrene boxes (R Edgley, HK Govt Lab)
7 mins 20 mins 30 mins 60 mins
Smouldering was initiated with a cigarette applied to a torn corner of the box
40 cm
Smouldering combustion
Initiation of smouldering
• Contact with a smouldering source
• Contact with a hot surface
• Exposure to radiant heat
• Spontaneous combustion
Transition to flaming
Requires increase in temperature and in the mass flowrateof the fuel vapours. Mechanism poorly understood.
Smouldering fires
t = 0 min
t = much latert = 58 mint = 55 min
t = 53:30 mint = 43 mint = 21 min
Smouldering combustion
Premixed and Diffusion Flames
premixeddiffusion QQ
The rate of burning/rate of heat release
Approximate energy densities:
Diffusion flame 1 MW/m3
Premixed flame200 MW/m3
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of originDr. Björn Karlsson
61
Enclosure fires
Time (s)
Bu
rnin
g ra
te (
g/m
2.s
)
The development of the rate of burning of a slab of PMMA (0.76m x 0.76m) under confined conditions
This illustrates the key difference between a fire in the open and one in a confined space
Compartment fires
Fire development in a compartment – Temperature as a function of time
Time
Backdraft
Flashover
Growht toflashover
Temp
Tid
The Pre-flashover Fire
Mechanism for flashover:
Fire produces a plume of hot, smoky gases
Hot smoke layer accumulates under the ceiling
Hot smoke and heated surfaces radiate downwards
Flame spread rate over combustible surfaces increases
Rate of burning increases
Smoke accumulating under ceiling gets hotterFeedback loop
extQ
The Pre-flashover Fire)
“The Front Room Fire”
(BRE Video)
The Pre-flashover Fire
)
)
The Pre-flashover Fire
)
)
The Pre-flashover Fire)
Evolution of smoke
The Pre-flashover Fire
)
)
Ignition of exposed side of chair leg
The Pre-flashover Fire)
The Pre-flashover Fire
Smouldering fires
Produces cool smoke
Sticky smoke deposit at all levels
Deep charring at locus of origin
Smoke is “flammable”
Backdraft (backdraught in England)
Severely underventilated fires
Flaming may cease when [O2] < 8 – 10%
Smouldering will continue at [O2] ~ 4
Sticky smoke deposits on cold surfaces
Smoke is “flammable” – can produce abackdraught if ventilation is suddenly provided
Conditions required for backdraft ??
Can we identify if backdraft has occurred?
Content
1. Premixed and diffusion flames– Premixed flames, flammability limits, explosions– Diffusion flames, liquids, flashpoint and firepoint– Diffusion flames, solids, ignition temperature
2. Ignition of solids– Thermally thin solids– Thermally thick solids– Piloted ignition and spontaneous ignition
3. Smoldering combustion4. Enclosure Fires
– Pre-flashover fires– Post-flashover (or fully developed) fire
5. Fire and smoke spread beyond room of originDr. Björn Karlsson
73
The fully-developed fire
Temperatures achieved in the fully developed fire
Pettersson et al.
Showed that the temperature-time curves depended on:
Thermal properties of the boundaries
Fuel load (MJ/m2)
Ventilation parameter
tA
HA
(post-flashover)
Air in
Fire gases outNeutral plane
+ dp
- dp
Fuel loads (MJ/m2)
Temperature-time curves calculated by Pettersson et al. Restricted (low) ventilation (top left) associated with low temperatures.
For high ventilation, high temperatures are predicted, but the fire may become “fuel controlled”
12.0tA
HA
The fully-developed fire
Smoke and fire spread
Fire spread beyond the room of origin
Spread of smoke and hot gases through any opening above neutral plane (door, barrier penetration, window ..)
Spread of flames from compartment of origin by the same routes
Toxic products of combustion (mainly CO) can be carried far beyond the locus of the fire
Burning gases can flow along corridor ceilings, burning vigorously where they meet a fresh air supply
Smoke and fire spread
Fire spread beyond the room of origin
“The stack effect”
Neutral pressure plane
Tinside > Toutside
This is the situation that existed
in the Garley Building, Hong
Kong (November 1996) and led
to the deaths of 40 people on
the top floors of a 15-storey
commercial building
The fire started on the second
floor lift lobby and spread up
two open liftshafts
There was no fire damage
between the 4th and 11th floors
MGM Grand Hotel, Las Vegas,
1980: fire in Casino on the
ground floor led to the deaths
of 80 hotel guests who were in
their rooms on the upper floors
The smoke reached the upper
levels through breaches in
vertical integrity (partly arising
from the design of the building
which had to be earthquake-
resistant)
Enclosure Fires- Summary
Compartment boundaries strongly influence fire behaviour
Key factors are the amount of ventilation available and the area of combustible surfaces present
Above the neutral plane, positive over-pressure forces flame and hot gases through any openings
Temperatures > 1000oC can be achieved in a fully developed VC fire
Severely under-ventilated fires will self-extinguish, but can cause a backdraught if ventilation is provided