Annual Fire Safety Engineering Conference 2013 · Annual Fire Safety Engineering Conference 2013 Björn Karlsson Arnheim, 12-13 November 2013 1 Workshop 2: IGNITION AND FLAME SPREAD

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


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 Diffusion


Premixed

Premixed flames

• Fuel vapour and air intimately mixed before ignition

• “Flammability limits” apply

↑

Gas/air mixture


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)


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 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





5. Fire and smoke spread beyond room of originDr. Björn Karlsson

16


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.


“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.


q

Closed Cup Apparatus Open Cup Apparatus

n-decane CC Flashpoint = 46oC

(C10H22) OC Flashpoint = 52oC

OC Firepoint = 62oC

* *●


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


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


0oC

50oC

100oC

Temperature

Combustible Liquids

60oC

32oC

Flammable Liquids

Highly Flammable Liquids

UK USA

100oC 212oF

Temperature 60oC

200oF


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


0oC

50oC

100oC

Temperature

Combustible Liquids

60oC

32oC

Flammable Liquids


UK USA

100oC 212oF

Temperature 60oC

200oF


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


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


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






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


“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


“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)


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:


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)


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


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 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






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


Model of the smouldering process

Virgin cellulose Discoloration of cellulose

Black charMaximum temperature

Propagation

Residual ash/char

Smoke

Glowing char


Typical materials which smoulder:

Wood-based products

Cellulose

Viscose rayon

Dusts and fibres of vegetable origin

Rubber latex foam

Some leathers

Some polyurethane foams


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


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



premixeddiffusion QQ

The rate of burning/rate of heat release

Approximate energy densities:

Diffusion flame 1 MW/m3

Premixed flame200 MW/m3

Content






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)


)

)


)

)


Evolution of smoke


)

)

Ignition of exposed side of chair leg



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






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

Documents

Annual Fire Safety Engineering Conference 2013 · Annual Fire Safety Engineering Conference 2013 Björn Karlsson Arnheim, 12-13 November 2013 1 Workshop 2: IGNITION AND FLAME SPREAD