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A STUDY OF THE PRECURSORS TO BACKDRAFT (SMOKE

EXPLOSION) IN A MECHANICALLY-VENTILATED

COMPARTMENT AND THE ROLE OF SOOT IN THIS

PHENOMENON

PhD student : Alvin Si-Xian LOO

Supervisors: Prof. Alexis COPPALLE (CORIA), Philippe AÎNE (AREVA)

10ème Journée des Doctorants du CORIA 2013

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Lets start by examining the different stages of the fire power development in a

compartment during an accidental fire:

WHAT IS A B ACKDRAFT?

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Well-ventilated case


F i r e p o w e r ( W )

Time (t)

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Time (t)

1 Ignition

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Time (t)

1 Ignition

2 Fire growth

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Time (t)

1 Ignition

2 Fire growth

3 Burning if all combustible materiels

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Time (t)

1 Ignition

2 Fire growth


4 Fire decay due to lack of fuel

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Time (t)

1 Ignition

2 Fire growth



5 Extinction of fire

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Time (t)


Time (t)

Underventilated case + Backdraft

1 Ignition

2 Fire growth




1 2 3 4

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Time (t)


Time (t)


1 Ignition

2 Fire growth




1 2 3 4

1 Ignition

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Time (t)


Time (t)


1 Ignition

2 Fire growth




1 2 3 4

1 Ignition

2 Fire growth

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Time (t)


Time (t)


1 Ignition

2 Fire growth




1 2 3 4

1 Ignition

2 Fire growth

3 Fire decay due to lack of air

+ accumulation of combustible

vapor and soot

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Limited ventilation during an enclosure fire can lead to the production of large

amounts of unburnt pyrolysis products. When an opening is suddenly

introduced, the inflowing air forms a gravity current and begins to mix with

the unburned pyrolysis products,creating a combustible mixture of gases in

some part of the enclosure. Any ignition sources, such as a glowing ember, can

ignite this combustible mixture. Resulting in an extremely rapid burning of

gases/pyrolysis products forced out through the opening and causes a fireball

outside the enclosure. 15-- Fleischmann/Pagni/Quintiere

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Limited ventilation during an enclosure fire can lead to the production of large

amounts of unburnt pyrolysis products. When an opening is suddenly

introduced, the inflowing air forms a gravity current and begins to mix with

the unburned pyrolysis products,creating a combustible mixture of gases in

some part of the enclosure. Any ignition sources, such as a glowing ember, can

ignite this combustible mixture. Resulting in an extremely rapid burning of

gases/pyrolysis products forced out through the opening and causes a fireball

outside the enclosure. 16-- (Fleischmann/Pagni/Quintiere)

Backdraft is sometimes called ‘smoke explosion’

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CONTENTS OF THIS PRESENTATION

What is a Backdraft?

Context and Objectives of this study

Experimental apparatus for underventilated fires

Some Results of underventilated fires

Experimental apparatus for smoke deflagration

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CONTEXT OF THIS STUDY

To prevent radioactive substance leak, compartments are

sealed to ensure air-tightness. Mechanical ventilation

systems maintain an underpressure and renew the air of

the compartment

Study fire scenarios in nuclear installations.

Air

Smoke

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the compartment


Problem: Maintain confinement within thecompartments during fires Air

Smoke

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the compartment


Problem: Maintain confinement within thecompartments during fires

- Radioactive substance leak to the outside

- Damage of safety equipments in the compartment or in the ventilationnetwork (filters, doors, dampers…etc)

What could happen:

Air

Smoke

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the compartment


Problem: Maintain confinement within thecompartments during fires

- Radioactive substance leak to the outside

- Damage of safety equipments in the compartment or in the ventilationnetwork (filters, doors, dampers…etc)

Feared events: - Overpressure caused by the fire

- Temperatures inside the compartment becomes too high

- Backdraft

What could happen:

Air

Smoke

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Overpressure and high temperatures can be caused by the fire itself:

(Internal energy of the compartment)

Backdraft can happen:

- Smoke dilution in the ventilation network

- Opening of doors by rescue personnels

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So it is important to be able to predict correcly the power generated by the fire.




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- Interaction between the fire and its confined environment

which is under-oxygenated and polluted.

- Aeraulic pertubation caused by the mechanical ventilation




Difficulty:




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Difficulty:




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Difficulty:




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1. Behavior of underventilated fires in a

mechanically-ventilated compartment.• Underventilated fires in confined

compartments must occur before the

conditions favourable to Backdraft can

be created.

• Experimental study using a reduced-

scale 1m3 compartment (fire chamber)

OBJECTIVES OF THIS STUDY

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Diagram of the experimental apparatus for studying underventilated fires in a

reduced-scale mechanically-ventilated compartment (fire chamber)

EXPERIMENTAL APPARATUS FOR FIRES

• Compartment made of steel - 1m2 steel plates are fixed onto a frame

• Braids are crushed between the steel plates and the frame to ensure air-tightness

• Refractory ceramic fibres on the inner wall to ensure thermal insulation.

•Water seal between the scale and the fuel to ensure air-tightness

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Fuel Mass Loss Rate (MLR) estimation of pool fires (Hamins 1996) :

Smoke

Fuel

flameQ

reflect Q

smokeQ

SOME RESULTS FOR UNDERVENTILATED FIRES

One example of study: Determination of the fuel’s Mass Loss Rate J o urn

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Fuel’s mass loss rate is caused by

the heat flux from various sources:

flame, smoke, compartment walls

The fuel reflects and reradiatessome part of the heat away.

wall Q

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SOME RESULTS FOR UNDERVENTILATED FIRES

ARR = 5

ARR = 0,3

ARR =11

MLR at Free-burning (fire in non-enclosed area)

- Flames are extinguished due to lack of air

- MLR in these cases do not reach free burning rates

Development of the fuel’s Mass Loss Rate (MLR) at different ventilation rates:

Fuel : Heptane

Size of fuel pan: 2 x 10.8cm

*ARR = Air Renewal Rate

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0.2s 0.2s 0.2s

Expanding flames

SOME RESULTS FOR UNDERVENTILATED FIRES Visual results:

Flames extend outside

the perimeter of the fuel

pan periodically

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0.2s 0.2s 0.2s

Expanding flames

Wandering flames:

Ghosting flames:

0.2s 0.2s 0.2s

0.2s 0.2s 0.2s

SOME RESULTS FOR UNDERVENTILATED FIRES Visual results:

Flames extend outside

the perimeter of the fuel

pan periodically

Flames move around the

compartment towards

the opposite side of the

air inlet

Flames are lifted above

the fuel pan and float

around

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Fresh air

Smoke from the fire chamber

EXPERIMENTAL APPARATUS FOR B ACKDRAFT

Diagram of the experimental apparatus (under construction):

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Methane gas

Fresh air

Smoke from the fire chamber


Diagram of the experimental apparatus (under construction):

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E B

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What do we wish to study?

(thesis of Courty 2012)

1) Conditions of Backdraft :- Concentration of the different species

- Temperature

- Soot properties (size distribution, mass concentration)Gas analyser Testo 350

Thermocouples

2) Flame propagation

- Dynamic pressure (pressure increase during the

deflagration)

- Flame speed-Ombroscopy technique (thesis of Courty 2012)

-PIV technique (thesis of Varea 2013)

(thesis of Varea 2013)

Dynamic pressure sensor

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E B

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Role of soot -- What do we wish to determine?

We suspect that during the deflagration, the thermal radiation of soot cancontribute to the heating the fresh gases, thus increasing the flame speed.

+

?

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Heating of the fresh gas can be caused by radiation

Combustion productsFresh gas

SL

Pre-heating zone Reaction zone

Flame propagation mecanism:

+

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Heating of the fresh gas can be caused by radiation

The radiation emission and

absorption of soot increase the

radiated heat received by the freshcombustible mixture

More powerful deflagration ?

Combustion productsFresh gas

SL

Pre-heating zone Reaction zone

Flame propagation mecanism:

+

?

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