1. Compartment Fire Dynamics Ventilation Controlled Compartment Fires
Firefighting Course

2. Aim
The aim of this lesson is to impart a good understanding of compartment fire behaviour in a ventilation controlled burn regime

3. Objectives To define ventilation controlled compartment fires
To provide the student with the skills to recognise a VCF
To describe the factors that influence VCF development
To explain the four possible outcomes of a ventilation controlled fire
To describe the effects of the reintroduction of oxygen on a VCF
To describe and define backdraught
To identify the signs of backdraught

4. Fire Growth Curve Fuel Controlled Compartment Fire
Fully Developed Fire
Flashover Period
Decay Period
TEMPERATURE
Early Stages
TIME

5. VENTILATION CONTROLLED FIRE
Definition:
When there is no longer sufficient oxygen in the compartment to combust the pyrolised gases formed. The fires heat release rate is then controlled completely by the amount of oxygen available. The fire is now described as Ventilation Controlled

6. MODERN BUILDING CONSTRUCTION

7. Modern Building Construction
Compartments designed to save energy
Draught proofing
High Levels of insulation
Effectively “sealed compartments”
Will ultimately limit available oxygen as the fire develops

8. VENTILATION CONTROLLED FIRES
rtmp://video.learnprouk.com/vod/mp4:Fire_92_DublinFireandRescueService_8007_Dash_Roll_Draft.f4v
HTML - Vent Controlled Fires

9. Review Fires in compartments generally start off as Fuel Controlled
These fires tend to present as Ventilation Controlled Fires by the time the fire service arrives
Why ?

10. POSSIBLE OUTCOMES OF A VENTILATION CONTROLLED FIRE
Fire Goes Out spontaneously
Fire Resumes Development when opening occurs
Fire Auto ignites when opening occurs
Fire Backdraughts when opening occurs
GRAB

11. Outcomes Dependant on Fuel Load v Compartment size
Stage Fire Developed to when oxygen deficiency occurs
Fire Gas Temperature when opening occurs
Size of opening occuring
What we do !

12. Fire Goes out Spontaneously
Oxygen deficiency will affect:
Heat Release Rate +Mass Loss Rate
Temperature drops causing smouldering combustion
Insufficient temperature to trigger further pyrolysis
Most common Scenario
Easy to Extinguish
Extensive smoke damage
Probable occupant fatality

13. Fire Goes out Spontaneously
Oxygen Deficiency
TEMPERATURE
TIME

14. Fire Resumes Development
Fire had more oxygen/fuel available initially:
Larger room volume : fuel quantity
Higher Heat Release rate achieved before decay phase reached
Large amount of warmer unburnt gases
If opening occurs ………..How ?
Gravity Current will form at opening
Neutral Zone will rise
Fire may return to fuel controlled state
Fire may reach Flashover stage (ventilation induced!)
Fire may reach full development

15. Gravity Current

16. Fire Resumes Development
Oxygen Deficiency
TEMPERATURE
Opening occurs
TIME

17. Fire Gases Auto Ignite
Fire has developed to much later stage before oxygen consumed
Fire Gases have reached very high temperature:
500ºC - 600ºC plus
Fire Gases temperature is above spontaneous ignition point but outside flammable range
Opening occurs
Air/Fuel mixture auto ignites
Gases may ignite on contact with air
Fire Fully Develops

18. Fire Gases Auto Ignite Fully Developed Fire 850ºC Oxygen deficiency
TEMPERATURE
TIME

19. Backdraught Large amount of hot unburnt gases Opening occurs ……
High Velocity Gravity Current
Air Current forms premixed fuel/O2 mixture area
Location of Ignition Source determines time delay
Premixed Flame ignite and moves very quickly
Turbulent Deflagration
Powerful Expansion of Gases
Fire Ball outside opening
Backdraught can lead to fully developed fire
Backdraught sometimes expels all fire gases leaving only localised combustion

20. BACKDRAUGHT

21. Backdraught Fully Developed Fire Backdraught Opening Occurs
TEMPERATURE
Oxygen deficiency
TIME

22. Fire Gas Pulsations
Fire Gas Pulsations at openings may occur at any ventilation controlled fire
Heat Release Rate limited by available oxygen
As oxygen level drops:
Temperature drops, volume of gas drops
“Fire draws Breath”
Fire gases and oxygen drawn in
Heat Release Rate increases
Gas Volume increases causing pulses
Cycle triggers next pulsing effect

23. Smoke Pulsations
Oxygen Deficiency
TEMPERATURE
TIME

24. Flow chart - Backdraught
Initial fire
Developing fire
Smouldering
fire
Fully developed fire
Mini backdraught
Too rich
mixture
Backdraught
Flashover

25. Definition -'Backdraught'
Limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and unburned pyrolysis products.
If these accumulate then the admission of air when an opening is made to the compartment can lead to a sudden deflagration.
This deflagration moving through the compartment and out of the opening is a Backdraught

26. SIGNS OF POTENTIAL BACKDRAUGHT

27. Review Define Ventilation controlled fires
What are the four possible outcomes?
What does G.R.A.B. Stand for ?
What is a Backdraught ?
What are the signs of a Ventilation Controlled fire ?
What are the signs of a potential Backdraught ?