Compartment Fire Dynamics Ventilation Controlled Compartment Fires Firefighting Course

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

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

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

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

MODERN BUILDING CONSTRUCTION

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

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

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 ?

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

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 !

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

Fire Goes out Spontaneously Oxygen Deficiency TEMPERATURE TIME

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

Gravity Current

Fire Resumes Development Oxygen Deficiency TEMPERATURE Opening occurs TIME

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

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

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

BACKDRAUGHT

Backdraught Fully Developed Fire Backdraught Opening Occurs TEMPERATURE Oxygen deficiency TIME

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

Smoke Pulsations Oxygen Deficiency TEMPERATURE TIME

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

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

SIGNS OF POTENTIAL BACKDRAUGHT

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 ?