Principles of Fire Behavior

Additional information for MINA

What is Fire?

• An uncontrolled chemical reaction producing light and energy sufficient to damage skin

• Also identified as combustion – a chemical reaction involving fuel and an oxidizer in the air

• Think kilowatts

Natural Causes of Fire

• Lightning

• Extreme weather

• Earthquake

Man Made Causes

• Open flames

• Explosive environments

• Inattention

• Chemical reactions

Types of Fires

• Diffusion flames

• Smoldering

• Spontaneous combustion

• Premixed flames

Diffusion Flames

• A combustion process in which the fuel gas and oxygen are transported into the reaction zone due to concentration differences – Fick’s Law

• Move from high to low concentration in the mixture

• Natural flaming fires are diffusion flames– Match or candle

Diffusion Flames

• Terms identifying diffusion flames– Pyrolysis – decomposition process as in wood– Laminar – pure molecular diffusion as in a

candle– Buoyancy – gravity or lack of gravity with

hotter or lighter gases– Jet flame – high pressure created by gaseous

fuel sources

Smoldering

• Slow combustion process• Charring - >1000 degrees• Solid fuel source• Produces deadly levels of carbon monoxide• Examples

– Cigar– Upholstered chair– Charcoal grill

Spontaneous

• Begins with a slow oxidations in a fuel exposed to air

• Very little heat lost • Thermal runaway• Examples

– Haystacks– Sawdust bin– Oily rags piled

Premixed Flames

• Mixing of fuel gas and air or oxygen– Mixed before ignition and propagation

• Controlled would be jet engines and oxyacetylene torch

• Uncontrolled examples– Confined space – Methane leak

Premixed Flames

• Concentration limits

• Gaseous fuels will ignite within limits– Upper (UFL)– Lower (LFL) – temperature just before fuel

condenses to a liquid at the LFL is call the flashpoint

– Autoignition temp (AIT) – lowest temp to cause spontaneous ignition

Heat Transfer

• Conduction – a molecular phenomenon

• Convection – conduction in a moving fluid

• Radiation – an electromagnetic phenomenon

• Heat Flux – the flow rate of heat – key to assessing the potential damage by a fire

Ignition

• Piloted – process of initiation and flame propagation in premixed fuel systems – e.g., welding, jet engines, gas stoves

• Autoignition – no spark or flame source

Ignition

• Evaporation – the process of gas molecules escaping from the surface of a liquid

• Humidity – higher the humidity, less evaporation in the air – lower, more

• Flashpoint – temp of a liquid fuels and the LFL – point of piloted ignition

• Boiling point – temp that liquid can evaporate under normal atmospheric conditions

Flame Spread

• The process in which the perimeter of the fire grows

• Wind aided

• Opposed flow

• Pyrolysis or vaporization region

• Burning rate

Flame Spread

• Surface flame

• Downward/lateral wall spread – opposed flow flame spread

• Spread through porous solid arrays – brush/debris

• Spread on liquids – viscous/surface tension

Burning Rate

• The mass of solid or liquid fuel consumed per unit time

• Mass loss rate – the mass of fuel vaporized but not necessarily burned per unit time

Fire Plumes

• Turbulent fire plumes – the flame and gases emanating from a burning object

• Buoyancy – effective force on fluid due to density or temperature differences in a gravitational field

• Entrainment – the process of air or gases being drawn into a fire, plume or jet

Fire Plumes

• Flame height – the vertical measure of the combustion region – Pool fires– Line fires

• Eddies – rotating regions of a fluid

• Vortex – a ring of eddies

• Flame lengths – depends on energy released

Combustion Products

• Products – chemical compounds produced by fire

• Yield – the mass of product produced per unit mass of fuel supplied

• Concentrations – the percentage of material per unit mass (or volume) of its mixture

Concentration

• The primary hazard of fire is the composition and associated concentrations of the smoke

• Hazards– Vitiation – reduction in oxygen

– Narcotic gases – narcosis

– Irritant gases – acid, hydrocarbon byproducts

– Smoke visibility

– Hyperthermia – heat stress

Compartment Fires

• Stages of fire development– Developing fire– Flashover– Fully developed

Compartment Flow Dynamics

• Layers and vent flows

• Smoke filling

• Smoke movement

Fire Analysis

• Fire safety design– Detection and design– Mitigation of growth and suppression– Egress– Continuity of operations– Structural integrity– Refuge and rescue

Resources

• Firedynamics.com

• www.campusfire.org

• www.campusfiresafety.com

• www.pp.okstate.edu/esh/Fire.htm

• www.fpemag.com

• www.ul.com

Resources

• American Fire Journal

• Fire House

• Fire Chief

• Fire Engineering

• Fire Technology

• USFA – Fire Data Analysis Handbook