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