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Understanding Principles of Fire Behavior for Safety

Learn about the natural and man-made causes of fire, types of fires, heat transfer mechanisms, ignition processes, flame spread, combustion products, and compartment flow dynamics to enhance fire safety. Access valuable resources for fire analysis and prevention.

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Understanding Principles of Fire Behavior for Safety

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  1. Principles of Fire Behavior Additional information for MINA

  2. 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

  3. Natural Causes of Fire • Lightning • Extreme weather • Earthquake

  4. Man Made Causes • Open flames • Explosive environments • Inattention • Chemical reactions

  5. Types of Fires • Diffusion flames • Smoldering • Spontaneous combustion • Premixed flames

  6. 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

  7. 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

  8. Smoldering • Slow combustion process • Charring - >1000 degrees • Solid fuel source • Produces deadly levels of carbon monoxide • Examples • Cigar • Upholstered chair • Charcoal grill

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. Flame Spread • The process in which the perimeter of the fire grows • Wind aided • Opposed flow • Pyrolysis or vaporization region • Burning rate

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. Compartment Fires • Stages of fire development • Developing fire • Flashover • Fully developed

  23. Compartment Flow Dynamics • Layers and vent flows • Smoke filling • Smoke movement

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

  25. Resources • Firedynamics.com • www.campusfire.org • www.campusfiresafety.com • www.pp.okstate.edu/esh/Fire.htm • www.fpemag.com • www.ul.com

  26. Resources • American Fire Journal • Fire House • Fire Chief • Fire Engineering • Fire Technology • USFA – Fire Data Analysis Handbook

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