Chapter 4 - Fire Dynamics Flashcards
Fire science
All fires involve a heat-producing chemical reaction between some type of fuel and an oxidizer. Oxidizers are not combustible but will support or enhance combustion.
Common oxidizers
Oxygen
Calcium hypochlorite (chlorination of swimming pools)
Chlorine (water purification)
Ammonium nitrate (fertilizer)
Hydrogen peroxide (industrial bleaching)
Methyl ethyl ketone peroxide (catalyst in plastics manufacturing)
Oxidation
Chemical reaction involving the combination of an oxidizer, such as O2 in the air, with other materials. Can be slow such as oxygen with iron to form rust, or can be rapid as in combustion of methane (natural gas).
Energy (measured in Joules)
Capacity to perform work. Forms of energy are either potential (amount of energy that an object can release at some point in the future) or kinetic (the energy that a moving object possesses).
Chemical
Thermal
Mechanical
Electrical
Light
Nuclear
Sound
All energy can change from one type to another.
Heat of combustion
Potential energy available for release in the combustion process.
Thermal energy
As the heat increases, these molecules vibrate more and more rapidly.
British thermal unit (BTU)
1 Btu = 1055 J
Exothermic reactions
Reactions that emit energy as they occur. Fire is an exothermic reaction that releases energy in the form of heat and sometimes light.
Endothermic reactions
Reactions that absorb energy as they occur. Converting water to steam.
Fire triangle
Fuel, oxygen and heat. Necessary for combustion to occur.
Fire tetrahedron
Reducing agent (fuel), Oxidizing agent (mostly O2), heat and chemical chain reaction.
Ignition
Fuels must be in a gaseous state to burn.
Piloted ignition
Most common form of ignition and occurs when a mixture of fuel and oxygen encounter an external heat source with sufficient heat or thermal energy to start the combustion reaction.
Autoignition
Occurs without any external flame or spark to ignite the fuel gases or vapours. The fuels surface is heated to the point at which the combustion reaction occurs.
Pyrolysis
Chemical decomposition of a solid material by heating. Precedes combustion of a solid fuel.
Vaporization
Physical process that changes a liquid into a gaseous state. The rate of vaporization depends on the substance involved, heat, pressure, and exposed surface area.
Autoignition temperature (AIT)
The minimum temp at which a fuel in the air must be heated in order to start self-sustained combustion. Always higher than its piloted ignition temp.
Combustion
Chemical process of oxidization that occurs at a rate fast enough to produce heat and usually light in the form of either a glow or flame. Nonflaming and flaming.
Nonflaming combustion
Occurs more slowly and at a lower temp, producing a smouldering glow.
Flaming combustion
Referred to as fire. Produces visible flame above materials surface. Requires a liquid or solid fuels to be converted to a gas phase through the addition of heat. Ignition is where the combustion process begins.
Density of the hot combustion products is less than the surrounding air, and the combustion products float on the dense cool air surrounding the fuel, thus creating layers of smoke and fuel gases that fill a compartment.
Entrained
To draw in
Smoke
Product of incomplete combustion. Smoke = fuel! Products of combustion are heat and smoke. Smoke is an aerosol comprised of gases, vapor, and solid particulates. When air supply is limited, the level of incomplete combustion is higher which produces more smoke.
Carbon based fuels
Wood, cotton.
Hydrocarbon fuels
Plastics, synthetic fabrics
As part of the chemical reaction..
The consumed oxygen combines with carbon in the smoke to form CO or CO2.
Common products of combustion
CO - toxic, flammable product of the incomplete combustion of organic (carbon containing) materials. Colorless, odourless gas.
Hydrogen cyanide (HCN) - toxic and flammable substance produced in combustion of materials containing nitrogen. 35 more times more toxic than CO. Prevents body from using O2 at cellular level.
CO2 - Product of complete combustion of organic materials. Not toxic but displaces existing O2.
Formaldehyde - Suspected carcinogen, pungent and colorless gas
Nitrogen Dioxide - Reddish brown gas or yelllow ish brown fluid
Particulates - Small particles
Sulfur dioxide - Colorless gas
Pressure
Force per unit of area applied perpendicular to a surface. Gases always move from areas of higher pressure to areas of lower pressure. Heat from a fire increases the pressure of surroundings gases. Will seek to expand to areas of lower. Heated gases will rise and remain aloft (buoyant) and generally travel up and out. At same time, fresh air will generally travel inward toward the fire. Creates a convective flow.
Heat
Is the thermal kinetic energy needed to release the potential chemical energy in a fuel. As heat begins to vibrate the molecules in a fuel, the fuel begins a physical change from a solid or liquid to a gas. Temp is a measure of heat. 1 candle burns at same temp as 10 but the heat release rate is 10 times greater than 1. Boiling point of water 100C. Room temp 20C. Freezing point of water 0C. Normal human body temp 37C.
Heat flux
The measure of the rate of heat transfer to or from a surface. Typically expressed in kW/m2.
Heat release rate
Total amount of heat release per unit time. kW or MW.
