Combustion & Heat Transfer
OMBUSTION CHEMISTRY
Well... I lied. Don't hate me. I held off on this as long as I could, and I'll make it as easy as I know how. Don't worry about the chemical equations, or all the different reactions and physical changes for now. What you need to know is the end result. Here's a little vocab to start us off:
Exothermic - Relating to a chemical reaction that releases heat. (If something gets hotter as the reaction occurs, it's exothermic)
Endothermic - Relating to a chemical reaction that absorbs heat. (If something gets colder as the reaction occurs, it's endothermic)
Oxidation- The combination of a substance with oxygen.
Combustion- Rapid oxidation, accompanied by heat and light.
A fire will always be changing its environment. The chemical reactions will be very complex, vast amounts of heat and light will be produced, and the byproducts produced will be ever-changing based on the different fuels that are burning. Don't sweat it, because it doesn't change a darn thing when it comes to what we need to know right now.
PRODUCTS OF COMBUSTION
This is the stuff that's left over after it's burned. With technology the way it is today, when a house burns down there could be hundreds of different gases in the air. Byproducts from burning plastics, petroleum products, and Lord knows what else could be present. These gases are usually a result of incomplete combustion. If a fire burns absolutely perfectly, there will be no byproduct. In reality, this just plain does not happen. The odds of your fuel and air being at the perfect ratio while being at the perfect temperature are almost mathematically impossible. Modern cars get closer and closer to reaching that perfect ratio, but we won't see perfect combustion in our lifetime.
All airborne products from incomplete combustion are referred to as smoke. All particles, gases, and vapors fall into this category as long as they're airborne. It's perfectly common for smoke to be found as a superheated gas, or be down as low as room temperature. The particles themselves are tiny solid pieces of fuel (usually), which can be burned or unburned. Vapors are the tiny liquid droplets within the smoke, and Gases make up the rest.
COMMON BYPRODUCTS
Ammonia- Clear, colorless gas with a strong odor. Nonflammable, can cause burns on contact. (Chemical Formula: NH3)
Carbon Dioxide- Colorless, odorless gas. Relatively nontoxic and noncombustible. Heavier than air and may asphyxiate by the displacement of air. (Chemical Formula: CO2)
Carbon Monoxide- Colorless, odorless, and tasteless gas. Acts as a powerful asphyxiant because hemogloben in the blood will bind with this gas instead of oxygen. (Chemical Formula: CO)
Chlorine- Greenish-yellow gas with a pungent, suffocating odor. Toxic by inhalation. Extremely poisonous, corrosive, and a strong oxidizing agent. (Chemical Formula: Cl)
Hydrogen Chloride- A colorless gas with a sharp, pungent odor. Both poisonous and corrosive. Also called Hydrochloric Acid. (Chemical Formula: HCl)
Hydrogen Cyanide- A clear colorless liquid with a faint odor of bitter almonds. Fatal if inhaled, ingested or absorbed through skin. (Chemical Formula: HCN)
Nitrogen Dioxide- Reddish brown gas or yellowish-brown liquid when cooled or compressed. Poisonous, corrosive, and a strong oxidizer. (Chemical Formula: NO2)
Phosgene- Colorless gas with an odor of new-mown hay or green corn. Poison, corrosive, and extremely toxic. (Chemical Formula: Cl2CO)
HEAT TRANSFER
Heat actually spreads in 3 different ways. Most fires will have all 3 of these methods working at the same time:
Conduction - The process of transferring heat by direct contact on a molecular level. Metals are notorious for their ability to conduct heat, whereas wood or fiberglass are poor conductors. A common example of heat conduction would be putting a metal rod into a fireplace. The rod will grow red hot from the heat it absorbs through conduction.
Convection - The movement of heat through a fluid medium. A "fluid medium" does not mean a normal fluid, but instead anything that can move freely. Air, water, anything. If you hold your finger two inches over a lit match, you'll be burned through convection. By far, this method of heat transfer is the easiest to observe in a normal fire. In a fluid medium (such as air), heat will rise to the ceiling and begin to spread outward. Smoke will usually follow convection streams.
Radiation - The transfer of energy through electromagnetic waves. In our case, heat is the energy transferred. All energy received from our Sun is radiation. A sunburn is causes by heat transfer through radiation. It is not uncommon for a large fire to cause sunburn in the same manner if a person isn't properly protected. Radiation is also the primary culprit when it comes to exposure fires. For example, a house that is on fire burns so hot that the house next to it begins to burn.
Tip: These three forms of heat transfer can be difficult for a test to convey visually, so my best advice is this: If you're shown direct contact, that's conduction. If you see heat rising or spreading along a ceiling (with or without smoke), that will likely be convection. Radiation will be shown without smoke, and likely be shown moving in a direction other than upward (most examples use the heat rising to show convection).