Physics of Fire

Jared Blatt

G

March 19th 2017

Physics of Fire

• A candle is a system for making hydrocarbon molecules react with oxygen to produce heat and light, carbon dioxide and water.

• Howard Ross stated, "There are literally thousands of reactions that go on from the moment the fuel vapor is produced and leaves the wick to the time it actually burns and produces CO2 and water."
• As melted wax rises through the wick, its long hydrocarbon molecules are vaporized and cracked apart by heat emanating from the flame.
• Some of the fragments migrate outward. Some are transformed into ring-shaped molecules called polycyclic aromatic hydrocarbons; those clump together, forming large particles of soot, which drift upward and are burned, or escape from the top of the flame as smoke.

• Most of the heat but not the light is released at the surface of the flame, where fuel vapor diffusing out from the wick meets oxygen diffusing in from the surrounding air.
• The various carbon compounds and the oxygen molecule O2 have weaker bonds but more potential energy than do CO2 and H2O.
• When the carbon and oxygen combine to form CO2 and water, some of the energy difference is released as heat.
• The visible light of a candle flame is caused by two different processes
• incandescence
• chemiluminescence.
• The bright yellow light in the tongue of the flame comes from incandescent soot particles.
• The faint blue light around the bottom comes from furiously vibrating CH and C2, still on their way to being burned.

• An ordinary fire doesn't sit passively waiting for oxygen to diffuse toward it.
• As air heated by the flame rises, cool oxygen air flows in at the bottom.
• It is said that a forest fire can generate winds of more than 100 miles per hour.
• A well made candle usually doesn't flicker unless it's buffeted by external air currents.
• If the fuel is distributed in a regular way, the burning pool will pulse as regularly as a clock.

Thuillard Experiment

• A small, round, shallow dish of ethanol on the floor of flameproof lab, which is a mostly empty concrete cube 30 feet high and 30 feet across.
• A colleague of Thuillard holds a lighter to the ethanol and ignites it.

• What's happening in that flickering flame is cool air rushing over the flame is creating a wave on it.
• The flame wave travels from the edge of the pool toward the center, where it becomes a flame mushroom that billows upward and outward.
• When there are gusty winds fanning the fire, or when it's a heap of burning logs in your fireplace instead of a pool of ethanol, the flicker becomes irregular.
• But Thuillard has found that, either way, flames still contain hidden mathematical regularities that make it possible to distinguish them from any other light source, such as sunlight that flickers on a wall because it is irregularly blocked by blowing foliage outside.