Although a precise definition is elusive, a flame is generally recognized as a visible sign of combustion and the release of heat at high temperature. A flame involves chemical processes, in the form of exothermic (heat releasing) reactions that energize molecules and produce light. It also involves the physical processes of transfer of both matter and energy. The heat release arises through changes in chemical bonding. Each molecule of fuel and oxidant is made up of atoms bound together by forces of an electrostatic nature. When the bonds are broken, the atoms become free to rearrange themselves and bind together into a different pattern, releasing their excess binding energy in the process. When the natural gas methane (CH4), for example, is burned with oxygen, the following reaction takes place:
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The total energy of the product gases is less than that of the fuel and oxygen before combustion. This energy appears as heat and light. The above combustion equation represents the initial and final materials only; the combustion reaction itself is made up of a continuous series of steps, with the momentary formation of a whole variety of products of partial oxidation, which are themselves unstable and lead to further reaction steps. Flame color is largely determined by these partial products and, in some cases, can be used to identify the material being burned.
Types of flame
Flames are usually classified as either premixed or diffusion. In the former case, the fuel and oxidant are mixed beforehand in ambient conditions and are then introduced to the flame, where they burn rapidly and, if the mixture ratio is suitable, completely. In the diffusion flame, on the other hand, the fuel meets the oxidant only at the flame itself, and the rate of combustion at the interference is then largely controlled by the rate of physical mixing, rather than the rate of chemical reaction.
Both types of flame can be demonstrated with a Bunsen burner. If insufficient air for complete combustion is induced by adjusting the air supply to the gas jet at the base, the gas–air mixture burns with a conical premixed flame which, being fuel-rich, gives rise to an outer diffusion flame owing to subsequent mixing with the atmospheric air. In fact, the two flames may be drawn apart by means of a Smithell’s separator, which consists essentially of a glass tube fitted over the top of a Bunsen burner. A wick in a candle or a wick fed from a liquid fuel supply also supports a flame of the diffusion type, the combustion heat that is released being sufficient to promote a continuous supply of fuel vapor from the wick surface.
The most common types of fuel in general use are compounds of hydrogen and carbon, sometimes with oxygen, in gaseous, liquid, or solid states. Their premixed flames tend to be blue, owing to the final phase of combustion of carbon monoxide (CO) to carbon dioxide (CO2) and their diffusion flames yellow, luminous, and sometimes smoky, owing to carbon particles in the flame. Free carbon is formed by cracking, a disruption of the fuel molecules at high temperature in the absence of sufficient oxygen for immediate combustion. Highly radiant flames are required in heat-transfer devices, such as boilers and furnaces, but are undesirable in work transfer devices, such as heat engines, where the energy is required to remain in the gas stream rather than be transferred to the walls.
