A flame is the visible, gaseous part of a fire, representing a rapid chemical process called combustion. This process involves a fuel reacting with an oxidizer, typically oxygen, which releases energy as heat and light. The flame is a self-sustaining exothermic reaction, generating enough heat to continually vaporize more fuel, thus sustaining the reaction.
The color and structure of this glowing gas result directly from the combustion’s efficiency and temperature. Understanding the different regions within a flame reveals why the temperature varies significantly.
The Three Zones of a Typical Flame
A standard flame, such as one from a candle, is classified as a diffusion flame and exhibits three distinct zones.
The innermost zone, known as the dark zone, is the least hot and is located immediately surrounding the wick. This area consists primarily of hot, unburnt fuel vapor but contains almost no oxygen to support combustion.
The middle zone is the luminous zone, recognizable by its bright yellow or orange color. Here, partial combustion begins due to limited oxygen supply. The heat causes fuel molecules to break down, forming small, incandescent carbon particles (soot) that glow brightly and give the zone its luminosity.
The outermost zone is the non-luminous zone, forming a thin, often barely visible mantle. This is where the flame interacts directly with the surrounding atmosphere. This region is often blue or faintly colored and is distinct from the brighter yellow middle section.
Locating the Maximum Temperature
The hottest part of a typical diffusion flame is located in the outermost, non-luminous zone. This is the boundary where the gaseous fuel mixture meets the maximum supply of oxygen. For a common candle flame, the temperature in this outer section can reach 1,400 degrees Celsius (about 2,550 degrees Fahrenheit).
The middle, luminous zone is moderately hot, reaching temperatures around 1,200 degrees Celsius. The innermost dark zone remains the coolest part of the flame, with temperatures around 1,000 degrees Celsius, because it contains mostly uncombusted fuel vapors. The non-luminous blue color indicates higher heat than the bright yellow.
Why Combustion Efficiency Creates Temperature Differences
The temperature gradient across the flame is determined by the efficiency of the combustion reaction in each zone. Chemical energy stored in the fuel is released most effectively during complete combustion, which requires a sufficient amount of oxygen.
The outer zone is hottest because it has unrestricted access to atmospheric oxygen. This allows the fuel to be fully oxidized into carbon dioxide and water vapor, releasing the maximum amount of thermal energy.
In the middle zone, the limited oxygen supply prevents complete combustion, resulting in an incomplete reaction. This less efficient reaction produces byproducts like carbon monoxide and unburnt carbon particles, which are the source of the yellow light and soot. Since the fuel is not completely oxidized, less energy is released as heat, making this zone cooler. The innermost dark zone is the coolest because it is fuel-rich but oxygen-starved, and combustion has not yet begun.