A flame is the visible, gaseous part of a fire, resulting from a rapid chemical reaction that releases heat and light. This process, known as combustion, involves the rapid oxidation of a fuel.
Understanding Flame Zones
A flame is not uniform; instead, it consists of distinct regions or zones, each with unique characteristics. A typical flame, such as that from a candle, displays an inner, a middle, and an outer zone. The innermost zone, often appearing dark or black, is found directly around the wick and contains unburnt fuel vapors.
Surrounding this inner region is the yellow middle zone, the brightest part of the flame, due to hot, glowing soot particles. The outermost zone is a faint blue.
Why Flame Temperatures Vary
The hottest part of a flame is the outer blue zone, where temperatures can reach 1,400 °C (2,552 °F) to 1,600 °C (2,912 °F). This region experiences complete combustion due to an optimal oxygen supply from the surrounding air. Complete combustion efficiently reacts fuel with oxygen, producing carbon dioxide and water vapor, and releasing maximum heat.
In contrast, the yellow middle zone is cooler, with temperatures around 1,200 °C (2,192 °F), and the innermost dark zone is the least hot, at approximately 1,000 °C (1,832 °F). The yellow color indicates incomplete combustion, which occurs due to a limited oxygen supply within that region. This incomplete process results in the formation of unburnt carbon particles, or soot, which glow incandescently, giving the flame its characteristic yellow appearance.
Flame Color and Heat
Flame color indicates its temperature. As temperature increases, color shifts from red to orange, then yellow, and finally to blue or bluish-white. Blue flames are the hottest, indicating efficient combustion, while red flames are cooler, typically 600 °C to 800 °C.
The visible light emitted by a flame is primarily a result of blackbody radiation and spectral band emission. Hot soot particles in a flame emit light across a spectrum, with cooler particles appearing red or orange and hotter ones shifting towards yellow and white. Blue light, however, often comes from the emission of excited molecular radicals within the flame, which are present during more complete combustion.
What Influences Overall Flame Temperature
Beyond the internal zones, several external factors can significantly influence a flame’s overall temperature. The type of fuel used plays a major role, as different fuels possess varying chemical energy content. For example, acetylene burns at higher temperatures than methane, reaching approximately 3,100 °C in oxygen compared to methane’s 2,810 °C in oxygen.
Oxygen supply is another determinant of flame temperature. An optimal supply leads to a hotter flame, while a limited supply results in a cooler one. Impurities in the fuel or the surrounding air, such as nitrogen, can also lower flame temperature by absorbing heat without contributing to the combustion process.