Inspiration can be found everywhere we look, and if you happen to have been staring at a candle last night you might have been inspired by the intriguing colors of a flame.
The color and temperature of a flame are dependent on the type of fuel and amount of soot present in the combustion. A red flame is the coolest part, then as the temperature rises it changes to orange, yellow, and finally to white, being the hottest part. A blue flame will appear if the concentration of oxygen is high enough and creates enough energy to excite and ionize gas molecules in the flame. It can be seen near the base of candles where airborne soot is less concentrated.
Photo by wikipedia
Different flame types of a Bunsen burner depend on oxygen supply. On the left a rich fuel with no premixed oxygen produces a yellow sooty diffusion flame; on the right a lean fully oxygen premixed flame produces no soot and the flame color is produced by molecular radicals, especially CH and C2 band emission. The purple color is an artifact of the photographic process.
Flame color depends on several factors, the most important typically being blackbody radiation and spectral band emission, with both spectral line emission and spectral line absorption playing smaller roles. In the most common type of flame, hydrocarbon flames, the most important factor determining color is oxygen supply and the extent of fuel-oxygen "pre-mixture", which determines the rate of combustion and thus the temperature and reaction paths, thereby producing different color hues.
Photo by young_einstein
In a laboratory under normal gravity conditions and with a closed oxygen valve, a Bunsen burner burns with yellow flame (also called a safety flame) at around 1,000°C. This is due to incandescence of very fine soot particles that are produced in the flame.
Photo by Jeremy Brooks
With increasing oxygen supply, less blackbody-radiating soot is produced due to a more complete combustion and the reaction creates enough energy to excite and ionize gas molecules in the flame, leading to a blue appearance. The spectrum of a premixed (complete combustion) butane flame on the right shows that the blue color arises specifically due to emission of excited molecular radicals in the flame, which emit most of their light well below ~565 nanometers in the blue and green regions of the visible spectrum.
Flame temperatures of common items include a blow torch at 1,300°C, a candle at 1,400°C, or a much hotter oxyacetylene combustion at 3,000°C. Cyanogen produces an ever-hotter flame with a temperature of over 4525°C (8180°F) when it burns in oxygen.
Photo by matildaben
Generally speaking, the coolest part of a diffusion (incomplete combustion) flame will be red, transitioning to orange, yellow, and white the temperature increases as evidenced by changes in the blackbody radiation spectrum. For a given flame's region, the closer to white on this scale, the hotter that section of the flame is. The transitions are often apparent in TV pictures of fires, in which the color emitted closest to the fuel is white, with an orange section above it, and reddish flames the highest of all. Beyond the red the temperature is too low to sustain combustion, and black soot escapes. A blue-colored flame only emerges when the amount of soot decreases and the blue emissions from excited molecular radicals become dominant, though the blue can often be seen near the base of candles where airborne soot is less concentrated.
From the CL Library
Header photo by Cade
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