A Staff Report from the Straight Dope Science Advisory Board

What is the hottest part of a flame?

April 27, 2004

Dear Straight Dope:

OK, Uncle Cecil, we need a bet settled. The other night while trying to light a fire in the backyard, my wife claimed that the hottest part of her lighter flame was the blue part close to the lighter. I corrected her, letting her know the tip of the flame is the hottest. I distinctly remember being told that the closer (brighter?) the flame is to white, the hotter it is. Please settle this bet so I can avoid doing laundry for two months.

Two problems, Damien. First, Uncle Cecil is busy, so you don't have him answering your question, you've got me. Second, you wrote this question in 2001. That laundry must be really piling up.

I'll try to make this answer worth the wait. Analysis of a flame can be quite complex. Coal and solid fuel flames (my specialty) are the most complex, followed in difficulty by liquid fuel flames, and finally gaseous fuel flames. I'll assume that for your backyard experiments with the wife you're using a butane or propane lighter with a simple gas flame. That means we're dealing with a premixed laminar flame - in lay terms, "a flame that has its fuel mixed with air before burning and a relatively steady shape."

Although a lighter or candle flame appears to be a solid mass of light, it's actually hollow - the luminous outer layer is typically less than 1 mm thick. The core of the flame consists of the fuel gas and air pushing steadily outwards in the "flame" shape until they reach the thin combustion zone. The hottest portion of the flame typically is in and immediately outside this zone, which is filled with the immediate products and partial-products of the chemical reaction known as combustion.

Which part of the combustion zone is the hottest? Peak temperatures are more uniform than you might expect along the length of the flame. Measurements of a 79 mm methane flame, a 107 mm methane flame, and an 88 mm ethylene flame (see Santoro below) all generally showed slightly higher temperatures (by 20-50 degrees K) near the base. Pitt's work cited below shows substantially the same thing and has some nice graphs of temperature versus height along the flame.

Not to be outdone by Pitt, I took Santoro's measurements of a symmetrical methane flame, which were available in a spreadsheet file on the National Institute of Standards and Technology website, and generated a graph showing the temperature versus the distance from the centerline of the flame. Several temperature curves are shown, measured at different heights above the flame's base. The magenta curve corresponds to a level near the base of the flame, and the light blue curve corresponds to a level near the tip. You can see from the graph that the peak temperatures at the top of the flame are slightly lower than those at the middle and base.

Temperature is only part of the equation, though. More relevant if you're trying to light a fire is the total heat available at different spots in the flame. That's a function of the volume of fuel and air and the temperature. In graphical terms, you're looking for the part of the flame with the most area under the curve. In a typical flame that's near the tip - look at the area under the curve of the 70 mm line compared to the other lines. Why is there more heat in the tip? Because the non-burning center of the lower part of the flame is relatively cool, whereas all of the tip is aflame and thus uniformly hot throughout. We'll come back to this in a moment.

Apart from incombustible elements, the color of a hydrocarbon flame is primarily dependent on the richness of the flame - that is, on how much oxygen there is to combust the fuel. In practice*, when the mixture is slightly lean (has more oxygen than required for complete combustion), the color of the combustion zone is generally blue-violet due to large amounts of high-energy radical carbon and hydrogen compounds. When the mixture is slightly rich (slightly too much fuel and not enough oxygen), the color is sometimes green due to C2 molecules breaking free, and the high-temperature products can glow red from the CO2 and H2O produced during combustion. When the mixture is very fuel rich (a poor flame, with not enough oxygen to burn properly), carbon particles form and an intense yellow radiation results from their being heated in the flame. In very rich flames - often you see this in candles - soot particles may impart a black color to the outer edge of the yellow flame. Flame color comes from the energy released by the electrons of the atoms of burning gas as they are raised to higher energy states during combustion, then fall back to lower energy states. Some of this energy is released in the form of visible light. The color corresponds to frequency, which is a function of the amount of energy released. (Work with me on this.) Low energy, low-frequency light is red; medium-frequency, medium-energy light is orange, yellow, or green; and high-energy, high-frequency light is blue or violet. If the energy levels are spread over a wide range of the visible spectrum, the light will appear as white.

Provided your backyard lighter flame is free of contaminants that might skew the color, a slightly lean violet-blue flame is the hottest.  Blue-violet = high frequency = high energy = high temperature. A white flame has its visible radiation energy spread out more evenly across the spectrum and isn't peaking on the high-energy blue end. That indicates lower overall energy,  and thus lower temperature, than a blue flame.

What I'm telling you is that your wife is technically right. The blue part of the flame is the hottest, not the white part. But a manly man is more concerned about the real world - as we saw above (show her the graph), there's more total heat in the tip of the flame, so that's the part you want to use to light the fire. In other words, while she's got a better grasp of theory, you're going to get the fire lit sooner. I'm not saying you won't get an argument as far as your bet goes. But put it to her this way: The price of clean laundry is raw steak.

* Many thermodynamics and chemistry texts state that adiabatic flame temperature is highest when the flame is at perfect stoichiometry (exactly enough air to burn the fuel). Since mixing and other practical effects require extra air to ensure combustion, the hottest flames in practice tend to be slightly lean (slightly more oxygen than needed).

References

Pitts, William M., "Thin-Filament Pyrometry in Flickering Laminar Diffusion Flames,"  Twenty-Sixth Symposium (International) on Combustion, Combustion Institute (1996), pp.1171-1179

Metcalfe, H. Clark; Williams, John E.; Castka, Joseph F.; Modern Chemistry (Teacher's Edition), 1978

Kuo, Kenneth Kuan-yun. Principles of Combustion, 1986.

Puri, R.; M. Moser, R.J. Santoro, and K.C. Smyth, Twenty-Fourth Symposium (International) on Combustion, pp. 1015-1022 (1992).

Puri, R.; Santoro, R.J.; and Smyth, K.C.; Combustion and Flame, 97:125-144 (1994).

R. C. Weast, editor, CRC Handbook of Chemistry and Physics, 67th edition, 1983.

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