flame

Sooty flame

A candle flame in the plate capacitor , which was alternately charged to 25 kV voltage and then discharged again. The flame is deformed in the electric field because it is a plasma and therefore contains free charge carriers that react to an electric field.

Candle flame in weightlessness

Dimethylamine-oxygen flame with different fuel / oxidizer ratios stabilized in low pressure at 40 mbar

As a flame is of, generally as part of a fire outgoing, the area of burning or otherwise exothermically reacting referred to gases and vapors, in the light emitted is.

process

The radiation released during a reaction is caused by the light emission of the molecular bands and the atomic line spectrum of the molecules and atoms involved in the combustion as well as by solid particles and aerosols . Solid particles such as soot or ash emit a radiation spectrum that corresponds to that of a black body at the flame temperature. As far as solid particles are contained in the flame, their thermal radiation predominates.

In most technical applications, the term “flame” refers to the visible reaction of a fuel with the oxidizing oxygen . The reaction area comprises the preheating zone, the reaction zone and the equilibrium zone. The flame forms out of the reaction zone. This usually leads to an intense glow that sharply delimits the reaction area ( flame front ) and can take on different colors.

Flame color

A significant proportion of the flame color is caused by various components in the reaction zone:

yellow to orange: through (glowing) soot particles (size of a few 10 nm). Their emission spectrum corresponds approximately to that of a black body
blue: by excited CO ₂ , CH radicals
turquoise: due to C ₂ molecules.

In contrast, the reaction products of the combustion of hydrocarbons ( CO ₂ and H ₂ O ) tend to radiate in the (invisible) infrared spectral range. If there are any impurities in the flame, the result is intense flame coloration , the color of which depends on the contents. Here the radiation from energetically low-lying resonance transitions (first excited state) mainly contributes to the flame lighting. A particularly simple color change to yellow can be achieved through the sodium content in the table salt . This possibility is used for fireworks that react in all colors of the color wheel.

characterization

There are several ways to characterize a flame. This includes the type of flow ( laminar or turbulent ), the ratio of fuel and oxidizer and whether these are already mixed or not mixed before combustion.

With “premixed flames”, for example, there is a homogeneous mixture of fuel and oxidizer before the combustion process takes place (for example blowtorch and gasoline engine ). In the case of “non-premixed flames”, fuel and oxidizing agent only meet in the reaction zone and react there with one another. The combustion process takes place at the interface where the gases mix (for example, candles , campfires, aircraft turbines and pressure atomizer oil burners ).

In addition, a flame can be described by its fuel-oxidizer ratio. Flames with an excess of fuel are referred to as “fat” flames, while flames with excess oxidizer are “lean” flames. A more precise indication of which mixture is present is made using the equivalence ratio Φ (chemical name) or the air ratio λ (technical name).

A flame with a low proportion of oxygen is referred to as a reducing flame ( luminous flame ). In the pyrolytic combustion reaction, CH radicals are formed from the hydrocarbon molecules . These react (among other reactions) with the oxygen radicals formed to form water vapor. Due to the lack of oxygen ( combustion air ratio ), more carbon monoxide and elemental carbon are formed in the form of soot ; both can be oxidized in the heat through oxygen uptake. The flame has a reducing effect , oxygen-releasing substances held in the flame are reduced (see also borax pearl ). The emissivity of the soot is responsible for the intense glow of the flame, for its yellow color the relatively low combustion temperature (around 1,000–1,200 ° C). The soot from such exhaust gases can soot the inner walls of chimneys or soot the ceiling if candles and oil lamps are used intensively. ${\ displaystyle \ lambda <1}$

Oxidation flames contain excess oxygen. In the combustion reaction, the bound carbon of the hydrocarbons (from CC and CH bonds) is oxidized to carbon oxides . Oxidation flames are hotter than reduction flames. Because of the low proportion of soot, they only shine weakly and, due to chemiluminescence, have blue flame fronts (chemiluminescence of CH radicals near 314, 390 and 431 nm, of OH radicals with a peak near 309 nm, of CO ₂ radicals around 415 nm and C ₂ near 510 nm).

A flash of flame arises as soon as an oxidizable, pressurized gas mixture can suddenly combine with oxygen. The activation energy of this reaction must be achieved by an external ignition source, especially if the ignition temperature of the respective reaction mixture is exceeded.

Trivia

The hottest flame to date is produced by a reaction between dicyanoethine and ozone at 40 bar pressure and reaches a flame temperature of around 6000 ° C. The theoretical combustion temperatures of hydrocarbons with air are around 2000 ° C. Such temperatures, which are possible under ideal conditions, are by no means reached in everyday flames, since the gas cools down during combustion due to the radiation emission. Very hot flames also occur in the launch vehicles for space satellites from.

The derived meaning "flame" has been used metaphorically since the 18th century for a girl with whom one is in love and for whom one is therefore inflamed. Compare also the lyrics to the lyrics No fire, no coal can burn so hot, // As secretly silent love that nobody knows anything about. (Folk song, 18th century.)

The heat of a flame can be estimated with the help of "spectral glasses" (a children's toy): The spectral glasses break down the light rays of the flame color into their spectral components, and the temperature can be deduced from the size of the light spots

Web links

Wikiquote: Flame - Quotes

Wiktionary: Flamme - explanations of meanings, word origins, synonyms, translations

Commons : Flame - collection of images, videos and audio files

Norbert Peters: Technical Combustion - Lecture reprint, RWTH Aachen University (PDF; 3.6 MB)

Individual evidence

↑ Jürgen Warnatz, Ulrich Maas, Robert W. Dibble: Combustion - Physico-chemical basics, modeling and simulation, experiments, generation of pollutants. Springer, Berlin / Heidelberg 2001, ISBN 978-3-540-42128-3 , doi : 10.1007 / 978-3-642-56451-2 .
↑ Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, (full text)
↑ Johannes Eichmeier: Combined combustion of fuels mixed inside the combustion chamber with different ignitabilities investigated using the example of diesel and gasoline. Logos Verlag Berlin GmbH, 2012, ISBN 978-3-832-53172-0 , p. 59 ( limited preview in the Google book search).
↑ Maurizio De Leo, Alexei Saveliev, Lawrence A. Kennedy, Serguei A. Zelepouga: OH and CH luminescence in opposed flow methane oxy-flames. Another important sources of chemiluminescence; , 2007, cited by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)
↑ Madleine M. Kopp, Olivier Mathieu, Eric L. Petersen: Rate Determination of the CO2 * Chemiluminescence Reaction CO + O + M <--> CO2 * + M , 2014, cited by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)
↑ Eric Petersen, Madleine Kopp, Nicole Donato: Assessment of Current Chemiluminescence Kinetics Models at Engine Conditions , 2011, quoted by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)
↑ Zoltán Faragó: Stoking the fireplace properly - looking at flames through spectral glasses

[1] Jürgen Warnatz, Ulrich Maas, Robert W. Dibble: Combustion - Physico-chemical basics, modeling and simulation, experiments, generation of pollutants. Springer, Berlin / Heidelberg 2001, ISBN 978-3-540-42128-3 , doi : 10.1007 / 978-3-642-56451-2 .

[2] Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, (full text)

[3] Johannes Eichmeier: Combined combustion of fuels mixed inside the combustion chamber with different ignitabilities investigated using the example of diesel and gasoline. Logos Verlag Berlin GmbH, 2012, ISBN 978-3-832-53172-0 , p. 59 ( limited preview in the Google book search).

[4] Maurizio De Leo, Alexei Saveliev, Lawrence A. Kennedy, Serguei A. Zelepouga: OH and CH luminescence in opposed flow methane oxy-flames. Another important sources of chemiluminescence; , 2007, cited by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)

[5] Madleine M. Kopp, Olivier Mathieu, Eric L. Petersen: Rate Determination of the CO2 * Chemiluminescence Reaction CO + O + M <--> CO2 * + M , 2014, cited by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)

[6] Eric Petersen, Madleine Kopp, Nicole Donato: Assessment of Current Chemiluminescence Kinetics Models at Engine Conditions , 2011, quoted by Krzysztof Adam Grabinski: Experimental and numerical kinetic study on charged and excited species in oxyfuel combustion for CO2 capture , Norwegian University of Science and Technology, 2016, page 14 (full text)

[7] Zoltán Faragó: Stoking the fireplace properly - looking at flames through spectral glasses