The flame zone of a premixed flame may be less than 1 mm thick. As the volume of the hot burnt gas is greater than that of the same mass of cold unburnt gas, the flame front is pushed outwards from the ignition point, like the skin of an inflating balloon. Not every mixture of air and fuel will burn.
Depending on the type of fuel and oxidant involved air or pure oxygen, for example , a mixture initially at room temperature and pressure will only burn if the concentration of fuel lies between certain well-defined limits, called flammability limits. The figures quoted for limits of flammability may vary as a number of factors may slightly alter the value: pressure, temperature, dimensions of the test apparatus, direction of flame propagation and moisture content of the mixture all have some effect.
In general, the range between limits widens with increased temperature. For each mixture of fuel and air between the flammability limits, there is a characteristic burning velocity at which a premixed flame will propagate through a stationary gas.
Burning velocity is dictated by the chemical processes involved: how quickly the fuel reacts with the oxygen. The methane and oxygen molecules do not simply combine instantaneously to form carbon dioxide and water vapour, but form free radicals and intermediates such as formaldehyde and carbon monoxide along the way to completing the reaction.
If the premixture flows into a flame with a laminar flow whose velocity is equal to the burning velocity of the mixture, the flame can be held stationary.
This is how premixed flames on Bunsen burners, domestic gas rings, etc. Local air currents and turbulence caused by obstacles can cause a flame to move at speeds much higher than the burning velocity. The speed at which a flame moves relative to an observer is the flame speed, which is different to the burning velocity. For example, the burning velocity of a methane-air flame is about 0.
Thus, consideration of laminar flame shapes is undertaken in the following, emphasizing conditions where effects of gravity are small, due to the importance of such conditions to practical applications.
Another class of interesting properties of laminar diffusion flames are their laminar soot and smoke point properties i. These are useful observable soot properties of nonpremixed flames because they provide a convenient means to rate several aspects of flame sooting properties: the relative propensity of various fuels to produce soot in flames; the relative effects of fuel structure, fuel dilution, flame temperature and ambient pressure on the soot appearance and emission properties of flames; the relative levels of continuum radiation from soot in flames; and effects of the intrusion of gravity or buoyant motion on emissions of soot from flames.
An important motivation to define conditions for soot emissions is that observations of laminar jet diffusion flames in critical environments, e. Another important motivation to define conditions where soot is present in diffusion flames is that flame chemistry, transport and radiation properties are vastly simplified when soot is absent, making such flames far more tractable for detailed numerical simulations than corresponding soot-containing flames.
In order to fix ideas here, maximum luminous flame boundaries at the laminar smoke point conditions were sought, i. Document ID. Document Type. Dai, Z. Michigan Univ. The brightness of the flame on the left comes from chains of microscopic soot particles numerous enough to emit a significant amount of heat; they incandesce like so many tiny light-bulb filaments.
To obtain the dimmer image on the right, the camera aperture had to be left open 30 times longer. Photographs courtesy of the authors. Combustion requires fuel, which in the cases of interest is most often made from long, complex chains of carbon and hydrogen atoms. When a flame is lit, the heat breaks apart these hydrocarbons in a process called pyrolysis.
The smaller chunks that result are often radicals, highly inclined toward chemical reactions, in particular oxidation: Oxygen combines with the carbon and hydrogen radicals to produce carbon dioxide and water, releasing heat in the process. However, some of the radicals react with one another, rather than with oxygen, forming rings of carbon called polycyclic aromatic hydrocarbons.
These newly formed compounds continue to grow into carbon-rich lattices and then into full-fledged particles, which agglomerate into long chains that resemble strings of beads. As these soot masses travel upward inside a flame, they react with oxygen molecules, which can break off pieces and cause the particles to incandesce more brightly, creating the flame's bright yellow glow.
Whether or not the soot will be fully burned in this way completely transformed into carbon dioxide and water before leaving the flame depends on the type of fire.
If not completely burned, the residual soot is released as smoke. Tall, buoyant flames with relatively lazy mixing of the fuel and air promote the formation of soot, because there is ample residence time within these flames for fuel molecules to pyrolyze and then recombine.
The larger hydrocarbon compounds made in this way eventually come together and form carbonaceous particles. The thermal radiation given off by this soot is important to sustaining and initially growing pool fires. Indeed, the presence of soot allows up to 50 percent of the energy in a flame to be transferred back to the pool of fuel, where it acts to vaporize the liquid and sustain the reaction.
In large fires, thermal radiation from the flames provides almost all of the energy transferred to the liquid fuel. Ironically, it is the radiant emission given off by the hot soot that results in the thermal quenching of flames at the top and around the periphery of the fire, leading to the release of the cool, light-absorbing soot. The radiant heat transfer exceeds kilowatts per square meter for pools 2 meters across but is less for larger pools, which tend to produce thick layers of smoke around the fire.
In comparison, solar radiation on a clear, dry day with the sun directly overhead is 1 kilowatt per square meter. As important as they are in accidental fires, soot particles also influence people's lives in other ways. Commercial combustion products, called carbon blacks, have long been ingredients in rubbers and inks, and soot was the original source of the soccer-ball-shaped carbon molecules called fullerenes, as well as carbon nanotubes, both of which are of great interest in the field of nanotechnology.
Soot plays the benefactor in boilers and furnaces that rely on flame radiation to transfer heat to the walls to generate steam, for example , but the same mechanism makes these particles harmful for internal-combustion engines, where such heat losses decrease efficiency and require that high-temperature materials be used.
Soot emissions to the atmosphere from industrial smokestacks and automobile tailpipes contribute to the formation of light-absorbing haze and have recently received attention for their influence on climate. This connection arises because the more sunlight that is reflected back out into space, the cooler the Earth becomes, whereas the more sunlight absorbed in the lower atmosphere or on the surface, the more the planet is warmed.
Hence the effect of soot particles on climate strongly depends on their optical properties, specifically how they scatter and absorb sunlight. Most climatologists believe that atmospheric soot enhances global warming, but some studies have suggested that soot can have a cooling effect, too, because these particles can act as nucleation points for molecules of water vapor, allowing more sunlight-reflective clouds to form. Soot also influences climate because it is deposited on snow and ice surfaces, darkening them and thus increasing their rate of melting.
The absorptivity of cold soot is important in determining not only how effectively it takes in light, but also how effectively hot soot emits light, by virtue of a principle called Kirchoff's law: The emissivity of a substance at thermal equilibrium is always equal to its absorptivity. Quantifying the optical properties of soot has proven to be difficult, because it is such a heterogeneous substance. Its atomic bonding is dependent on the environment in which it is formed, thereby affecting its optical properties.
Just think of the contrast in optical properties between diamond and graphite, both pure forms of carbon. Indeed, the quest for a better understanding of this subject has quite a long history.
An appreciation of soot's optical properties began, appropriately, in London early in the industrial age, when the city was choked with smoke from the widespread burning of coal. It was in this era that Faraday carried out research on flames.
He soon understood the importance of the optical properties of soot. Faraday's knowledge of this subject was built on the first extensive studies of combustion initiated by his mentor at the Royal Institution, Sir Humphry Davy, when Davy was developing the coal miner's safety lamp in Faraday was a popular lecturer, and for many years during the Christmas holidays of the mids, he gave a lecture series for young audiences titled "The Chemical History of a Candle," in which he demonstrated that a flame without soot produces a very weak glow.
Faraday clearly saw the irony: "Is it not beautiful to think that such a process is going on, and that such a dirty thing as charcoal can become so incandescent? Figure 3. British physicist Michael Faraday was one of the first scientists to appreciate the role of soot in flames.
A popular lecturer, Faraday often gave talks to packed audiences in which he would perform dramatic demonstrations, as depicted in this lithograph from One series of lectures that Faraday gave for many years was on the chemistry of a burning candle. He was able to show his audience that a flame without soot produces a very dim glow. Faraday concluded, "It is to this presence of solid particles in the candle-flame that it owes its brilliancy.
After Faraday, the optical properties of soot did not receive much attention until the early 20th century, when combustion-related research began to develop as an active field. A number of German investigators examined the radiation properties of soot in an attempt to determine flame temperature and to understand the contribution these particles made to the transfer of heat in furnaces and boilers. In the United States in the s, Hoyt C. Hottel, a professor researching heat transfer in furnaces at the Massachusetts Institute of Technology, improved on the optical pyrometer, an instrument much like a thermometer except that it measures temperature without contacting the surface.
The device uses a filter to determine the amount of energy being radiated from a sample in a single wavelength, or color, which is compared with a table of known temperature-color relations. Hottel developed the first two-color device for measuring the temperature of soot and demonstrated its superiority to conventional single-color methods. The advantage is that one can determine temperature from the ratio of the two light intensities, whereas a single-color instrument needs to measure absolute amounts and is thus difficult to calibrate.
Later on, Hottel measured the angular distribution of light scattering by soot inside a flame and in the early s collected some of the first images of soot particles by transmission electron microscopy TEM , showing a highly agglomerated structure of primary particles about 25 nanometers in diameter.
At about the same time, Roger C. Millikan, a researcher at General Electric, measured the spectral variation of soot absorption above a horizontally flat flame and also analyzed the atomic ratio of hydrogen to carbon in the soot. He found that with increasing height above the burner, this ratio decreased, as did the spectral variation of optical absorption.
This observation gave the first concrete evidence that the absorption properties of soot varied with position within a flame and were linked to the chemical composition of the soot. In , Adel F. Sarofim, a former graduate student of Hottel's who joined him on the faculty of MIT, published the results of measurements of the index of refraction of soot collected from propane- and acetylene-fueled flames.
The index of refraction is the Holy Grail of material optical properties, allowing the calculation of both absorption and scattering of various shapes of the material, including round particles of a known size. The index is a measurement of the path that light will take within a particular material. The value is expressed as a complex number, with the real component relating to the velocity of light within the material relative to the speed of light in vacuum.
This component also indicates how much a ray of light will be bent, or refracted, when it passes through the material. The imaginary term indicates how much of the light is lost due to absorption. Sarofim reported the index of refraction of the soot he measured to be 1. This result was an improvement over past attempts to measure the optical properties of soot, and Sarofim had the convenient result of finding a similar refractive index for both fuel sources and for a range of visible wavelengths.
Also, a number of succeeding determinations produced values close to Sarofim's. As a consequence, his value for the refractive index was widely adopted in both combustion- and fire-research communities as the refractive index of soot, and it has largely retained that status all the way to the present.
A few investigators pointed out long ago that the method Sarofim and others had used to determine the index of refraction was subject to significant errors. The technique required soot aggregates to be pressed into a smooth surface, a virtually impossible task to perform with hard, agglomerated nanoparticles.
In , Jay Janzen from Phillips Petroleum Company made the prescient observation that not only were there unavoidable errors in this approach, but that the derived refractive-index values were inconsistent with well-established measurements of the amount of light a given volume of soot particles extinguish through absorption and, to a lesser extent, scattering.
Janzen performed measurements of the spectral extinction of small, uniform particles of carbon black and calculated a refractive index that was quite different 2. Unfortunately, this work was published in a journal that was not typically read by combustion and fire researchers, so it went unnoticed.
For several years now, we have been investigating large-scale pool fires, both experimentally and numerically, because of the risk they pose during transport accidents. In the course of developing and validating our computational models, we've acquired a keener appreciation for soot in determining how fires spread. In particular, knowledge of the soot concentration, temperature and optical properties within these fires is required to quantify the amount of heat transferred. Figure 4. Candles and oil lamps are called diffusion flames because the air and fuel are not mixed before burning but rather meet and react through diffusion.
In contrast, premixed flames, such as those in gasoline engines, have the fuel and air combined before ignition. Diffusion flames have distinct regions where different chemical reactions take place, as shown in this schematic by different colors.
The surface that encloses the entire flame, the flame sheet, is the location of highest temperature. Fuel, made of complex chains of hydrocarbons, first enters the fuel-pyrolysis zone, where it is heated to high temperatures and decomposes into fragments. Some of the fragments combine to form larger molecules, particularly carbon-ring structures. These molecules then flow into the soot-inception zone, where they grow in size and polymerize.
These chains form liquid-like soot-precursor particles gray spheres , which have no internal structure and do not absorb or emit visible light. But as these precursor particles continue into the soot-growth zone, they give up H 2 gas and form solid, light-absorbing particles, which rapidly agglomerate into clusters.
As the soot aggregates are driven to the top of the flame, the soot-oxidation zone, they are consumed by reactions with oxygen and hydroxyl radicals, shrinking in size as they give off energy. The final combustion products, emitted from the flame sheet, are water and carbon dioxide.
Over the past 15 years, several investigators have shown through TEM and optical measurements that when soot begins to form in a flame, it first develops liquid-like precursor particles, which have little absorptivity.
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