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Contents
  1. 1Definition and comparison with diffusion flames
  2. 2Structure of a premixed flame
  3. 3Burning velocity
  4. 4Flame propagation in ducts and enclosures
  5. 5Premixed flames in fires and explosions
  6. 6Limits of flammability and stability
  7. 7See also
  8. 8Notes
  9. 9References

A premixed flame is a flame in which the fuel gas and the air, or another oxidant, are mixed before the mixture reaches the flame and is ignited.[1][2] A gas cooker, a Bunsen burner and the cylinder of a petrol engine burn premixed flames.[1][3] Because fuel and oxygen are already mixed, the flame does not have to wait for them to meet.[2] It moves into the unburnt mixture at a characteristic speed, the burning velocity.[4]

Most unwanted fires burn as diffusion flames, in which fuel and air only mix at the flame.[3] Premixed flames still matter in fire safety engineering. They govern the ignition of flammable vapours above a liquid or a heated solid, gas and dust explosions, backdrafts and smoke gas explosions.[5][6][7] A premixed flame can only exist between the flammability limits of the mixture.[8] In a closed space, it can raise the pressure to about seven to nine times the initial value.[5][9]

Definition and comparison with diffusion flames

Flames are the visible part of a combustion process, in which gaseous fuel reacts with oxygen. The fuel gas is either premixed with air at the source, or it mixes with air at the flame by diffusion or turbulence.[1] Introduction to Fire Safety Engineering defines the premixed flame as a flame whose air and fuel mixture is already mixed before it is ignited.[1] NUREG-1805 describes a flame in which oxygen is mixed with the combustible gas by a mechanical device before combustion.[10]

The difference from a diffusion flame can be expressed in time scales. In a premixed flame, fuel and oxygen are mixed before they reach the combustion zone, so the times for transport and for reaction are of similar size.[2] In a diffusion flame, transporting fuel and oxygen to the reaction zone takes much longer than the chemical reaction itself.[2] A premixed flame can therefore travel through a flammable mixture on its own.[11] A diffusion flame, by contrast, stays in the zone where fuel and oxygen meet from opposite sides.[10]

Turbulent premixed flames occur mostly in engineered combustion systems. Examples are boilers, furnaces, process heaters, gas burners, oxyacetylene torches, petrol engines and domestic gas cookers.[3] In such a burner, air or pure oxygen is mixed with the gas at the base, so that the fuel is consumed almost instantly at the mouth of the burner.[12] The hottest part of a Bunsen burner flame reaches about 1,600 °C. An oxyacetylene cutting torch reaches about 3,500 °C, because of its higher oxygen content.[12]

The two flame types also occur together. If the gas mixture in a Bunsen burner contains too little oxygen to burn all of the fuel, the outer sheath of the flame is essentially a diffusion flame.[4] Conversely, a jet of fuel gas burning as a diffusion flame is anchored by a small pocket of premixed gas just above the burner rim.[13]

Structure of a premixed flame

A Bunsen burner flame has a luminous inner cone, surrounded by an outer sheath of hot combustion gases.[4] The inner cone marks the end of the primary reaction zone, where a fast oxidation reaction takes place and the temperature rises very rapidly.[4] In a lean mixture, the outer sheath is indistinct, because the mixture already contains enough oxygen to burn the fuel completely.[4]

Bunsen burner: gas and air enter at the base and mix in the tube; above it a sharp inner cone and a larger outer sheath of hot gases.
Bunsen burner flame: fuel gas and air mix in the tube; the inner cone marks the end of the primary reaction zone, surrounded by an outer sheath of hot combustion gases. Based on Simmons (2002), p. 1-144, and Iqbal and Salley (2004), p. 3-9.

Measurements through a flat laminar flame show several distinct zones. In the preheating zone, the gas is heated by conduction from the hotter parts of the flame. Chemical reaction does not start until the temperature reaches about 700 K.[14] In the primary reaction zone, free radicals diffusing back from the hot gas attack the fuel molecules. A rapid build-up of radicals then produces the luminous flame front.[14][15] In the secondary reaction zone, most of the carbon monoxide is converted to carbon dioxide and the temperature rises further, more slowly, to its final value.[15]

The highest temperature is usually reached a little downstream of the inner cone. In a large flame, the calculated adiabatic flame temperature is nearly reached in the middle of the gas stream.[4] In a burner flame of 7.5 % natural gas in air, the highest measured temperature contour is 1,755 °C.[16] Small premixed flames and turbulent flames of all kinds normally fall short of the adiabatic value, because they lose appreciable heat.[16]

The chemistry is complex. Detailed calculations of a one-dimensional laminar flame typically involve more than 120 elementary reaction steps, even for simple hydrocarbon fuels.[15] For engineering use, such schemes are reduced to a few global steps. A three-step mechanism, for example, represents a stoichiometric methane and air flame well.[15]

Burning velocity

The key property of a premixed flame is its burning velocity. Simmons defines it as the velocity with which a premixed flame moves, normal to its surface, through the adjacent unburnt mixture.[4] A flat flame on a burner is only stable when the flow velocity of the incoming mixture equals the burning velocity. If the flow is too fast, the flame is blown off; if it is too slow, the flame flashes back into the burner.[4] A conical flame, like that of a Bunsen burner, is stable over a much wider range of flows. The area of its cone adjusts to keep the burning velocity and the flow velocity normal to the flame front in balance.[4]

Three panels of a flat flame above a burner: stable when burning velocity equals gas velocity, blown off when slower, flashing back when faster.
A flat premixed flame is stable when the burning velocity Su equals the gas velocity v0; it blows off when Su is smaller and flashes back when Su is larger. Based on Simmons (2002), p. 1-144 (Figure 1-9.1, redraw, do not copy).

The burning velocity depends on the fuel, the oxidant and the composition of the mixture. All saturated hydrocarbons have about the same maximum burning velocity in air.[11] For propane and air, most measured maximum values lie at 41 ± 2 cm/s.[11] Hydrogen and acetylene burn much faster. Burning in pure oxygen is faster still, for example 360 cm/s for propane instead of 41 cm/s in air.[11]

Bar chart on a logarithmic scale: burning velocities from 41 cm/s for propane in air to 1,180 cm/s for hydrogen in oxygen.
Maximum laminar burning velocities of fuel and air mixtures (blue) and fuel and oxygen mixtures (orange) at room temperature and atmospheric pressure. Based on Simmons (2002), p. 1-146.
Maximum laminar burning velocities at room temperature and atmospheric pressure[11]
Mixture Maximum burning velocity (cm/s)
Propane and air 41
Ethene and air 68
Acetylene and air 175
Hydrogen and air 320
Propane and oxygen 360
Acetylene and oxygen 1,120
Hydrogen and oxygen 1,180

Zalosh lists slightly different laminar burning velocities in air, for example 40 cm/s for methane, 46 cm/s for propane, 166 cm/s for acetylene and 312 cm/s for hydrogen.[9] Hotter flames burn faster. Raising the initial temperature of a propane and air mixture from 300 to 480 K doubles its maximum burning velocity.[11] Inert gases such as nitrogen, argon and carbon dioxide lower the flame temperature. If the burning velocity falls far enough, the flame is extinguished.[15]

Turbulence increases the speed of a premixed flame. In laminar flow, the flame front is sharply defined. At higher Reynolds numbers it becomes a blurred "brush", because local velocity fluctuations wrinkle the front and increase its area.[11] Flow in a tube is always laminar below a Reynolds number of 2,300 and usually turbulent above 3,200. For propane, ethene and acetylene flames, the burning velocity roughly doubles at a Reynolds number of 40,000. Theory suggests an increase of up to five times in very intense turbulence.[11]

Flame propagation in ducts and enclosures

When the flow is too slow to hold a stationary flame, a premixed flame travels through the mixture, provided that its composition lies within the limits of flammability.[11] In a duct, the area of the flame front is larger than the cross-section of the duct. The visible flame therefore moves faster than the burning velocity.[5] Simmons relates the two through the ratio of the areas:[5]

Duct section with a curved flame front whose area is two to three times the duct cross-section, moving into unburnt mixture.
A premixed flame in a duct: the curved flame front has two to three times the area of the duct section, so it moves along the duct two to three times faster than the burning velocity. Based on Simmons (2002), p. 1-147.

V=SuAfAd

Symbols in the flame speed relation
Symbol Description Unit
V linear speed of the flame along the duct m/s
Su burning velocity of the mixture m/s
Af area of the flame front m²
Ad cross-sectional area of the duct m²

The relation applies to a flame that moves along a duct away from an open end at a fairly uniform speed. The flame front area is always larger than the duct section, typically by a factor of two or three.[5] For a propane and air mixture with a burning velocity of 0.41 m/s, the flame therefore moves along the duct at about 0.41 × 2 = 0.82 m/s to 0.41 × 3 = 1.2 m/s.[11][5] For hydrogen and air, at 3.2 m/s, the same factors give about 6.4 to 9.6 m/s. Simmons stresses that this difference between flame speed and burning velocity matters in the design of automatic protection.[5]

If the hot products cannot escape, they push the flame and the unburnt gas forward with increasing speed. This creates turbulence ahead of the flame, which increases the burning velocity further.[5] In a long duct the flame front can then act as an accelerating piston. A shock wave forms, and the flame becomes a detonation that travels at 1,500 to 3,000 m/s.[5] Behind a detonation, the pressure can reach 20 bar for gases initially at atmospheric pressure, and up to 100 bar where it is reflected from the end of the duct.[5]

A premixed flame that propagates more slowly than sound is called a deflagration. NUREG-1805 gives the speed of sound in air at 20 °C, about 330 m/s, as the upper bound for a deflagration and about 2,000 m/s for a detonation.[17] A flame can also be quenched. If it tries to pass through a gap that is too small, heat loss to the walls and the removal of free radicals stop it. Flame traps use this effect.[5]

Premixed flames in fires and explosions

Even fires that later burn as diffusion flames usually begin with a premixed flame. Simmons notes that the initial ignition of a liquid or solid fuel involves a mixture of fuel vapour and air in the boundary layer above its surface.[5] The flash point of a liquid is the temperature at which its vapour pressure just gives a lean limit mixture with air. A somewhat higher temperature, the fire point, is needed before the fuel keeps burning.[5]

Premixed burning is the mechanism behind gas and vapour explosions. Combustible gases, vapours and dusts present an explosion hazard when they are mixed with air in proportions between the lower and upper flammable limits.[18] The hazard lies in the premixing of fuel and oxidant in a confined space before an ignition source appears. On ignition, the flame spreads through the mixture, and the confinement prevents the free expansion of the hot products.[18] NUREG-1805 names three conditions that must exist together: a flammable mixture, a means of ignition and an enclosure.[17]

The resulting pressure follows from the temperature rise of the gas. A stoichiometric hydrocarbon and air mixture at 300 K has an adiabatic flame temperature of about 2,200 K. In a closed vessel, the pressure can therefore reach a little over 7 bar.[5] Heat losses to the walls make the real maximum somewhat lower, but a vessel that was not designed for such pressures will rupture. Connected vessels are worse, because burning in the first vessel raises the pressure in the second before the flame arrives there.[5]

Zalosh obtains the pressure rise from the ideal gas law. The ratio of final to initial pressure equals the ratio of the number of moles times the absolute temperature after and before the deflagration.[19] The stoichiometry of the mixture matters, but the worst case lies slightly on the rich side. Calculated maximum pressures occur at equivalence ratios of 1.1 to 1.2 and reach 8 to 9.6 atm.[19]

Near the flammability limits, measured pressures are much lower than calculated ones.[19] The flame may burn only part of the mixture, and slow flames lose heat to the walls.[9] In lean hydrogen and air mixtures, for example, the burnt fraction of the hydrogen rises from zero at 4 % to almost one at 8 % hydrogen.[9] Measured maximum explosion pressures of most gases and vapours lie between 6.8 and 8.1 bar above atmospheric pressure. Acetylene, at 10.6 bar, and ammonia, at 5.4 bar, are the exceptions.[9]

Deflagration data for selected gases in air[9]
Gas Maximum pressure (bar g) Laminar burning velocity (cm/s)
Methane 7.1 40
Propane 7.9 46
n-Butane 8.0 45
Hydrogen 6.8 312
Acetylene 10.6 166

Fires can create premixed mixtures of their own. A backdraft occurs when fresh air enters a closed, oxygen-starved fire room and mixes with the accumulated fuel-rich smoke. NUREG-1805 notes that it has the characteristics of a premixed fuel and air deflagration.[6] Introduction to Fire Safety Engineering describes the smoke gas explosion in the same way. A typical condition is that air and pyrolysis gases are reasonably premixed and lie between the lower and upper explosion limits.[7]

Limits of flammability and stability

A stable premixed flame can only be obtained over a limited range of mixture compositions and flow rates.[4] The composition limits are the lower and upper flammable limits. Methane in air, for example, can carry a flame between 5 and 15 % by volume at normal temperature and pressure.[8] At the lower limit, the calculated adiabatic flame temperature of most fuels containing only carbon, hydrogen and oxygen is about 1,600 K.[20] The limits are measured for upward flame propagation in a tube about 1 m long and 5 cm wide, because this gives the widest range.[11]

The limits widen as the temperature of the mixture rises.[5] Adding inert gas narrows them. For propane, the oxygen content of an air and nitrogen mixture must fall below 12 % before no mixture with propane can burn.[15] Bromine compounds act differently: they are chemical inhibitors, and even traces markedly reduce the burning velocity of a premixed flame.[13] Very rich premixed flames, like diffusion flames, can also form soot.[21]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Van Rossum (2027), §4.3.
  2. ↑ 2.0 2.1 2.2 2.3 Tien et al. (2002), p. 1-86.
  3. ↑ 3.0 3.1 3.2 Iqbal and Salley (2004), p. 3-12.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 Simmons (2002), p. 1-144.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 5.15 Simmons (2002), p. 1-147.
  6. ↑ 6.0 6.1 Iqbal and Salley (2004), p. 15-4.
  7. ↑ 7.0 7.1 Van Rossum (2027), §4.10.
  8. ↑ 8.0 8.1 Beyler (2002b), p. 2-172.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Zalosh (2002), p. 3-407.
  10. ↑ 10.0 10.1 Iqbal and Salley (2004), p. 3-10.
  11. ↑ 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 11.10 Simmons (2002), p. 1-146.
  12. ↑ 12.0 12.1 Iqbal and Salley (2004), p. 3-9.
  13. ↑ 13.0 13.1 Simmons (2002), p. 1-151.
  14. ↑ 14.0 14.1 Simmons (2002), p. 1-148.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 Simmons (2002), p. 1-150.
  16. ↑ 16.0 16.1 Simmons (2002), p. 1-145.
  17. ↑ 17.0 17.1 Iqbal and Salley (2004), p. 15-3.
  18. ↑ 18.0 18.1 Zalosh (2002), p. 3-402.
  19. ↑ 19.0 19.1 19.2 Zalosh (2002), p. 3-406.
  20. ↑ Beyler (2002b), p. 2-177.
  21. ↑ Iqbal and Salley (2004), p. F-55.

References

  • Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Simmons, R. F. (2002). Premixed burning. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-144–1-154). National Fire Protection Association; Society of Fire Protection Engineers.
  • Tien, C. L., Lee, K. Y., & Stretton, A. J. (2002). Radiation heat transfer. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-73–1-89). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Zalosh, R. (2002). Explosion protection. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-402–3-421). National Fire Protection Association; Society of Fire Protection Engineers.
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