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Smouldering-to-flaming transition

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Revision as of 00:22, 2 November 2026 by Mark (talk | contribs) (FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5))
Contents
  1. 1Definition
  2. 2Conditions for the transition
  3. 3Role of air supply and geometry
  4. 4Observed times and hazard
    1. 4.1Worked example
  5. 5Detection and design practice
  6. 6Related phenomena
  7. 7Limits and uncertainty
  8. 8See also
  9. 9Notes
  10. 10References

The smouldering-to-flaming transition is the change of a fire from slow, flameless smouldering combustion to flaming combustion, in which fast reactions take place in the gas phase.[1][2] It occurs when gases above the smouldering zone form a mixture within their flammability limits and a heat source hot enough to ignite that mixture is present at the same place and time.[2] A typical case is a cigarette that starts smouldering in upholstery or bedding. In tests on chairs and beds, about two-thirds of the items turned to flaming between 22 and 306 minutes after the smouldering began.[1][2]

The transition matters because smouldering offers a path to flaming from heat sources far too weak to produce a flame directly.[1] Before the transition, the main threat is carbon monoxide; after it, a burning armchair can create fatal conditions within minutes.[3] The timing is very hard to predict, and design methods in Australia and New Zealand therefore handle smouldering fires with prescribed measures, such as smoke detection, rather than by calculation.[4][5][6]

Definition

Ohlemiller defines smouldering as a slow, low-temperature, flameless form of combustion. It is sustained by the heat released when oxygen attacks the surface of a solid fuel directly.[1] Introduction to Fire Safety Engineering likewise distinguishes two forms of combustion, flaming and smouldering, which release heat and light as a flame or as a glow.[7] In most smouldering materials the main heat source is the oxidation of char, the carbon-rich residue that forms when the fuel decomposes.[1] The non-flaming decomposition of a material under an external heat flux is not smouldering but forced pyrolysis, because little or no oxidation takes place.[1]

Flaming combustion, by contrast, involves fast exothermic reactions in the gas phase.[2] The transition marks the start of the development phase of a fire, which Introduction to Fire Safety Engineering defines as the period from the onset of flaming combustion to flashover.[8] The smouldering period before it belongs to what design guides call the incipient phase. The Australian data sheets for fire engineering describe this phase as a smouldering fire with a very low heat release rate that can readily change into a growing flaming fire.[9] The commentary on the New Zealand Verification Method C/VM2 places the start of the growth phase at the moment when radiation from the flame governs the burning rate.[10]

The transition links two stages of fire development, but not every fire passes through it. The International Fire Engineering Guidelines note that a smouldering fire may or may not develop into a flaming fire, and that many fires have no smouldering phase at all.[11][12] A smouldering design fire that never makes the transition typically has a maximum heat release rate below 5 kW.[11]

Conditions for the transition

Flaming ignition of the smouldering zone needs two things at once. The gases and air must form a mixture within their flammability limits, and a heat source must be strong enough to ignite this mixture.[2] Both conditions must be met at the same location and at the same moment. Any factor that raises the net rate of heat generation, or lowers the net rate of heat loss, pushes the smouldering material towards flaming. Such a factor raises the local temperature and the rate at which pyrolysis gases are produced.[2]

Ohlemiller names three such factors. The first is a better supply of oxygen. The second is a larger scale, which usually means smaller surface heat losses per unit volume of smouldering material. The third is an increasingly concave smoulder front, which reduces radiative losses and lets the fuel gases build up.[2] All three may act together in smouldering upholstery and bedding, and photographs of a smouldering chair before and during the transition are consistent with this idea.[2]

Cellulosic materials have an additional mechanism, called secondary char oxidation. Intense reaction fronts, probably hotter than 1070 K, then move intermittently through the low-density char left behind by the main smoulder front.[2] A front the size of a single fibre is unlikely to ignite flammable gases, but larger fronts of 1 to 10 mm may well do so. A comparable process has occasionally caused flaming in smouldering cellulosic insulation without retardant.[2]

The location of the transition depends on the material. Smouldering solid wood turns to flaming readily in a configuration that limits heat losses, and the transition was inferred to occur when its surface temperature reached about 950 to 1000 K.[2] In polyurethane foam smouldering under a forced air flow, the transition did not start at the foam surface. It started within the depth of the char left by the smoulder front, where continued reactions opened holes for vigorous gas-phase reactions.[2]

Role of air supply and geometry

A forced increase in oxygen supply can induce the transition, as anyone who has lit a campfire from tinder and sparks will recognise.[2] Palmer first studied this quantitatively for air flowing over horizontal layers of sawdust. He saw flaming only when the air flowed in the same direction as the smoulder spread, at air speeds of about 0.9 to 1.7 m/s depending on the material.[2] No flaming developed when the mean particle size was below 1 mm.[2]

Later studies confirmed the importance of the flow direction. Fibrous insulation turned to flaming at air speeds of about 2 m/s, again only for forward smoulder.[2] When smoulder zones ran in both directions at once, flaming appeared at 4 m/s, and only after the smouldering had created a substantial cavity in the fuel surface. Ohlemiller explains the weak response of reverse smoulder by heat transfer effects at the leading edge of the reaction zone. In forward smoulder these effects intensify the reaction, creating small cavities a few centimetres across that act as flame initiation regions and flame holders.[2]

Geometry can concentrate the air supply. Small mock-up studies suggest that a chimney-like flow develops in the crevice between the seat and back cushions of a smouldering chair.[13] The extra air presumably speeds up local char oxidation until the char can ignite the pyrolysis gases. Ohlemiller calls this mechanism plausible but notes that it has not been demonstrated in real upholstery or bedding.[2]

In a three-sided wooden channel with a controlled air flow, stable smouldering occurred only for inlet air speeds of 0.05 to 0.20 m/s. Below this range the smouldering stopped, and above it flaming eventually broke out.[14] Large piles of self-heating material show a related pattern. Hot gases rise through chimneys of partly burnt material, and when a chimney reaches the edge of the pile, smoke appears first and then the inflow of air causes flame.[15] The Australian data sheets note that a smouldering fire can readily change to flaming, particularly when the ventilation conditions change.[9]

Observed times and hazard

Babrauskas and Krasny reviewed the published data on smouldering upholstered chairs and beds. About two-thirds of the tested items turned to flaming, after times ranging from 22 minutes to 306 minutes.[2] In tests with a cotton mattress in a room 2.4 m across, the transition occurred in two of five tests after 65 to 80 minutes. That is close to the time at which the total exposure to carbon monoxide was estimated to be lethal.[14] In a larger room, flaming usually needed 2 to 3 hours of smouldering first. A review of all available data concluded that a lethal carbon monoxide dose and a transition to flaming are about equally likely from 1 to 2.5 hours after a cigarette starts the smouldering.[14]

Purser describes armchair tests in a simulated apartment at the National Institute of Standards and Technology. Cigarettes caused smouldering for about an hour, which decomposed about 1 kg of polyurethane foam. Carbon monoxide in the burn room rose from 180 ppm to 1000 ppm, enough to cause incapacitation after just over an hour.[3] When flaming began, the chair burned very rapidly. An occupant of a target room down the corridor would have lost consciousness within 1 minute and received a fatal dose within 2 minutes.[3] For the chair with standard foam, death would have followed within 1 minute of the spontaneous transition to flaming after 75 minutes.[16]

Smouldering itself is slow, and its smoke is seldom dense, so an alerted occupant has ample time to escape. Sleeping occupants, however, may be overcome after a long exposure.[3] Fires that burned for 30 minutes or more before discovery have probably involved long-term smouldering. For victims aged 19 to 49, deaths are 20 times more likely in such fires than in fires discovered within 5 minutes.[3]

A later study of the fire risks of upholstered furniture used an indirect method. It implied that the toxic exposure from a smouldering chair was rarely fatal and that deaths came with the transition to flaming. Ohlemiller notes that the data are not yet sufficient to resolve this question.[13]

Worked example

Few models exist for the incipient phase.[9] The International Fire Engineering Guidelines give a model for the mass loss of a smouldering fire that is sometimes used in design.[17] For time t in minutes, the pyrolysis rate in grams per minute is:[17]

dmdt=0.10t+0.0185t2(0<t<60 min),dmdt=73(60<t<120 min)

Integrating the first part gives the mass lost up to time t: m=0.05t2+0.00617t3 grams. After 22 minutes, the shortest transition time in the review of Babrauskas and Krasny, about 90 g has smouldered away. After 60 minutes the total is about 1.5 kg, and after 75 minutes, the transition time in the armchair test, it is about 2.6 kg. The model's 1.5 kg at one hour is of the same order as the 1 kg of foam decomposed in just over an hour in the armchair tests.[3]

Symbol Description Unit
m mass lost by smouldering since the start g
dm/dt pyrolysis (mass loss) rate g/min
t time since the start of smouldering min

The smouldering area at the moment of transition can be small. In the mattress tests, the smoulder front spread outward at about 6.3 × 10−5 m/s.[14] After 65 to 80 minutes the smouldering zone therefore had a radius of only 0.25 to 0.30 m and an area of about 0.19 to 0.29 m². The Australian data sheets point out that experimental data may be more suitable than the model when activation times of smoke detectors or times to untenable conditions are needed.[9]

Detection and design practice

Smoke from a smouldering fire is hard to detect. Residential detectors respond well to the air speeds above 0.25 m/s that flaming fires create.[18] They may fail to respond when the rising column of a smouldering fire is slower than 0.15 m/s.[18] A smoke detector that uses the photoelectric principle tends to respond faster to smouldering fires, while one that uses ionisation responds somewhat faster to open flaming fires.[19] Purser adds that detection and warning can overcome the dangers of the smouldering phase, but the siting of the detectors matters.[16]

Performance-based codes treat the smouldering phase with a dedicated design scenario. In New Zealand, Verification Method C/VM2 (second edition, 2025) contains design scenario SF, a smouldering fire close to a sleeping area. For buildings with a sleeping use it requires an automatic smoke detection and alarm system throughout the building, installed to NZS 4512, and no further analysis.[5] In Australia, the Fire Safety Verification Method Standard 2022 contains the same scenario with the required outcome of a safe sleeping area. Its typical solutions are separating elements that prevent all smoke ingress, usually with a pressure difference, or automatic smoke detection throughout the sleeping and adjoining spaces.[6]

Neither code asks for a prediction of the transition. The C/VM2 commentary of 2013 calls the incipient phase extremely complex and too unpredictable to be included in design.[10] The International Fire Engineering Guidelines state that the time before a smouldering fire turns to flaming is difficult to calculate with certainty. Because many fires have no smouldering phase, a flaming design fire is commonly assumed to start growing at time zero without one.[12] The Australian data sheets add that untenable conditions in smouldering fire experiments were mainly caused by carbon monoxide in a closed room of fire origin.[9]

Related phenomena

Smouldering can also trigger fast combustion of gases that have accumulated in a closed space. A backdraft is the explosive ignition of pyrolysis gases when oxygen suddenly enters a room in which the fire has used up much of the oxygen.[20] A smoke explosion involves gases that are already premixed with air between their flammability limits. Such a mixture can be ignited by an ignition source such as a spark or glowing particles from the fire.[21] In a reduced-scale study of smoke explosions with a smouldering wood crib, the transition from smouldering to flaming was proposed as the trigger, although the evidence was indirect.[2]

Retardants can suppress the transition, but only in part. Boric acid, a smoulder retardant, and borax, a flame retardant, each eliminated the transition to flaming when treated cellulosic insulation was the only fuel.[2] Their effect dropped sharply when the smouldering insulation touched untreated wood, as it usually does in houses, because heat from the smoulder zone readily ignited the wood. Layers of fine dust that do not turn to flaming themselves can likewise ignite adjacent flammable materials.[2]

Extinguishing a smouldering fire requires removing heat from the fuel. Removing oxygen is not enough unless it continues until the fuel has cooled so far that fresh oxygen no longer causes reignition.[14] In self-heating materials, the oxygen demand before flaming is very low. By the time flames appear, broad channels of destroyed material usually give air ready access.[22]

Limits and uncertainty

No model predicts the moment of transition reliably. Hall notes that the timing of the transition from smouldering to free burning is not well described by any model and is subject to enormous variability.[4] Ohlemiller adds that the mechanism of the spontaneous transition has not been investigated in detail. Whether smouldering turns to flaming is even less related to the factors that start the smouldering than whether the smouldering keeps spreading.[1][13]

Many observations depend on the test set-up. The air speed limits for stable smouldering in the wooden channel are probably specific to those tests.[14] Sato and Sega found a range of conditions in which both smouldering and flaming are possible for cellulosic materials, with hysteresis in the switch between the two modes.[13] Statistics can also hide the transition. Many smouldering fires become flaming fires before they are detected, so the true number of deaths during the smouldering phase is difficult to know.[3]

The mass-loss model in the worked example describes one set of smouldering experiments. The International Fire Engineering Guidelines attribute it to Quintiere and co-workers in 1985, while the Australian data sheets give 1982 for the same work.[17][23] Neither source gives a method to predict whether or when the transition occurs, so fire safety designs rely on detection and separation rather than on a predicted transition time.[5][6]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Ohlemiller (2002), p. 2-200.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 Ohlemiller (2002), p. 2-208.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Purser (2002), p. 2-134.
  4. ↑ 4.0 4.1 Hall (2002c), p. 1-202.
  5. ↑ 5.0 5.1 5.2 Ministry of Business, Innovation and Employment (2025b), p. 42.
  6. ↑ 6.0 6.1 6.2 Australian Building Codes Board (2022), p. 13.
  7. ↑ Van Rossum (2027), §4.1.
  8. ↑ Van Rossum (2027), §4.11.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Australian Building Codes Board (2020a), Data Sheet B1, p. 9.
  10. ↑ 10.0 10.1 Ministry of Business, Innovation and Employment (2013), p. 53.
  11. ↑ 11.0 11.1 National Research Council of Canada et al. (2005), p. 1.2-29.
  12. ↑ 12.0 12.1 National Research Council of Canada et al. (2005), p. 1.4-5.
  13. ↑ 13.0 13.1 13.2 13.3 Ohlemiller (2002), p. 2-207.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Ohlemiller (2002), p. 2-206.
  15. ↑ Gray (2002), p. 2-225.
  16. ↑ 16.0 16.1 Purser (2002), p. 2-135.
  17. ↑ 17.0 17.1 17.2 Quintiere et al. (1985), as cited in National Research Council of Canada et al. (2005), p. 2.4-11.
  18. ↑ 18.0 18.1 Iqbal and Salley (2004), p. 11-2.
  19. ↑ Iqbal and Salley (2004), p. 11-5.
  20. ↑ Van Rossum (2027), §4.9.
  21. ↑ Van Rossum (2027), §4.10.
  22. ↑ Gray (2002), p. 2-224.
  23. ↑ Australian Building Codes Board (2020a), Data Sheet B1, pp. 9, 23.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
  • Gray, B. (2002). Spontaneous combustion and self-heating. 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. 2-211–2-228). National Fire Protection Association; Society of Fire Protection Engineers.
  • Hall, J. R., Jr. (2002c). Statistics. 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. 1-193–1-202). 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.
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Ohlemiller, T. J. (2002). Smoldering combustion. 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. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
  • Purser, D. A. (2002). Toxicity assessment of combustion products. 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. 2-83–2-171). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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