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Fuel-controlled fire

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Contents
  1. 1Definition
  2. 2What limits the heat release rate
  3. 3Fuel control during the course of a fire
  4. 4Distinguishing fuel and ventilation control
  5. 5Use in design and modelling
  6. 6Limits and common misconceptions
  7. 7See also
  8. 8Notes
  9. 9References

A fuel-controlled fire is a fire whose heat release rate is limited by the fuel that is burning, not by the supply of air. There is enough oxygen to burn all the combustible gases that the fuel releases.[1][2] Other terms in the sources are fuel-limited fire and overventilated or well-ventilated fire.[2][3][4] The opposite is a ventilation-controlled fire, in which the air supply limits the burning.[1]

In a room, every fire starts as a fuel-controlled fire. At first the fire is small compared with the room, and there is plenty of oxygen.[1] A fire in the open air is always fuel controlled.[1] In the growth phase of a compartment fire, the fire is fuel controlled until flashover. After flashover the fire is usually ventilation controlled, and in the decay phase it may become fuel controlled again.[5][6]

The distinction matters for fire safety engineering, because it decides whether the size of a fire follows from the fuel or from the openings. It also affects how much smoke and carbon monoxide is produced, and whether fire models and design methods can be applied.[7][8][9] Design guidance therefore calculates both the fuel-controlled and the ventilation-controlled heat release rate and uses the lesser of the two.[10]

Definition

Introduction to Fire Safety Engineering distinguishes two types of fire, with the amount of oxygen as the deciding factor. In a fuel-controlled fire, the development of the fire depends mainly on the fuel, and oxygen is present in sufficient quantity.[1] The Fire Dynamics Tools of the US Nuclear Regulatory Commission describe a fuel-limited fire as a compartment fire in which the air supply is enough to maintain combustion. The amount of fuel that is burning then limits the size of the fire.[2]

The SFPE Handbook uses the same idea. Walton and Thomas state that fires with sufficient oxygen for combustion are said to be fuel controlled, and that in such fires there is enough air to react with all the fuel within the compartment.[11][12] The NIST investigation of the World Trade Center fires defines an overventilated fire in the same way. Enough air is available near the fire to react with all of the gaseous fuel being generated.[3]

Two room cross-sections: left, a small fire under a thin smoke layer with air flowing in; right, a large fire with a deep layer and flames out of the door.
Left: a fuel-controlled (overventilated) fire burns completely inside the room below the smoke layer. Right: a ventilation-controlled fire with unburned gases burning outside the opening. Based on Gottuk and Lattimer (2002), p. 2-55 and Walton and Thomas (2002), pp. 3-172 to 3-173.

A related term is the free-burning fire. Walton and Thomas define it as a fire whose pyrolysis rate and heat release rate depend only on the burning of the fuel itself, not on the room environment. It is comparable to a fire burning out of doors on a calm day.[12] A fuel-controlled fire in a room is not necessarily free burning. Wall linings and fuels in the hot upper layer are likely to be influenced by the room environment before flashover, and radiation feedback can enhance the burning rate.[12][13]

What limits the heat release rate

In practice, fuel control means that the fire is limited by the rate at which combustible pyrolysis gases are released from the fuel. Introduction to Fire Safety Engineering points out that there is often more than enough combustible material present, but not all of it is directly available as pyrolysis gases.[1] The heat release rate equals the mass burning rate of the fuel times its effective heat of combustion.[12] In a fuel-controlled fire, both depend on the fuel and on the heat it receives, not on the air flow.

The burning rate of a fuel-controlled fire is hard to predict from theory. According to the International Fire Engineering Guidelines, it depends largely on the nature and geometric arrangement of the fuel.[13] Burning rates of uniform fuels such as timber cribs can be estimated with published formulae. Timber cribs are, however, a rather unique arrangement, and the formulae should be used with caution for other fuels.[13] Lie describes the same regime as a fire controlled by the surface area of the materials that can take part in the burning.[14]

Measured burning rates of furniture and other fuel packages are therefore the main data source. They come from free-burn tests and reflect fuel-controlled burning. The guidelines warn that in a building the burning may be inhibited by ventilation or enhanced by radiation feedback, so engineering judgement is needed.[13] Babrauskas notes the same for liquid pools. Burning rates from the literature hold only in a very large, well-ventilated room, and radiation from a hot layer increases the burning rate.[15]

Fuel control during the course of a fire

In the fire development stages, fuel control is typical of the growth stage. Introduction to Fire Safety Engineering describes the development phase of a room fire as the period from flaming ignition to flashover. The fire is still local and involves one or a few objects, so its heat release rate is limited by the pyrolysis gases produced.[5] Walton and Thomas likewise describe the growth stage as one in which the fire grows mainly as a function of the fuel itself, with little or no influence from the compartment.[11]

Schematic heat release rate curve: growth, plateau after flashover and decay, with growth and decay shaded as fuel controlled.
Typical course of a compartment fire: fuel controlled during growth, usually ventilation controlled after flashover, and fuel controlled again during decay. Based on Van Rossum (2027), §4.11 and Walton and Thomas (2002), pp. 3-171 to 3-172.

Flashover usually marks the change. After flashover the whole room is involved, there is an abundance of pyrolysis gases, and the fire is usually ventilation controlled.[5][16] More fuel is then pyrolysed than can burn with the oxygen available in the compartment, and the unburned fuel may burn outside the openings.[6] In the decay stage, as fuel is consumed, the fire may change back from ventilation control to fuel control.[6]

The Australian Fire Safety Verification Method Data Sheets describe how this can unfold in a fully developed fire. Just after flashover, small items and easily pyrolysed materials produce a very high pyrolysis rate. The fire is then strongly ventilation controlled, with long flames from the openings. Later the pyrolysis rate falls, the flames outside shrink, and at some stage the fire becomes fuel controlled with limited flame extension from the openings.[17] Changes in ventilation, such as breaking windows, can shift the balance during the fire.[17][7]

A fully developed fire can also remain fuel controlled throughout. Whether a fire is controlled by the fuel surface or by the ventilation depends on the amount of combustible contents and on the size of the windows.[14] In experiments with wood cribs in a 29 m2 compartment, both window areas gave enough air for the lower fire load to burn at its free-burning rate. With the doubled fire load, the burning rate did not double, because the window restricted the ventilation.[18] In the World Trade Center, NIST characterised the fires in WTC 2 as fuel controlled and those in WTC 1 as oxygen controlled. In the WTC 2 simulations, a higher burning rate of the furnishings therefore raised the total heat release rate noticeably.[19][20]

Distinguishing fuel and ventilation control

A simple check compares the heat release rate of the fire with the largest heat release rate that the air flowing through the openings can support. For most fuels, about 3,000 kJ of heat is released for each kilogram of air consumed.[12] The air flow through an opening is roughly proportional to its area times the square root of its height, the ventilation factor of the opening. This leads to a stoichiometric heat release rate of 1,500 kW per unit of ventilation factor.[21] The ratio of the actual heat release rate to this value is the equivalence ratio φ:[22]

ϕ=Q˙1500A0H0

Symbol Description Unit
ϕ equivalence ratio, actual heat release rate relative to the stoichiometric one dimensionless
Q˙ heat release rate of the fire kW
A0 area of the opening m2
H0 height of the opening m
1500 stoichiometric heat release rate per unit ventilation factor kW/m5/2

At stoichiometry φ equals 1. A value below 1 means fuel-lean burning, which corresponds to a fuel-controlled fire, and a value above 1 means fuel-rich burning.[22] The relation is an approximation for fires near flashover and after flashover, when the hot layer reaches close to the floor and the air inflow depends mainly on the opening. It assumes a fuel that releases about 3,000 kJ per kilogram of air.[23][12]

As a first worked example, the 2013 commentary on C/VM2 shows a space of 15 m by 10 m with a vent 8.0 m wide and 3.5 m high. A fast t-squared fire grows to 20 MW, nearly unaffected by the space, and the upper layer never reaches the flashover temperature of 500 °C.[24] The ventilation factor is 28 × √3.5 = 52.4 m5/2, so the stoichiometric heat release rate is about 78.6 MW. The equivalence ratio is 20 / 78.6 = 0.25, well below 1, so the fire is fuel controlled.

As a second example, Walton and Thomas use a room with a door opening of 1.08 m2 and 1.8 m high.[25] Its stoichiometric heat release rate is 1,500 × 1.08 × √1.8 = 2,173 kW. A fire of 500 kW gives φ = 0.23 and is clearly fuel controlled. Babrauskas's method predicts flashover in this room at about 1,087 kW, half of the stoichiometric value.[25] A fast t-squared fire would reach that size after about 152 s, so fuel control in a small room can end within minutes.[26]

Straight line of equivalence ratio against heat release rate for a room with a 1.08 m² door; markers at 500 kW, at flashover (1,087 kW) and at φ = 1 (2,173 kW).
Equivalence ratio φ against heat release rate for a room with a door of 1.08 m² and 1.8 m high: fuel controlled below φ = 1. Based on Walton and Thomas (2002), pp. 3-181, 3-184 and 3-185.

For design, the International Fire Engineering Guidelines recommend calculating both the ventilation-controlled and the fuel-controlled heat release rate. The lesser of the two is used as the peak heat release rate of the fully developed fire.[10] The Australian Data Sheets state the same: the maximum heat release rate is the lesser of the ventilation-controlled and fuel-controlled values.[27] In New Zealand, C/VM2 (second edition, 2025) defines the ventilation limit through fire modelling. It is the heat release rate at the moment the predicted energy release first diverges from the design fire because of a lack of oxygen.[28]

Use in design and modelling

Most design fire curves describe a fuel-controlled fire. In C/VM2 (second edition, 2025), the design fires are intended to represent free-burning fires. They must be modified during the analysis to account for building ventilation and for automatic suppression.[29] In the OZone software, the heat release rate, mass loss and fire area curves entered by the user likewise correspond to a fuel-controlled fire, the fire that would occur without any influence of the compartment.[30]

Fire models must then decide what happens when oxygen runs short. OZone offers several combustion models. As long as oxygen is present in the compartment, the fire is fuel controlled and all the fuel lost releases its energy inside the room.[31] In the decay phase the fire returns to the fuel-controlled regime.[32] Introduction to Fire Safety Engineering notes that most models handle ventilation-controlled fires poorly or not at all. Users must therefore specify whether unburned gases burn inside, burn outside the room or stop burning.[9]

Fuel control also affects the products of combustion. In an overventilated compartment fire, the plume stays in the lower layer, combustion is nearly complete and little carbon monoxide is formed.[8] Free-burning, well-ventilated fires are generally of little concern for toxic species.[4] C/VM2 (second edition, 2025) reflects this with separate yields. Before flashover it uses a soot yield of 0.07 kg/kg and a carbon monoxide yield of 0.04 kg/kg. After flashover or once the fire is ventilation limited, these rise to 0.14 and 0.40 kg/kg.[33][28]

For structural fire design, the distinction appears in the Eurocode parametric fire curve. As described by the Australian Data Sheets, the choice depends on a time calculated from the fire load density and the opening factor. This time is compared with a limiting time of 25 minutes for slow, 20 minutes for medium and 15 minutes for fast fire growth. If the calculated time is at least the limiting time, the fire is ventilation controlled. Otherwise it is fuel controlled, and a modified heating curve is used.[34] Models for a travelling fire in large open floors are also commonly built from a series of localised fuel-controlled fires.[35]

Limits and common misconceptions

A common misconception is that a fuel-controlled fire is small or harmless. The commentary example above reaches 20 MW while remaining fuel controlled.[24] In small rooms, flashover often occurs while the fire is still fuel controlled. Babrauskas found that test fires reached flashover at between 0.3 and 0.7 times the stoichiometric heat release rate.[25] Fuel control describes what limits the fire, not how large or dangerous it is.

A second misconception is that fuel control depends on the total amount of fuel present. It depends on how fast the fuel releases combustible gases, which is governed by its nature, arrangement and exposed surface.[1][13] In the compartment fire experiments described by Walton and Thomas, doubling the fire load did not double the burning rate once the window limited the air supply.[18] For ventilation-controlled fires, a higher fuel load mainly extends the duration of the fire.[36]

Assuming ventilation control where a fire is actually fuel controlled can also mislead. Drysdale notes that calculating a fire's temperature history with the ventilation-controlled burning rate overestimates the severity of fuel-controlled fires with large openings.[37] Lie adds that in shallow rooms of limited size, more combustible gas burns outside as ventilation increases, so that the temperature may fall.[38] Conversely, burning rates measured under free-burning conditions may underestimate the burning in a hot room, where radiation feedback increases it.[13][15]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Van Rossum (2027), §4.8.
  2. ↑ 2.0 2.1 2.2 Iqbal and Salley (2004), p. 2-4.
  3. ↑ 3.0 3.1 Pitts et al. (2005), p. 43.
  4. ↑ 4.0 4.1 Gottuk and Lattimer (2002), p. 2-54.
  5. ↑ 5.0 5.1 5.2 Van Rossum (2027), §4.11.
  6. ↑ 6.0 6.1 6.2 Walton and Thomas (2002), p. 3-172.
  7. ↑ 7.0 7.1 National Research Council of Canada et al. (2005), p. 1.4-6.
  8. ↑ 8.0 8.1 Gottuk and Lattimer (2002), p. 2-55.
  9. ↑ 9.0 9.1 Van Rossum (2027), §8.4.
  10. ↑ 10.0 10.1 National Research Council of Canada et al. (2005), p. 2.4-14.
  11. ↑ 11.0 11.1 Walton and Thomas (2002), p. 3-171.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 Walton and Thomas (2002), p. 3-173.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 National Research Council of Canada et al. (2005), p. 2.4-15.
  14. ↑ 14.0 14.1 Lie (2002), p. 4-202.
  15. ↑ 15.0 15.1 Babrauskas (2002b), p. 3-26.
  16. ↑ Van Rossum (2027), §4.7.
  17. ↑ 17.0 17.1 Australian Building Codes Board (2020a), Data Sheet B1, p. 14.
  18. ↑ 18.0 18.1 Walton and Thomas (2002), p. 3-175.
  19. ↑ McGrattan et al. (2005), p. 111.
  20. ↑ Prasad and Baum (2005), p. 230.
  21. ↑ Walton and Thomas (2002), p. 3-184.
  22. ↑ 22.0 22.1 Walton and Thomas (2002), p. 3-181.
  23. ↑ Walton and Thomas (2002), p. 3-174.
  24. ↑ 24.0 24.1 Ministry of Business, Innovation and Employment (2013), p. 61.
  25. ↑ 25.0 25.1 25.2 Walton and Thomas (2002), p. 3-185.
  26. ↑ Iqbal and Salley (2004), p. B-3.
  27. ↑ Australian Building Codes Board (2020a), Data Sheet B1, p. 13.
  28. ↑ 28.0 28.1 Ministry of Business, Innovation and Employment (2025b), p. 21.
  29. ↑ Ministry of Business, Innovation and Employment (2025b), p. 57.
  30. ↑ Cadorin and Franssen (2003), p. 402.
  31. ↑ Cadorin and Franssen (2003), p. 420.
  32. ↑ Cadorin and Franssen (2003), p. 421.
  33. ↑ Ministry of Business, Innovation and Employment (2025b), p. 20.
  34. ↑ Australian Building Codes Board (2020a), Data Sheet B3, p. 40.
  35. ↑ Australian Building Codes Board (2020a), Data Sheet B1, p. 16.
  36. ↑ Iqbal and Salley (2004), p. 8-1.
  37. ↑ Drysdale (2002), p. 1-95.
  38. ↑ Lie (2002), p. 4-203.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Babrauskas, V. (2002b). Heat release rates. 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. 3-1–3-37). National Fire Protection Association; Society of Fire Protection Engineers.
  • Cadorin, J.-F., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires – OZone V2. Part 1: Pre- and post-flashover compartment fire model. Fire Safety Journal, 38, 395–427. https://doi.org/10.1016/S0379-7112(03)00014-6
  • Drysdale, D. D. (2002). Thermochemistry. 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. 1-90–1-98). National Fire Protection Association; Society of Fire Protection Engineers.
  • Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. 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. 2-54–2-82). 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.
  • Lie, T. T. (2002). Fire temperature-time relations. 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. 4-201–4-208). National Fire Protection Association; Society of Fire Protection Engineers.
  • McGrattan, K. B., Bouldin, C., & Forney, G. P. (2005). Computer simulation of the fires in the World Trade Center towers (NIST NCSTAR 1-5F). National Institute of Standards and Technology.
  • 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.
  • Pitts, W. M., Butler, K. M., & Junker, V. (2005). Visual evidence, damage estimates, and timeline analysis (NIST NCSTAR 1-5A). National Institute of Standards and Technology.
  • Prasad, K., & Baum, H. (2005). Fire Structure Interface and thermal response of the World Trade Center towers (NIST NCSTAR 1-5G). National Institute of Standards and Technology.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Walton, W. D., & Thomas, P. H. (2002). Estimating temperatures in compartment fires. 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. 3-171–3-188). National Fire Protection Association; Society of Fire Protection Engineers.
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