Contents
Time to ignition is the interval between the start of a specified exposure and the onset of the defined ignition event. For an externally heated solid, this interval includes the material's thermal response, the formation of a combustible gas mixture and initiation of combustion. It depends on both the specimen and the conditions of exposure.[1]
A reported ignition time therefore requires more than a material name. Heating intensity, specimen geometry, surface condition and the presence of a pilot flame or spark influence the result. The observation must also distinguish a brief flame from flaming that persists when external heating is removed.[1][2]
In engineering calculations, ignition delay helps describe the initiation of a fire and its spread to another fuel item. Simplified correlations can estimate this delay for particular heating arrangements. Their material parameters, thermal assumptions and ignition criteria remain part of the prediction, rather than optional details.[3][4]
The processes before a flame appears
An externally heated solid first responds to the supplied energy. Its surface warms while heat moves into the material and is lost to the surrounding environment. Thermal decomposition can then produce combustible vapours. Kanury describes this solid-phase response as one of three broad contributions to the observed ignition delay.[1][5]
The released vapours must mix with an oxidant, usually oxygen in the surrounding air. Their production alone does not establish that the mixture will ignite. The local temperature and composition influence how rapidly combustion reactions develop. The observed event follows the interaction between heating, mixing and chemical reaction, rather than a single isolated temperature change.[1]
In piloted ignition, a spark or small flame initiates combustion in the prepared mixture. The pilot's role differs from the external heating that produces fuel vapours from the solid. Kanury describes it as a local trigger for a reaction that can propagate through the mixture. Its presence changes the conditions required for ignition.[1]
Without a pilot, ignition is termed spontaneous ignition or autoignition. The gas mixture must then develop combustion through its own temperature and chemical conditions. NUREG-1805 explains that piloted ignition generally requires a lower temperature than autoignition in the examples considered. An ignition delay measured with a pilot is therefore not automatically applicable to an unpiloted exposure.[1][6]
For the solid-fuel calculations discussed in NUREG-1805, the gas-phase ignition process is generally much faster than solid heating. The report identifies low ambient oxygen as an exception to that simplified reasoning. This allows conduction-based models to represent many of the considered cases. It does not eliminate gas-phase effects when the surrounding conditions change.[7]
The material's physical response also influences what is being heated. Kanury distinguishes solids that form char from thermoplastics that melt and decompose to produce vapours. These are different pathways to a combustible mixture. A time associated with one pathway cannot be understood solely from the chemical label of the starting solid.[1]
Defining and measuring the event
The start and end of the measured interval must refer to a stated exposure and a stated ignition event. Kanury distinguishes transient ignition from persistent flaming. A flame can appear during external heating and disappear when that heating is withdrawn. A longer exposure can establish flaming that continues after the external source is removed.[1]
These observations answer different experimental questions. The first records the appearance of flaming under the imposed conditions. The second concerns whether the reaction can continue when the imposed heating changes. Treating both as one unspecified ignition time can hide a substantial difference in the thermal preparation of the specimen.[1]
A cone calorimeter provides controlled radiant exposure and records ignition time among other fire measurements. Tewarson also describes ignition measurements in the fire propagation apparatus. The historical apparatus comparison identifies a spark igniter for the cone calorimeter and a pilot flame for the fire propagation apparatus. The source refers to the test editions described in its 2002 handbook chapter.[8]
Tests at several applied fluxes provide more information than one ignition time. Tewarson describes correlating the inverse square root of the measured delay against external flux. The fitted slope and intercept represent different aspects of the material's ignition response. The fitting procedure uses observations away from the critical-flux region.[2]
NUREG-1805 also reproduces measured examples from Quintiere's earlier work. At an exposure of 20 kW/m², its table lists 150 seconds for gypsum-board paper and 250 seconds for wood particle board. These are separate reported material observations under the tabulated exposure, rather than results of the worked calculation below. The table supplies limited specimen detail and does not establish a common value for all products bearing either description. Its useful comparison is that the same stated radiant flux can accompany different ignition delays. The exposure value alone therefore does not identify the response of an otherwise unspecified solid.[9]
Surface preparation is part of this measurement context. The SFPE chapter describes blackening specimens to control their radiation absorption. It reports different fitted response parameters for blackened and non-blackened polymethylmethacrylate surfaces. Comparisons of ignition delays therefore require the surface condition as well as the applied flux and specimen dimensions.[2]
Failure to observe ignition within the test period is also a bounded observation. Tewarson describes a threshold-screening procedure with about 15 minutes of exposure without ignition. That duration belongs to the procedure being described. It cannot be silently converted into an observation that ignition remains impossible for every longer exposure or changed condition.[2]
Thermal response and material thickness
The rate at which a solid surface heats depends on conductivity, density and specific heat. In the thermally thick model discussed by NUREG-1805, these properties combine as thermal inertia, kρc. The square root of that product enters the thermal response parameter. A temperature rise from ambient to the ignition condition supplies its other principal factor.[5][4]
This response parameter connects the heating process to the selected ignition condition. It is not simply the specimen's ignition temperature. Two specimens can require similar surface temperatures while taking different times to reach them. Their ability to conduct and store heat changes the transient response under the same imposed heating.[5]
The distinction between thermally thin and thick materials describes how temperature varies through the specimen during the exposure. A thin-body approximation can represent nearly uniform temperature. A thick-body treatment retains an interior that is cooler than the exposed surface. Kanury's conduction analysis distinguishes surface temperature from the material's mean temperature.[10]
The mean temperature does not generally equal the surface temperature in a thick specimen. Ignition can occur near the heated surface while much of the interior remains cooler. This matters when interpreting a model based on a critical surface temperature. Substituting a whole-body mean temperature would change the criterion and the associated heating problem.[10]
For thin specimens, NUREG-1805 identifies weight or thickness as a significant part of ignition behaviour. Its displayed thick-material correlation is therefore not a universal formula for every sheet or coating. The thermal regime has to match the method. Material name alone does not establish whether that match exists.[4]
Surface losses also affect the heating history. Kanury discusses convection losses alongside radiation input and internal conduction. Reducing those losses changes the temperature history even if the nominal incident radiation remains fixed. The ignition delay consequently reflects the boundary conditions as well as the solid's internal thermal properties.[10]
A correlation for constant radiant heating
NUREG-1805 gives the following estimate for a thermally thick material exposed to constant radiant flux. The applied flux must exceed the critical heat flux for ignition in the selected correlation. The time depends on the response parameter divided by the excess flux. The report's numerical factor is retained explicitly.[4]
| Symbol | Description | Unit |
|---|---|---|
| tig | Calculated ignition delay | s |
| q″e | Constant external radiant heat flux | kW/m² |
| CHF | Critical heat flux associated with the fitted model | kW/m² |
| TRP | Thermal response parameter in the NUREG convention | kW·s1/2/m² |
Increasing the response parameter increases the delay when the fluxes remain fixed. Increasing the excess of applied flux over CHF reduces the delay. Both effects are squared in this expression. The equation thus distinguishes resistance during the heating period from the threshold separating the represented ignition and non-ignition regimes.[4]
The expression applies during the transient heating period before steady conditions are reached. At the critical value, its denominator approaches zero and the predicted time grows without bound. Substitution below the critical value does not create a valid finite ignition prediction. The squared expression must remain subject to its physical condition q″e > CHF.[4]
The report discusses other correlations whose fitted material parameters are method-specific. The SFPE chapter also prints a different normalisation of the parameter called TRP. A calculation therefore requires a compatible equation and parameter set. The example below uses both from NUREG-1805, without transferring numerical coefficients from another convention.[11][2]
Worked example for polypropylene
NUREG-1805 lists polypropylene with a critical flux of 15 kW/m² and a response parameter of 193 kW·s1/2/m². Select a constant exposure of 50 kW/m² for an illustrative calculation. The excess flux is then 35 kW/m². The selected exposure is an input to the example, while the two material parameters come from the source table.[12][4]
Substitution gives approximately 23.8819 seconds before rounding. Reporting this as 23.8 seconds rounds the calculated ignition delay down to three significant figures. The result describes the stated correlation and inputs. It is not a measured ignition time for an unidentified polypropylene product or a complete prediction of a developing compartment fire.[4][12]
The same calculation at 30 kW/m² gives approximately 130.024 seconds, reported as 130 seconds when rounded down. Here the excess flux is only 15 kW/m². The response parameter has not changed, but the exposure is much closer to the threshold. The larger delay follows from the inverse-square dependence on excess flux.[4][12]
These two results are separate constant-exposure scenarios. They do not describe a specimen first exposed to 30 kW/m² and then to 50 kW/m². In that changing exposure, the first stage would establish a different initial thermal state for the second stage. The constant-flux calculation does not itself provide the intervening temperature history.[4][10]
The table also illustrates why one threshold cannot determine the delay. Its Douglas fir entries have the same critical flux, but different response parameters for ordinary and fire-retardant material. In the displayed model, changing that response parameter changes the calculated time even when the exposure is identical. Ignition comparisons therefore require both parameters and their conditions.[4][12]
From specimen ignition to fire development
Ignition delay is one part of a fire-development sequence. NUREG-1805 connects secondary ignition with the rate at which fire spreads between fuel items. Their position, spacing and orientation affect that sequence. A specimen calculation alone does not describe how the exposing fire changes or how the next target is arranged.[3][7]
The report illustrates orientation with a curtain. Ignition near its lower edge can lead to rapid upward spread, while ignition near its top leads to slower downward spread. The ignition location therefore influences the subsequent fire growth. Two cases with an ignition event do not necessarily have the same development after that event.[7]
Fuel placement near a wall can also change the surrounding flow and flame geometry. NUREG-1805 explains that reduced access for cool entrained air can produce higher temperatures and taller flames. Those changes affect exposure of nearby material. The imposed constant flux in a specimen model is consequently a representation of a broader fire scenario.[7]
The NUREG methods were developed for the United States Nuclear Regulatory Commission's nuclear power plant fire protection inspection programme. Their use here illustrates an engineering calculation, without assigning a regulatory pass or fail time. The document's original setting and edition remain relevant when interpreting its methods and tabulated data.[13]
Limits and interpretation
Ignition temperature is itself an approximation in a simplified model. NUREG-1805 notes that it can vary with the heating rate. Kanury also discusses the influence of geometry and gas-phase conditions on ignition. A single temperature threshold cannot make all resulting ignition times independent of the exposure and surrounding atmosphere.[6][14]
Hot gases with insufficient oxygen can damage a solid through thermal decomposition without establishing flaming. Kanury uses this example to distinguish heating from the complete ignition process. Oxygen availability, mixing and chemical reaction still matter. A conduction calculation that reaches its selected surface temperature does not automatically resolve those gas-phase conditions.[14]
The distinction between transient and persistent flaming also remains relevant after a numerical time has been obtained. A model or test must identify the event associated with that time. Removing the external heat source can change whether burning continues. The calculated onset of one event is therefore not interchangeable with every measure of sustained fire growth.[1]
See also
Notes
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Kanury (2002), chapter 2-11, p. 2-230.
- ↑ 2.0 2.1 2.2 2.3 2.4 Tewarson (2002), chapter 3-4, pp. 3-83–3-84.
- ↑ 3.0 3.1 Iqbal and Salley (2004), p. 6-1.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Iqbal and Salley (2004), p. 6-5.
- ↑ 5.0 5.1 5.2 Iqbal and Salley (2004), p. 6-4.
- ↑ 6.0 6.1 Iqbal and Salley (2004), pp. 6-1–6-2.
- ↑ 7.0 7.1 7.2 7.3 Iqbal and Salley (2004), p. 6-3.
- ↑ Tewarson (2002), chapter 3-4, pp. 3-84–3-85.
- ↑ Quintiere (1997), as cited in Iqbal and Salley (2004), p. 6-4, Table 6-2.
- ↑ 10.0 10.1 10.2 10.3 Kanury (2002), chapter 2-11, p. 2-237.
- ↑ Iqbal and Salley (2004), p. 6-7.
- ↑ 12.0 12.1 12.2 12.3 Iqbal and Salley (2004), p. 6-6, Table 6-3.
- ↑ Iqbal and Salley (2004), title page and chapter 6.
- ↑ 14.0 14.1 Kanury (2002), chapter 2-11, pp. 2-239–2-240.
References
- 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.
- Kanury, A. M. (2002). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., chapter 2-11). National Fire Protection Association.
- Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., chapter 3-4, pp. 3-82–3-161). National Fire Protection Association.