Piloted ignition

Often combustion is initiated in a mixture of fuel and oxidizer by a localized source of energy. This source might be an electric arc (or spark) (moving charged particles in a fluid or a plasma) or a small flame itself. Because the spark or small flame would locally raise the temperature of the mixture (as 7 X did in the autoignition case), this case is defined as piloted ignition. The bulk of the mixture remains at T, well below the AIT. 

Hence the minimum value of 8 to cause ignition is smaller for a system under the heating of a pilot compared to autoignition, e.g. Equation (4.18). 

U-shaped curve, we have mixtures that can be ignited for a sufficiently high spark energy. From Equation (4.25) and the dependence of the kinetics on both temperatures and reactant concentrations, it is possible to see why the experimental curve may have this shape. The lowest spark energy occurs near the stoichiometric mixture of XCUi =9.5%. In principle, it should be possible to use Equation (4.25) and data from Table 4.1 to compute these ignitability limits, but the complexities of temperature gradients and induced flows due to buoyancy tend to make such analysis only qualitative. From the theory described, it is possible to illustrate the process as a quasi-steady state (dT/dt = 0). From Equation (4.21) the energy release term represented as 

In dimensionless terms, there is a critical value for S (Damkohler number) that makes ignition possible. From Equation (4.23), this qualitatively means that the reaction time must be smaller than the time needed for the diffusion of heat. The pulse of the spark energy must at least be longer than the reaction time. Also, the time for autoignition at a given temperature T is directly related to the reaction time according to Semenov (as reported in Reference [5]) by 

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Ignition controls shall include upstream (in flare header prior to knockout drum) dual flow sensing equipment which shall start the automatic flare purge, pilot ignition and the flare ignition cycle. N P Refinery will be responsible for the wiring between the flow sensors and the ignition control panel. 

On proving the pilot ignition, the main gas valve opens and the burner ignites. Once alight, the main burner will modulate to the temperature set by the room thermostat. 

Minimum energy required for piloted ignition of wood, melting of plastic tubing... 

Determination of the minimum radiant heat flux to sustain piloted ignition. Harkleroad, Quintiere and Walton (3) determined q"n from... 

I. A preheat region in which the heat transfer from the flame brings the unbumed mixture to its critical temperature for ignition, T[g. This is much like what occurred in describing auto and piloted ignition, except that the the heat is supplied from the flame itself. 

Figure 6.2 Dynamics of piloted ignition for a liquid fuel in air under convective heating from the...

Example 6.1 Estimate the minimum piloted ignition temperature for methanol in air... 

The minimum temperature for piloted ignition is given by A) = Xg(7L)- From Equation (6.18),... 

The convective heat transfer coefficient may be approximated as that due to heat transfer without the presence of mass transfer. This assumption is acceptable when the evaporation rate is small, such as drying in normal air, and for conditions of piloted ignition, since XL is typically small. Mass transfer due to diffusion is still present and can be approximated by... 

Let us just consider the piloted ignition case. Then, at Tpy a sufficient fuel mass flux is released at the surface. Under typical fire conditions, the fuel vapor will diffuse by turbulent natural convection to meet incoming air within the boundary layer. This will take some increment of time to reach the pilot, whereby the surface temperature has continued to rise. 

Finally, we estimate the order of magnitude of the time of the heated solid to achieve Tpy at the surface. This is primarily a problem in heat conduction provided the decomposition and gasification of the solid (or condensed phase) is negligible. We know that typically low fuel concentrations are required for piloted ignition (XL 0.01-0.10) and by low mass flux (mv 1-5 g/m2 s) accordingly. Thus, a pure conduction approximation is satisfactory. A thermal penetration depth for heat conduction can be estimated as... 

Figure 7.3 Piloted ignition of wood particle board (vertical) by radiant heating [3]...

Table 7.1 lists heat flux levels commonly encountered in fire and contrasts them with perceptible levels. It is typically found for common materials that the lowest heat fluxes to cause piloted ignition are about 10 kW/m2 for thin materials and 20 kW/m2 for thick materials. The time for ignition at these critical fluxes is theoretically infinite, but practically can be (9(1 min) (order of magnitude of a minute). Hashover, or more precisely the onset to a fully involved compartment fire, is sometimes associated with a heat flux of 20 kW/m2 to the floor. This flow heat flux can be associated with typical... 

Figures 7.8(a) and (b) display piloted ignition results for a metalized polyvinyl fluoride (MPVF) film of 0.2 mm thickness over a 25 mm fiberglass batting [14]. The MPVF film was bonded to a shear glass scrim (with no adhesive or significant thickness increase) to prevent it from stretching and ripping. The unbounded MPVF film was also tested. This shows several features that confirm the theory and also indicate issues.

Figure 7.8 (a) Piloted ignition of glass-bonded MPVF and ripping times for unbonded MPVF... 

Table 7.3 Piloted ignition properties of wood species [ 11 ...

Scatter in these data indicate accuracy, but they suggest that the earliest ignition time would be piloted, followed by glowing and then autoignition. This still may not preclude the effect of smoldering on piloted ignition at very low heat fluxes, as occurs with autoignition at 40 kW/m2. 

Table 7.4 clearly shows that the ignition temperature for autoignition is considerably higher than for piloted ignition. In conformance with this behavior, Boonmee [15] indicates that the corresponding fuel mass fractions needed are about 0.10 and 0.45 0.15 for the pilot and autoignition of redwood respectively. 

Table 7.5 Piloted ignition and flame spread properties from ASTM 1321 [18]...

A flame radiates 40 % of its energy. The fuel supply is 100 g/s and its heat of combustion is 30kJ/g. A thin drapery is 3 m from the flame. Assume piloted ignition. When will the drapery ignite The ambient temperature is 20 °C and the heat transfer coefficient of the drapery is... 

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