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Ignition Handbook.pdf

The Alcohol Ignition Interlock Program is another tool in our fight against drinking and driving. The ignition interlock is a proven technology that is helping to reduce drinking and driving in other parts of Canada and around the world.

Ignition Handbook.pdf

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The process of spotting occurs in wildland fires when fire-lofted embers or hot particles land downwind, leading to ignition of new, discrete fires. This common mechanism of wildland fire propagation can result in rapid spread of the fire, potentially causing property damage and increased risk to life safety of both fire fighters and civilians. Despite the increasing frequency and losses in wildland fires, there has been relatively little research on ignition of fuel beds by embers and hot particles. In this work, an experimental and theoretical study of ignition of homogeneous cellulose fuel beds by hot metal particles is undertaken. This type of well-characterized laboratory fuel provides a more controllable fuel bed than natural fuels, and the use of hot metal particles simplifies interpretation of the experiments by reducing uncertainty due to unknown effects of the ember combustion reaction. Spherical steel particles with diameters in the range from 0.8 mm to 19.1 mm heated to temperatures between 500C and 1300C are used in the experiments. A relationship between the size of the particle and temperature required for flaming or smoldering ignition is found. These results are used to assess a simplified analysis based on hot-spot ignition theory to determine the particle size-temperature relationship required for ignition of a cellulose fuel bed.

The work presented here is a combined experimental and theoretical study of fuel bed ignition by hot particles. Inert steel spheres are used to approximate firebrands/heated particles to remove uncertainty introduced with burning embers (ember temperature, char layer thickness, combustion characteristics, thermal properties, etc.). Similarly, powdered cellulose is used as the target fuel because it is homogeneous in composition and has known properties. Finally, a simplified analytical treatment based on the classical hot spot theory is reviewed and its predictive capabilities are assessed.

Using single glowing embers of Douglas Fir (5 mm or 10 mm diameter, 51 mm and 76 mm length, respectively) under air flow of 0.5 m/s or 1 m/s, Manzello et al. [7] found that smoldering ignition would occur in shredded paper but no ignition would occur in pine needles or hardwoods. For flaming embers under the same conditions, flaming ignition would occur in all fuels except hardwood mulch at 11% moisture content. Using four glowing embers, smoldering ignition could be achieved in dry hardwood mulch. However, four flaming embers were not capable of igniting hardwood at 11% moisture content. Similar results are found using disk shaped embers [4, 5] where flaming ignition occurred only when flaming embers are dropped. It was also observed that multiple flaming embers resulted in flaming ignition where single embers would cause no ignition. In general, smoldering particles are not capable of igniting fuels, whereas flaming embers will likely result in ignition of thin, dry fuels.

Electrically heated hot-spots were used by Caine et al. [8] to study the ignition of four porous fuels. As hot-spot size was increased, the power required to ignite the material was observed to increase which is believed to be explained by the existence of a critical temperature for ignition. No obvious relationship between the hot-spot diameter and the temperature required for ignition in any of the four fuels was found.

These experimental studies differ from the experiments reported in this paper because rather than inert particles, combusting embers or particles artificially maintained at elevated temperatures were used. Therefore, ignition is influenced both by the combustion reaction and heat transfer between the particle and the fuel bed.

The depth to which the hot spheres penetrated the fuel bed was not controlled and was found to vary depending on the size of the sphere. Large spheres were more likely to be partially embedded while smaller particles could be completely embedded. The depth of the particle below the surface is likely to have an effect on ignition, but this was not studied in this work.

Experiments are conducted to identify the effect of particle size and temperature on ignition of powdered cellulose. Depending on the particle characteristics, both flaming and smoldering ignition were observed. In flaming ignition, a flame kernel initiated around the hot particle; if the particle was hot enough, this flame would propagate across the free surface of the sample and eventually extinguish. In-depth smoldering would then be seen to continue for several hours.

In the case of smoldering ignition, a smolder front would be established around the hot particle. This front would then propagate laterally as well as in depth. In all cases when smoldering was ignited, the sample was seen to burn to completion. Transition from smoldering to flaming was not observed.

Figure 4 shows the ignition propensity as a function of particle size and temperature. Triangles represent direct flaming ignition, circles are smoldering ignition, and crosses represent no ignition. The data clearly show a demarcation between no ignition, smoldering ignition, and flaming ignition. It can be seen that for both flaming and smoldering ignition, smaller particles require higher temperatures than larger particles. The trends in Figure 4 are qualitatively consistent with the data of Stokes and Rowntree [2, 3]. Due to the experimental method for delivering the hot particles, it was not always possible to ensure the particles were exactly the same temperature upon landing on the fuel and obtained the same level of submergence in the cellulose. This results in some overlap between the ignition types in some cases.

For the range of particles tested, the minimum particle temperature at which smoldering could be initiated was 550C, and the minimum temperature at which flaming ignition occurred was 650C. In both cases, this was for a particle diameter of 19.1 mm. Flaming could only be observed for particles larger than 2.4 mm heated to 1200C.

Figure 5 shows energy plotted against particle size where energy is calculated using the specific enthalpy for a given particle temperature and mass of the sphere. It can be seen that a correlation between particle energy and ignition is not sufficient to explain the observed results. For example, an energy of 200 J will result in flaming ignition for particles 9.5 mm and 12.7 mm but only smoldering ignition for particles of 15.9 mm and 19.1 mm. This suggests that the ignition process is complex and governed not only by the energy of an ember but also the temperature and size in agreement with the comments by Babrauskas [24].

It can be seen from Figure 6 that the hot spot theory presented above, when provided with the input parameters given in Table 2, qualitatively reproduces the experimental data but is not quantitatively accurate. However, the hot spot theory is conservative for particles smaller than 2.4 mm in diameter that have temperatures greater than 850C because it suggests that ignition occurs for some size/temperature combinations but the experiments show that ignition does not occur.

The mechanism of flaming ignition may be somewhat different from that described by hot spot theory (thermal runaway inside the condensed phase). It is possible that a heated particle sitting on top of the cellulose bed acts as a localized heat source, causing the powdered cellulose to pyrolyze at a rate that is sufficient to produce a flammable mixture in the vicinity of the heated particle, which in turn acts as an ignition pilot. Gas-phase ignition (outside of the fuel bed) may occur if the residence time of the combustible gas mixture flowing by the heated particle is comparable to the ignition delay time of that mixture at the particle temperature. Ignition would occur in the gas-phase, and a diffusion flame then becomes anchored to the surface.

The results show as particle size is reduced, increased temperature is required for ignition. For a particle size of 2.4 mm, temperatures of 1200C were required for flaming ignition and this was reduced to 650C for particles of 19.1 mm. 350c69d7ab


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