Ignition Handbook.pdf
Ignition Handbook.pdf ===> https://shurll.com/2tavkA
The hot spot theory suggests a rather complex relationship between particle temperature, size, and energy required to ignite the particle. This is supported by the results of the experiments with glowing embers, where the embers were not all ignited at the same time and under similar conditions. Furthermore, an ember must be dropped from a specific height to ignite it. This suggests that the ignition of a particle is a complex process that includes local heating of the particle surface and the flame front propagating into the particle. This is in agreement with the finding that multiple flaming embers are required to ignite a particle and with the identification of the ignition as an independent process from the combustion process of the fuel. However, the complex nature of the ignition process is also supported by the finding that the ignition of a particle can be achieved with particle temperatures between 600°C and 1200°C, where the temperature is not sufficient to initiate combustion of the fuel.
The prediction of Babrauskas [24] that a spherical particle will only ignite around and above the critical size is observed in the experimental data. As seen in Figure 6, ignition is only predicted for particles smaller than 2.4 mm at particle temperatures higher than 850°C. Furthermore, the prediction of a critical particle size is also true for the theoretical prediction using the hot spot theory. For example, using the hot spot theory to predict the ignition of a particle with diameter 0.8 mm at temperatures between 700°C and 1200°C, the critical energy is ~2400 J. Figure 7 shows that for particles larger than ~2.4 mm, energy above ~2400 J is required to ignite the particle. The hot spot theory suggests that the critical energy for ignition is independent of particle size, where about 25% of the surface area of the sphere is hot. The energy required to ignite particles decreases with increasing particle diameter. This decrease is not due to the decrease in hot spot area because the total surface area available for ignition is relatively constant, but rather a decrease in the actual temperature of the hot spot. As the hot spot temperature is decreased, the threshold energy required to ignite the particle is increased. The effect of temperature on the critical energy is shown in Figure 8, where the hot spot energy required to ignite a sphere of diameter 2.4 mm is predicted at 1100°C, 630°C, and 500°C. These three temperatures are representative of the experimental data obtained from the experiments described in the previous section.
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