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Show then rises above the fluid temperature during char combustion. The maximum temperature difference increases with particle size, ranging from about 1 K for the 5 J-Lm particle (A) to 20 K for the 100 J-Lm particle (D). The higher temperatures of the larger particles deserves comment because this characteristic may appear to be contrary to intuition. Turnbull, et al. [13], were the first to explicitly state that such temperature differences can occur under certain conditions because of the relatively higher heat loss for smaller particles as a result of their greater surface curvature. This observation agrees with earlier calculations reported by Field [7] and also with 2-color pyrometer measurements of particle temperature reported by Shaw and Essenhigh [11]. The values predicted in Fig. 9, ranging from a fraction of a degree to 20 K, however, are much lower than measured values reported by Obloza, et al [9], which were as high as 400 K. It confirms that large particles can sustain temperatures much higher than the neighboring fluid temperature, whereas the temperature difference between small particles and the neighboring fluid temperature is very marginal. Particle temperature history, which depends on the trajectory, determines the onset of pyrolysis and the rate at which it occurs, as shown in Figs. 10 and 11. Figure 11 gives the fraction of volatiles released for three 5 J-Lm particles (A, B, C) and three 100 J-Lm particles (D, E, F) injected at (dimensionless) radial locations of .1, .2, and .3, respectively. Particle A is the first to begin pyrolyzing, followed by B, D, E, C, and F. From Fig. 10, this order is consistent with the order in which particles reach the required temperature for pyrolysis (about 800 to 900 K; normalized particle temperature .35 to .4). Those particles ~ that are injected closer to the center of the air jet (A, B, D, E) reach pyrolysis temperature sooner, as illustrated in Fig. 10, because they are traveling significantly faster downstream towards the peak flame temperature. As indicated by the greater slopes for particles A, B, D, and E in Fig. 11, higher rates of pyrolysis also occur for those particles injected closest to the centerline of the burner because of their higher particle temperatures during pyrolysis. Figure 12 shows the unburned fraction of char, Ub , for three 5 J-Lm particles (A, B, C) and three 100 J-Lm particles (D, E, F) injected at (dimensionless) radial locations of .1, .2, and .3, respectively. As expected, the reaction efficiencies, 1} = ( 1 - Ub ), for the smallest 10 |
