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Show however, that the initial particle size distribution may not represent the final distribution in the engine after atomization of the fuel (Holve, et al., 1985; Chigier and Meyer, 1984). During the atomization process, particles may agglomerate to form particles 2 or 3 times the sizes in the initial distribution and, then burn as a single large particle. The actual ignition and combustion characteristics in the cylinder, therefore, may reflect a distribution of particles with a larger mean size than the initial particle distribution of the fuel. For purposes of this study, a collection of discrete particle sizes were selected to represent the distribution of the coal fuel after atomization. Since the precise distribution in the cylinder is unknown, distributions were selected which would accentuate the ignition characteristics of the smaller particles and the burnout characteristics of the larger particles. One distribution considered was comprised of 20%(by mass) 10 micron, 60%(by mass) 15 micron, and 20%(by mass) 30 micron particles, yielding a sauter mean diameter (SMD) of 15 microns. Figure 6 shows the indicated thermal efficiency as a function of initial gas terapertature for the coal fuel with this distribution as a dashed line. Also, for reference, this figure shows results for raonosize 10 micron, 30 micron and SMD (15 micron) particles. As shown in the figure, the dashed line is slightly skewed from the SMD results indicating the ignition is characterised by the smaller particles at the low initial gas temperatures. At the higher temperatures, combustion is limited by the 30 micron particles. Other coal fuel distributions were studied and these Frueeslu lAtdsd iatriev erse ported by Bell and Caton (1985b). The ignition characteristics of the fuel and the engine performance were investigated utilizing diesel oil as an ignition improver for the CWM. Two scenarios for injecting the secondary fuel were considered. In both cases 30 micron particles were assumed. In the first, a small amount of diesel fuel was added to the slurry prior to injection. Thus a coa1/water/diesel mixture was injected into the cylinder. Figure 7 presents the indicated thermal efficiency of the engine as a function of the initial (at BDC) gas temperature for four different coa1/water/diesel mixtures. Figure 7 shows ignition and efficient engine operation may be attained by replacing a small amount of the water with diesel. For example, 5% (by mass) diesel oil additive results in a maximum efficiency for an initial gas temperature of 450 K whereas CWM (no diesel) fuel results in a maximum efficiency for an initial gas temperature of about 530 K. That is, as the amount of diesel additive is increased, the initial gas temperature required for ignition decreases. Figure 8 shows the computed ignition delay for each of the four coal fuels as a function of the initial gas temperature. This figure shows that the ignition delay decreases as the L initial gas temperature increases or the amount of diesel additive increases. As previously seen, for high initial gas temperatures the ignition delay attains a constant, minimum value. This minimum ignition delay is about 6 CA's for the conditions and fuels examined here. Similar results have been reported by Siebers and Dyer (1983). In the second scenario, the diesel oil was assumed not mixed with the CWM but to be separately injected as a pilot fuel. Two cases were considered: (1) 5% pilot diesel oil injected into the first 25% (first 5 CA's) of the coal fuel slurry spray, and (2) 5% pilot diesel injected into the last 25% (last 5 CA's) of the coal fuel spray. Figure 9 shows the indicated thermal efficiency as a function of the initial gas temperature for these two cases as well as for the previous case where the diesel was mixed with the CWM. As shown, mixing the diesel with the coal slurry is the most advantageous for these conditions. The second best alternative is to use pilot-fuel combustion in the first 25% of the coal spray. The ignition delay was the shortest for the 5% diesel mixed case and was only slightly longer for injecting in the first 25% of the spray (Bell and Caton, 1985b). Overall, the use of diesel oil as a pilot fuel was effective in reducing the Cioganlit iPorno pdeerltiaeys for the conditions studied. The sensitivities of the ignition and combustion processes to the surface reaction rate kinetics and to the devolatilization rates were investigated. The results provide information on the impact of using coals with higher reactivities on the ignition and combustion characteristics. The results of this phase of the study were obtained for a 50/50 CWM with 15 micron monosize particles. Figure 10 shows the indicated thermal efficiency as a function of the initial gas temperature for three coals (i.e., the surface reaction rate kinetics were multiplied by factors of 50, 10, and 1). The initial gas temperature required for maximum performance decreased for increases in particle reactivity (e.g., for the coal with a factor of 50 rate increase, the required temperature was about 325 K where as for the coal with no rate increase (factor of 1), the required temperature was about 400 K for peak performance). In general, the maximum efficiency was not affected by the coal reactivity showing only a slight increase with increase in reactivity. This indicates the majority of the combustion process is diffusion limited. At a given temperature, however, it was found that increasing the reactivity of the coal could slightly lower the performance. For example, at 400 K the indicated thermal efficiency with the mid-reactivity fuel was above«that of the highest reactivity fuel. This was a consequence of constant air entrainment rates for all three cases which, for the high reactivity coal, resulted in less total air entrainment due to the later fuel injection. That is, ignition occurred earlier in the injection process, before much of the air is mixed into the jet. This is analagous to reducing \ J |
