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Show < temperature IK I : *. • ^ ^ • --Jii • u -10 0 20 60 100cm FIGURE 9: TYPE-1 FLAME; COMPUTED AND MEASURED TEMPERATURE AND OXYGEN INSIDE THE AAS BURNER; ASM COMPUTED, K-e COMPUTED; x - MEASURED VALUES (-lOcm.Ocm lOOcm-Distancc From The Furnace Front Wall) Tvoe-1 Flames And Corresponding Isothermal Flows When considering type-1 flames the interaction between a non-swirling, particle-laden stream and the swirling combustion air stream is an important phenomenon. Capabilities of the turbulence models to predict this interaction are examined here using an ambient-air flow measured by Hagiwara et al., (1986). Figure 8 shows a comparison between the measured, k-e computed and ASM-computed flow patterns. A primary non-swirling jet of Up=5 m/s inlet velocity interacts with a swirling annular flow of the same axial velocity but of 1.5 inlet swirl. The measured full penetration of the primary air through the IRZ is not predicted when the k-e model is used (detailed comparison of the measured and computed velocities and turbulence can be found in Visscr, 1991.) The discrepancies between the k-e flow predictions and the measured flow may have large consequences for the predicted characteristics of the solid phase in swirling pulverized coal flames. A simulation with the k-e model would indicate that most of the coal particles, injected with the primary air, do not penetrate the IRZ. A simulation with the A S M would indicate substantial penetration. Figure 9 shows the measured and predicted temperature and oxygen fields in the burner zone of a type-1 pulverized coal flame. Generally, the agreement between the measurements and predictions is worse than for the type-2 flame. Clearly, the A S M results being far from perfect, simulate the primary jet penetration better than the k-e predictions. Figure 7 shows a type-2 flame of 1070 ppm NOx (3% O2) flue emissions while in Figure 9 a type-1 flame of 430 ppm N O x is shown. Both flames were generated using the A A S Burner firing a Coal Valley coal. The only difference between the burner input conditions for the two flames was the positioning of the coal injector. In the lypc-2 flame, the coal gun was positioned at the -Uk • , , , 1 , Ul_^ _x ^ ^l_ 500 1000 1500 (K) 0 5 10 15 20 2 5 % femperature oxygen FIGURE 10: ALTERNATIVE SOLUTIONS FOR TYPE-1 F L A M E ; ASM-PREDICTIONS A T T H E Q U A R L O U T L ET (0cm From The Furnace Front Wall- sec Figure 9) solution shown in Fig. 9; alternative solution burner quarl inlet while in the type-1 flow the coal injector was inserted 0.4 quarl diameter inside the burner quarl. Examining Figures 8 and 9 may lead to a conclusion that a second-order turbulence model is required for predicting type-1 flames. That would certainly be suggested if it had not been experienced that for the type-1 flame considered another mathematical solution, different to that presented in Figure 9, could be found. This alternative solution is shown in Figure 10. These two fully converged ASM-predictions arc obtained with identical inlet and boundary conditions. Moreover, it is possible to transform the two solutions into each other by applying a "little" disturbance to the flame region located in the close vicinity of the fuel injector as demonstrated by Visser et al., (1990). Needless to say, the alternative solution (Figure 10) which was obtained with the A S M applied, is very similar to the k-e results shown in Figure 9. The question arises whether the existence of the two mathematical solutions, one indicating a full penetration type-1 flame (the A S M Predictions shown in Figure 9) and the second one showing a partial penetration type-1 flame (the k-e predictions shown in Figure 9 or the A S M predictions of Figure 10) represents reality. Morgan and Dekker (1988) have experimentally confirmed that full penetration and partial penetration flames could be generated using the A A S Burner. Which type of the penetration was observed was dependent on whether the final burner set-up was achieved by pushing the coal injector into the burner quarl or by pulling it back from the far-insertion position. Thus, within a certain range of burner operational parameters both the full penetration and partial penetration flames could be stabilized due to hysteresis. The above discussion clearly indicates that designing a burner which would produce type-1 flames, cannot be based entirely on the mathematical model predictions. This is because the degree of IRZ penetration by the primary air jet is not only a function of the burner set-up but is also strongly dependent on the hysteresis effect present. In general, the flow field of type-1 flames is very difficult to compute with the necessary confidence. TEMPERATURE, OXYGEN AND CARBON MONOXIDE Predictability of temperature, oxygen and carbon monoxide fields are examined using flames of hvBb coal of the following ultimate analysis (daf): C 0.754, H 0.045. N 0.009. S 0.003. 10 11-11 |
