OCR Text |
Show between x = 1.8 to 4.6D, the 0 2 data taken across the furnace diameter do not collapse to form a single radial profile indicating that the flame is not fully symmetric as presumed. Overall, the measured combustion characteristics are captured quite well by the simulation as shown by the solid curves in Figures 4 and 5. For axial distance less than 4.6 D, both the measured and predicted radial profiles indicate the presence of a high temperature fuel-rich zone (high C O , low 02) near the furnace axis, a transition zone at radial distances between 0.05 and 0.06 m where the flame front is located (with sharp changes in C O and 02) and ultimately a region far away from the axis primarily filled with combustion products (flat 0 2 profile and negligible C O ) . The flamelet model is able to predict the 0 2 depletion near the axis but generally under-estimates the C O levels (see Figure 4). Perhaps this region is partially premixed and can not be adequately represented by the non-premixed flamelet model. Also, the fast chemistry assumption in the flamelet model may introduce some inaccuracy in accounting for the slower C O reactions. Consistent with the under-prediction of C O , the calculated temperature level in this region is higher than that measured (see Figure 5). Figure 6a shows the comparison between predicted and measured temperature pdfs on the furnace axis at x = 1.8 D. Both the predicted pdf shape and mean temperature are different from that measured. However, at the transition zone near the flame sheet, the predicted and measured temperature pdfs are almost perfectly matched (see Figure 6b). The predicted 0 2 and C O levels at this transition region are also very close to the measured values as shown in Figure 4. This is not surprising because the flame sheet region is where the non-premixed flamelet model should work well. At regions far away from burner and the flame (e.g. x = 12 D or radial distance greater than 0.2 m ) , the flamelet model is able to capture the complete combustion limit. The mean temperature (see Figure 5 and 6c), 0 2 and C O levels (see Figure 4) are all reasonably well predicted. The pdf measured far off axis (shown in Figure 6c) is broader than predicted possibly because the C A R S temperature measurement technique is limited to a resolution of 40-50 K. In Figure 7, the predicted N O concentration is seen to increase sharply across the thin flame front. The increase is attributed to the high temperature in the flame front and the availability of O-atoms to initiate the thermal N O mechanism (Rl to R3). Also, the H C N and N created by the reaction between C H and N 2 (see R4) in the sub-stoichiometric environment can be oxidized by O and OH-radicals to form N O (see R5 to R 7 and R3). Once the N O formation achieves a stable level through the flame front, the N O species is convected to the rest of the furnace. Because there is no fuel-rich region beyond the jet flame for N O reduction, the N O distribution becomes very uniform beyond the flame front. These N O characteristics predicted by the N O model are substantiated by measurements as shown in Figure 7. For x = 1 to 1.8D, the predicted sharp increase of N O through the flame front in the radial direction is confirmed by the measured values. From an axial distance of x = 2.5D onward, the measured N O radial profiles become fairly flat while the prediction continues to show a sharp change until x = 3.9D. Possibly, at these axial distances the tail end of the jet flame may be unstable and begin to 'flap' and hence make the measurement of N O at the furnace centre higher than reality. Nevertheless, the N O model is able to predict accurately the uniform N O level beyond the flame front. At x = 12D, the predicted and measured N O levels are virtually identical (3 p p m difference). 7 |