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Show Ultra Low N0X Burner Test Case By design, this burner should produce a long, slow-moving flame with a large fuel-rich region to reduce N O x . This is achieved by starving the fuel introduced through the burner tile of 02. To complete combustion, tertiary air is introduced from the two sides of the tile (see Figure 2). The radial profiles of 0 2 and C O , measured across the plane that cuts through the two tertiary air jets, are shown in Figure 8 at selected axial distances away from the burner. Indeed a large fuel-rich region (high C O and low 02) of about 0.2 m in diameter is observed along the furnace central axis at most of the measurement ports. In addition, the two prominent tertiary air jets are clearly displayed by the 0 2 measurements. A comparison between predicted and measured C O and 0 2 values is also shown in Figure 8. The simulation captures the overall 0 2 variation very well whereas the predicted peak C O levels at the furnace axis are consistently lower than the experimental data. A s mentioned before, the application of a non-premixed flamelet model in this partially premixed fuel-rich region has limitations on C O prediction. Aside from these limitation, another factor may have contributed to the. discrepancy. The simulation assumes that the secondary air enters straight through the 4 slots in the burner tile (see Figure 2) in uniform proportion. In reality, the secondary air m a y enter in a different fashion, allowing the natural gas stream to better preserve its momentum to produce a peak C O along the furnace central axis whereas the uniform assumption results in a rather flat C O profile across the fuel-rich region. The measured temperature profiles in Figure 9 reflect the flame characteristics observed in the C O and 0 2 concentration plot. The predicted temperature values are close to the measured ones with a slight over-prediction along the furnace axis (possibly due to the incorrect CO prediction there). The quality of the predicted temperature pdfs is the same as discussed in the High N O x Burner test case. In Figure 10a, a typical comparison between the measured and calculated temperature pdf along the central axis shows that the simulation predicts a higher mean temperature. In regions near the flame front (see Figure 10b), the measured pdf is actually 'bi-modal'. This is due to the flame moving in and out of the measurement zone during the experiment. W h e n the local flame is in the measurement zone the mean temperature is high because of the presence of a mixture of burned and unburned gases - resulting in a broad, high temperature pdf. W h e n the recirculated gases are in the measurement zone the mean temperature is lower and the mixture would be of a more homogeneous temperature mixture - resulting in a narrow low temperature pdf as seen in Figure 10b. The simulation predicts the higher temperature pdf of the flame front very well. Further off the central axis (see Figure 10c), the measured temperature pdf is again 'bi-modal' because the flame occasionally drifts into the measurement zone creating a small high temperature pdf overlayed on the low temperature pdf. The correct temperature distribution is the lower temperature pdf which is captured by the simulation. The measured and calculated N O x characteristics are shown in Figure 11. Both the measurements and predictions indicate a large low N O region occupying the centre of the furnace corresponding to the size of the fuel-rich zone described above. This is expected because by burning the fuel in an oxygen-lean environment, the overall flame temperature drops thus reducing the effectiveness of the thermal N O mechanism. Also, O-atoms are scarce within the fuel-rich zone and the key N O formation steps Rl and R5 are inhibited. In addition, CHj radicals are plentiful in this region to consume any N O present through reactions R 8 and R9. Across the thin transitional zone between the fuel-rich region and the tertiary air jets, the N O concentration is predicted to rise rapidly because the temperature at this interface is high (see Figure 9) and O- 8 |