OCR Text |
Show not significant at the centre line where there is a low concentration of oxygen atoms, however after the i ntroduct i on of the secondary air, the oxygen atom concentrat ion increases di ffus i ng towards the centre 1 i ne, leading to the formation of NO. The thermal NO concentration reaches its maximum value in the areas near the flame front, however the concentration diminishes considerably in the region just downstream inside the furnace. Th is is due to d i ffus i on of NO from regi ons where it is formed. The concentration of thermal NO becomes more uniform downstream inside the furnace reducing to 1270 ppm at the end part of the furnace. In general, the resul ts i ndi cate that the thermal-NO · product i on rate is 1 i mi ted to narrow regions around the local maximum temperature because of the large activation energy of thermal-NO kinetics combined with the sharp temperature maximum in the fluctuating diffusion flame1et. Therefore the NO production will depend on the maximum temperature of the flame and less dependent on the turbu1 ent mean temperature. Figure 6b shows that the production of prompt-NO reaches its maximum value in the regions close to primary air injection port and around the centre 1 ine of the burner. It is assumed that high CH concentration in this region contributes to the format i on of prompt-NO. However the contri but i on of prompt-NO to the level of total NO emitted is less than 10%. Figures 7{a-c) show the effects of removing both 1 iners from the burner and changing the inlet velocities of the preheated air on thermal, prompt and total NO format ion. There is a small reduct ion in max i mum fl arne temperature, however because of the fl ame shape and 1 oca 1 i sed hot region and removing the secondary air the total NO concentration has increased by around 4%. It is interesting to observe that the contribution of the prompt-NO to total NO increases by 6-8% indicating the effect of secondary air on NO formation. In general the rate of formation of NO follows the velocity and temperature field coupled with the distribution of oxygen and fuel concentration inside the burner. In order to invest i gate the affects of superequi 1 i bri um 0 and OH radical on the rate of NO formation, the model was used without imposing the 0 and OH radicals overshoot and assuming a partial equilibrium cond it ion. The resu1 ts shown in Fi g. 8 for concentrat i on of thermal-NO, indicate that the model underpredicts the level of NO emissions by approximately 28%, in particular in the flame zone. Comparing experimental resul ts wi th computat ion resu1 ts, the importance of i ncorporat i ng superequi 1 i bri um radi cal s concentrat ion is obvi ous. The affect of superequilibrium on NOx formation rates in the extended Zeldovich mechanism is not very strong, because high 0 and OH concentrations are compensated by a lower flame t~mperature. One way to account for the importance of fl uctuat ions of speci es concentrat i on and temperature in rate of NO product ion is to study the calculated pdf at various flame positions. by careful evaluation, one is able to detect any unrealistic result predicted by the model and thus to identify their applicable regions. Figure 9 shows some of the calculated pdf's at various flame positions for thermal-NO prediction when VI and V2 were temperature and oxygen mass fraction respectively, and the associated mean values. Figure 10 shows the rate of production of thermal and prompt-NO when there was no turbulence (lOa, c) and with turbulence (lOb, d) respectively. The effect of turbulence has caused a reduction in the production rate of thermal-NO and a shift towards the flame region. Figures 10(c-d) show that the 9 |