OCR Text |
Show conditions, air-side dilution is more effective than fuel-side dilution. Comparing the results for fixed fuel mass flux (Cases 2 and 4), however, we see that fuel-side dilution is now more effective than air-side dilution. TABLE III. Contributions of Various Factors to NO Emission Index (CH4-air, T = 300 K) Parameter Case No. Fuel Consumption (moVcm2-s) NO Production (moVcm2-s) NO Destruction (moVcm2-s) Net NO Formation (moVcm2-s) Ratio of Production to Destruction NO Emission Index (g/kg) ---uo = 50 cm/s --No Dilution Air Dilution Fuel Dilution (Z=O) (Z=0.15) (Z=0.15) 1 5.87.10-5 35.9.10-9 23.6.10-9 12.3.10-9 1.52 0.39 2 5.00.10-5 16.4.10-9 9.55.10-9 6.85.10-9 1.72 0.25 3 4.20.10-5 13.6.10-9 7.08.10-9 6.52.10-9 1.92 0.29 -uo = 128cm/sFuel Dilution (Z = 0.15) 4 6.3410-5 3.10.10-9 1.17.10-9 3.93.10-9 3.50 0.086 Insights into the differential effects of air-side versus fuel-side dilution are obtained by looking at the various factors involved in the NO emission index calculation. To detennine the NO emission index, both the net NO fonnation rate and fuel consumption rate are required, i.e., the numerator and denominator of Eqn. 1, respectively. Table III shows these quantities for the four conditions considered here. Also shown in Table III is a breakdown of the net NO formation into production and destruction components for each flame condition. These two components are the positive and negative areas, respectively, under the volumetric NO production rate-versus-distance curves shown in Fig. 6. From Table III, we see for both air- and fuel-dilution at the Uo = 50 cm/s condition that the fuel consumption rates are decreased from the no-dilution case, with the greater decrease associated with fuel dilution. These reduced fuel consumption rates tend to increase the NO emission indices. However, since the net NO fonnation rates are also decreased, the combined effect is a lowering of the emission indices, as expected with dilution. Note that since the NO fonnation rates are nominally equal for air- and fuel-dilution (Cases 2 and 3), the factor causing the fuel-dilution to be less effective is the decreased fuel consumption rate. For fuel dilution with a fixed fuel mass flux (Case 4), however, the fuel consumption rate increases. This has the effect of decreasing the NO emission index. The steeper concentration 8 |