| OCR Text |
Show present experiments, particularly for the low-Stp regime. Several scenarios can be envisioned to produce a source of unburned hydrocarbons. First, it is possible that, at some locations within the flame, combustion reactions are quenched by very rapid mixing with cold air. A second possibility is that, prior to any combustion taking place, the rapid mixing produced by the precessing jet results in locally premixed conditions beyond the lean flammability limit A third scenario is that, .. as a result of the precessing motion, some fuel may be swept away from the fuel jet very close to the nozzle exit Regardless of the exact physical mechanism responsible, the data clearly suggest that some unburned fuel is available to react at relatively low temperatures, say, 1000-1200 K (Hori et al., 1992), to form CO and promote the conversion of NO to N02. Hori et al., and others, have shown that the dominant mechanism for the oxidation of NO to NCh when hot product gases are mixed with air is NO + H02 ¢:> N02 + OH . (3) With hydocarbons present in the product gases, the supply of H02 radicals is greatly enhanced and NO conversion promoted, if temperatures are within an appropriate band (Hori et al 1992). Hori et ale (1992) found that C3Hg is more than seven times as effective as Cf4, on a per-carbon-atom basis, in enhancing the of conversion NO to ND2. Although unburned hydrocarbons were not measured in the present experiments, it seems likely that the above mechanism is at work. As can be seen in Fig. 9, near unity N(h-to-NOx ratios were observed over a wide range of Strouhal numbers for the C3Hg (de = 10 mm) precessing jet flames; while for the CRt (de = 10 mm) flames, NCh-to-NOx ratios fell to values near 0.6 at the highest Strouhal numbers. For the de = 3 mm precessing flames, the N02-to-NOx ratios are also much higher for the C3Hg flames in comparison to the Cf4 flames. These observations are consistent with the. hypo~esis of hydrocarbon-enhanced NO-to-NCh conversion and the greater effectiveness of C3Hg over Cf4 In practical applications, where the availability of oxygen in the flame is, by design, limited, high ratios of NCh to NOx are not likely to occur, since temperatures will always be much higher than the required 1000-1200 K range. Experimental measurements from a cement kiln (Nathan and Manias, 1995), where the ratio of N02 to NOx was found to be of the order of 2-5% when the concentration of 02 at the back of the kiln was about 2%, support this contention. N Ox Emission Indices NOx emission indices are presented in Fig. 10, which shows the effects of Strouhal 10 |
