OCR Text |
Show CH was detennined in a rich flame. Fig 11 shows a profile 9 mm above the burner exit tube supporting the premixed flame. Here only LIF is detected. The cut lies below the tip of the premixed flame, thus encompassing both flames. The CH appears only in the rich premixed flame. It does not survive past the flame front, so there is no initiation of prompt NO formation downstream of that point In this case, the CH signal profiles have not been corrected for quenching. As seen below, the CH exists in a region of sharp temperature gradient Changes in quenching might occur along this profile, due to density and composition changes along this temperature gradient It is useful to know where the CH exists within the temperature gradient marking the premixed flame front. Because Rayleigh scattering depends on the total gas density, which is inversely proportional to the temperature, it can be used to approximately map the temperature profile. In Fig. 12, the monochromator is tuned to pass both LIF and Rayleigh scattering. The region shown is through one of the premixed flame fronts cut by the laser beam. The Rayleigh signal decreases before the CH peak arises, thus indicating that the major density drop takes place before the CH appears. Furthermore, because the rate coefficient for the CH + N2 reaction is highly temperature dependen~ the exact unbumtgas rich intercone exhaust 14 12 fr-CH Rayleigh + _ 10 Background :j >. \ .!. ii 8 ~\ c Q Cij 6 • \ 4 ~ 2 0.0 0.2 0.4 0.6 relative position [em I Figure 12. Simuttaneous measurement of the Rayleigh and CH signal along the premixed flame front. The diagram shows the density drop due to the temperature rise and the appearance of CH. positioning of the CH peak within this temperature gradient will significantly influence prompt NO production. CONCLUSION Two important tools, detailed computer modeling and laser diagnostics, have been applied to the understanding of flame structure and prompt NO formation in natural gas flames. A key issue is the observation of the CH radical, which is responsible for the breakage of the N -N bond in air nitrogen. LIP measurements in spatially resolved low pressure flames yield absolute concen- 16 |