OCR Text |
Show From the slopes we deduce lifetimes of 65 ± 3 ns and 54 ± 3 ns for the A and B states, respectively. The corresponding radiative lifetimes are 540 ns and 350 ns, so that even at 40 Torr only about 15% of the molecules radiate. Extrapolating to annospheric pressure, we expect a lifetime of 3 ns, so the quenching cannot be directly measured under those conditions with a 10 ns long laser pulse. In that case, quenching must be estimated by knowledge of the flame gas composition and individual quenching rate coefficients for each species present The geometrical factor D was detennined by two independent means, Rayleigh and Raman scattering. These are spontaneous scattering processes unaffected by collisions; scattering cross sections in N2 and H2, the gases used here, are well known. It is important to use the same laser, geometry, and detection system for both the calibration and the LIF signals. Not shown in Eq. (2) is an additional small correction, about 30%, that we must apply which accounts for overlap of the finite bandwidths of the laser and the absorption line. With two different band systems, and two types of scattering calibrations for each, we have available four independent measurements of the absolute CH concentration. These are averaged to produce the result shown in Fig. 5. In the stoichiometric flame, the measured concentration is 5.8 ± 1.5 ppm. The predicted value of 8 ppm is in good agreement In the rich flame the agreement is not so good: 7.8 measured vs. 13 predicted. Sensitivity analysis shows that the predicted NO concentration is directly related to CH concentration through the CH + N2 reaction. The model is consistent because it provides excellent agreement for NO in the stoichiometric flame: 15 ppm predicted vs. 14 ppm measured. However, the prediction is too high in the rich flame: 30 ppm predicted and 19 ppm measured. We conclude that the chemistry of NO fonnation is reasonably well understood, but some rate coefficients still need some improvement. Likely possibilities include the reactions CH + N2 ~ HeN + N and CH + H20 ~ H + CH20. ATMOSPHERIC PRESSURE FLAME EXPERIMENTS Most residential and commercial appliances employ burners featuring partially premixed flames, flames of a Bunsen type. These flames consist of two different burning stages, a rich premixed flame forming an inner cone and a diffusion flame constituting an outer cone. Such a flame forms a complex system of chemical and flow processes, so that understanding NO formation in these flames requires detailed infonnation on the flame structure. Due to the complexity of the system, modeling must go hand in hand with the experimental studies. 12 |