Laser-induced fluorescence measurements and modeling of nitric oxide in high-pressure premixed flames

Laser-induced fluorescence (LIF) has been applied to the quantitative measurement of nitric oxide (NO) in premixed, laminar, high-pressure flames. The chemistry of these flames was also studied using three current chemical kinetics schemes so as to determine the predictive capabilities of each mechanism with respect to NO concentrations. The flames studied were low-temperature (1600 $\u3c$ T $\u3c$ 1850 K) $\rm C\sb2H\sb6/O\sb2/N\sb2$ and $\rm C\sb2H\sb4/O\sb2/N\sb2$ flames, and high-temperature (2100 $\u3c$ T $\u3c$ 2300 K) $\rm C\sb2H\sb6/O\sb2/N\sb2$ flames. The NO formed in the low-temperature flames was predominantly prompt-NO, while the NO formed in the high-temperature flames was predominantly thermal-NO. It was initially desired to use laser-saturated fluorescence (LSF) to measure the NO concentrations. However, while the excitation transition was well saturated at atmospheric pressure, the fluorescence behavior was basically linear with respect to laser power at pressures above 6 atm. It has been demonstrated through measurements and calculations that the fluorescence quenching rate variation is negligible for a set of LIF measurements of NO at a given pressure. Therefore, linear LIF could be used to perform quantitative measurements of NO concentration in these high-pressure flames. For the low-temperature flames, it was found that the equivalence ratio corresponding to the peak (NO) at a given pressure shifted towards leaner conditions with increasing pressure. Chemical kinetics calculations using the coupled species-energy equations gave generally acceptable NO predictions for the ethane flames, but less acceptable results for the ethylene flames. The LIF measurements of (NO) in the high-temperature flames were also compared to NO predictions using the energy solution; however, the input temperature profile found from the energy solution was scaled to a post-flame temperature measured using Rayleigh scattering. These results showed acceptable agreement between the measurements and predictions, but demonstrated the need for more precise temperature measurements. The transportability of a calibration factor from one set of flame conditions to another also was investigated by considering changes in the absorption and quenching environment for different flame conditions. Finally, the feasibility of performing LIF measurements of (NO) in turbulent flames was studied; the single-shot detection limit was determined to be 2 ppm