Laser-induced fluorescence measurements and modeling of nitric oxide and methylidyne in laminar, counter-flow partially premixed flames at high pressure

Oxides of nitrogen (NOx) have considerable environmental impact while the methylidyne radical (CH) plays a crucial role in basic kinetic processes leading to nitric oxide (NO) formation in flames. Implementation of complicated CFD codes in the design of future gas-turbine combustors mandates development of reduced kinetic models for NO under partially premixed conditions to enable reasonably accurate predictions of NOx emissions. For these reasons, we report in this work quantitative spatially-resolved, quenching-corrected LIF measurements of NO and CH concentrations in laminar, counter-flow partially premixed and non-premixed flames at pressures up to 15 atm. Three partially premixed (&phis;B = 1.45, 1.6 and 2.0) methane-air flames and a single non-premixed methane-air flame are investigated at a global strain rate of 20 s−1. Excitation occurs at 226.03 nm for NO and at 431.5 nm for CH in their corresponding A-X (0,0) systems. Fluorescence is monitored in a 3-nm window centered at 236 nm in the A-X (0,1) band for NO and in a 10-nm window centered at 430 nm in the A-X (0,0) band for CH. The LIF signals are quantified by using calibration factors determined by seeding pure nitric oxide (for NO) and by employing cavity ring-down spectroscopy (for CH) in flames at 1 atm. Computed spectral overlap fractions for the chosen transitions are utilized to transport atmospheric calibration factors to higher pressures. The LIF measurements are investigated primarily by using pathway and sensitivity analyses with different comprehensive kinetic mechanisms. Spatial locations as well as absolute concentrations of NO and CH are predicted well when using GRI 3.0 for partially premixed flames throughout the pressure range. These trends are consistent with dominance by the thermal and N2O mechanisms in these flames. However, the kinetic mechanisms fail to predict even qualitatively measurements in non-premixed flames between 2 and 5 atm. A significant contribution from prompt NO occurs in these diffusion flames which leads to strong interactions between NO and CH kinetics. Some modifications for critical elementary reactions will be necessary in future efforts so as to match measured LIF data, thus hopefully producing a kinetic scheme which predicts NO concentrations more accurately over a wide range of pressures