Quantitative fluorescence measurements of nitric oxide and the hydroxyl radical in high pressure methane flames

A high-pressure combustion facility has been constructed to study laser-induced fluorescence of OH and NO in the burnt gas region of non-premixed {dollar}rm CHsb4/Osb2/Nsb2{dollar} flat flames, at pressures up to 10 atm. A method for quantitatively inferring the laser-induced fluorescence measurements as concentration measurements was developed. This method involved modeling the fluorescence signal per unit absorber (OH or NO) mole fraction as a function of temperature, pressure, laser spectral bandwidth, and flame equivalence ratio. Planar laser-induced fluorescence images and single-point measurements were made from 1 to 10 atm, and 1670 to 2080 K.; The model was validated using single-point measurements of the {dollar}rm OH Asp2Sigmasp+gets Xsp2Pi (1,0) Psb1(8){dollar} transition and the NO {dollar}rm Asp2Sigma+gets Xsp2Pi (0,0) Psb{lcub}21{rcub}+Qsb1(14)/Rsb{lcub}12{rcub}+Qsb2(21)+Psb1(23){dollar} feature. Experimental values of the collision broadening for both species are used in the model to determine absorption lineshapes. Analytical expressions for the electronic collision cross sections of the two radicals are used in formulating the fluorescence yield. The developed model indicates that the only pressure-dependent component of the fluorescence signal is the overlap or convolution of the laser and absorption lineshapes, when the concentration is in terms of mole fraction of the absorbing species.; Planar fluorescence images were acquired for both OH and NO, from 1 to 10 atm. The model was applied so that the resulting images were quantitative mole fraction images of these radicals in the post-flame region.; For fuel-lean flames, with large excess oxygen levels in the product gases, substantial interference with the NO (0,0) band from the O{dollar}sb2{dollar} Schumann-Runge system was found. This interference was minimized by using the NO {dollar}rm Psb{lcub}21{rcub}+Qsb1(14)/R12+Qsb2(21)+Psb1(23){dollar} feature near 226 nm. However, the magnitude of this interference at elevated pressures can be substantial. The ratio of NO to O{dollar}sb2{dollar} fluorescence was also modeled as a function of temperature, pressure, equivalence ratio and relative NO and O{dollar}sb2{dollar} mole fractions. This model is useful in understanding the range of conditions where the O{dollar}sb2{dollar} interference in problematic and in designing collection filtering strategies to discriminate the NO signal from the O{dollar}sb2{dollar} signal.