Laser-Saturated Fluorescence Measurements of Nitric Oxide in Laminar, Flat, C₂H₆/O₂/N₂ Flames at Atmospheric Pressure

Abstract

We have performed both laser-saturated fluorescence (LSF) and linear laser-induced fluorescence (LIF) measurements of NO in lean and rich atmospheric-pressure C₂H₆/O₂/N₂ flames. Unlike previous LSF measurements of OH, NH, and CH, the LSF measurements of NO require a broadband detection scheme, and thus we include a comprehensive theory for broadband LSF. Saturation of NO is found to be easily attainable at atmospheric pressure. When high laser energies are used to insure saturation of NO, background fluorescence often occurs from additional flame species; hence, a subtraction technique is introduced to eliminate this fluorescence from the NO signal. Calibration of both the LSF and LIF techniques was accomplished by doping lean flames with known quantities of NO. A comparison of the LSF and LIF signals from postflame gases at ~ 1700 K as a function of equivalence ratio suggests that the influence of stoichiometry on fluorescence quenching is nearly negligible. Finally, we discuss the relative merits of the LSF and LIF methods, which both give a detection limit of ~1 ppm in atmospheric-pressure flames.

Key Words:

• nitric oxide
• laser-induced fluorescence