Effects of signal corrections on measurements of temperature and OH concentrations using laser-induced fluorescence

Temperature and OH concentrations derived from OH laser-induced fluorescence (LIF) are known to be susceptible to effects such as collisional quenching, laser absorption, and fluorescence trapping. In this paper, a set of analytical and easy-to-implement methods is presented for treating these effects. The significance of these signal corrections on inferred temperature and absolute OH concentration is demonstrated in an atmospheric-pressure, near-stoichiometric CH₄-air flame stabilized on a Hencken burner, for laser excitation of both the A²Σ⁺←X²Π (0,0) and (1,0) bands. It is found that the combined effect of laser attenuation and fluorescence trapping can cause considerable error in the OH number density and temperature if not accounted for, even with A–X(1,0) excitation. The validity of the assumptions used in signal correction (that the excited-state distribution is either thermalized or frozen) is examined using time-dependent modeling of the ro-vibronic states during and after laser excitation. These assumptions are shown to provide good bounding approximations for treating transition-dependent issues in OH LIF, especially for an unknown collisional environment, and it is noted that the proposed methods are generally applicable to LIF-based measurements.