Absolute Temperature Fields in Flames by 2D‐LIF of OH Using Excimer Lasers and CARS Spectroscopy

Abstract

A tunable KrF‐excimer laser was used to obtain temperature fields in a laminar, premixed atmospheric pressure flame by 2D‐LIF of OH. Relative temperatures were obtained first from the intensity ratio of two sequentially excited rotational lines of the OH (A²Σ⁺ – X²H) transition in the (3–0) band near 248 nm. Because the ν' = 3 level is predissociative, this excitation scheme is less sensitive to collisional quenching than lower vibrational levels of OH (A²Σ⁺). Fluorescence from lower lying, non‐predissociative vibrational levels of OH (populated by vibrational transfer) was effectively suppressed by narrow spectral filtering of the (3–2) band of OH. Absolute temperatures were determined by calibration at one point with broadband CARS measurements; the excellent agreement of LIF and CARS profiles suggests that for these conditions and at the present level of accuracy no further corrections are needed. The uncertainty in the 2D‐LIF measurements of 7% (for a spatial resolution of 300 μm) is due mainly to shot‐to‐shot fluctuations of the beam profile. Based on the photon shot noise contribution (less than 1%), we estimate that, provided the beam energy and profile are accounted for, single‐shot measurements should be feasible with a spatial resolution of 300 μm and a precision of 2%.