Quantitative imaging of OH concentrations in a swirling methane jet flame via single-pulse laser-induced predissociative fluorescence

A quantitative imaging method using 2-D single-pulse laser-induced predissociative fluorescence (LIPF) of OH concentrations is developed to study the flame structure in a swirling methane jet flame. A narrowband tunable KrF excimer laser is used to excite the P₂(8) rotational line of the A²Σ ← X²Π (3,0) transition at λ = 248.46 nm. Though this transition produces a relatively weak signal, LIPF is much less sensitive to collisional quenching in atmospheric flames. Therefore, it is suitable for generating quantitative data. OH concentration data are obtained by careful calibration against flat flame burner data of known fuel-air equivalence ratios using an identical optical setup. Because the distribution of OH concentration has a good correspondence with the flame, the measured 2-D imaging of OH indicates that instantaneous shape of the reaction zone. In the upstream section of the swirling flame, combustion is found to take place in three regions: the shear layer of the fuel jet, the recirculation vortex inside the recirculation bubble, and the thin layer between the recirculation zone and the ambient air. High OH concentration is found in the upstream central portion inside the fuel region. The temperature and radicals of the recirculated hot products appear to accelerate the initial decomposition and radical-generating processes after strong turbulent mixing with fresh fuel and air. This is believed to be superequilibrium OH, because its intensity is higher than that from the recirculated burnt gas and the measured local temperature is low (less than 1200 K). The measured OH concentration structure is strongly influenced by the characteristic swirling flow and flame structures and is also closely related to the NO_x formation and flame stabilization in the swirling flame.