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 predisso ciative fluorescence (LIPF) of OH concentrations is developed to study the flame structure in a swirling methane jet flame. A narrowband tunable KrF e xcimer laser is used to excite the P-2(8) rotational line of the A(2)Sigma <-- (XII)-I-2(3,0) transition at lambda = 248.46 nm. Though this transition produces a relatively weak signal, LIPF is much less sensitive to collisio nal quenching in atmospheric flames. Therefore, it is suitable for generati ng quantitative data. OH concentration data are obtained by careful calibra tion against flat flame burner data of known fuel-air equivalence ratios us ing an identical optical setup. Because the distribution of OH concentratio n has a good correspondence with the flame, the measured 2-D imaging of OH indicates the instantaneous shape of the reaction zone. In the upstream sec tion of the swirling flame, combustion is found to take place in three regi ons: the shear layer of the fuel jet, the recirculation vortex inside the r ecirculation bubble, and the thin layer between the recirculation zone and the ambient air. High OH concentration is found in the upstream central por tion inside the fuel region. The temperature and radicals of the recirculat ed 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 highe r than that from the recirculated burnt gas and the measured local temperat ure is low (less than 1200 K). The measured OH concentration structure is s trongly influenced by the characteristic swirling flow and flame structures and is also closely related to the NO, formation and flame stabilization i n the swirling flame. (C) 2000 Society of Photo-Optical Instrumentation Eng ineers.