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

Abstract

A quantitative imaging method using 2D 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 A2(Sigma) implied by (Chi) 2(Pi) (3,0) transition at (lambda) equals 248.46 nm. Though this transition produces a relatively weak signal, LIPF is much less sensitive to collisional quenching in atmospheric flames and suitable for generating quantitative data. OH concentration data are obtained by careful calibration against the flat flame burner data of known fuel-air equivalence ratio and temperature using identical optical setup. In this experiment, the OH fluorescence signal is imaged onto an intensified CCD camera. Since the distribution of OH concentration has good correspondence with the flame, the measured 2D imaging of OH indicates the instantaneous shape of the reaction zone. In the present experiment, the flame structure is first identified from the flame visualization. In the upstream of the swirling flame, combustion is found to take place in two regions; the recirculation vortex inside the recirculation bubble and the thin layer between and the thin layer between the recirculation zone and the ambient air. Higher intensity of OH is found early in recirculation zone. It is superequilibrium OH, because its intensity is higher than that from the recirculated burnt gas and the measured temperature is low. The measured OH concentrations have a direct influence on flame stabilization and NOx pollutant formation.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.