Imaging and characterization of OH structures in a turbulent nonpremixed flame

Abstract

Planar laser-induced fluorescence imaging of the hydroxyl radical (OH) is used to investigate spatial structures in a number of highly turbulent (Re D ≈2300 to 50,000) nonpremixed hydrogen jet flames burning in air. Hydroxyl marks the flame zone and is also expected to mark large vortical structures in the flame. At each experimental condition, more than 80 OH images are recorded within 8 seconds, permitting the compilation of statistical measurements at more than 120,000 spatial locations. Several image analysis techniques are presented. Each technique is applied to individual images within a data set, and then statistics are compiled for the complete set. Two-dimensional Fourier transform techniques are used to calculate spatial autocorrelations on each instantaneous image, from which length scale information is extracted. Two orthogonal correlation lengths are determined for each image. The correlation length along the flame exhibits a Reynolds number invariance for high Re D (>2×10 4 ). The autocorrelation technique also produces a clear, mathematically-defined flame angle. The measured flame angles indicate increased angular fluctuation of the jet at high Reynolds number. The dependence of lift-off height on jet velocity is also measured. The lift-off results agree well with previous measurements based on flame emission and schlieren photographs, with the OH measurements producing slightly lower lift-off heights.