THE STRUCTURE OF OH FIELDS IN HIGH REYNOLDS-NUMBER TURBULENT JET DIFFUSION FLAMES

Planar laser-induced fluorescence (PLIF) imaging of the OH radical is used to investigate the structure of the OH fields in turbulent H-2/Ar jet diffusion flames. Flames are investigated at moderate to high Rey nolds number (Re-d=3x10(4), 7.5x10(4), 1.5x10(5)), and PLIF images are obtained over the full length of the flames. For the lowest Re-d case , in agreement with previous studies, the images reveal OH zones that appear as thin filaments connected by diffuse regios. For the highest Re-d cases, over the entire length of the flame, the OH zones are broa der and appear to be more influenced by small-scale turbulence. The im ages also show that for the highest Re-d cases the large-scale turbule nt structure, as defined by OH, appears to be less prevalent upstream of the flame tip, but after the flame tip a clear large-scale apparent ly helical instability is present. Statistics generated from the OH PL IF signals show clear changes with increasing Reynolds number. RMS OH signal radial profiles show that over the entire extent of the flame, fluctuations are largest for the low Re-d case, owing to large fluctua tions in the thin OH zones. At high Re-d for the lower half of the fla me, the RMS profiles exhibit a double peak which results from the dist ributed OH zones combined with small-scale mixing on the rich and lean sides of the reaction zone.