The Structure of OH Fields in High Reynolds NumberTurbulent Jet Diffusion Flames

Abstract

Planar laser-induced fluorescence (PLIF) imaging of the OH radical is used to investigate the structure of the OH fields in turbulent H2/Ar jet diffusion flames. Flames are investigated at moderate to high Reynolds number (Red =3×l04, 7.5×l04, 1.5× 105), and PLIF images are obtained over the full length of the flames. For the lowest Red case, in agreement with previous studies, the images reveal OH zones that appear as thin filaments connected by diffuse regios. For the highest Red cases, over the entire length of the flame, the OH zones are broader and appear to be more influenced by small-scale turbulence. The images also show that for the highest Red cases the large-scale turbulent structure, as defined by OH, appears to be less prevalent upstream of the flame tip, but after the flame tip a clear large-scale apparently helical instability is present. Statistics generated from the OH PLIF 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 Red case, owing to large fluctuations in the thin OH zones. At high Red for the lower half of the flame, the RMS profiles exhibit a double peak which results from the distributed OH zones combined with small-scale mixing on the rich and lean sides of the reaction zone.