AN EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF COMPRESSIBILITY ON A TURBULENT REACTING MIXING LAYER

Experiments were conducted to investigate the effect of compressibilit y on turbulent reacting mixing layers with moderate heat release. Side -and plan-view visualizations of the reacting mixing layers, which wer e formed between a high-speed high-temperature vitiated-air stream and a low-speed ambient-temperature hydrogen stream, were obtained using a combined OH/acetone planar laser-induced fluorescence imaging techni que. The instantaneous images of OH provide two-dimensional maps of th e regions of combustion, and similar images of acetone, which was seed ed into the fuel stream, provide maps of the regions of unburned fuel. Two low-compressibility (M-c = 0.32, 0.35) reacting mixing layers wit h differing density ratios and one high-compressibility (M-c = 0.70) r eacting mixing layer were studied. Higher average acetone signals were measured in the compressible mixing layer than in its low-compressibi lity counterpart (i.e. same density ratio), indicating a lower entrain ment ratio. Additionally, the compressible mixing layer had slightly w ider regions of OH and 50% higher OH signals, which was an unexpected result since lowering the entrainment ratio had the opposite effect at low compressibilities. The large-scale structural changes induced by compressibility are believed to be primarily responsible for the diffe rence in the behaviour of the high-and low-compressibility reacting mi xing layers. It is proposed that the coexistence of broad regions of O H and high acetone signals is a manifestation of a more biased distrib ution of mixture compositions in the compressible mixing layer. Other mechanisms through which compressibility can affect the combustion are discussed.