Experiments were conducted to investigate the effect of
compressibility on turbulent
reacting mixing layers with moderate heat release. Side-
and plan-view visualizations
of the reacting mixing layers, which were 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 technique. The instantaneous images of OH provide
two-dimensional maps of
the regions of combustion, and similar images of acetone,
which was seeded into the
fuel stream, provide maps of the regions of unburned fuel.
Two low-compressibility
(Mc=0.32, 0.35) reacting mixing
layers with differing density ratios and one high-compressibility
(Mc=0.70) reacting mixing layer
were studied. Higher average
acetone signals were measured in the compressible mixing
layer than in its low-compressibility counterpart
(i.e. same density ratio), indicating a lower entrainment
ratio. Additionally, the compressible mixing layer had
slightly wider 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 difference
in the behaviour of the high- and low-compressibility
reacting mixing layers. It is
proposed that the coexistence of broad regions of OH and
high acetone signals is a
manifestation of a more biased distribution of mixture
compositions in the
compressible mixing layer. Other mechanisms through which
compressibility can affect the combustion are discussed.
Intense sound radiation by high-speed flow: Turbulence structure, gas properties, and near-field gas dynamics
