Double-pulse imaging using simultaneous OH/acetone PLIF for studying the evolution of high-speed, reacting mixing layers

A technique for recording the evolution of large-scale structures in high-speed nonpremixed combustion flows is demonstrated. A single frequency-doubled dye laser system that can produce two pulses separated by tens of microseconds is used to simultaneously excite fluorescence from acetone, seeded into the fuel stream, and from OH, which marks the combustion gases. Thus, four images, an OH/acetone pair for each of the two pulses, are recorded on four separate cameras. The accuracy of the double-pulse technique is examined in a low-speed methane-air flame, which is essentially frozen during the measurement time. The average difference, or error, between corresponding pixels in a double-pulse pair of OH images shows little spatial biasing and primarily results from signal shot-noise fluctuations. The double-pulse OH/acetone technique is demonstrated in a nominally two-dimensional, high-speed reacting mixing layer consisting of an ambient-temperature, low-speed hydrogen-containing fuel stream and a high-temperature, high-speed oxidizing stream. Two mixing layer conditions are presented: a low compressibility case, with convective Mach number M c =0.32, and a more compressible case, M c =0.70. Reasonable quality images, having minimum noise levels of 11–14%, were obtained in both cases. The peak OH levels are near 2000 ppm, and the acetone seeding is roughly 3500 ppm. The regions of the flow defined by the interface between the mixing layer and the acetone-seeded fuel stream are relatively stable during the 30–50 μs measurement times and have convective velocities slightly higher than the speed of the fuel stream. The regions of the mixing layer marked by OH move faster and exhibit greater distortion.