Instantaneous dual-tracer PLIF measurements of molecular mixing in axisymmetric jets

A dual-tracer PLIF technique for making instantaneous, quantitative measurements of molecularly mixed fluid fraction is presented and demonstrated in an axisymmetric jet in a mild co-flow. The two-camera, two-laser technique uses PLIF of nitric oxide (NO) seeded in a nitrogen jet to mark unmixed jet fluid fraction, while PLIF of acetone seeded into the low velocity air co-flow marks total co-flow fluid fraction. By combining data from these two simultaneous images, quantitative measurements of molecularly mixed jet fluid fraction can be made on a pixel-by-pixel basis, while the "resolution-free" nature of the technique allows simultaneous study of overall flowfield characteristics and large-structure behavior. By calculating the jet fluid mixing efficiency, regions of subresolution stirring can also be visualized. Instantaneous images and probability density functions, calculated at x/d, = 2 and 3.5, are presented for Re_jet, = 1000, 10,000, and 50,000. For the fully turbulent (Re_jet = 50,000) conditions, stirring at subresolution scales is detected primarily on the jet side of the mixing layer. This creates a hybrid PDF behavior (stationary on the jet side of the mixing layer, marching on the co-flow side) that is not shown by passive scalar methods at equivalent image resolution.