Instantaneous, quantitative measurements of molecular mixing in the axisymmetric jet near field

Abstract

Results from a dual-tracer planar laser-induced fluorescence (PLIF) technique for making instantaneous, quantitative measurements of molecularly mixed fluid fraction are presented for an axisymmetric jet in a slow co-flow. The two-camera, two-laser technique uses PLIF of nitric oxide seeded in a nitrogen jet to mark the unmixed jet fluid fraction, while PLIF of acetone seeded into the low velocity air co-flow marks the 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 simultaneously allowing visualizations of large-structure behavior and regions of subresolution stirring. Instantaneous images of molecularly mixed jet fluid fraction and jet fluid mixing efficiency, probability density functions (PDFs) of mixed jet fluid fraction, and associated statistics are presented for Rejet=1000, 5000, 10,000, 50,000, and 100,000. For fully turbulent conditions (Rejet⩾30,000), 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.