A novel method for instantaneous, quantitative measurement of molecular mixing in gaseous flows

We describe a novel method for making instantaneous, quantitative, planar measurements of fluid mixed at the molecular level in gaseous flows. The method relies on the effective oxygen quenching of the phosphorescence of luminescent tracers, such as acetone and biacetyl. The tracer’s fluorescence emission is used to obtain information about the passive scalar, regardless of its molecular mixing state, whereas the phosphorescence emission from the same tracer displays mixing-state-dependant behavior and reveals the presence of molecularly unmixed fluid. By combining the information from both fluorescence and phosphorescence signals, the instantaneous, quantitative measurements of molecularly mixed fluid fraction can be obtained at each pixel of the detector. This method accomplishes the same objectives as previous dual-tracer LIF methods, but with a single tracer and a much-reduced burden on the instrumentation and the experimental setup. The new technique is demonstrated in a study of mixing in a forced acetone-seeded nitrogen jet discharging into ambient air. The instantaneous spatial maps of molecularly mixed jet fluid fraction and jet fluid mixing efficiency are presented. The capability of the present technique to identify stirring at sub-resolution scale is also demonstrated.