A common approach for mixing studies is the use of Laser Induced Fluorescence (LIF) diagnostics for the quantitative mapping of a passive scalar field. In LIF a fluorescent tracer is premixed into one of the two mixing streams and the concentration of the tracer, after excitation by a laser, is measured using a CCD detector. LIF measures the average concentration of the fluorescent tracer contained within a small sampling volume determined by the spatial and temporal resolution characteristics of the measuring apparatus (i.e. detector pixel size, image ratio, etc.). The difficulty arises if the sampling volume is larger than the smallest mixing scales in the flow, as is usually the case in high Reynolds number flows. Under these circumstances, it is impossible to determine the true extent of molecular mixing within the measurement resolution volume and, as a result, the passive scalar technique tends to overpredict the actual amount of molecularly mixed fluid (Breidenthal, 1981; Koochesfahani and Dimotakis, 1986). High-resolution passive scalar LIF studies have been carried out while resolving the smallest mixing scales (Dahm et al., 1991; Su and Clemens, 1997). These are done by imaging a small region in the flow at the expense of sacrificing the overall view of the flow field and its large-scale structures.

The difficulties associated with the finite sampling volume can be solved by using diagnostics that rely on chemically-reacting techniques. In this case, the average amount of chemical product measured over the finite sampling volume gives the true indication of the extent of molecular mixing within that sampling volume. ( Breidenthal, 1981 and Koochesfahani and Dimotakis, 1986 for liquid-phase applications and Mungal and Dimotakis, 1984 for gaseous mixing). In most chemically-reacting approaches only the product of the reaction is measured and not the distribution of the passive scalar reactants. Using fast chemistry and the probability density function (pdf) description of the scalar field, "flip experiment" methods have been devised to extract certain statistical properties of the scalar field from chemical product information (e.g. Koochesfahani et al., 1985; Koochesfahani and Dimotakis, 1985).

To study molecular mixing in gaseous flows Paul and Clemens (1993) and Clemens and Paul (1995) introduced a new method they called "cold chemistry." This method relies on the significant quenching of NO LIF signal by oxygen to provide a resolution-free measurement of the level of molecular unmixedness. The NO tracer is premixed into one of the two mixing streams containing no oxygen, whereas the other stream is ambient air. Since mixing of NO with even trace amounts of oxygen causes a large reduction of NO fluorescence intensity, the measured fluorescence intensity gives the amount of pure unmixed NO within each pixel of the detector. The instantaneous distribution of the amount of molecularly mixed fluid is not available in this method since a zero LIF signal could imply two streams that are completely mixed or simply pure unmixed fluid from the unseeded stream. In another approach to directly image molecular mixing, Yip et al. (1994) described the method of sensitized phosphorescence. In this method the excited state molecules of one species (the donor) transfer energy through collisional interactions to another species (the acceptor), which then phosphoresces. Donor-acceptor pairs such as acetone-biacetyl or toluene-biacetyl have been considered. The quantitative utilization of this method is somewhat complex and requires detailed attention to the molecular interactions and energy transfer between the donor and acceptor molecules.

Recently, King et al. (1997, 1999) introduced a dual-tracer planar technique to provide instantaneous planar maps of molecular mixing in gaseous flows. The technique, which is a cold chemistry approach, relies on the simultaneous imaging of the LIF signals of acetone and NO tracers. As before, NO seeded into a nitrogen jet mixes with the oxygen in an air coflow and the resulting LIF signal quenching provides information on the pure unmixed jet fluid. Simultanesously, acetone LIF is used to obtain the concentration of the coflow fluid in the sampling volume, regardless of its molecular mixing state. By combining the information from these two LIF signals the instantaneous, quantitative measurements of molecularly mixed jet-fluid can be obtained. This method requires the use of two tracers, two laser sources for excitation (one at 226 nm for NO and the other at 266 nm for acetone), generation of two co-planar laser sheets, and two spatially-aligned detectors with appropriate optical filters to minimize the cross contamination of the two LIF signals. Even though the experimental setup is relatively involved, the technique is quite powerful and has been effectively used in several studies.

In the present study, a novel technique is developed for making instantaneous, quantitative, planar measurements of fluid mixed at the molecular level. The method relies on the effective phosphorescence quenching by oxygen of luminescent tracers such as acetone and biacetyl. All previous methods based on fluorescence quenching rely on information obtained from the "intensity axis" of the emission process. In our approach, we rely also on the information contained in the "time axis" of the emission process, as oxygen quenching leads to several orders of magnitude reduction in the phosphorescence lifetime. This method accomplishes the same objectives as the dual-tracer LIF method of King et al (1999, Physics of Fluids, Vol.11, No. 2, 403-416), but with a single tracer and a much reduced burden on the instrumentation and experimental setup. The unmixedness information, which is derived from NO quenching in the dual-tracer method, is obtained here from the phosphorescence quenching of the same tracer that is used for scalar concentration measurements.

For the present novel method, Fluorescence signal of the tracer, which is almost not quenched by oxygen in air, is used to represent the behavior of passive scalar. This is the same as conventional LIF technique. Phosphorescence signal of the same tracer, which is greatly quenched by the oxygen in air, displays mixing-state-dependant behavior. By combining the information from both fluorescence and phosphorescence signals, the instantaneous, quantitative planar measurements of molecularly mixed fluid fraction in the gaseous flow can be achieved. The implementation and application of the present new technique are demonstrated in a study of mixing in a forced acetone-seeded nitrogen jet discharging into ambient air. The instantaneous maps of molecularly-mixed jet fluid fractions and sub-resolution stirring (in the form of mixing efficiency) have been presented in the near field of the forced jet flow. The results demonstrate the capability of this novel technique for obtaining instantaneous, quantitative measurements of molecular mixing, while at the same time visualizing large-scale mixing structures in the flow.

Selected Publications