Dual emission laser induced fluorescence for direct planar scalar behavior measurements

In this paper, a new method of measuring scalar behavior in bulk aqueous fluid flows is presented. Using a simple ratiometric scheme, laser induced fluorescence from organic dyes can be normalized so that direct measurements of a scalar in the flow are possible. The technique dual emission laser induced fluorescence (DELIF) relies on normalizing the fluorescence emission intensity of one dye with the fluorescence emission intensity of a second dye. Since each dye fluoresces at a different wavelength, one can optically separate the emission of each dye. This paper contains an overview of the basic ratiometric technique for pH and temperature measurements as well as the spectral properties of nine water soluble dyes. It also covers the three most significant sources of error in DELIF applications. To demonstrate the technique, steady state turbulent jet mixing and temperature fields in a thermal plume were quantified. The accuracy was camera limited at under 3% of the fluorescence ratio which corresponds to 0.1 pH units or 1.8 degrees C.