Characterization of tracers for two-color laser-induced fluorescence liquid-phase temperature imaging in sprays

Abstract

The variation of the fluorescence spectral signature of tracer solutions with temperature enables temperature imaging measurements in liquids and sprays by simultaneously recording and rationing the fluorescence intensity detected in two separate wavelength channels. In this work, we recorded fluorescence spectra of ethanol-based solutions of nine laser dyes used as tracers (PTP, stilbene 1, coumarin 152, coumarin 153, rhodamine B, rhodamine 101, pyrromethene 597, DCM, and pyridine 1) after excitation at either 266, 355, or 532 nm (depending on the dye) for temperatures between 298 and 348 K (close to the boiling point of the solvent), and for concentrations (depending on dye) around 10 mg/l (i.e., ~ 10^–5 mol/l). The influence of signal self-absorption was investigated for the tracers best suited for thermometry, rhodamine B and coumarin 152, where the latter is almost unaffected due to its large Stokes shift. In thin-film (100 µm) cells, possible concentration effects on the fluorescence spectrum were investigated in the absence of signal self-absorption in the 0.1–10 and 0.5–50 mg/l range for rhodamine B and coumarin 152, respectively. Sensitivities of the two-color intensity ratios were determined for two selected color detection channels for each tracer characterized by their center wavelength and spectral half width and conditioned on averaged intensities of larger than 10% of the spectral peak of their respective fluorescence spectrum. The use of coumarin 152 that showed the overall best spectroscopic properties was demonstrated for temperature imaging in a burning ethanol spray.

Graphic abstract