Measurement of water film thickness by laser-induced fluorescence and Raman imaging

We present two non-intrusive, laser-based imaging techniques for the quantitative measurement of water fluid film thickness. The diagnostics methods are based on laser-induced fluorescence (LIF) of the organic tracer ethyl acetoacetate added to the liquid in sub-percent (by mass) concentration levels, and on spontaneous Raman scattering of liquid water, respectively, both with excitation at 266 nm. Signal intensities were calibrated with measurements on liquid layers of known thickness in a range between 0 and 500 μm. Detection via an image doubler and appropriate filtering in both light paths enabled the simultaneous detection of two-dimensional liquid film thickness information from both methods. The thickness of water films on transparent quartz glass plates was determined with an accuracy of 9% for the tracer LIF and 15% for the Raman scattering technique, respectively. The combined LIF/Raman measurements also revealed a preferential evaporation of the current tracer during the time-resolved recording of film evaporation.