Material and surface texture-dependent fluorescence behavior of liquid coolant wall films

The study investigates the liquid film thickness-dependent laser-induced fluorescence of a dye in a heat transfer oil affected by wall reflections at different solid surface materials (aluminum, copper, steel) and surface textures (polished and sandblasted). A specially designed fluorescence cell allows a precise adjustment of the film thickness at a fixed temperature and allows the investigation of various substrate materials and textures. Photo-dissociation free measurements are ensured due to a closed-loop circuit, driven by a pump. The LIF signal was generated by admixture of the fluorescent dye Nile red to the heat transfer oil Marlotherm LH. A CW laser at 532 nm was applied for excitation, and emissions were recorded by using a spectrometer. The use of a relatively low dye concentration (0.59 mg/l) ensures negligible reabsorption of the fluorescence and thus minimal spectral changes due to a variation in film thickness, which is indispensable for precise temperature measurements. A comparison of the dye fluorescence affected by reflections at different solid materials and surface treatments for a 1-mm film thickness reveals a similar trend for all investigated materials, except for copper. Copper leads to a surface texture-dependent spectral shift of the peak emission (polished: 3.8 nm, sandblasted: 4.3 nm) toward larger wavelengths in comparison with the remaining materials (peak always at 586.4 nm). This is attributed to the more distinct wavelength-dependent reflection behavior of copper evaluated in a theoretical study. Since the fluorescence signal experiences a stronger reflection in comparison with the incident laser beam, this leads to a spectral shift of the emission spectra toward larger wavelengths. A model approach is developed describing effects of direct and non-direct reflection of fluorescence for different materials and textures. A diffusive reflection leads to an overall decrease of reabsorption. This is caused by the reduced direct reflection of laser light, which passes through the liquid film a second time (or multiple times) and consequently less emission signal. Temperature-dependent measurements in combination with a two-color measurement approach showed the significant influence of wavelength-dependent reflection behavior on the temperature determination on liquid films.