Fluorescence Lifetime Imaging applied to the measurement of the temperature mixing in sprays

Minghao Wang1^{1}, Hadrien Chaynes1^{1}, Simon Bbecker1^{1}, Mehdi Stiti2^{2}, Guillaume Castanet1,^{1,\star}
^{\star} : guillaume.castanet@univ-lorraine.fr
1^{1} LEMTA Université de Lorraine
2^{2} Department of Physics, Lund University
Mots clés : Fluorescence, droplet, temperature measurement
Résumé :

Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications. In this study, time Correlated Single Photons Counting (TCT_CSPC) is applied to monitor the fluorescence decay and determine the fluorescence lifetime which depends on the temperature. One of the major advantages of using the fluorescence lifetime (instead of the fluorescence intensity) relates to the fact that it is an intrinsic molecular property, which is not affected by fluctuations in excitation of the laser source or in the transmission of the fluorescence signal across the spray and the optics. The aim of the present study is to demonstrate the advantage of such a measurement technique in real spray conditions. The technique is used for characterizing the mixing of two sprays which are injected with significantly different temperatures. Provided sufficiently different fluorescence lifetimes for the droplets of the two sprays, the fluorescence decay is expected to follow a multiple exponential decay. Different approaches are tested for measuring the temperature of the two sprays as well as their mixing fraction based on the analysis of the fluorescence decay. Both sprays are mounted on an automated platform allowing 3D scanning and motions which allows obtaining maps of the fluorescence decay. The out-of-field fluorescence, observed in dense sprays when fluorescence is induced by one-photon absorption, is suppressed by using a two-photon fluorescence excitation. This approach significantly improves the spatial resolution of the measurements. Both the droplet temperature and the mixing fraction are measured simultaneously using a single dye, namely RhB, whose fluorescence lifetime is temperature dependent. The fluorescence decay in the mixing zone of the two sprays is considered as a combination of two biexponentials. Results show that the volume fraction of a spray must exceed about 10% to make it possible to determine its temperature with an accuracy of about 2^{\circ}C-3^{\circ}C. Simultaneous measurements of the sprays ‘temperatures and volume fractions provides a means to calculate the mixing temperature (the average between the temperatures of the two sprays weighted by their volume fractions).

doi : https://doi.org/10.25855/SFT2023-119

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