Liquid sheet thickness measurements using multi-pass, time-gated femtosecond imaging
Résumé
The present work focuses on the development and application of a non-invasive technique for measuring the thickness of a flat liquid sheet. The technique consists in separating a 100 femtosecond (fs) laser pulse into an imaging pulse which passes through the liquid sheet and a gating pulse, that travels only in air and whose path length can be adjusted using a delay line. The time delay ∆τ between the imaging and gating pulse is directly proportional to the liquid sheet thickness h and can be measured using Second Harmonic Generation (SHG) based time gate (here a Beta Barium Borate crystal is used for SHG). In order to enhance the thickness measurement resolution, an original multi-pass configuration was designed where the imaging pulse is passing twice (or more if needed) through the medium which increases the time delay between imaging and gating pulse. As a first step, we have checked the reliability of this technique by conducting measurements for a glass plate of known thickness (h g =120 µm). The measured thickness value (117 µm) is in close agreement with the expected value. Then, attention has been paid on flat liquid sheets produced by single-hole fan spray nozzles with various water-glycerol solutions. The streamwise evolution of the measured thickness of the sheet exhibits good agreement with the semi-analytical model of Dombrowski et al. [1].
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