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The planar laser-induced fluorescence (PLIF) method has been widely applied for measuring the thickness of liquid films. To identify the liquid–gas interface, however, PLIF-based methods require an ar...tificial threshold value of brightness or a calibration curve between the thickness and the brightness, limiting its application in measuring unknown film thickness. To overcome the drawbacks, we propose a new method, time-variant PLIF (T-PLIF), which employs an index of time variance of brightness to detect the interface. We first establish the mathematical principle of T-PLIF, wherein the time variance of a phase-dependent variable becomes the maximum exactly at the time-averaged position of the wavy interface. We then perform experiments for a well-controlled downward annular liquid film flow to test the reliability of T-PLIF. We demonstrate that T-PLIF measures liquid film thickness of h > 0.2 mm with the accuracy of ε ≤ 10% to the theoretical reference and h ≤ 0.2 mm with ε = 20%.T-PLIF is able to quantify the film thickness with no need for any pre-/post-calibration or artificial threshold values. We further confirm the applicability of T-PLIF to the wavy film flow sheared by an airflow up to 30 m/s by measuring the phase velocity and wavelength, which well matches the theoretical results.続きを見る
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