Time-variant planar laser-induced fluorescence for thickness measurement of wavy liquid films: a calibration-free and threshold-free method

Abstract

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 artificial 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 \(h > 0.2\,\textrm{mm}\) with the accuracy of \(\varepsilon \le 10\%\) to the theoretical reference and \(h \le 0.2\,\textrm{mm}\) with \(\varepsilon = 20\%\). 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 \(30\,\text{m/s}\) by measuring the phase velocity and wavelength, which well matches the theoretical results.