Interface Identification in Falling-Film Flows by Structured Laser-Induced Fluorescence

A structured laser-induced fluorescence technique (S-PLIF) was developed towards the reliable time- and space-resolved measurement of film-thickness in gas-liquid annular flows. Recently, "conventional" techniques such as planar laser-induced fluorescence (PLIF) and brightness-based laser induced fluorescence (BBLIF) were shown to suffer from large errors induced by total internal reflection (TIR) and refraction at the gas-liquid interface, depending on the interface toplogy. With this novel variation of PLIF we exploit TIR in order to locate the free surface with a high degree of accuracy, and compare our results to simultaneously recovered PLIF data, as well as data from other techniques. Our experiments show that S-PLIF outperforms PLIF when the observation angle is set to 70 ^o, as any distortions caused by refraction of the emitted fluorescence are suppressed. Image-processing methodologies were developed based on the identification of gradient variations in the S-PLIF images, and the employment of a commercial 2D particle image velocimetry (PIV) code.