Structured planar laser-induced fluorescence (S-PLIF) for the accurate identification of interfaces in multiphase flows.

• A new optical technique is investigated that recovers the true thickness of thin-film flows. • The technique relies on an adaptation of PLIF (S-PLIF) by modulating the laser-light intensity. • S-PLIF is not vulnerable to errors due to light reflection or circumferential film non-uniformity. • Simultaneous S-PLIF and PLIF measurements are performed and the resulting data compared. • The simultaneous application S-PLIF and PIV for velocity information is also demonstrated. Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable and detailed (i.e., time- and space-resolved) measurement of film thickness in these flows continues to elude us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), which can suppress the errors in PLIF-derived film thickness measurements that arise due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on the periodic modulation of the laser-light intensity and angled illumination of the examined region of the flow, in order to generate fluorescence images with alternating bright and dark regions at an angle to the channel wall and film interface. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range R e L ≈ 150 − 1500. The results from this technique are compared to simultaneously recovered, "conventional" (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera's observation axis set to β = 70 ∘ and 90 ∘ is also performed, showing that the employment of β = 70 ∘ is highly advantageous in avoiding distortions caused by optical interactions of the emitted fluorescence at the free surface of circumferentially non-uniform films. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-PLIF is demonstrated in the presence of a gas-shear flow with gas entrainment in the liquid, and simultaneously with particle image velocimetry (PIV). [ABSTRACT FROM AUTHOR]