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

Abstract 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 time- and space-resolved measurement of film thickness in these flows still eludes 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), and with which we seek to suppress the errors in PLIF-derived film thickness measurements due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on a periodic modulation of the laser-light intensity along the examined region of the flow in order to generate fluorescence images with alternating bright and dark regions. 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 reflections of the emitted fluorescence at the film free-surface. 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).