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 Re 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).

Transient freezing of water between two parallel plates: A combined experimental and modelling study

“…These techniques are employed to overcome some important problems identified in the literature and early-stage experimental trials with planar laser-induced fluorescence (PLIF). It was found that temperature-driven gradients led to local refractive index variations that ultimately resulted in light distortion, which prevented locating the interface accurately [26,28,39,40]. PLIF measurements were also unsuitable due to reflections induced by the ice layer, hence lowering the accuracy of the ice layer tracking.…”

Simultaneous laser-induced fluorescence and capacitance probe measurement of downwards annular gas-liquid flows

“…Details of the previous experimental campaigns performed in this facility can be found in Refs. [11,13,14,17]. The test-section consists of a vertical ∼ 5 m long fluorinated ethylene propylene (FEP) pipe of radius, R = 16.2 ± 0.2 mm.…”

“…Häber et al [12] performed ray-tracing simulations, and revealed an overestimation in PLIF-derived instantaneous film-thickness measurement of up to 30% (in PLIF90), primarily caused by total internal reflection and refraction effects about the gas-liquid interface. Nevertheless, as discussed in the study by Charogiannis et al [13], PLIF90 measurements are also prone to a second source of error, associated with the imaging angle used, resulting in underestimation of the film thickness. Interestingly, the two errors were found to be of same order, but with opposite effects, which almost cancelled each other out during the time-averaging process in falling films (resulting in measurements close to theoretical predictions).…”

“…These methods, however, are not robust being underpinned by several assumptions, and are validated primarily against 'idealised' smooth falling films, even though downwards gas-liquid annular flows are typically cirumferentially non-uniform, with enhanced wave activity and entrainment. To obviate the need for a posteriori corrections, Charogiannis et al [13] proposed a new experimental method, 'Structured-PLIF' (S-PLIF), which unlike the aforementioned methods, does not depend solely on light intensity but also on the direction of the light path. The light is structured in an alternating dark/bright pattern, which reveals the change in the direction of the light when it hits the gasliquid interface.…”

Experimental investigations of upward-inclined stratified oil-water flows using simultaneous two-line planar laser-induced fluorescence and particle velocimetry

“…PLIF is a two-dimensional (2-D) planar technique used to extract information on the distribution of the phases and interface profiles, but also in other flows on scalar fields (e.g., Markides et al, 2016), while PIV/PTV can provide information on 2-D velocity vector-fields, conventionally in flow crosssections, but also over interfaces between immiscible phases (e.g., Charogiannis et al, 2016). In two-phase isothermal flows, PLIF has been used in falling-film flows (Charogiannis et al, 2015;2017a,b;, in gas-liquid annular vertical flows (Schubring et al, 2010;Zadrazil et al, 2014a,b;Charogiannis et al, 2019;Cherdantsev et al, 2018;, co-current liquid-liquid downward vertical flows (Liu et al, 2006), and co-current liquid-liquid horizontal flows (Morgan et al, 2012;2013), by employing a thin laser-sheet to illuminate a plane through the flow of interest. The addition of a fluorescence dye in one of the fluid phases enables the spatial identification of the seeded phase.…”