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This paper investigates non-invasive techniques for annular two-phase flow analysis, focusing on liquid film characterization to understand the interfacial phenomena that are crucial for heat and mass transfer. Limited methods allow the study of the temporal and spatial evolution of liquid film, such as Planar Laser-Induced Fluorescence (PLIF). However, this method possesses optical challenges, leading to the need for improved techniques to mitigate refraction and reflection, such as Refractive Index Matching (RIM). This study utilizes an experimental annular flow facility to analyze both RIM and non-RIM PLIF over a range of liquid Reynolds numbers from 4200 to 10,400. Three configurations—PLIF RIM90, PLIF RIM40, and PLIF nRIM40—are compared from both qualitative and quantitative perspectives. In the quantitative analysis, key variables of the liquid film are measured, namely mean film thickness, disturbance wave height, and frequency. Variations in the analyzed variables indicate minor deviations, which are not likely to be caused by the technique used. However, all three methodologies exhibited errors that are estimated to be within a maximum of 10%, with a mean value of approximately 8%. [ABSTRACT FROM AUTHOR]

• We study the effect that surface tension changes produce on disturbance and ripple waves in annular flow. • We analyzes the effect of the gas and liquid Reynolds numbers and the surface tension on the interfacial properties. • We study the change of the frequency and amplitude of the disturbance waves with the surface tension in annular flow. • Interface property changes are studied when varying the boundary conditions in annular flow. In this paper, we have performed an experimental study of the effect of changing the surface tension of the water in vertical annular concurrent two-phase flow (air + liquid) by adding small amounts of 1-butanol to the liquid phase. Adding small amounts of this chemical compound diminishes the surface tension of the water, while the dynamic viscosity remains practically unchanged. A set of experiments have been performed in the interval of surface tension values that ranges from 72 10-3N/m to 45 10-3 N/m, and with different values of the superficial velocities of the gas and liquid flows. In this broad range of surface tensions and boundary condition values the average amplitude and frequency of the disturbance waves have been measured for each boundary condition and surface tension value. In addition, we also measured the mean amplitude of the ripple waves, the mean value of the unperturbed base thickness, and the average film thickness. Also, we have obtained a set of correlations expressing all these magnitudes in non-dimensional form in terms of the Reynolds number of the gas, the Reynolds number of the liquid and the Kapitza number. Finally, we have performed a brief discussion and comparison with the results of other authors. [ABSTRACT FROM AUTHOR]