Experimental study of falling films by simultaneous laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry

measurement technique based on the simultaneous implementation of Laser-Induced Fluorescence (LIF), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) has been applied to the study of wavy liquid falling film flows characterized by low Reynolds (Re) and Kapitza (Ka) numbers. The presently examined Re number range was 2.2 – 8.2, while the Ka number range was 28.6 – 41.4. The experimental methodology was developed with the ultimate aim of allowing for the evaluation of the local and instantaneous film thickness, interfacial velocity and velocity field from within the illuminated liquid volume underneath the wavy interface. The major challenges associated with the simultaneous implementation of the two optical diagnostic techniques were, firstly, the development of a refractive index correction approach allowing for liquids of different properties (surface tension and viscosity) to be tested, secondly, the identification of the location of the two liquid boundaries (solid-liquid and gasliquid) in the LIF images, and lastly, the isolation of out-of-plane reflections from primary scattering regions in the raw PIV images. Following a detailed account of the novel practices formulated and utilized in tackling the aforementioned challenges, the efficacy of the proposed methodology is demonstrated through comparisons between laser-based measurements conducted in flat films, film thickness measurements performed with a micrometer, and the solution to the Navier-Stokes equation based on the assumptions of one-dimensional (1-D), steady and fully developed flow. In addition, sample film topology results are presented for a range of flow pulsation frequencies (1 – 8 Hz), while film thickness and interfacial velocity time traces were reconstructed and are presented along with film thickness and interfacial velocity statistical results for select flow conditions