Direct detection method for cusp thickness in wavy thin-film flow using an optical waveguide film

The wave crest (cusp) of the disturbance wave in thin liquid film flow is an important factor contributing to heat/mass transfer, e.g., fuel rods in boiling water reactors, stator/rotor blades in steam turbines, and cleaning/drying wafer processes in semiconductor manufacturing. We developed a new technique for directly detecting the thickness of wave cusps using array-based sensing with an optical waveguide film (OWF). This technique, based on geometrical optics assumptions, simultaneously obtains information on liquid films’ thickness and their interfacial shape, i.e., whether or not the local interface is convex upward. We first performed a pseudo-film flow measurement using a metal specimen to confirm the basic principle. According to the results, a meaningful signal indicating the wave-cusp passage, along with a thickness signal, was detected simultaneously. The OWF signal processing for cusp thickness detection was newly established based on this fact. We then applied this technique to a wavy liquid film flow formed on a flat plate in the entry region. A series of experiments were performed over a wide range of air speeds ( j G = 20–70 m/s). As a result, the cusp thicknesses of relatively large waves on the wavy interface were successfully extracted from the OWF output signal. Further, the major thickness variables (i.e., base film thickness, median film thickness, and cusp thickness) were compared with those of conventional thickness estimation methods, which showed reasonable agreement. This paper provides a framework for wavy thin-film flow measurements via OWF that is specialized for directly detecting local thickness profiles. Graphical abstract