New measurement method via optical waveguide film for droplet and liquid film flow

We developed a gas-liquid two-phase flow sensor with an optical waveguide film (OWF) to measure droplet impingement and wavy thin-film flow directly. The OWF is a flexible polyimide film about 0.1 mm thick with multiple built-in micro-optical sensors. The sensor with propagating laser light detects the reflected light from the interface, enabling the distinction between gas and liquid phases and the measurement of the thickness of the liquid phase. Adjusting the arrangement and function of adjacent sensors also enables the estimation of wave velocity and detection of interfacial waves' local shape. This study conducted application tests of droplet impingement and liquid film flow measurements using the OWF. In the droplet measurement test, we measured the impact behavior of water and ethanol droplets. In the liquid film measurement test, we measured the time-series thickness of shear-driven film flows in a vertical pipe and a horizontal rectangular duct. These results were compared with visualization images obtained from a high-speed camera to demonstrate the validity of droplet detection and measurements of local film thickness and wave velocity with the OWF. We expect this technique to significantly contribute to two-phase flow sensing in narrow channels, where the measurement is generally difficult.