Development of Liquid Film Thickness Measurement Considering Effect of Curved Gas-Liquid Interface Based on Optical Waveguide Film

This study aims at developing a liquid film sensor with an optical waveguide film (OWF-based sensor) that provides accurate measurements of liquid film thickness. The OWF measurement principle employs detection of light reflected at the liquid film surface with high spatial resolution. Since the curved surface of the liquid film reflects the light and increases measurement error, we proposed a signal processing method to remove the error factor in calculation of the time-averaged thickness. This method requires knowing an appropriate range for the thickness measurement and predicting the surface curvature. We experimentally simulated a liquid film curved surface with a metal test piece, and the test piece was moved with a constant velocity in an acrylic water vessel. We found that an output signal from the OWF-based sensor peaked when the wave passed above the sensor. We calculated the output signal with our 3D ray-tracing simulator of the OWF-based sensor. The simulation results showed that the effect of the curved surface up to the surface curvature of 5.0 mm−1 was negligible based on the good agreement of the liquid film thickness with that of the flat liquid film surface within 6 % difference. Furthermore, we found the surface curvature in the range of 0.1–20.0 mm−1 was able to be predicted using a characteristic of the output signal peak. Consequently, we confirmed the effectiveness of the developed sensor with the OWF-based sensor.