Relationship between interface shape and signal waveforms of an Optical-fiber-based Reflective Probe during film-thickness-measurements

Abstract

Liquid film flow widely appears in the industrial field, affecting the machinery's reliability and lifetime. The notorious erosion in the steam turbines is one example, which derives from coarse droplets atomized by the liquid film on the turbine blade. There are fewer methods for measuring the liquid film in such a condition directly. We have proposed an Optical-fiber-based reflective probe (ORP) for this issue. We demonstrated a thickness measurement for the liquid-film flow driven by airflow and found an uncertainty caused by the local curvature of the interface. In this study, we investigated the signals delivered by the ORP during the measurement of the wavy film flow. The ORP measured the wavy interface simulated by a stainless-steel specimen machined into a sinusoidal waveform. We also conducted high-speed visualization and LIF imaging to confirm their positional relationship; then, we compared the time-series variation of the film thickness and the ORP signals. As a result, we found the ORP has a significant sensitivity to the interface's angle even, which is hard to visualize. Further, the waveform of the ORP signal depends on the intensity distribution of the light emitted from its tip. These are beneficial information for optimization of the ORP for future studies.