2022 年 36 巻 1 号 p. 70-77
Liquid-film flow appears in widely industrial fields, and its characteristics can impact product reliability and efficiency. Steam turbines, for example, notorious erosion and moisture loss are significant problems due to water droplets generated on the blade surface under humid low-pressure environments. A method for directly measuring the liquid-film flow in the machine is required. However, measuring the liquid-film flow is difficult by conventional methods under high temperature and pressure. Moreover, the thin liquid-film flow is frequently accompanied by disturbance waves in such an extreme environment. This study demonstrated the thickness measurement of the thin and wavy liquid-film flow using an Optical-fiber-based Reflective Probe (ORP). This technique can measure local film thickness utilizing the relationship between the distance and the light intensity reflected at the interface. At first, we investigated the ORP signal of a pseudo liquid-film flow using a wavy-stainless steel surface. According to the results, the ORP signal tends to peak at the locally flat positions of the wave. The ORP signal processing was newly established based on this fact. After that, we performed the ORP measurement in the thin-liquid-film flow condition with its average thickness of less than 500 μm. The experiment was conducted in a blowdown wind tunnel. We flowed air (jG=41~75 m/s) into the rectangle test section and injected tap water (Γ=1~3x10-5 m2/s) from holes on the channel base. The ORP was fixed at this section to obtain time-series data of the liquid-film thickness. Furthermore, we visualized the liquid-film flow from the side via a high-speed camera, then estimated the film thicknesses. The results showed a good agreement, and consequently, we confirmed the ORP successfully evaluates the thickness even in the liquid-film flow with disturbance waves.
