To elucidate details of the fascinating nonlinear phenomena of gas-liquid interfaces in micro- and minichannels, precise spatiotemporal knowledge of the interface in gas-liquid two-phase flows is essential. This paper presents a new method for measuring the interface of a liquid film in microchannels using a laser focus displacement meter (LFD). The purpose of the study was to clarify the effectiveness of the new method for obtaining detailed information concerning interface displacement, especially in the case of a thin liquid film, in micro- and mini-channels. In the test, water and nitrogen gas were used as working fluids. To prevent the tube wall signal from disturbing that of the gas-liquid interface, a fluorocarbon tube with water box was used; the refraction index of this device was the same as that for water. With this method, accurate measurements of the liquid film interface were achieved in real time, with a sensitivity of 0.2 μm and 1 kHz. The error caused by refraction of the laser beam passing through the acrylic water box and fluorocarbon tube was estimated theoretically and experimentally. The formulated theoretical equation can derive the real interface displacement using measured displacement in a fluorocarbon tube of 25 μm to 2.0 mm I.D. A preliminary test using fluorocarbon tubes of 1 and 2 mm I.D. showed that the corrected interface displacement calculated by the equation agreed with real displacement within a 1% margin of error. We made simultaneous measurements of the interface in fluorocarbon tubes of 0.5 and 1 mm I.D. using the LFD and a high-speed video camera with a microscope. These showed that the LFD could measure the interface of a liquid film with high spatial and temporal resolution during annular, slug, and piston flow regimes. The data also clarified the existence of a thin liquid film less than 1 μm in thickness in slug and annular flow regimes.

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