Two-Phase Flow Holdup Meter Using a Double-FBG Viscous Force Sensor

In this article, we propose a double-fiber Bragg grating (FBG)-based sensor for measuring the holdup of two-phase flow. The sensor consists of a double-FBG and a flexible sheet that has tangential movement in the fluid. The tangential movement can be measured by FBG wavelength shift. The test results show that the flow rate, holdup, and temperature are related to the FBG wavelength shift. This means that the holdup can be calculated by the FBG wavelength signal when the flow rate and temperature are known. A double-FBG uses differential amplification to enhance sensitivity and eliminate the self-temperature effect. The flexible sheet, with a thickness of 0.1 mm and a size of $10\times 20$ mm, reduces the normal force and increases the tangential force. The sensor was verified to measure the holdup of the different holdup of oil-water mixtures in the pipeline, with an error of 6.69%–12.67%. The measurements introduce a novel principle for two-phase flow sensing, proposing possibilities for the advancement of optical fiber holdup sensors.