Optical fiber probing is very useful and reliable for bubbles/droplets measurement particularly in the gas-liquid two-phase flows that have dense dispersed phase and are impossible to be measured via usual visualization techniques. For the practical purpose of small- or medium-size bubbles/droplets measurement, one of the authors successfully developed a Four-Tip Optical-fiber Probe (F-TOP) and reported their excellent performance in industrial uses. Recently, particular demands for measuring properties of micro bubbles/droplets have increased in researches on multi-phase flows. However, no one succeeded in simultaneously measuring diameters and velocities of high-speed micro-droplets (velocity > 50 m/s; 50 μm < diameter < 500 μm). We made a challenge of measuring such tiny droplets via newly developed optical fiber probe equipped with two tips (Two-Tip Optical-fiber Probe: T-TOP). We have succeeded in this difficult measurement with it. Each optical fiber probe composing the T-TOP is made of a silica optical fiber (125 μm in external diameter, 50 μm in core diameter, 37.5 μm in clad thickness). The optical fiber was fine-drawn using a micro pipette puller, and this yielded a sub-μm-scale tip. The interval of the fiber axes and the gap of the tips were arranged depending on the droplets diameter range. In this paper, we demonstrate the performance of the T-TOP. First, we confirm its practicality in industrial use. The strength of the T-TOP is confirmed by exposure test of high-velocity and high-temperature steam flows. Second, we consider the influence of the flow on the measurement of T-TOP; the optical noise due to probe vibration by the high-velocity gas flow around the T-TOP is considered. Next, we confirm its performance using an orifice-type nozzle (300 μm < droplets diameter < 500 μm; droplets velocities < 40 m/s). We confirm the performance of the T-TOP; the results of T-TOP are compared with those of the visualization of the droplets by using an ultra-high-speed video camera. At the same time, we consider the process of droplet contact with the T-TOP via visualization of ultra-high-speed video camera.

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