Developmental Research of Multi-Fiber Optic Liquid Film Sensor

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Two kinds of fiber optic liquid film sensors have been newly developed, both of which are more versatile and more reliable than the conventional electric conductance and electric capacitance probes. A pickup (sensor head) of the first sensor, which we call a seven-fiber sensor, consists of seven fibers tightly joined each other in parallel, i.e. a centered fiber serving as a transmitting fiber and six receiving fibers surrounding it. A pickup of the second sensor, which we call a mono-fiber sensor, consists of only one fiber playing both roles of transmitting light and of receiving the reflected light from a film surface. Both these fiber optic sensors can be applied to almost all the fluid film flow except an opaque one.The characteristics of a static calibration and a dynamic response of these sensors to liquid film wave have been investigated by computer simulations based on the authors' measurement theory and by simulation experiments. After that, these sensors have been applied to actual air-water and air-Freon R113 two-phase annular flows in a vertical tube, and the local and instantaneous value of the film thickness has been measured using these sensors. The following facts have been as a result clarified. The mono-fiber sensor can measure the film thickness ranging from 0mm to about 2 mm and it can distinguish the crest and the trough from other parts in the wave-form of the liquid film and can measure the film thickness at these parts because the output signal is obtained from the mono-fiber sensor only at the crest and the trough. The mono-fiber sensor can therefore determine the overall wave-form of the liquid film. On the other hand, the seven-fiber sensor can measure the film thickness ranging about from 1 mm to 10 mm. It can also distinguish the crest and the trough in the wave-form of the liquid film and can measure the local and instantaneous value of the curvature as well as the film thickness at these parts.