A Newly Developed Technique for Detection of a Contact Position in Bubble Measurements by a Single-Tip Optical Fiber Probe

An optical fiber probe has been frequently employed to measure bubble diameters, velocities, and local void fractions simultaneously in gas-liquid two-phase systems. For the application of the probe to tiny-bubble measurement, one of the authors already developed a Single-Tip Optical fiber Probe (STOP). The purpose of this study is to rapidly improve the measurement accuracy of the S-TOP. A bubble chord length pierced by the S-TOP is obtained. Consequently, the chord length depends on the pierced position. The chord lengths measured by the S-TOP include an error owing to the random positions pierced by the S-TOP; i.e. the measured chord length becomes shorter than the bubble minor axis, with a shift of the contact position towards the outer edge of the bubble. The S-TOP axis crosses the direction of the bubble motion at a random angle. This also causes a miscalculation of the chord lengths. In order to correct these errors in the S-TOP measurement, we need to detect the contact position and the intersection angles. To realize this, using a pre-signal is quite effective. The pre-signal is generated clearly and intensively, only when the S-TOP sensing tip is ground in a wedge shape and the tip touches vertically the center region of the bubble frontal surface. The pre-signal becomes weak and indistinct under the other contact conditions. Making the smart use of these phenomena, we are able to solve the above defects of the S-TOP. First, the relationship between the intensity of the pre-signal and the pierced positions/angles is systematically quantified. Second, a signal processing to detect the pierced positions/angles, based on the relationship, is established. Third, we discuss a mechanism of the pre-signal. We determine the most suitable S-TOP size, tip diameter and wedge-angle, for the most accurate measurement. Finally, we demonstrate the effectiveness of our newly proposed method.