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.

This content is only available via PDF.
You do not currently have access to this content.