Reliability of Fiber-Optic Probe Measurement for Evaluating Bubble Rise Characteristics

Abstract

The rise characteristics of bubbles, such as rise velocity and chord length, in a liquid are measured using fiber-optic probes. The optical probe consists of three plastic optical fibers of identical diameter, 250-µ m OD, of which one is used as the probe tip and the other two are used as transmitting lines of incident and reflected lights. A pair of probes with different vertical tip locations (by ~1 mm) is utilized to register the time difference resulting from the bubble rise as well as a pair of bubble-piercing times, or loosely, the bubble residence time. These two types of times and the inter-probe distance are, in turn, used to estimate the bubble rise characteristics. Combined with high-speed imaging, it is found that the chord length or local "bubble height" can be estimated with reasonable accuracy when using the average bubble rise velocity measured based on the image data. When only the probe data are utilized, however, the local bubble rise velocity is overestimated— under certain bubble-piercing conditions— by as much as fivefold. The main reason for this discrepancy is found to be bubble deformation, especially near the bubble edge, triggered by the bubble-probe incidence and prolonged during the piercing of the rising bubble by the probe tip.