Computational analysis of responses of a wedge-shaped-tip optical fiber probe in bubble measurement

Optical-fiber probing is widely employed in bubble/droplet measurement in gas-liquid two-phase flows. Several types of optical fiber probes with a very high S/N ratio and high performance have been developed, but further improvement in the probes' measurement accuracy and reliability for industrial applications is desired. We tried to eliminate optical noise in the probe measurements, and we found that the signals include some peak signs that have potential for advanced measurement with optical-fiber probing. We developed a ray-tracing numerical simulator and identified the mechanisms underlying the generation of the signals. In order to numerically simulate the optical probing signals, the simulator must use 3D frameworks composed of incident beams, the reflection and refraction on the surfaces of the optical elements (i.e., an optical fiber, a sensing tip, an air phase, and a water phase), and beams returning from the sensing tip to the other tip through the fiber. We used all of these in a simple rendering framework based on a ray-tracing algorithm with Fresnel's law, and we observed the mechanism of some promising signals that may be useful for extracting the hidden potential of optical-fiber probing. To verify the simulator's performance, we carried out three comparative experiments with fundamental setups using a wedge-shaped single-tip optical fiber probe, examining: (1) the beam trajectories and energy leaking out from the sensing tip into the surrounding air phase or water phase, (2) the probing signals throughout penetration of the sensing tip at the air-water free interface in light of the three-dimensional deformation, and (3) the probing signals throughout penetration of the sensing tip into a bubble in light of the three-dimensional bubble shape. As a result, (a) we found that an optical fiber probe with a wedge-shaped tip has particular characteristics of beam emissions from the tip, and the emitting angles switched depending on the phases covering the tip. This phenomenon is very effective for further advanced measurement. (b) We observed numerically that the cutting angle of the sensing tip maximizing the air signal level was approximately 30°, and therefore this angle is the best for obtaining the highest S/N ratio. (c) We found that the meniscus shape clearly affected the probing signal optically. (d) We observed the mechanism of a pre-signal caused by the reflection at the frontal and rear interfaces of a bubble. The pre-signal is very useful for practical measurement because it appears only when the probe penetrates the center region of a bubble. We compared the above numerical results with the results of the three experiments, and there was satisfactory correspondence between the numerical and experimental results.