50 巻 (2014) 10 号 p. 703-711
In the context of the practical significance of flow-rate control as required in semiconductor manufacturing, the ultrasonic vortex flowmeter has been increasingly used in processing equipment. However, there still remains an issue related to the measurement of gas-liquid two-phase flows which are associated with chemical and biomedical engineering using ultrasonic vortex flowmeters. In this research, a visualization study using a high-speed camera was performed to examine the bubble behaviors behind a vortex shedder. Based on the visualization and the signal analysis, we determined the measurement limit of the liquid velocity for flows accompanied by bubbles, as a function of the void fraction. The high-speed tracking of bubbles revealed that most bubbles were gathered in the shedding Kármán vortices and formed into clusters, which should affect the ultrasonic wave transmitted across the pipe flow. We draw the following three major conclusions: (i), a low-frequency noise in the obtained signal was due to the interception of the ultrasonic wave by large bubble clusters deviating from the Kármán vortex; (ii), a high-frequency noise was induced by small bubbles; and (iii), the doubled frequency of the Kármán vortex array was caused by amplitude modulation by bubbles gathered into the Kármán vortex.