Monitoring of Void Fraction and Bubble Size in Narrow-Channel Bubbly-Flows Using Ultrasonic Pulses With a Super Bubble-Resonant Frequency

Small bubbles suspended in liquid in narrow gaps often adversely affect machines and their monitoring is required to avoid machine troubles. Traditional measurement method adopting continuous acoustic waves and the Wood's equation cannot detect such small bubbles because of multiple reflections within narrow gaps. This requires a short ultrasonic pulse to surmount multiple reflections. In this article, we proposed a method of instantaneously measuring a void fraction and a major bubble size in a narrow-channel bubbly-flow using an ultrasonic pulse. From results of numerical analysis of the pulse behavior, attenuation of the pulse was judged useful to estimate a projection void fraction. Furthermore, the projection void fraction can be converted into local void fraction via probability equation as a function of the bubble size. Meanwhile, phase and group velocities can be approximated by functions of the void fraction, the ultrasound frequency, and the bubble size. Here, because the frequency is predetermined, we can estimate the void fraction and the bubble size by solving simultaneous equations. To verify the efficacy of present method, we compared results of a demonstration of the proposed method and a simultaneous optical visualization of microbubbles. From the comparison, it is confirmed that the ultrasonic measurement utilizing attenuation and phase velocity of ultrasonic pulse provided reasonable results of projection void fraction, the void fraction and the bubble size.