Experimental Investigations of Pressure Wave Propagation by the Inception of a Laser-Induced Bubble in Bubbly Flows

Abstract

The pressure wave propagation caused by the breakdown of a focused laser beam in a bubbly channel flow is investigated experimentally. Measurements were conducted in the downstream of a convergent-divergent channel with rectangular cross-section; microbubbles are generated downstream the constriction of the channel. Optical observation and pressure measurement were conducted using a high-speed video camera and a hydrophone, respectively. There are two major pressure waves: one which propagates with the speed of sound in water known as a precursor wave, another which propagates more slowly. The slower pressure wave is associated with the volume elasticity of gas-liquid mixture which depends on the local void fraction and bubble oscillations. The propagation speed of the slower wave is calculated from the image processing, and decreases down to 60 m/s. The influence of wave length, bubble size, and surface tension on the void fraction evaluated from the propagation speed of the slower wave is discussed. As a result, the void fraction is quantitatively estimated from luminance value and the propagation speed of slower wave in an isothermal homogeneous two-phase mixture. It was shown that with increasing the void fraction, the amplitude of the precursor wave decreases drastically while that of the slower wave decreases weakly.