Effect of internal bubbly flow on channel vibration: Comparison between experiment and model

The effect of an internal turbulent bubbly flow on vibrations of a channel wall is investigated in this paper both experimentally and theoretically. Vibrations of an isolated channel wall and associated wall pressure fluctuations are measured using several accelerometers and pressure transducers along streamwise direction under various gas void fractions and characteristic bubble diameters. A waveguide theory based mathematical model, i.e. a solution to the 3D Helmholtz Equation in an infinite long channel, and the physical properties of bubbles is developed to predict the spectral frequencies of the vibration and the wall pressure fluctuation, the corresponding attenuation coefficients of spectral peak and propagated phase speeds. Results show that compared with the same flow without bubbles, the presence of bubbles substantially enhances the power spectral density of the channel wall vibrations and pressure wall fluctuations in the 250-1200 Hz by up to 27 dB and 26 dB, respectively, and increases their overall rms values by up to 14.1 times and 12.7 times, respectively. In the lower frequency range than the resonant frequency of individual bubble, i.e. 250-1200 Hz range, both vibrations and spectral frequencies increase substantially with increasing void fraction and slightly with increasing bubble diameter. The origin for enhanced vibrations and wall pressure fluctuations is demonstrated to be the excitation of the streamwise propagated acoustic pressure waves, which are created by the initial energy generated during bubble formations. The measured magnitudes and trends of the frequency of the spectral peaks, their attenuation coefficients and phase velocities are well predicated by the model. All the three variables decrease as the void fraction or bubble diameter increase. But the effect of void fraction is much stronger than that of bubble diameter. For the same void fraction and bubble diameter, the peaks at higher spectral frequencies decay faster.