ACOUSTIC DAMPING IN SMOOTH AND CORRUGATED PIPES WITH AND WITHOUT LIQUID INJECTION

From previous experiments, it was shown that small quantities of liquids reduce or eliminate the singing phenomenon in corrugated tubes. The singing mitigation was attributed to filling up of the corrugations, interference with the boundary layer or due to the increased acoustic damping. To evaluate the influence of the presence of liquids on the acoustic damping, this damping was measured in a smooth and a corrugated tube (internal diameter 49 mm). The damping was measured using an active source (loudspeaker) and measuring amplitude of sound waves on both sides of the pipe. This was done at different gas velocities (U = 0, 15, 30 m/s) and different liquid injection rates (0, 150, 250 350 ml/min) for a frequency range between 0-2200 Hz. The smooth pipe results compared well with literature models such as the Kirchhoff model. In the presence of liquid, the acoustic damping increased linearly with the liquid rate. This increase was higher than expected based on the assumptions of mist flow or overall pressure drop. In case of the corrugated tube, the measured damping is dominated by a large fluctuation due to the source behavior (negative damping) at flowing conditions. At low velocities a decrease in source strength was measured with increasing liquid At higher gas velocities, it was observed that liquid has almost no effect on the damping.