The effect of high-frequency oscillations on the disturbance waves in annular flow

In this paper, an analysis of susceptibility of wave structure in annular flow to small high-frequency oscillations is carried out. Downward adiabatic air-water flow at atmospheric pressure is studied in a vertical 11.7 mm pipe. The range of superficial gas velocities is 0 - 70 m/s and the range of wetting densities is 1.6 - 4.6 cm(2)/s. Spatiotemporal records of liquid film thickness are obtained with the Brightness-Based Laser-Induced Fluorescence technique. The region of interrogation is one longitudinal section of the pipe with a length of 470 mm, starting from the inlet. The oscillations are imposed by a solenoid-driven membrane causing periodic pulsations of liquid flow rate. The oscillation frequency range is 0 - 100 Hz. The amplitude of pulsations decreases with the oscillation frequency. The effect of oscillations is first tested on liquid films falling in absence of gas flow. In this case, the excited waves are of the same frequency as the imposed oscillations, and the stages of wave development are shifted closer to the inlet. In the case of liquid films under strong gas shear, the oscillations induce low-amplitude waves, which interfere with high-frequency initial waves, produced by the shear instability. Without excitation, groups of successive initial waves coalesce to produce the disturbance waves, which vary in speed and coalesce with each other. Within a certain range of excitation frequency, each period of low-amplitude wave produces one disturbance wave, so that coalescence nearly disappears and the waves become regular. Various criteria of susceptibility are proposed; the best ones are based on spectral analysis. The susceptibility range of excitation frequencies encompasses the 'natural' passing frequency of disturbance waves far downstream. The width of this range is of the order of 20 Hz but decreases at high frequencies when the amplitude of the pulsations decreases. Under excitation, the disturbance waves accelerate and grow in amplitude very fast, reaching saturated values near the inlet. The saturated values are not very different from the 'natural' ones. An increase in the excitation frequency within the range of susceptibility leads to a small decrease in the speed and amplitude of the disturbance waves. The entrained fraction shows only a minor increase due to the excitation.