Gradual contraction of pipe cross-section effects on transient behavior of air-water slug flow

The current experimental investigation was aimed at understanding the influence of reducing sections on the transient behavior of slug flow. Experiments were carried out using two concentric area cross-section reducers with 160 mm lengths; the first one was shaped as a conical frustum with a 44 mm inlet diameter reducing to a 30 mm outlet diameter and the second one as a frustum with the same inlet diameter converting to a rectangular cross-section (34 mm × 21 mm). Four different water superficial velocities (0.43, 0.55, 0.72 & 0.84 m s−1) and three different air superficial velocities (2.5, 3.6 & 4.9 m s−1) were applied in the course of the experiments. Visualization studies, as well as frequency distribution analyses, were used to observe flow field changes. The visualization study showed that the gas slug tail, liquid slug, and gas slug subsequent to passing through the reducers converted to non-homogeneous bubbly flow, several small slugs, and pseudo-slugs, respectively. The obtained frequency distributions, which were calculated using a Fourier transform tool, showed that unlike that of the dominant frequency, the power of other frequencies was amplified in the low-frequency range owing to the high formation rate of pseudo-slugs and small slugs. This change indicates that intermittent air-water slug flow with a specified dominant frequency subsequent to passing reducers varies to an uncertain flow with semi-uniform distribution in the low-frequency range. Estimation of Shannon entropy also showed that entropy increased by almost five times downstream of the reducers, which confirms that energy is distributed uniformly in a broad frequency bandwidth. Analyses of the essential bandwidth showed that the reducing section acted as a frequency distribution modifier whose performance decreased with liquid flow rate increments. Based on this achievement, it was found that both the circular and the rectangular reducers were able to generate the named conditions.