The purpose of this work is to analyze the influence of a sudden expansion on the upstream behavior of the horizontal gas-liquid slug flow. Measurements were made on a 40 mm ID pipe with and without a sudden enlargement of aspect ratio σ = 0.444. The experiments were carried out with two-phase air-water mixture. The slug lengths and frequencies were measured using a non-intrusive video technique. Upstream the sudden enlargement, it was observed that the mixture velocity has no influence on slug length. The variation of slug frequency is found proportional to the liquid superficial velocity for the two cases within this study. It was also observed that the behavior of the slug length and frequency was affected by the presence of the sudden enlargement. The comparison of the results obtained with various empirical correlations available in the literature showed that the latter are not worthwhile in the case where singularity is installed.

The transition from stratified to intermittent air-water two-phase flow was investigated experimentally, by flow visualization and pressure drop signals analyses, in a 30 mm ID pipe. The intermittent flow’s onset was found to be mainly dependent on the liquid superficial velocity and the pipe diameter. Plug flow, Less Aerated Slug (LAS) or Highly Aerated Slug (HAS) flows could be obtained on the gas superficial velocity grounds. The available models, compared to experiments, could not predict adequately the intermittent flow onset. The appearance of liquid slugs was revealed by peaks in the pressure drop signal. Furthermore, it was shown that the available slug frequency correlations were not valid in the zone of the onset of intermittent flow.

Static mixers are commonly used for process intensification in a wide range of industrial applications. For the design and selection of a static mixer, an accurate prediction of the hydraulic performance, particularly the pressure drop, is essential. This experimental study examines the pressure drop for turbulent single-phase and gas–liquid two-phase flows through a Komax triple-action static mixer placed on a horizontal pipeline. New values of friction factor and z-factor are reported for fully turbulent liquid single-phase flow (11,700 ≤ Re_L ≤ 18,700). For two-phase flow, the pressure drop for stratified and intermittent flows (0.07 m/s ≤ U_L ≤ 0.28 m/s and 0.46 m/s ≤ U_G ≤ 3.05 m/s) is modeled using the Lockhart–Martinelli approach, with a coefficient, C, correlated to the homogenous void fraction. Conversely, the analysis of power dissipation reveals a dependence on both liquid and gas superficial velocities. For conditions corresponding to intermittent flow upstream of the mixer, flow visualization revealed the emergence of a swirling flow in the Komax static mixer. It is interesting to note that an increase in slug frequency leads to an increase, followed by stabilization of the pressure drop. The results offer valuable insights for improving the design and optimization of Komax static mixers operating under single-phase and two-phase flow conditions. In particular, the reported correlations can serve as practical tools for predicting hydraulic losses during the design and scale-up. Moreover, the observed influence of the slug frequency on the pressure drop provides guidance for selecting operating conditions that minimize energy consumption while ensuring efficient mixing.

These recent years, several experimental investigations were carried out on horizontal gas-liquid two-phase sub-regimes including plug, Less Aerated Slug (LAS) and Highly Aerated Slug (HAS) flows. Such classification is based on the shape of the elongated bubble/liquid slug interface as well as the aeration in the liquid slugs. In this paper, an extensive experimental visualization was performed using air-water mixture in a 30 mm ID horizontal pipe covering a total of 189 operating conditions. It was reported, for the first time, that each sub-regime had a specific mechanism of aeration of the liquid slug flow. For instance, the presence of gas bubbles within liquid slugs, in case of plug flow, is due to the interfacial shear induced waves mechanism. The transitions to LAS and HAS flows are accompanied by the appearance of shearing-off and scooping mechanisms of bubbles, in the liquid film, into the liquid slug, respectively. In addition, the reported flow transition lines, as well as those existing in the literature, were analyzed using different existing coordinate systems. This analysis showed that the X-T system is the most suitable for predicting the three flow sub-regimes.