Elsevier

Chemical Engineering Science

Volume 202, 20 July 2019, Pages 392-402
Chemical Engineering Science

Transition from bubble flow to slug flow along the streamwise direction in a gas–liquid swirling flow

https://doi.org/10.1016/j.ces.2019.03.058Get rights and content

Highlights

  • Transition from bubble flow to slug flow under the effect of swirl was observed.

  • Variation of interface morphology along the streamwise direction is analyzed.

  • Mechanism for the transition to slug flow in the swirling flow is analyzed.

  • A model is developed to predict the axial position for the transition to slug flow.

Abstract

Transition from bubbly to slug flow is a well-known phenomenon. In the channel with a swirler, a transition to slug flow at a lower gas superficial velocity than in the same channel without the swirler was observed in this work. The transition phenomenon from bubble flow to slug flow under the effect of swirl in a circular pipe was observed and investigated. Compared to the flow pattern (maintaining the bubble flow along a streamwise direction) in a non-swirling flow, we observe that bubble flow is transformed to a gas column downstream of the swirler, then broken up, and finally separated into gas slugs in the streamwise direction. The dynamic condition for the occurrence of a liquid bridge is proposed, then combined with a swirl decay model of a gas–liquid two-phase swirling flow; thus, the axial position for the transition from a gas column to a slug flow along the streamwise direction can be predicted. Whether slug flow always remains along the streamwise direction is closely related to its size. Under the effect of swirl, the void fraction is decreased and the pressure drop is increased compared with that in a non-swirling flow, and gradually approximates the values of a non-swirling flow along the streamwise direction.

Introduction

The gas–liquid two-phase swirling flow has been widely used in many industries, such as in phase separator gas and liquid phase separation (Cloos et al., 2016), heat transfer enhancement (Manglik, 2004), and liquid-carrying-capacity improvement (Ali et al., 2003). In the presence of a centrifugal force in the swirling flow, flow characteristics in the gas–liquid flow are different from those in a non-swirling flow. These differences include phase distribution (Liu et al., 2018a), pressure drop (Kanizawa et al., 2011, Kanizawa and Ribatski, 2012), droplet entrainment and deposition (Fryer and Whalley, 1982), turbulent quantities (Gomez et al., 2004b), and liquid-film flow rate and thickness (Kataoka et al., 2009, Matsubayashi et al., 2011, Falcone et al., 2003). Centrifugal force can be induced by tangential inlets (Shakutsui et al., 2010, Hreiz et al., 2014), helical channels, inserted swirlers (Lim et al., 2017), and rotating devices (Cloos et al., 2016, Bluemink et al., 2009).

Because bubble flow is used in many pieces of industrial equipment, the effects of swirl on bubble flow have been researched. The trajectory of a single bubble is greatly influenced by the value of angular velocity in a swirling flow (Magaud et al., 2003, Yin et al., 2017). For elongated bubbles, a gas column occurs, and the flow changes to an annular swirling flow just out of the swirl device (Rocha and Ganzarolli, 2015). Dispersed bubbles in bubble flow move to the center of the pipe where the gas column occurs (Akhavan-Behabadi et al., 2009, Lucas and Mishra, 2005). This phenomenon benefits the separation of bubbles in a gas–liquid two-phase flow (Zonta et al., 2013, Whalley, 1979, Junlian et al., 2018). However, the development of a gas column in the streamwise direction is unclear, and is closely related to the length of the gas column. Further, because centrifugal force greatly depends on swirl intensity, this factor has a significant effect on the formation and development of a gas column in a swirling flow.

The stability of the air column formed in a low-pressure region is affected by swirl intensities (Gupta et al., 2008). The air column is stable around the centerline for high values of swirl intensity, and a whipping gas-core phenomenon occurs at low swirling intensities (Gomez et al., 2004a, Gomez et al., 2004b) as the centrifugal force which enhances the stability of a liquid film becomes weak (Chen, 2007, Chen et al., 2004). When the swirl intensity is weak, the bubble flow fails to form a gas column (Shakutsui et al., 2010). The swirling flow can be classified as a continuous swirl flow, with swirl occurring over the entire length of the pipe and a decaying swirl flow (Manglik, 2004). Swirling flow generated by any swirling device decays moving in the downstream direction (Liu and Bai, 2015a). In a single-phase decaying swirl flow, the swirl intensity of a single-phase swirl flow decays more rapidly, in particular at the initial segment (Smithberg and Landis, 1964, Kreith and Sonju, 1965, Najafi et al., 2005, Wu et al., 2000). A turbulent swirl was observed that decayed to approximately 10%–20% of its initial intensity at a distance of approximately 50 pipe diameters (Kreith and Sonju, 1965). In a decaying gas–liquid two-phase swirling flow, swirl also decays rapidly, centrifugal force becomes weak along the streamwise direction, and flow patterns return to a non-swirling gas–liquid two-phase flow (Liu and Bai, 2015b). However, the effects of swirl decay on the gas column in a bubble flow along the streamwise, which is important for maintaining the length of the gas column in a swirling flow, have not been investigated in detail.

The present study mainly focused on the effects of swirl on bubble flow and the development of a gas column in the swirling flow. Further, a transition phenomenon from bubble flow to slug flow in the gas–liquid two-phase swirling flow was observed and investigated. We observed that a gas column occurred and was separated into gas slugs. The transition mechanism from gas column to slug flow was analyzed, then a dynamic condition was proposed to predict the axial position of the transition from gas column to slug flow along the streamwise direction.

Section snippets

Flow loop

The flow loop facility of the State Key Laboratory of Multiphase Flow in Power Engineering at Xi’an Jiaotong University was used for the gas–liquid two-phase flow studies. The facility mainly comprised a gas-input section, liquid-input section, vertical test section (Perspex pipe), and return section. Air and water at room temperature were used in the setup. Water from a water tank was measured by three flow meters, two Coriolis mass flow meters with a range of 0–30 t/h and 0–10 t/h, and one

Transition from bubble flow to slug flow

When the flow pattern upstream of the swirler is bubble flow, the axial development of interfacial geometric configurations downstream of the swirler was observed. The bubble flow upstream of the swirler was concentrated on the pipe center because of the presence of the centrifugal force induced by the swirler, resulting in the formation of a gas column downstream of the swirler. Because swirl decay occurs along the streamwise direction, the gas column breaks up into gas slugs and transforms

Breakup of gas column

As a gas column develops, in the redistribution of bubbles, a liquid bridge forms because of the decay of the swirling flow. This bridge leads to the breakup of the gas column. The mechanism for the formation of a liquid bridge is important for predicting the axial position where a gas column transforms into a slug flow along the streamwise direction in a swirling flow. Therefore, the hydrodynamic mechanism underlying the breakup of a gas column is investigated here.

The phenomenon of liquid

Conclusions

In the channel with a swirler, bubble flow is transformed to slug flow at a lower gas superficial velocity than in the same channel without the swirler was observed in this work. The transition phenomenon under the effect of swirl in a circular pipe was experimentally and theoretically investigated. The following conclusions were drawn.

  • 1.

    Compared to the continuously maintained bubble flow in a non-swirling flow, bubble flow is transformed into a gas column in the decaying swirl flow downstream of

Conflict of interest

There is no conflict of interest.

Acknowledgment

The financial support of the National Natural Science Foundation of China (No. 51706024 and 51425603) is appreciated.

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      Recently, swirling flow patterns generated by a vane-type swirler have been investigated as well. In vertical pipes: bubbles in the bubbly flow move to the pipe center leading to the transition to a gas column flow (Liu and Bai 2019b; Liu et al. 2020a); in the slug flow, dispersed bubbles in the liquid slug move to the pipe center and Taylor bubbles in the gas slug are stretched (Liu et al. 2020c); churn flow can transform to swirling annular flow at a lower gas velocity (Liu et al. 2019; Kataoka et al. 2009); annular flow transforms to swirling annular flow characterized by thicker liquid film and less waves at the gas core surface compared to those in the annular flow without swirl, leading to a higher pressure drop (Liu et al. 2020b; Funahashi et al. 2018; Kataoka et al. 2009). Swirling flow regimes map generated by vane-type swirler in a vertical pipe has been proposed by our previous work (Liu and Bai 2014; Liu and Bai 2019a; Wang et al. 2019a).

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