Pattern transition of a gas–liquid flow with zero liquid superficial velocity in a vertical tube

https://doi.org/10.1016/j.ijmultiphaseflow.2019.06.004Get rights and content

Highlights

  • An experimental study on the flow pattern transitions of the gas–liquid flow with zero liquid superficial velocity (ZLSV) in a vertical tube is carried out.

  • Current flow pattern transition models are evaluated with experimental data.

  • A new bubble-to-slug flow transition criterion is proposed.

  • A new slug-to-churn flow transition criterion is developed.

  • Predicted results by the new model are consistent with the experimental data.

Abstract

The flow pattern transition is of importance in the analyses of gas–liquid two-phase flows. However, few studies have been carried out on the pattern transition of the gas–liquid flow with zero liquid superficial velocity (ZLSV). We carried out an experimental study on the ZLSV flow with air and water as working fluids in a vertical pipe having a length of 12 m and internal diameter of 100 mm.

The void fraction at the bubble-to-slug flow boundary is in the range of 0.06–0.10. The gas superficial velocity at the slug-to-churn flow transition boundary is approximately 0.30 m/s. The predicted results of these two transitions by the current models are larger than the experimental results.

For the bubble-to-slug flow transition, we propose a transition mechanism based on the coalescence of central bubbles. Considering the radial velocity distribution in the bubble flow, a new flow pattern transition criterion is proposed. For the slug-to-churn flow transition, the transition mechanism is the breakdown of liquid slug caused by the strong wake effect. By combining the hydraulic model of the slug flow with the wake effect, the slug-to-churn flow transition criterion is proposed. The predicted results are consistent with the experimental data. This study provides valuable insights into the flow pattern transitions and paves the way for further studies on ZLSV flows.

Introduction

The gas–liquid two-phase flow has attracted considerable attention over the past decades owing to its wide application in engineering fields such as the petroleum, chemical, nuclear power, process, and related industries (Carvalho, 2006, Morgado et al., 2016). The flow pattern classification is crucial in the modelling of different flow phenomena, heat and mass transfer rates, pressure drop, and momentum loss as the flow characteristics are considerably different among various flow patterns. Consequently, a large number of experimental and theoretical studies have been carried out on flow pattern transitions (Wu et al., 2017).

Generally, for a two-phase flow in a vertical tube, the flow patterns are classified into bubble, slug, churn, and annular flows (Cheng et al., 1998, Taitel et al., 1980). The bubble-to-slug flow transition plays an important role in the vertical gas–liquid two-phase flow. Griffith and Snyder (1964) carried out experiments in vertical tubes having diameters of 19.1 and 25.4 mm and reported that the bubble flow converted to the slug flow in the void fraction range of 0.25–0.3. Taitel et al. (1980) regarded the bubble as a spherical shape. At the transition boundary, the spacing between adjacent bubbles was sufficiently small so that the coalescence rate sharply increased. They assumed that the critical distance between bubbles was half of the bubble radius and proposed a criterion for the bubble-to-slug flow transition:εg=0.25

According to the theory proposed by Taitel et al., 1980, Hasan and Kabir, 1988 derived an expression with measurable parameters:εg=usgC0um+uwhere um, usg, and ɛg are the mixture velocity, gas superficial velocity, and void fraction, respectively, C0 is the velocity distribution coefficient in which the effects of the non-uniform flow and concentration profiles were considered by Zuber and Findlay (1965), and u is the terminal rising velocity of an individual bubble in a stagnant liquid column expressed by Harmathy (1960):u=1.53[gσ(ρlρg)ρl2]0.25

This criterion is widely used to predict the flow pattern owing to its simple form (An et al., 2000). However, the transition criterion based on void fraction is controvertible as the critical void fraction varies with the experimental conditions. Omebereiyari et al. (2007) carried out experiments in a 189-mm vertical tube with naphtha and nitrogen as working fluids and reported that the critical void fraction distinguishing the bubble flow from other flows is 0.68, which is different from 0.25.

The bubble dynamic approach can also be used to understand the flow pattern transition, which is increasingly employed as it considers the fluid viscosity, surface tension, flow rates, and other experimental conditions (Das and Pattanayak, 1994, Majumder, 2016, Wang et al., 2012). A transition criterion was proposed based on a population balance model, which described the bubble behaviours and size distribution to demonstrate the processes of bubble coalescence and breakage (Das and Pattanayak, 1994, Wang et al., 2005). Another similar method of bubble dynamics models the interfacial area transport process by solving interfacial area transport equations (Wang et al., 2012). Although further studies are required to complete this theory, the bubble dynamics provide constructive insights into the bubble-to-slug flow transition (Zhang et al., 2017).

The slug-to-churn flow transition is another critical transition of the gas–liquid two-phase vertical flow (Montoya et al., 2016). Currently, there are four mechanisms proposed to predict the slug-to-churn transition boundary:

  • (1)

    Flooding mechanism. The slug-to-churn flow transition was connected with the flooding phenomenon (Jayanti and Hewitt, 1992, Mcquillan and Whalley, 1985, Nicklin, 1962). This theory states that large interface waves are formed in the liquid film surrounding a Taylor bubble. When the gas flow rate increases, the transition is triggered when the falling film is destroyed and gas-lifted by the Taylor bubble.

  • (2)

    Slug aeration mechanism. Kaichiro and Ishii (1984) proposed a transition criterion based on the high aeration in the liquid slug. They reported that the churn flow occurred when the mean void fraction in the slug unit became larger than that in the Taylor bubble region.

  • (3)

    Slug breakup mechanism. This mechanism is attributed to the slug-to-churn flow transition to bubble coalescence (Brauner and Barnea, 1986). The gas phase exists in the liquid slug in the form of dispersed bubbles owing to the effect of the turbulent force. When the void fraction in the liquid slug reaches 0.25, bubbles approach each other and coalesce, destroying the liquid slug and leading to the churn flow.

  • (4)

    Entrance effect mechanism. In this theory, the entrance length is defined as the pipe length that is sufficiently long for the formation of a stable slug. If the pipe length is smaller than the entrance length, a stable slug flow cannot form and thus a churn flow eventually occurs (Taitel et al., 1980).

The predictions of the slug-to-churn flow transition based on the above four models are plotted in Fig. 1. Significant differences exist between these models; even the line trend is inverse (comparison of the Kaichiro's model with the other three models), which indicates the difficulty in the choice of a reasonable model to predict the slug-to-churn flow transition. A new mechanism is required to understand the flow pattern transition.

Zero liquid superficial velocity (ZLSV) flow is a special type of gas–liquid two-phase flow characterised by the gas flowing through a liquid column but no liquid flows out of the pipe. It exists in many engineering applications, particularly in petroleum and natural gas fields (An et al., 2000, Guo et al., 2005), such as liquid loading, relief well killing, and pumping oil well. This type of two-phase flow can also occur in bubble columns. Besagni et al. (2017b) and Guédon et al. (2017) achieved considerable contributions to the understanding of the flow pattern transition in bubble columns. Although the lift force has been widely studied to explain the flow pattern transition from a homogeneous flow to a heterogeneous flow in bubble flow (Besagni et al., 2017a, Lucas et al., 2005, Mena et al., 2005, Ruzicka et al., 2001a, 2008, Ruzicka et al., 2001b, Shah et al., 1982, Wilkinson et al., 1992), further investigations on pattern transitions with higher gas flow rates (e.g., bubble-to-slug and slug-to-churn flows) are required. A pattern transition criterion for the ZLSV flow is needed to further understand the hydrodynamic behaviour of the ZLSV flow.

An experiment on the pattern transition of the ZLSV flow is carried out in a vertical transparent tube having a length of L = 12 m and inner diameter of D = 0.1 m. It should be noted that the patterns and hydrodynamic properties of a two-phase flow in a large-diameter tube differ from those in a small-diameter tube (Besagni and Inzoli, 2016b). The dimensionless diameter, D*, is used to distinguish large- and small-diameter tubes (Isao and Mamoru, 1987):D*=Dσ/g(ρlρg)

Tubes with dimensionless diameters larger than the critical value of 52 are considered large-diameter tubes (Brooks et al., 2012), where the Taylor bubble is not sustained owing to the Rayleigh–Taylor instabilities. In this experiment, the dimensionless diameter of the vertical tube is 36.79. Therefore, the slug flow is expected to appear.

The rest of this paper is organised as follows. The mechanism models for flow pattern transitions are proposed in Section 2. The experimental arrangement and procedure are described in Section 3. In Section 4, the experimental results are discussed and the comparison with the mechanism model is presented. Conclusions are presented in Section 5.

Section snippets

Transition mechanism

When the superficial gas velocity is larger than the critical value, the bubble flow disappears and the slug flow occurs in the whole test section (Bouré and Mercadier, 1982, Cheng et al., 1998, Song et al., 1995). In the bubble flow, when two or more bubbles coalesce, a Taylor bubble could form. Moreover, in the ZLSV flow, the turbulence force is very small owing to the small Reynolds number of the liquid flow and thus hardly leads to bubble breakdown, which increases the possibility of bubble

Experimental arrangement and procedure

To investigate the characteristics and pattern transitions of the ZLSV flow, a flow loop was constructed including a 12.5 m × 100 mm (internal diameter) vertical transparent tube as shown in Fig. 5. The test section is made of polyvinyl chloride with a maximum pressure resistance of 10 MPa which enables the observation of flow characteristics. The gas is supplied by an air compressor and then is dried by a scrubber. At a low flow rate, it is pumped into the flow loop through an orifice plate

Bubble-to-slug flow transition

  • a.

    Analysis of the deviation between the published model and experimental data

Fig. 10 shows that the flow pattern transition criterion based on void fraction is in the poor agreement with experimental data. The void fraction at the transition boundary is in the range of 0.06–0.10, considerably smaller than 0.25.

The transition criterion based on void fraction considers that the bubble spacing at the transition boundary is half of the bubble radius, which corresponds to a void fraction of 0.25 (

Conclusions

An experimental study on pattern transitions of the ZLSV flow was carried out. The bubble-to-slug and slug-to-churn flow transition criteria were proposed. The predicted results are in good agreement with the experimental data.

The ZLSV flow pattern transitions occurs more easily. The bubble-to-slug and slug-to-churn flow transition boundaries predicted by current models were larger than the experimental results. The void fraction at the bubble-to-slug flow boundary was in the range of

Acknowledgements

The authors acknowledge the supports of the National Natural Science Foundation–Outstanding Youth Foundation (51622405), the Shandong Natural Science funds for Distinguished Young Scholar (JQ201716), the National Key Basic Research Program of China (973 Program, 2015CB251200), the National Science and Technology Major Project (2016ZX05028-001-003) and Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R58).

References (62)

  • R. Das et al.

    Bubble to slug flow transition in vertical upward two-phase flow through narrow tubes

    Chem. Eng. Sci.

    (1994)
  • G.R. Guédon et al.

    Prediction of gas–liquid flow in an annular gap bubble column using a bi-dispersed Eulerian model

    Chem. Eng. Sci.

    (2017)
  • S. Guet et al.

    Influence of bubble size on the transition from low-Re bubbly flow to slug flow in a vertical pipe

    Exp. Therm. Fluid Sci.

    (2002)
  • K. Isao et al.

    Drift flux model for large diameter pipe and new correlation for pool void fraction

    Int. J. Heat Mass Transfer

    (1987)
  • S. Jayanti et al.

    Prediction of the slug-to-churn flow transition in vertical two-phase flow

    Int. J. Multiph. Flow

    (1992)
  • M. Kaichiro et al.

    Flow regime transition criteria for upward two-phase flow in vertical tubes☆

    Int. J. Heat Mass Transfer

    (1984)
  • T. Liu et al.

    Structure of air-water bubbly flow in a vertical pipe—I. Liquid mean velocity and turbulence measurements

    Int. J. Heat Mass Transfer

    (1993)
  • T.J. Liu et al.

    Structure of air-water bubbly flow in a vertical pipe—II. Void fraction, bubble velocity and bubble size distribution

    Int. J. Heat Mass Transf.

    (1993)
  • D. Lucas et al.

    Influence of the lift force on the stability of a bubble column

    Chem. Eng. Sci.

    (2005)
  • K.W. Mcquillan et al.

    Flow patterns in vertical two-phase flow

    Int. J. Multiph. Flow

    (1985)
  • P. Mena et al.

    Effect of solids on homogeneous–heterogeneous flow regime transition in bubble columns

    Chem. Eng. Sci.

    (2005)
  • G. Montoya et al.

    A review on mechanisms and models for the churn-turbulent flow regime

    Chem. Eng. Sci.

    (2016)
  • A.O. Morgado et al.

    Review on vertical gas–liquid slug flow

    Int. J. Multiph. Flow

    (2016)
  • R. Mudde et al.

    Liquid velocity field in a bubble column: LDA experiments

    Chem. Eng. Sci.

    (1997)
  • D.J. Nicklin

    Two-phase bubble flow

    Chem. Eng. Sci.

    (1962)
  • S. Nogueira et al.

    Flow patterns in the wake of a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids: an experimental study

    Chem. Eng. Sci.

    (2006)
  • R. Omar et al.

    Fluid structure behaviour in gas-oil two-phase flow in a moderately large diameter vertical pipe

    Chem. Eng. Sci.

    (2018)
  • N.K. Omebere-Iyari et al.

    A study of flow patterns for gas/liquid flow in small diameter tubes

    Chem. Eng. Res. Des.

    (2007)
  • M. Ruzicka et al.

    Effect of bubble column dimensions on flow regime transition

    Chem. Eng. Sci.

    (2001)
  • M. Ruzicka et al.

    Effect of surfactant on homogeneous regime stability in bubble column

    Chem. Eng. Sci.

    (2008)
  • M. Ruzicka et al.

    Homogeneous–heterogeneous regime transition in bubble columns

    Chem. Eng. Sci.

    (2001)
  • Cited by (0)

    View full text