Flow-Pattern Transition and Hydrodynamic Modeling of Churn Flow
- J.Ø. Tengesdal (Pennsylvania State U.) | A.S. Kaya (U. of Tulsa) | Cem Sarica (Pennsylvania State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 342 - 348
- 1999. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion, 4.1.5 Processing Equipment, 4.6 Natural Gas
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A thorough review of the existing models for slug-churn transition in vertical and upward inclined two-phase flow is presented. A new transition model based on the drift flux approach and experimental data has been proposed. The new transition model performed the best when it was compared with the existing models and agreed very well with the published experimental data. Two separate hydrodynamic models are presented to predict pressure drop and liquid holdup in churn flow. One is a modified slug model, and the other is a model based on drift flux approach. The performances of the hydrodynamic models are evaluated against a well databank that contains 46 churn flow cases including laboratory and field data.
Two-phase flow is of practical concern in many disciplines, including petroleum, chemical, civil and nuclear engineering. The essential characteristic of two-phase flow is the existence of flow patterns, which can be identified by a typical geometrical arrangement of the phases in the pipe. Inherent to each flow pattern is a characteristic spatial distribution of the interface(s), flow mechanisms and distinctive values for such design parameters as pressure gradient, holdup, and heat transfer coefficients.
It is virtually impossible to develop a generalized solution that is applicable to all flow patterns. Nevertheless, each flow pattern may show unique hydrodynamic behavior. In this regard, the accepted approach consists of predicting the flow pattern existing in the pipe and applying a separate flow model to determine the hydrodynamic parameters of pressure drop and holdup for each of the flow patterns.
Two-phase gas-liquid flow in vertical and deviated pipes exhibits four basic flow patterns: bubble, slug, churn and annular flows. Slug flow consists of a series of unit cells, consisting of a gas pocket called Taylor bubble, a plug of liquid called liquid slug and a film of liquid around the Taylor bubble that is flowing downward. Churn flow is a chaotic flow pattern consisting of Taylor bubbles and liquid slugs that are distorted in shape (see Fig. 1 ). No continuity of the phases appears to be present, because the slug is repeatedly destroyed by very high local gas concentrations and/or Taylor bubble collapses due to high liquid concentrations.
In this study, we briefly review the existing transition mechanisms and propose a new transition model based on the drift-flux approach. The comparisons of the different transition models against the experimental data are presented. For the pressure and holdup predictions in churn flow, two hydrodynamic models based on modified slug flow model and drift-flux approach, respectively, are proposed and evaluated against a well databank that contains 46 churn flow cases.
Flow Pattern Transition
Review of Existing Transition Models.
In the literature, the four major mechanisms, entrance effect, flooding, wake effect, and bubble coalescence have been proposed for the slug to churn transition. Jayanti and Hewitt1 presented very informative assessment of the models based on the above four different mechanisms. Here, brief descriptions of each mechanism are given.
Taitel et al.2 considered churn flow as an entrance phenomenon. It is seen as a part of the process of formation of the stable slug further downstream in the pipe. They propose that when gas and liquid are introduced at the inlet, short liquid slugs and Taylor bubbles are formed. Since short liquid slugs are unstable, they will collapse down the tube, coalesce with the next slug and form bigger slugs that can retain their identity longer before they eventually collapse. The flow between the entrance and the point where stable slugs are formed appears to be churn flow because of the oscillatory motion of the rising and collapsing liquid slugs.
The slug to churn transition can be caused by the flooding of the liquid film surrounding the Taylor bubble in slug flow. When the liquid breaks down due to the large interfacial waves in countercurrent flow, flooding is encountered. Several correlations and calculation methods exist for the prediction of flooding velocities (Wallis,3 Aziz et al.,4 Bankoff and Lee,5 McQuillan and Whalley,6 and Jayanti and Hewitt1).
The slug-churn transition can be attributed to the collapse of the liquid slug caused by the wake effect of the Taylor bubbles. Close to the slug-churn transition, the liquid slugs are very short, and consequently the Taylor bubbles would be very close to each other. This would destabilize the liquid slug and destroy it, because of the strong wake effect. Under these conditions, the mean void fraction over the Taylor bubble region would be equal to the average void fraction in the pipe. Mishima and Ishii7 propose that the transition from slug to churn flow would occur when the void fraction in the pipe is greater than the mean void fraction over the Taylor bubble region.
Brauner and Barnea8 found the transition from slug to churn flow to occur from the formation of highly aerated liquid slugs. According to them, the gas phase is entrained in the liquid slugs, where it is kept in the form of dispersed bubbles because of the turbulence within the slug. When void fraction in the liquid reaches 0.52, bubble coalescence increases significantly. This will result in a destruction of the identity of the liquid slug, leading to a transition from slug to churn flow.
Wake Effect and Bubble Coalescence.
Chen and Brill9 postulate that just before the slug to churn flow transition, the developed region of the slug almost disappears and the turbulent wake behind the preceding Taylor bubble begins to affect the nose of the following Taylor bubble. That is where the void fraction in the liquid slug reaches its maximum, and approaches that in the wake region. At the same time the dimensionless liquid slug length reaches its minimum. Under this condition, small increases in gas flow rate can result in a collapse of the liquid slug and a deformation of the Taylor bubble.
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