Two-Phase Flow Patterns in Vertical Tubes
From Thermal-FluidsPedia
Two-phase flow in a vertical tube tends to be more symmetric, since gravity acts equally in the circumferential directions. The gravitational force plays a more dominant role in the liquid phase – and therefore in the whole of the two-phase flow – in a vertical channel. The following flow patterns can be encountered in vertical upward flow (Hewitt, 1998; Thome, 2004):
• Bubbly flow. Vapor bubbles are dispersed in a continuous liquid phase. The size of the bubbles varies widely but is generally small compared to the diameter of the tube.
• Slug or plug flow. As the bubble size increases and the bubbles start to consolidate, plugs can form. The plugs are often bullet-shaped in an upward flow and may be separated by regions occupied by liquid with a dispersion of small bubbles. These bullet-shaped plugs are commonly referred to as “Taylor bubbles” after the Taylor instability.
• Churn flow. As the slug bubbles grow larger, they start to break up, leading to a more random and unstable flow. Although the vapor continuously flows upward, the liquid phase may experience intermittent upward and downward motion, because the shear force from the vapor phase can just balance the imposed pressure gradient and the downward gravitational force. This oscillatory pattern is termed churn flow, and it is an intermediate regime between slug flow and annular flow. When the diameter of the vertical tube is small, the flow pattern can change directly from slug flow to annular flow without going through the churn flow pattern.
• Annular flow. At relatively high quality, the thin liquid layer flows along the inner wall of the tube and the central core of the flow consists of the vapor (gas) phase. Annular flow results when the interfacial shear of the high velocity gas or vapor on the liquid film becomes dominant over gravity. Liquid is then expelled from the center of the tube to form a film on the tube wall. Since the vapor core velocity is much higher than the liquid velocity, the vapor core may ripple the liquid layer and cause waves on the liquid film. It is also possible that some of the liquid phase may be entrained as small droplets in the gas core, or that some bubbles may be entrained in the liquid film.
• Wispy annular flow. At a high liquid flow rate, the concentration of the liquid drops in the vapor (gas) core increases. The merging of these liquid droplets can lead to large lumps, breaks, or wisps of liquid in the gas core.
In different regimes of two-phase flow, the pressure drop and heat transfer characteristics are significantly different, so it is necessary to identify the conditions corresponding to the flow regimes described above. A number of approaches have been developed to allow two-phase flow regime prediction; these include analytical and numerical models and the use of experimental and operational data that have been collected over the decades by various investigators. The models based on experimental data have resulted in semi-empirical correlations that have had some degree of success in predicting two-phase flow. The analytical models that are based on physical concepts are often used to derive one-dimensional continuity, momentum, and energy balances for simple boundary conditions and steady state systems. In transient systems and more complex systems where boundary conditions are not exact, numerical models based on physical concepts are used to predict two-phase flow. This information and/or experimental details have been transformed into readily available and simple tools that can develop a number of graphic diagrams, referred to as flow maps. These diagrams represent the flow in terms of important and useful parameters providing information about flow rate and operating parameters such as heat transfer coefficient and pressure drop. Based on these data, flow regimes representing different fractions of liquid and gas can be observed. Then, for a given set of operating conditions, the expected flow regime can quickly be determined.
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The accuracy of each flow map depends on the methods used to develop it. For example, a flow map developed from experimental data for one working fluid may not necessarily be representative of another working fluid. On the other hand, when a large amount of data for various fluids and operating conditions are used, a more representative flow map might be possible. One of the limitations of experimental or operational data is the subjectivity in the data as reported by an observer. Furthermore, it is not always easy to distinguish one flow regime from another. Discrepancies in the reporting of flow regimes can translate into errors in a given flow map. Conversely, a flow map based on accurate physical models will provide a better representation of the actual flow regimes.
Two-phase flow is generally a complex phenomenon; therefore, to obtain a reasonable analytical prediction, a number of approximations must be made while solving the momentum and energy balances. Generally, the pressure drop across the flow channel and/or heat transfer coefficients are calculated to determine the flow characteristics at a given point and allow prediction of the flow regime. To that end, prediction methods based on heat transfer coefficients and pressure drop correlations are developed.
The flow regime map is usually given in terms of superficial velocities or some other generalized flow parameters for liquid and vapor flow. The different regimes are separated by lines that represent the conditions for transition between flow regimes. A flow regime map proposed by Hewitt and Roberts (1969), which represents a fairly wide range of experimental data for upward two-phase flow, is shown in Fig. 1. The horizontal and vertical coordinates of the map are the superficial momenta of the liquid and vapor phases, respectively. It can be seen that the conditions for different flow regimes and the boundaries that separate them can be expressed as a combination of the superficial momentum fluxes of the liquid and vapor phases. The boundaries between different flow regimes are the criteria that identify points of flow regime transition and are often of interest in practical applications. In addition to the flow map developed by Hewitt and Roberts (1969), there are many flow maps available in the literature. Since the identification of the flow pattern, and even the name of the flow regime, is subjective and may differ from one observer to another, it is very challenging to present every correlation and flow pattern for all of the different flow regimes and configurations. A very detailed review of the boundaries between the different flow regimes, and the corresponding empirical correlations, can be found in Thome (2004).
References
Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA
Hewitt, G.F., 1998, “Multiphase Fluid Flow and Pressure Drop,” Heat Exchanger Design Handbook, Vol. 2, Begell House, New York, NY.
Hewitt, G.F., and Roberts, D.N., 1969, “Studies of Two-Phase Flow Patterns by Simultaneous X-ray and Flash Photography,” AERE-M 2159, HMSO.
Thome, J.R., 2004, Engineering Data Book III, Wolverine Tube, Inc., Huntsville, AL.