Multiphase flow pattern and Flow pattern maps

The behavior and shape of the interfaces between phases in a multiphase mixture dictates what is referred to as the ‘flow regime’ or the ‘flow pattern’. 

There are competing forces or mechanisms occurring within the multiphase fluid at the same time. The balance between these forces determines the flow pattern.

Factors dictating the flow pattern of a multiphase flow in a conduit are - 

• Phase properties, fractions and velocities.
• Operating pressure and temperature.
• Conduit diameter, shape, inclination and roughness.
• Presence of any upstream or downstream pipe work (e.g. bends, valves, T-junctions).
• Type of flow: steady state, pseudo steady state or transient.

Flow pattern classifications were originally based on visual observations of two-phase flow experiments in the laboratory. The experimental observations were mapped on 2D plots called ‘flow-pattern maps’.

The factors governing the interfacial distribution (flow regimes) in a gas– liquid flow include -

• surface tension, 
• wetting, 
• dispersion,
• coalescence, 
• body forces and 
• heat flux effects. 

There various flow regimes range from 100% liquid to 100% gas and are associated with the pipe orientation (i.e. horizontal versus vertical) due to the ever-present effects of the earth’s gravity.

Flow Regimes for Horizontal Pipe

The regimes in Horizontal gas–liquid flows are illustrated in Figure above . The regimes are as follows:

• Bubble flow - the phase is composed of bubbles dispersed in the liquid phase. However, due to the effect of buoyancy forces on the bubbles, they tend to accumulate in the upper part of the pipe.
• Stratified flow - This regime occurs when the gravitational separation is complete. The liquid flows along the bottom of the tube and the gas along the top part of the tube.
• Stratified Wavy flow - As the gas velocity is increased in stratified flow, waves are formed on the gas–liquid interface giving the wavy or stratified-wavy flow regime.
• Plug flow - Horizontal plug flow is characterized by bullet-shaped bubbles.These bubbles tend to flow along the top of the tube due to buoyancy forces.
• Slug flow - There are large (often frothy) surface waves signifying a large fluctuation in liquid delivery along the pipe. This regime is characterised by the passage along the channel of frothy ‘slugs'.
• Annular flow - The liquid flows on the wall of the tube as a film and the gas flows in the center. The liquid film tends to be much thicker at the base of the tub.

Flow regimes for a Vertical Pipe

The regimes in vertical gas–liquid flows are illustrated in Figure above. The regimes are as follows.

1. Bubble flow - Here, the liquid phase is continuous and a dispersion of bubbles flows within the liquid continuum. The bubbles are subject to complex motion within the flow, maybe coalescing, and are generally of non-uniform size.
2. Slug (or plug) flow - This flow pattern occurs when the bubble size is that of the channel, and characteristic bullet-shaped bubbles are formed, often interspersed with a dispersion of smaller bubbles.
3. Churn flow - At higher flow velocities, the slug flow bubbles breakdown leading to an
   unstable flow regime in which there is, in wide bore tubes, an oscillatory motion of the liquid, hence the name churn flow.
4. Annular flow - Here, the liquid flows on the wall of the tube as a film and the gas flows in the centre. Usually, some of the liquid phase is entrained as small droplets in the core; at high flows, it is also common for bubbles of gas to be entrained in the liquid film.

Horizontal and Vertical Flow Maps - 

Horizontal Flow Map

Vertical Flow Map

Even a very small variations in pipe inclination can cause important variations in the flow pattern map of a given mixture, all the rest staying the same.