The behaviour of a combined gas and liquid flow through a channel, duct, or pipe can take on an almost infinite number of possible forms. There are many descriptions used to define the possible flow patterns and there is often confusion through the subjective way in which flow patterns are characterized. In general, depending on the conditions of flow of the two phases, one phase may be considered to be the continuous phase while the other is the discontinuous phase. An example is the flow of a mist or fine dispersion of liquid droplets in a gas phase. The smaller the liquid droplet, the higher are the surface tension effects. Distortion of the discontinuous phase causes the shape to become non-spherical. There is a tendency for the liquid phase to wet the wall of the pipe and for the gas phase to congregate at the centre. An exception to this is in evaporators such as in refrigeration where nucleate boiling occurs on the pipe surface resulting in a vapour film or bubbles forming at the surface with a central core of liquid. The flow of fluids through pipes and over surfaces can be broadly described as being:

1. 1 Steady flow in which flow parameters at any given point do not vary with time.

2. 2 Unsteady flow in which flow parameters at any given point vary with time.

3. 3 Laminar flow in which flow is generally considered to be smooth and streamline.

4. 4 Turbulent flow in which flow is broken up into eddies and turbulence.

5. 5 Transition flow, which is a condition lying between the laminar and turbulent flow regimes.

Flow regime maps are charts representing the various flow regimes and flow patterns that are possible for two-phase gas–liquid flow in both horizontal and vertical pipes and tubes. There are many types of flow regime map that have been developed. The simplest form of map involves a plot of superficial velocities or flow rates for the two phases with the most widely used generalized flow regime map for horizontal flow having been developed by (p. 152) Baker in 1954. More complex maps plot the volume fluxes, mass fluxes, momentum fluxes or similar quantities for both the liquid and gas or vapour flows. The maps are populated with experimental data in which lines are drawn to represent the boundaries between the various distinguishable regimes of flow. These include stratified flow, intermittent flow, annular flow and bubble flow. Maps may also include identification of other regimes including slug, plug, wavy and annular flow. The boundaries between the various flow patterns are due to the regime becoming unstable as it approaches the boundary with the transition to another flow pattern. As with the transition between laminar and turbulent flow in a pipe, the transitions in a flow regime are unpredictable. The boundaries are therefore not distinctive lines but loosely defined transition zones. A limitation of the maps is that they tend to be specific to a particular fluid and pipe.