When two-phase flow occurs in pipes, many different flow patterns are created, depending on fluid propeties, the relative rates of fluids and the pipe inclination.

There are large differences in flow behavior between horizontal, inclined, and vertical pipe flow.  In horizontal pipes, three patterns are produced.  Segregated flow is a relatively smooth flow, where the boundaries between the gas and liquid are divided in a laminar fashion.  Segregated flow is further divided into stratified flow, wavy flow, and annular flow.  Intermittent flow consists of plug and slug flow.   Distributive flow occurs when the gas and liquid are well blended.  At higher velocities, froth flow can occur.  Distributed flow consists of bubble, and mist flow.  The photograph below shows the flow loop in a horizontal position.
 

Uphill flow must overcome the force of gravity.  With lower gas velocities, slug flow occurs in the pipe.   Liquid holdup plays a large role in inclined pipes.  As gas velocity is increased, the flow behavior shows rolling waves, comparable to that on a beach.
 

Four uphill flow regimes have been observed; these are bubble, slug, churn, and annular flow.  Each of these occur in progression with increasing gas rate for a given liquid rate.  Bubble flow exists in a continuous liquid phase with dispersed bubbles of gas.  Slug flow occurs as the gas flow increases.  Slugs of liquid which contain smaller gas bubbles mixed in, flow between the large gas bubbles that flow through the pipe.  The larger gas bubbles begin to coalesce with increasing gas rates.  This transitional state of flow forms churn flow.  Highly turbulent flow can exist.  Churn flow can be characterized by oscillatory, up and down motions of the liquid.  As gas rates get even higher, gas becomes the continuous phase.  Liquid flows in an annulus around the edge of the pipe with liquid droplets mixed in the gas phase.