An important distinction in single phase flow is whether the flow is laminar or turbulent, or whether flow separation or secondary flows exist. This information helps in modeling specific phenomena because one has an indication of the flow character for a particular geometry. Analogously in multiphase flow probably the key toward understanding the phenomena is the ability to identify the internal geometry of the flow; i.e., the relative location of interfaces between the phases, how they are affected by pressure, flow, heat flux and channel geometry, and how transitions between the flow patterns occur.
There are two fundamental types of flow patterns (Figure 1.1) one can identify, stratified and dispersed. A stratified flow pattern is one in which the two phases are separated by a continuous interface at a length scale comparable to the external scale of the flow; e.g., a liquid film on a wall with a gas or another immiscible liquid in the center of the channel. The complete separation of the two phases usually occurs due to density differences (horizontal flow) combined with a relatively low mass flowrate of the phase near the wall compared to the other phase in the center of the channel (e.g., vertical annular flow). These separated flow patterns can occur when the phases flow in the same direction (co-current flow) or in opposite directions (counter-current flow). The transition between these two types of stratified flow is governed by the balance between buoyancy and inertial forces.
A dispersed flow pattern is one in which one or more phases are uniformly dispersed within a continuum of another phase with a length much smaller than the external scale; e.g., gas bubbles or solid particles in a liquid or liquid droplets in a gas or another immiscible liquid. In this case the dispersed phase forms into nearly regular shaped particles with their stable size-governed again by a balance of buoyancy, inertial and surface tension forces. The transitional flow regimes between these two fundamental types can take on may geometries. Some of the more common transitional flow patterns are churn-turbulent and slug flow; i.e., dispersed-stratified flows where the discontinuous phase begins to form a continuum near the wall (bubbly-film) or in the center of the channel (wispy-annular). These flow patterns will be discussed in more detail in a subsequent section.