Two-phase Flow Capabilities
- Complete thermodynamics: phases appear and disappear as conditions warrant.
Two-phase Mixture Capabilities
- Mixtures of up to 26 liquids and/or gases
- Optional condensable /volatile component in mixture, including effects such as diffusion-limited condensation.
- Optional dissolution of any number of gaseous solutes into any number of liquid solvents, including homogeneous nucleation models
Types of Two-phase Flow
- Gas – liquid
- Gas – solid
- Liquid – solid
- Gas liquid system
- Orientation of flow
- Whether horizontal or vertical flow
Gas Liquid System
Two -phase Flow Regimes and Characteristic Linear Velocity
|Annular flow occurs at relatively lower gas velocities than dispersed flow. The liquid phase forms an annulus about the circumference of the pipe with the gas flowing through the central core. There is significantly more slip with annular flow than with dispersed flow. |
|Stratified flow occurs only in horizontal pipes when the gas phase velocity is not great enough to maintain an annulus of liquid about the circumference on the pipe. One form of stratified flow, called “wavy flow” is characterised by the formation of waves on the surfaces of the liquid phase. Wavy flow is formed close to the transition point where stratified flow can be transformed into slug flow with a further increase in gas velocity. |
|Slug flow is characterised by an intermittent pattern of alternating liquid phases and gas phases along the length of the line. The entire pipe cross-section area can be occupied by a slug of either liquid or gas at different points along the flow path. |
|Plug flow occurs when the liquid phase forms a nearly continuous phase with large elongated bubble plugs of gas located within the liquid phase.|
|Bubble or Froth Flow|
Two-phase Flow Types
- Each flow regime behaves differently.
- Each flow regime has its own set of empirical correlations for predicting flow behaviour.
- The most often used method to determine the flow regime is the baker
- Baker plot horizontal axis:
1. Steam condensate in return lines flashing into steam
|2.Two-phase feed lines entering distillation columns|
|3. Process plant refrigeration return lines.|
|Depending upon the flow regime, the liquid in a two-phase flow system can be accelerated to velocities approaching or exceeding vapour velocities. In some cases these velocities are higher than desirable for a process piping system. Such high velocities can cause a phenomenon known as “erosion corrosion” in equipment and piping systems. There are no general correlations that predict the rate of erosion corrosion in piping systems, but Coulson has proposed an index based on velocity head to determine the range of mixture densities and velocities below which erosion corrosion should not occur. The index takes the form |