Two-phase flow

Flow of gas and liquid phases

Two-phase flow refers to the simultaneous flow of two distinct phases, typically a gas and a liquid, within a conduit or over a surface. This phenomenon is prevalent in various natural and industrial processes, including boiling, condensation, and piping systems in chemical engineering.

Characteristics

Diagram of two-phase flow patterns

Two-phase flow is characterized by the interaction between the two phases, which can lead to complex flow patterns and behaviors. The flow can be described by parameters such as the void fraction, which is the volume fraction of the gas phase, and the superficial velocity, which is the velocity of each phase if it alone occupied the entire cross-section of the conduit.

Flow Patterns

The flow patterns in two-phase flow depend on factors such as the flow rates of the phases, the properties of the fluids, and the geometry of the conduit. Common flow patterns include:

• Bubbly flow: Discrete gas bubbles are dispersed in the liquid phase.
• Slug flow: Large gas bubbles, known as Taylor bubbles, are separated by liquid slugs.
• Annular flow: A thin liquid film coats the walls of the conduit, with the gas phase flowing in the core.
• Stratified flow: The gas and liquid phases flow in separate layers, typically with the gas above the liquid.

Phase Interactions

The interaction between the phases can lead to phenomena such as interfacial waves, mass transfer, and heat transfer. These interactions are crucial in applications like heat exchangers and reactors.

Applications

Two-phase flow is encountered in many engineering applications, including:

• Nuclear reactors: Coolant flow in nuclear reactors often involves two-phase flow, especially in boiling water reactors.
• Oil and gas industry: The transportation of oil and gas through pipelines often involves two-phase flow.
• Refrigeration: The operation of refrigeration cycles involves two-phase flow during the evaporation and condensation processes.

Modeling and Analysis

A bubble rising in a liquid

The analysis of two-phase flow is complex due to the interactions between the phases and the variety of possible flow patterns. Several models are used to predict the behavior of two-phase flow, including:

• Homogeneous flow model: Assumes that the phases are perfectly mixed and have the same velocity.
• Separated flow model: Considers the phases as separate entities with different velocities and properties.
• Drift flux model: Accounts for the relative motion between the phases and is often used for vertical flows.

Challenges

The study of two-phase flow presents several challenges, including:

• Measurement difficulties: Accurately measuring parameters such as void fraction and phase velocities is challenging due to the dynamic nature of the flow.
• Complexity of models: Developing models that accurately predict two-phase flow behavior across different conditions is complex.

Related pages

• Multiphase flow
• Fluid dynamics
• Heat transfer
• Mass transfer