Multiphase Flows

Multiphase flow is a type of flow in which two or more physical phases (gas, liquid, solid) coexist and move in dynamic interaction with each other. In such flows, the phases are distinctly separated above the molecular level, with boundaries between them observable at the macroscopic scale. Multiphase flows are prevalent in both natural processes and engineering applications and require specialized methods of investigation, analysis, and modeling due to their complex nature.

Multiphase flows can adopt a wide variety of configurations depending on the nature of the phases and the manner of their interaction. Common phase combinations in these systems include gas–liquid, gas–solid, liquid–solid, and gas–liquid–solid (three-phase) systems. In these flow types, the phases do not fully mix and remain separated by observable physical boundaries. Structurally, multiphase flows are primarily classified into two categories:

• Dispersed flows: One phase is distributed within the other as small particles, droplets, or bubbles.
• Separated flows: The phases form distinct layers or regions.

Multiphase flows can be classified according to the phases involved and the flow pattern:

• Gas–Liquid Flows
• • Bubbly flow: Small gas bubbles dispersed in a continuous liquid phase
  • Slug flow: Alternating sequences of large gas bubbles and liquid slugs
  • Stratified flow: A horizontal layered structure where gas and liquid phases are separated
  • Annular flow: A liquid film flowing along the pipe wall with a central gas core
• Gas–Solid Flows
• • Pneumatic conveying systems
  • Fluidized beds
  • Particle-laden flows in gas
• Liquid–Solid Flows
• • Slurry transport systems
  • Hydrotransport: Solid particles suspended in a liquid
• Three-Phase Flows
• • More complex systems where gas, liquid, and solid phases are transported simultaneously
  • Example: Flows in oil production systems carrying gas, water, and sediment

The primary physical mechanisms governing multiphase flows are:

• Cavitation: Occurs when vapor bubbles form due to low pressure in a liquid phase and subsequently collapse abruptly. It is described by the Rayleigh–Plesset equation. This phenomenon can cause erosion in pumps and turbines but can also be beneficially applied in fields such as ultrasonic surgery and lithotripsy.
• Bubble and Droplet Dynamics: Deformation, coalescence, and breakup of bubbles and droplets are critical components of multiphase flow behavior. These processes directly influence energy distribution and transport characteristics.
• Particle–Turbulence Interaction: The distribution of solid particles within turbulent flows can either dampen or enhance turbulence structures. This interaction depends on particle size, density ratio, and loading rate.

Modeling multiphase flows requires the combined use of theoretical and numerical methods. The main modeling approaches are:

• Lagrangian Trajectory Models: The individual motion of particles, droplets, or bubbles in the dispersed phase is tracked separately. Interactions such as drag, lift, and added mass forces are accounted for.
• Eulerian Two-Fluid Models: Each phase is treated as a continuous medium, with separate mass, momentum, and energy equations written for each phase. Interphase interactions are modeled using averaged quantities.
• Key challenges in these models include defining interphase boundary conditions and the statistical representation of discrete phases.

Multiphase flows are employed in a wide range of industrial and scientific fields:

• Engineering Applications: Steam boilers, chemical reactors, pneumatic conveying systems, fluidized bed technologies
• Energy Sector: Oil and natural gas transmission pipelines, coolant systems in nuclear power plants
• Environmental Applications: Sediment transport, slurry flows, environmental modeling
• Medical Technologies: Modeling of blood flow, ultrasonic lithotripsy applications, sonoluminescence