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Drag between phases in ANSYS Fluent

Consider the multiphase flow of gas-liquid, in which bubbles rise up in liquid like bubbles in a gas column or beverage. The difference in velocity of the bubbles and the liquid causes the friction between the phases or the transition between the phases. In this process, the velocity of the bubbles is reduced by the fluid and the fluid is accelerated by the bubbles.

Dragging between dispersed phase particles, small droplets or bubbles and continuous phases depends on the size and shape of the dispersed phase components, the flow regime, and the characteristics of the liquid. Empirical relationships are used to calculate forces. The size and shape of solid particles usually do not depend on changing the flow conditions. Small droplets between bubbles can be deformed by the forces of the liquid, or they break down or become larger droplets and bubbles. The deformation of the bubbles and small droplets is such that the small bubbles that are spherical in shape, when they grow larger they become elliptical.

 

The exact form of this type depends on the history of the phases and the type of flow, so the empirical relationships (drag rules) are used to calculate the drag coefficient. Another important phenomenon in multiphase flows is the mass transfer between phases, which can occur for the following reasons:

The change in the density of one of the phases due to the thermal effect or local pressure variation Like:

  • boiling
  • melting
  • freezing
  • Sublimation
  • Condensation
  • Cavitation
  • Water turbulence

 

Transitions of species from one phase to another in multiple systems occur due to the difference in concentration.

Species transport is often achieved using mass transfer coefficients and concentration equilibrium models, such as the Raoult and Henry law.

 

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