Related information is provided in Particle Transport Theory in the CFX-Solver Theory Guide.

Multiphase flow refers to the situation where more than one fluid is present. Each fluid may possess its own flow field, or all fluids may share a common flow field. Unlike multicomponent flow, the fluids are not mixed on a microscopic scale in multiphase flow. Rather, they are mixed on a macroscopic scale, with a discernible interface between the fluids. Ansys CFX includes a variety of multiphase models to enable the simulation of multiple fluid streams, bubbles, droplets, solid particles and free surface flows.

Two distinct multiphase flow models are available in Ansys CFX: an Eulerian-Eulerian multiphase model and a Lagrangian Particle Tracking multiphase model. Additional information on the Eulerian-Eulerian model is available in Multiphase Flow Modeling.

The Particle Transport model is capable of modeling dispersed phases which are discretely distributed in a continuous phase. The modeling involves the separate calculation of each phase with source terms generated to account for the effects of the particles on the continuous phase. Water droplets dispersed in air from the liquid sprays of a cooling tower, and solid particles dispersed in air from the pneumatic transport of solids, or transport of airborne particulates are such practical occurrences.

The implementation of particle transport modeling in Ansys CFX can be thought of as a multiphase flow in which the particles are a dispersed phase. Instead of using an Eulerian transport model for the dispersed phase, a Lagrangian transport model is used. All continuous phases must use the Eulerian model.

With the dispersed phase, each particle interacts with the fluid and other particles discretely. Therefore, another method is required to calculate the particle behavior. The most widely applied method available to determine the behavior of the dispersed phase is to track several individual particles through the flow field. Each particle represents a sample of particles that follow an identical path. The behavior of the tracked particles is used to describe the average behavior of the dispersed phase. This method is called separated flow analysis.

The separated flow analysis has been implemented as a Lagrangian tracking model to characterize the flow behavior of a dispersed phase and is available in Ansys CFX.

The term particle is used to describe an individual discrete element of the dispersed phase. The particle could represent a solid, droplet, or bubble.

Particle Number Rate  —  For applications involving tracking of discrete particles, it is not practical to track all physically existing particles. Instead representative particles, or parcels, are used to track these discrete particles. Each representative particle characterizes a certain number of actual particles. The actual number of particles represented by the representative particle is called the Particle Number Rate. The Particle Number Rate is determined from the mass flow rate assigned to the representative particle divided by the mass of an actual particle.

The known mass flow rate of particles can be specified on inlet, opening, wall and fluid-solid interface boundaries. This allows Ansys CFX to know how many particles the tracked particles represent.

All the particles represented by the sample particle will follow exactly the same track, even if the turbulent particle dispersion is used.

Initialization of the particle properties in the domain is not required.