7.5.4. Fluid Pairs

The models described in this section are selected on the Fluid Pairs tab when creating a domain containing particles in CFX-Pre. For details, see Fluid Pair Models Tab in the CFX-Pre User’s Guide.

7.5.4.1. Particle Fluid Pair Coupling Options

Particles can be either fully coupled to the continuous fluid or can be one-way coupled. Fully coupled particles exchange momentum with the continuous phase, allowing the continuous flow to affect the particles, and the particles to affect the continuous flow.

Full coupling is needed to predict the effect of the particles on the continuous phase flow field but has a higher CPU cost than one-way coupling. One-way coupling simply predicts the particle paths as a post-process based on the flow field and therefore it does not influence the continuous phase flow field. For details, see Interphase Transfer Through Source Terms in the CFX-Solver Theory Guide.

The choice of one-way or full coupling for particles depends on the mass loading, that is, the ratio of the mass flow rate of particles to the mass flow rate of fluid. One-way coupling may be an acceptable approximation in flows with low mass loadings where particles have a negligible influence on the fluid flow. If the particles influence the fluid flow significantly, then you should use full coupling.

To optimize CPU usage, you can create two sets of identical particles. The first smaller set should be fully coupled and is used to allow the particles to influence the flow field. The second larger set should use one-way coupling and provides a more accurate calculation of the particle volume fraction as well as local forces on walls. When post-processing these types of cases, you should not, for example, sum the forces on the wall from both sets of particles because each set fully represents all the particles.

The CPU cost of tracking particles is proportional to the number of particles tracked multiplied

by the number of times tracked. One-way coupled particles are tracked only once, at the end of the solver run. The number of times fully coupled particles are tracked depends on the iteration frequency set on the Solver Control tab and the number of iterations required for the simulation to converge.

You can define multiple sets of one-way coupled particles without affecting the flow field. For example, if you were conducting a parametric study with various different particle sizes, you can create multiple particle materials with the same properties and then use each one to define a set of particles with different diameters. This is not true of fully coupled particles, because each set influences the flow field.

7.5.4.2. Drag Force for Particles

There are three ways in which the drag forces between the continuous phase and the particle phase can be modeled:

– Use the Schiller-Naumann, Ishii-Zuber, or Grace correlations.

– Use Particle Transport Drag Coefficient and specify the drag coefficient using one of the following options:

o Drag Coefficient – specify a constant value (CEL expressions are not permitted)

o User Defined – specify a drag correlation using a particle user routine. See Particle User Source Example for an example of how to do this.

– Set the drag to None and set your own drag force using a particle user routine.

A description of these particle models is available in Interphase Drag for the Particle Model, along with models for Euler-Euler flows.

7.5.4.2.1. Particle User Source Example