Just as the fluid affects the particle behavior through forces and, for example, convective heat transfer, there is a counteracting influence of the particle on the fluid. This effect is termed as coupling between phases. If the fluid is allowed to influence trajectories but particles do not affect the fluid, then the interaction is termed one-way coupling. If the particles also affect the fluid behavior, then the interaction is termed two-way coupling.

The flow prediction of the two phases in one-way coupled systems is relatively straightforward. The fluid flow field may be calculated irrespective of the particle trajectories. One-way coupling may be an acceptable approximation in flows with low dispersed phase loadings where particles have a negligible influence on the fluid flow.

Two-way coupling requires that the particle source terms are included in the fluid momentum equations. The momentum sources could be due to turbulent dispersion forces or drag. The particle source terms are generated for each particle as they are tracked through the flow. Particle sources are applied in the control volume that the particle is in during the timestep.

The particle sources to the fluid momentum equations are obtained by solving transport equations for the sources. The generic equation for particle sources is:

(6–5)

Where are the contributions from the particles that are linear in the solution variable and contains all other contributions, including when appropriate a mass transfer term . This equation has the same form as the general particle transport and is solved in the same way as outlined above.

The source, S, to be added to the continuous phase is then multiplied by the number flow rate for that particle, which is the mass flow rate divided by the mass of the particle.

In CFX, the particle source terms are recalculated each time particles are injected. The source terms are then retained in memory in order that they may be applied each time the fluid coefficients are calculated. Thus, the particle sources may be applied even though particles have not been injected in the current flow calculation.