When the fluid changes the particles, there will be corresponding effects on the fluid. For example, when drag force acts on a particle, the exchange of momentum can change the fluid flow. When simulating particles using DPM, you can choose whether or not to include these effects in the flow solution; these alternatives are called Coupled and Uncoupled DPM. See Options for Interaction with the Continuous Phase for details about how to specify whether your simulation uses Coupled or Uncoupled DPM.

In Uncoupled DPM, the only purpose of the DPM particles is for postprocessing, and so particles are not tracked except when you request them, for example to calculate and display particle tracks. The particles can still change by heat and mass transfer, but the corresponding changes (such as vapor from an evaporating droplet) do not affect the flow solution.

In a Coupled DPM simulation, the effects of the particles are used to influence the flow solution. These effects are transmitted to the flow as DPM Sources. The DPM solution and the flow solution should reach a converged, self-consistent solution. Therefore, there are several options for running these solutions together—see Solution Strategies for the Discrete Phase.

As described in Steps for Using the Discrete Phase Models, to set up a DPM simulation you specify the starting conditions of a set of particles by defining an injection. By specifying boundary conditions and physical sub-models, you also specify how these particles interact with other zones in the geometry and how they eventually leave the model—for instance, they might bounce off some walls but be trapped by others.

You must also specify how Fluent is to track the particles you have defined. If Steady Tracking is enabled, then, as soon as a particle is released, it is tracked until it reaches its final destination according to the specified boundary behavior (or until a fixed number of particle time steps have been used). Therefore, each particle typically travels through many cells of the model, interacting with the flow and (in a Coupled DPM simulation) changing the DPM Sources in each cell. These sources influence the flow solution for a defined number of iterations or time steps—the flow solution can be steady or unsteady. Then, if required, a new set of particle trajectories is tracked, the DPM Sources are updated, and the sequence is repeated. An example of using Steady DPM with unsteady flow is when the chosen flow models require a transient simulation, although the final goal is a steady solution.

If Unsteady Tracking is enabled, then each particle is advanced by a specified number of particle time steps, not necessarily reaching a final destination, before the flow solution is updated. When Unsteady DPM is coupled to unsteady flow solution, the particles and the flow develop in time together concurrently, although different time steps can be used for DPM and flow.

Unsteady DPM can also be coupled to steady flow solution; this makes sense if there is a continuous source of DPM particles that pass through the system. For steady or unsteady flow, there are several DPM models where Unsteady Tracking is required. For example, in spray coalescence and collision models, particles change with time on the basis of interactions with other particles, so they must be tracked simultaneously

Especially when using Coupled DPM, the mass flow rate of a particle injection will often be a required and relevant input parameter since it determines the absolute value of the DPM Sources. This mass flow rate could be converted into the number of particles injected per unit time. However, it is often prohibitive to track that number of particles in a simulation. Strictly speaking, the model therefore tracks a number of ‘parcels’, and each parcel is representative of a fraction of the total continuous mass flow rate (in Steady tracking) or a fraction of the total mass flow released in a time step (in Unsteady tracking).

It is still sometimes helpful to refer to each parcel as a representative particle, because it has a specified particle diameter, and its trajectory in fluid flow uses the relaxation time appropriate for a single particle. (The relaxation time is a ratio of particle momentum to drag force). However, the parcel’s mass (or mass flow rate) becomes important when calculating the DPM Sources: for example, if a representative droplet loses a small amount of vapor by evaporation, the overall effect from the whole parcel will typically be much larger. Other models also use the parcel mass (or mass flow rate) to calculate total concentrations of DPM material—in particular, the Dense Discrete Phase Model (DDPM) uses this concentration to feed into the volume fraction of the Eulerian phase that represents the same material. See Dense Discrete Phase Model in the Fluent Theory Guide for details of the Dense Discrete Phase Model.

The concept of parcels is particularly important in the Discrete Element Method (DEM), where parcels occupy a finite volume and obstruct other DEM parcels. The volume occupied by a parcel is calculated directly from the mass that it represents (so that a realistic density is created when parcels pack together). The equivalent ‘parcel diameter’ is used for calculating parcel-parcel contacts and forces. However, for trajectories through fluid, it is still the ‘particle diameter’ that is used. See Discrete Element Method Collision Model in the Fluent Theory Guide for details of the Discrete Element Model.

The number of parcels in a DPM model is chosen in the model settings, and not defined by the true number of particles. There are several inputs that can be used to adjust the number of parcels when defining initial conditions such as the number of injection locations and (for Unsteady Tracking) the injection frequency (Setting Initial Conditions for the Discrete Phase) Other inputs in sub-models include: the number of sizes in a size distribution (Using the Rosin-Rammler Diameter Distribution Method); the number of stochastic tries in turbulent dispersion (Specifying Turbulent Dispersion of Particles); and the breakup characteristics of some sprays (Breakup). A high number of parcels can be computationally expensive, but it is often helpful for convergence, so that no single parcel has an overwhelming effect on the flow In general you should arrange for enough parcels to produce a statistical sample, representative of the full range of particle behavior.

The discrete phase formulation used by Ansys Fluent contains the assumption that the second phase is sufficiently dilute that particle-particle interactions and the effects of the particle volume fraction on the gas phase are negligible. In practice, these issues imply that the discrete phase must be present at a fairly low volume fraction, usually less than 10–12%. Note that the mass loading of the discrete phase may greatly exceed 10–12%: you may solve problems in which the mass flow of the discrete phase equals or exceeds that of the continuous phase. See Modeling Multiphase Flows for information about when you might want to use one of the general multiphase models instead of the discrete phase model.

This limitation is relaxed for some variants of DPM. For example, the Dense Discrete Phase Model (DDPM) adds effects due to friction and volume fraction, so that the concentration can approach the packing limit. Where high local concentrations of spray droplets cause coalescence and collision, these phenomena can be included in some spray models. Parcel-parcel contacts between solid particles are modeled in detail in Discrete Element Models, so that parcels can pack together closely.

In general, the discrete phase model is limited to the continuum regime where the mean free path of the continuous phase gas molecules is much smaller than the particle diameter (Knudsen Number ).

For situations where the mean free path of the gas phase is the same order of magnitude as the particle diameter, you can use the Stokes-Cunningham drag law. See Stokes-Cunningham Drag Law in the Fluent Theory Guide for additional information on this drag law. The discrete phase model should not be applied to the free-molecular flow regime (). Note that by default, Ansys Fluent limits the smallest particle diameter to 10 nm. If you want to model particles below this limit, you can set the minimum particle diameter using the define/models/dpm/options/set-minimum-particle-diameter text command.

The steady-particle Lagrangian discrete phase model is suited for flows in which particle streams are injected into a continuous phase flow with a well-defined entrance and exit condition. The Lagrangian model does not effectively model flows in which particles are suspended indefinitely in the continuum, as occurs in solid suspensions within closed systems such as stirred tanks, mixing vessels, or fluidized beds. The unsteady-particle discrete phase model, however, is capable of modeling continuous suspensions of particles. See Modeling Multiphase Flows for information about when you might want to use one of the general multiphase models instead of the discrete phase models.

When using rotating particles, note the following limitations:

The following restrictions exist on the use of other models with the discrete phase model: