Especially when using the Coupled Discrete Phase Model (Coupled DPM), the mass flow rate of a particle injection source is usually a necessary and relevant input parameter that must be set, as it determines the absolute value of the DPM source. This mass flow rate can be converted into the number of particles injected per unit of time. However, tracking such a quantity of particles in the simulation is usually prohibitive. Strictly speaking, the model tracks a group of “parcels,” and each parcel represents a part of the total continuous mass flow rate [in steady-state tracking], or each parcel represents a part of the total mass flow rate released in a single time step [in unsteady tracking].
It is still helpful to refer to each parcel as a representative particle because it has a specified particle diameter, and the trajectory of each parcel in the fluid flow uses the relaxation time suitable for a single particle. (The relaxation time is the ratio of the particle’s momentum to the drag force). However, when calculating the DPM source, the mass of the parcel (or the mass flow rate) becomes particularly important: for example, if a representative droplet loses a small amount of vapor through evaporation, the overall effect of the entire parcel will usually be much greater. Other models also use parcel mass (or mass flow rate) to calculate the total concentration of DPM materials, especially the Dense Discrete Phase Model (DDPM) uses this concentration to calculate the volume fraction of the Eulerian phase representing the same material. For more information on the Dense Discrete Phase Model, please refer to the Dense Discrete Phase Model in the Fluent Theory Guide.
In the Discrete Element Method (DEM), the concept of parcels is particularly important because parcels occupy a finite volume and hinder other DEM parcels. The volume occupied by the parcel is calculated directly based on the mass it represents (to create a real density when parcels are stacked together). The equivalent “parcel diameter” is used to calculate the contact and force between parcels. However, for the trajectory through the fluid, the “particle diameter” is still used. For more information on the Discrete Element Model, please refer to the Discrete Element Method Collision Model in the Fluent Theory Guide.
The number of parcels in the DPM model is chosen in the model setup, not defined by the number of real particles. When defining initial conditions, there are several inputs available to adjust the number of parcels, such as the number of injection locations and injection frequency [for unsteady tracking]. Other inputs in sub-models include: the number of sizes in the size distribution; the number of stochastic tries in turbulent dispersion; and some breakup characteristics in some spray situations. The calculation of a large number of parcels requires a lot of computing resources, but it is more conducive to convergence, so that a single parcel will not have an overwhelming impact on the flow field. Generally, you should arrange enough parcels to generate a statistical sample representing the full range of particle behavior.