When simulating particulate flows using the dense discrete phase model (DDPM) or discrete element method (DEM) in ANSYS Fluent software, you might want to initialize the case with a certain region filled with particles. Examples of this include an initial static bed in a bubbling fluidized bed simulation and a partially filled rotary drum in a powder mixing simulation. Although it is possible to use standard DPM injection options to continuously inject particles until the desired quantity is reached, this approach is computationally expensive and impractical when the amount of particulate mass to be injected is large. Additionally surface injection from “interior” cell facets is not recommended as this option does not provide good control over injection and may lead to numerical instability.
So what are the options currently available?
One option is to first generate an injection file using, for example, a FORTRAN™ or Python® program and then use that injection file to inject particles. A second option is to directly inject particles at the first time step using a user-defined function (UDF). We do have a Python file as well as a UDF that can be shared with interested users.
Whichever option you use, it is important to consider following factors before patching the particles.
Number of computational particles or parcels to inject: This directly affects the computational cost. Moreover in unsteady simulations if the particles are being continuously injected at each time step through other locations, the number of tracked particles will rapidly increase. A workaround is to inject fewer but stronger parcels as explained in a Knowledge Resource 1017 on the ANSYS Customer Portal.
Parcel size compared to cell size in a mesh: For flows involving interaction with the fluid phase, parcel size must be sufficiently smaller than the cell size in a mesh. This is a requirement of the volume fraction calculation method and convergence difficulties will arise if this condition is not met. However for DEM simulations with no interaction with fluid phase, this requirement can be ignored.
Regular or staggered arrangement of particles: A staggered arrangement of particles is preferable over a regular arrangement as the staggered arrangement avoids the possibility of most particles crossing cells at the same time, leading to a large value of source terms for the fluid phase.
Constant or varying number of particles per parcel: Varying the number of particles per parcel is useful for meshes with a large variation in cell volume (i.e. the ratio of maximum to minimum cell volume). This option avoids the jump in volume fraction that may occur when the constant option is used.