Add Particle Injection

Introduction

This functionality allows users to define custom particle injection regions with local injection rates, particle size distributions, and initial particle compositions that differ from those defined on the particle parent. Whereas children geometry attached to the parent inherit the size distribution, composition, and injection characteristics defined by the parent, particle injection zones allow for differentiated particle injection characteristics within a particle family.

Particle injections can have children geometry that localize the action of the injection region to specific regions of the simulation domain. Since the particle injection zone is a child to the particle parent, any children geometry attached to the particle injection region can be conceptualized as grandchildren to the particle parent. These grandchildren geometry do not inherit properties from the grandparent particle family—they inherit properties from their parent (which in this case is the particle injection region).

In the first example below, a single bubble parent contains two distinct injection zones. We use two injection zones to introduce separate gas streams with distinct compositions. Each injection zone has a single child. The first injection zone introduces pure CO₂ bubbles to the left-hand side of the vessel, while the second introduces pure O₂ bubbles to the right-hand side of the vessel. Because both injection zones are governed by the same parent, all bubbles—regardless of origin—can undergo coalescence with one another. As a result, coalescence events can produce bubbles containing mixed gas compositions. In this case, both injection zones produce bubbles of identical diameter; however, in practice, the initial bubble size or size distribution may differ between zones. Interfacial mass transfer is modeled for both gas species, leading to localized increases in dissolved CO₂ and O₂ concentrations within the surrounding fluid.

Download Sample File: Bubble Injection Region

In this second example, a single DEM particle parent contains two distinct injection zones. Here, the goal is to introduce particles with different diameters into different regions of the vessel. The first injection zone introduces 2 mm diameter particles into the left side of the vessel, while the second introduces 1 mm diameter particles into the right side. An impeller at the bottom of the vessel acts to homogenize the particles through mixing.

Download Sample File: DEM Particle Zone

These injection profiles, as demonstrated in both examples, cannot be realized using child geometry alone. Because child geometry inherits its properties from the parent, it is not possible to vary particle size, composition, or injection rate between child objects. In contrast, injection zones provide the flexibility needed to define and control these properties independently and in different parts of the vessel within a shared parent structure.

Property Grid

General

This option controls the particle injection type. Consider both the number and the location of the particles when adding particles to the system. The injection options are listed below and discussed in greater detail in the injection options overview. These options are the same as the particle parent, except they do not include density.

For massless tracers, the following injection options are prohibited: Volume Feed, Initial Mass Fraction, Initial Volume Fraction, and Total Mass.

Injection Option

This setting contains the different selections for introducing particles into a system. The number of particles added to the system is determined by an injection rate and an injection duration.

Dump

A single impulse addition of particles into the injection geometry. The dump defines the total number of particles added to the system at a single user-defined time.

Feed

A continuous feed of particles into the injection geometry at a user-defined number flow rate over a user-defined period. The feed rate can vary per user-defined expression.

Volume Feed

A continuous feed of particles into the injection geometry at a user-defined volume flow rate for a user-defined period.

Initial Mass Fraction

A single impulse addition of particles into the injection geometry at a user-defined fraction of the mass of the injection geometry.

Initial Volume Fraction

A single impulse addition of particles into the injection geometry at a user-defined fraction of the volume of the injection geometry.

Total Mass

A single impulse addition of particles into the injection geometry at a user-defined total mass.

No Injection

The particle feed rate is not directly controlled by the user. Particles are dynamically generated across the system via free-surface Eularian-Lagrangian conversion, immiscible two fluid Eularian-Lagrangian conversion, or volumetric particle generation.

Initial Size Distribution

Particle sizes are described by the diameter. The specified size distribution of particles can be previewed by clicking on the plot distribution icon in the property grid. This is the same as the parent.

Distribution Type

The following are the options for initial particle size distribution.

Single

The diameter of the particles is set by the user and assigned to all particles. This condition represents the addition of monodispersed particles.

Uniform

The diameter of each particle is randomly sampled at runtime from a uniform probability density function with a user-set minimum and maximum diameter.

Normal

The diameter of each particle is randomly sampled at runtime from a normal probability density function with a user-set mean and standard deviation.

Log Normal

The diameter of each particle is randomly sampled at runtime from a log-normal probability density function with a user-set log mean and log standard deviation.

Rayleigh

The diameter of each particle is randomly sampled at runtime from a Rayleigh probability density function with a user-set scale parameter.

Discrete Diameter Distribution

The diameter of each particle added to the system is randomly sampled at runtime from a user-specified particle diameter probability mass function.

Discrete Superquadric Distribution

A discrete superquadric (SQ) is a mathematical shape used to represent particles that are not perfectly spherical.

Discrete Volume Distribution

The diameter of each particle added to the system is randomly sampled at runtime from a user-specified particle volume probability mass function.

Initial Diameter UDF

This UDF defines the initial diameters of the particles entering the system.

If Interfacial Mass Transfer is on and a scalar is added, the following section will launch:

Advanced

Injection Downsampling

This parameter reduces the number of particles added to the system relative to the user-specified dump value, feed rate, initial volume fraction, initial mass fraction, or initial mass. Specifying a downsampling value transforms the particles into sampling parcels. These parcels are characterized by a parcel number scale and parcel diameter. The number scale quantifies the number of equal-sized particles contained within each parcel. The parcel diameter is equal to the particle diameter, as defined by the size distribution. To ensure that the mass represented by the parcels equals the user-specified dump or feed rate, all parcels entering the system are assigned an initial number scale equal to this injection downsampling parameter. The amplifying effects of the number scale on all particle-to-fluid forces, local particle volume fraction calculations, and interfacial mass transfer rates are handled internally and automatically.

With injection downsampling, not all particles are represented explicitly, so systems with injection downsampling are typically paired with the parcel-based breakup and coalescence representation. Thus the model underpredicts pair-wise collisions and explicit coalescence events.

The purpose of the downsampling is to reduce the memory requirements of a simulation. Increasing the number scale (the number of equal-sized particles contained within each parcel) will have the effect of reducing the number of explicitly tracked parcels in the system. The downsampling number should be kept as small as possible, which can minimize the effects of improper course graining and undersampling. In practice, we recommend keeping the number of parcels within the system order-of-magnitude consistent with the lattice count.

This parcel-based representation is also applied to the parcel-based breakup and coalescence representation and discussed in our webinars and reference literature.

Initial Velocity Type

By default, particles enter the system with zero velocity. Users may override this setting by assigning the particles a non-zero velocity. This velocity can be specified as either a constant value or a user-defined function of local fluid properties, particle properties, system variables, etc.

Constant

Under this option, all particles in the family entering the system will have the initial velocity defined below. By default, this value is set to a zero vector, implying no initial velocity.

Initial Velocity

m/s | This is the initial translational velocity vector of the particles entering the system. The three fields define the x, y, and z components of the velocity vector. These vectors are aligned with the basis x, y, and z unit vectors within the system.

Custom UDF

Under this option, all particles in the family entering the system will have an initial velocity computed at runtime from the UDF.

Initial Velocity UDF

m/s | The UDF defines the initial velocity of particles as they are added to the system. Three outputs must be defined within the UDF: floating point variables vx_p, vy_p, and vz_p. These output variables define the components of the initial velocity vector of the particles entering the system. This is a particle-based local UDF, calculated on a particle-by-particle basis using the local particle/fluid properties.

Download Sample File: Initial Velocity

Note

For injections at inlet boundary conditions, the initial particle velocity may be set to the inlet velocity to model a set of pre-entrained particles.

Location

The Add Particle Injection function is found on the following context-specific toolbars:

bubbles Gas Bubbles