Many students and even some professionals struggle to differentiate between particle stream and parcel used in Discrete Phase Model (DPM) simulations. This confusion is understandable, as the parcel concept is an abstraction designed to balance computational cost with an estimated representation of particle behavior. In literature, it is said that ANSYS Fluent predicts the trajectory of a discrete phase particle by integrating the force balance on the particle.
Nevertheless, from now on, we must bear in mind that it is not exactly the particle itself! In essence, a parcel in DPM represents a group of identical particles rather than individual particles themselves (see Fig. 1). This approach allows for the simulation of large numbers of particles without the computational overhead of tracking each one individually.
Figure 1: DPM Parcel concept illustration
Discrete Phase Model Parcel Formula
As stated earlier, a parcel concept in Discrete Phase Model (DPM) refers to a group of similar particles. The question arises here: How ANSYS Fluent calculate the number of particle in a parcel?
The answer is the following equation:
![\[ N_p = \frac{m_s \cdot \Delta T}{m_p} \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-501637d66e113627995de2165e095519_l3.png "Rendered by QuickLaTeX.com")
Where:
- Np is the number of particles in a parcel
- ms represent mass flow rate of particle stream
- Delta(T) is time step
- mp is particle mass
*** ms is defined by the user in DPM injection panel as “Flow Rate”.
*** mp is automatically calculated by the software considering particle diameter and density.
what are different parcel methods in DPM model?
Understanding the governing relation of parcel concept, now the question is How to introduce number of parcels in DPM Fluent? In the “set injection properties” panel of ANSYS Fluent, there is a separate tab titled “Parcel,” in which you are given four parcel release methods.
Standard Parcel Release Method
By default, ANSYS Fluent adopts standard method that was investigated in the previous section.
Constant Number Parcel Release Method
By employing constant number method, user is asked to define particle number in parcel. Nevertheless, the software calculate the number of parcels to satisfy the mass flow rate and particle size distribution. In fact, we ourself are specifying Np. Notably, this method is recommended for sprays application.
Constant Mass Parcel Release Method
If you utilize mass parcel release method, you have to define exact parcel mass. Then, the software determines the number of parcels to satisfy mass flow rate and particle size distribution.
Constant Diameter Parcel Release Method
Last but not least, constant diameter parcel definition requires setting parcel diameter. This method is highly recommended in DEM cases where we need to ensure that parcel diameter does not exceed the minimum cell size.
Figure 2: DPM Parcel definition methods
Difference Between Particle Parcel and Stream
There is another confusion for users about the definition of a particle stream in DPM (see Fig. 3). The number of streams corresponds directly to the number of locations where parcels are injected. According to the discrete phase model formula, the relationship between stream mass flow rate, injection time interval, and the number of particles in a parcel is clearly defined. The number of particles in each parcel can be fractional, allowing for more precise simulations. However, this definition belongs to the unsteady (transient) simulation. For Steady DPM simulations, the concept slightly differs. Instead of using the number of particles in a parcel, ANSYS Fluent employs “strength,” which represents the number of particles in a parcel per second. This approach allows for continuous particle injection in steady-state simulations.
** It’s worth noting that Ansys Fluent’s user documentation often refers to parcels as “particles,” which can sometimes lead to confusion. Understanding these distinctions and relationships is key to effectively utilizing DPM in CFD simulations.
Figure 3: Number of particle stream in DPM Fluent
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