When we run a CFD simulation with particles, we often use the Discrete Phase Model (DPM) in Ansys Fluent. If you want to learn the basics of DPM, you can find many helpful articles on our multiphase blog page. When we start a DPM simulation, the first step is very important. We must tell the software what kind of particle we are using. This is called the Particle Type. For example, is our particle a solid piece of sand, or is it a liquid water droplet that can turn into vapor? Choosing the right Particle Type is the most important decision because it sets the physics rules for the particle. This choice tells the simulation if the particle will heat up, change its mass, or react. If you select the wrong type, your results will be incorrect. To see these ideas in action, you can check out our library of ready-to-use DPM CFD Simulation tutorials.
Figure 1: DPM Injection panel in ANSYS Fluent indicating particle type options
Comparing DPM Particle Types: A Quick Guide
Ansys Fluent gives you five main particle types to choose from. Each type has a different purpose and follows different physics rules. Before we explain them one by one, let’s look at a quick comparison.
The table below summarizes the five particle types. It shows the main job of each particle and gives a common example of when to use it. This table will help you make a fast decision for your simulation.
| Particle Type | Main Job (What it does) | Best For (Example Use) |
| Massless | Follows the fluid perfectly. It has no mass or size. | Seeing how long fluid stays in a system (residence time). |
| Inert | Moves and can get hot or cold. It does not change mass. | Simulating solid dust, sand, or other solid particles. |
| Droplet | A liquid drop that gets hot and turns into gas (evaporates). | Water sprays or simple fuel injection simulations. |
| Multicomponent | A liquid drop made of many parts. The parts can evaporate. | Complex fuels or chemical solutions that evaporate. |
| Combusting | A particle that releases gas and can burn. | Simulating the burning of coal, wood, or biomass. |
This table is a good start. In the next sections, we will explore each of these particle types in more detail to help you understand them better. Figure 2 shows how the physics becomes more complex as we move from one particle type to the next. Think of it like building blocks. An Inert particle is the base, with only forces and heat transfer. The Droplet type adds a new block: mass transfer from evaporation. The Combusting particle adds the most complex blocks: chemical reactions like devolatilization and char combustion. This image helps you see that each particle type builds on the physics of the simpler ones.
Figure 2: A summary of DPM particle types and their main physical behaviors, from Inert to Combusting particles.
More About Each Particle Type
Now, we will look at each particle type in more detail. We will explain what each particle does, its physics, and when you should use it in your simulation.
Massless DPM Particle
A massless particle is the simplest type in Fluent’s DPM. Think of it as a marker that just floats along with the fluid. It has no mass and no size. Because it has no weight, it is not affected by gravity or drag forces. It perfectly follows the exact path of the fluid flow.
We use this particle type for one main reason: to measure residence time. Residence time tells us how long it takes for the fluid to travel from an inlet to an outlet.
You should choose the massless particle type only when you want to track the fluid’s path and calculate its travel time. It is not used to simulate real particles.
Inert Particles
An inert particle is a solid particle that has mass and size. Unlike a massless particle, it is affected by forces. The fluid can push it (drag), and gravity can pull it down. Because of this, an inert particle has its own trajectory and does not follow the fluid path perfectly.
This particle type can also exchange heat with the fluid, so it can become hotter or colder. However, the word “inert” means it does not change in other ways. It cannot change mass by evaporating, and it cannot have a chemical reaction.
The inert particle type is the correct choice when you simulate solid particles that do not react, like dust, sand, or medical powders. A perfect example of this is simulating a medical inhaler. The goal is to see where the solid drug particles go inside the lungs. You can see a full example of this in our Dry Powder Inhaler in Lungs DPM Simulation tutorial, which uses inert particles to track the medicine.
Figure 3: Dry powder inhaler as a great example performed by CFDLAND using Inert DPM particle type
Droplet DPM Particle
A droplet is a small particle of liquid, like a water drop or a simple fuel drop. Like an inert particle, it has mass and size, and it exchanges heat with the fluid around it.
But the most important thing about a droplet is that it can lose mass. This happens through two processes:
- Evaporation: The liquid slowly turns into a gas when heated.
- Boiling: If the droplet gets hot enough to reach its boiling point, it turns into gas very quickly.
To simulate this, you must tell Fluent what the liquid is (for example, water) and what its gas form is (water vapor).
You should use the Droplet DPM Fluent model for any simulation of simple liquid sprays that evaporate. Note that, in order to activate droplet DPM particle type, you should enable species transport model to define a mixture of species. At least two species: one as the main material and the other one as evaporating species. Furthermore, you can manually specify the boiling and evaporating temperature from Material panel, as shown below.
Figure 4: Defining boiling and vaporization temperature at material`s panel
A great use for this is studying cooling systems. For example, you can see how a fine mist of water spray evaporates to cool down a hot surface. Our Evaporative DPM Cooling with Mist Spray tutorial shows exactly how this works.
Figure 5: Evaporative cooling of a mist spray – a great application of droplet injection type
Combusting DPM Particle
A combusting particle is a particle that can burn. This is the most complex particle type in DPM. It is used for solid fuels like coal, wood, or biomass. The burning process happens in several steps:
- First, if the particle is wet, the liquid (like water) evaporates.
- Next, as the particle gets hotter, it releases a flammable gas. This step is called devolatilization.
- Finally, the solid material that is left, called char, burns on its surface. This is a chemical reaction with a gas from the flow, like oxygen.
To set up a combusting particle injection in Fluent, you must define several important parameters:
- Material: You must choose a material (like coal) that has combustion properties defined.
- Devolatilizing Species: The gas that is released during devolatilization.
- Oxidizing Species: The gas in the main flow that is needed for the char to burn (usually oxygen, O2).
- Product Species: The gas that is created after the char burns.
If the fuel is wet, you must also turn on Wet Combustion and define the Evaporating Species (like H2O). The combusting particle type is the correct choice for any simulation where you need to model the burning of solid fuel particles.
Figure 6: Setting up a Combusting Particle in Fluent. This window shows where you define the material (anthracite coal), the species for devolatilization, and the species for the final char combustion
Multicomponent Particles
A multicomponent particle is a special type of droplet. It is a single liquid drop that is made from a mixture of different liquids. For example, diesel fuel is not one single liquid; it is a mix of many different hydrocarbon liquids.
The key feature of this particle type is that each liquid inside the droplet evaporates at its own rate. This is very important for accurately simulating complex liquid fuels or chemical solutions.
When you set up a multicomponent particle in Fluent, you need to tell the software about the mixture. In the Components tab, you must:
- Define each liquid component in the droplet.
- Set the starting Mass Fraction for each component. This tells Fluent how much of each liquid is in the drop at the beginning.
- Define the Evaporating Species for each component. This is the gas that each liquid turns into when it evaporates.
You should use the multicomponent particle type when your droplet is a mixture of liquids that evaporate at different rates. This is essential for high-accuracy simulations of fuels like gasoline, diesel, or kerosene.
Figure 7: The Components tab in Fluent for a multicomponent particle. Here you define the mass fraction of each liquid inside the droplet, like water (h2o) and urea (co2).
Conclusion: Choosing the Right DPM Particle
Choosing the correct particle type is a very important step in your DPM simulation. Your choice tells Fluent what physics to solve. This blog post helped you understand the five main particle types in Ansys Fluent.
Let’s quickly review the most important points:
- Use a Massless particle only to track the fluid’s path and find its travel time.
- Use an Inert particle for solid particles that have mass but do not change, like sand or dust.
- Use a Droplet particle for simple liquid drops that can evaporate, like a water spray.
- Use a Multicomponent particle for complex liquid drops made of a mixture, like diesel fuel.
- Use a Combusting particle for solid fuels that burn, like coal or biomass.
By understanding these options, you can now confidently select the right particle type for your CFD project and get accurate, meaningful results.