Non-premixed combustion is the driving force behind many power systems, from diesel engines to gas turbines. In these systems, fuel and air enter the chamber separately and must mix to burn. The process becomes much more complex when we use Liquid Fuel Combustion. The liquid fuel, such as Pentane (C5H12), is injected as a spray of tiny droplets. These droplets must first heat up and evaporate into gas before they can mix with air and ignite.

Understanding this sequence—injection, evaporation, mixing, and burning—is vital for designing clean engines. A Fuel Droplet Combustion CFD simulation is the best way to study these physics. It allows engineers to track individual droplets and see how they interact with the flame. This tutorial demonstrates a Fuel DPM Spray Fluent simulation where we model the injection of liquid pentane. We utilize the Discrete Phase Model (DPM) to track the droplets. For more resources on reacting flows, please explore our Combustion CFD Simulation category.

Figure 1: C5H12 spray combustion inside the chamber simulated using the DPM model in Fluent.

Simulation Process: DPM and Non-Premixed Modeling in Fluent

The simulation process began with creating a 2D axisymmetric geometry. Modeling only half of the chamber and using the axis of symmetry significantly reduces the calculation time while maintaining accuracy. We generated a high-quality structured mesh consisting of 5200 cells. A structured grid is preferred here because it aligns well with the flow direction, ensuring that the Fuel Droplet CFD calculations are precise.

The setup for this Liquid fuel combustion simulation in ANSYS Fluent involved two key models. First, we activated the Non-premixed Combustion model. This model handles the chemistry efficiently using a Probability Density Function (PDF) table. It calculates the temperature and species based on the mixture fraction, which tracks how much fuel is mixed with the air. Second, we enabled the Discrete Phase Model (DPM) to simulate the Fuel DPM spray. This uses a Lagrangian approach, meaning the software tracks the path of individual fuel particles. Crucially, we enabled two-way DPM coupling. This allows the droplets to affect the gas (by cooling it during evaporation and adding mass) and the gas to affect the droplets (by heating them and moving them). This two-way interaction is essential for a realistic Fuel Droplet DPM Fluent analysis.

Post-Processing: Analyzing Evaporation and Flame Structure

In this section, we analyze the results of the Fuel Droplet Combustion CFD simulation to understand the thermal and chemical behavior of the spray. The temperature contour in Figure 2a is the primary indicator of combustion performance. The red zone near the injector represents the heart of the flame. The simulation calculates a peak temperature of 2280 K in this region. This intense heat occurs because the evaporated fuel vapor is mixing with the air in near-perfect stoichiometric proportions. This indicates that the Fuel DPM spray is evaporating quickly and reacting efficiently. As the hot gases move downstream (to the right), the temperature gradually drops. This cooling is due to heat diffusion and the fact that the reaction is completing. This thermal map helps engineers predict if the combustor walls will overheat.

Figure 2: Contours showing a) Temperature and b) Velocity patterns inside the chamber from the non-premixed combustion simulation.

We gain deeper insights by looking at the interaction between the droplets and the reaction products in Figure 3. Figure 3a shows the Mass Fraction of H2O (water vapor), which is a main product of burning hydrocarbons. The peak concentration is 7.56e-02. Notice that this high concentration matches perfectly with the high-temperature zone in Figure 2. This confirms that water is being produced exactly where the fuel is burning.

Figure 3b displays the DPM Evaporation Rate. This contour is critical for understanding Liquid fuel combustion. The evaporation rate is highest (Red) in the upstream region, right inside the flame zone. This demonstrates the “coupling” physics: the heat from the combustion (2280 K) provides the energy to boil the liquid droplets. These droplets turn into vapor, which then burns to keep the flame hot. If the droplets did not evaporate here, the flame would extinguish. This Fuel Droplet CFD simulation successfully captures this self-sustaining cycle of evaporation and combustion.

Figure 3: Contours showing a) H2O Mass Fraction and b) DPM evaporation rate, highlighting the link between fuel evaporation and product formation.

Key Takeaways & FAQ

• Q: What is the Discrete Phase Model (DPM) in ANSYS Fluent?
  • A: DPM is a modeling approach used for multiphase flows where one phase (like liquid fuel) is sparsely distributed as particles or droplets in a continuous phase (like air). It tracks the trajectory, heating, and evaporation of these discrete particles using a Lagrangian framework.
• Q: Why is “Two-way Coupling” important in spray combustion?
  • A: In a spray flame, the gas heats the droplets (one way), but the droplets also cool the gas as they evaporate and add fuel vapor to it (the other way). Two-way coupling ensures the simulation calculates both effects, which is physically necessary for accurate temperature and evaporation predictions.
• Q: What is Non-premixed Combustion?
  • A: It is a type of combustion where fuel and oxidizer enter the chamber in separate streams. The burning rate is limited by how fast they can mix. This is common in diesel engines and direct-injection burners.