Mild Combustion, also known as Flameless Oxidation (FLOX) or High Temperature Air Combustion (HiTAC), is a revolutionary way to burn fuel. In this process, the incoming air is preheated to a very high temperature, and a large amount of hot exhaust gas is recirculated back into the chamber. This creates a unique environment where the reaction happens in a large, distributed zone rather than a thin, visible flame. This leads to lower peak temperatures and significantly reduces harmful pollutants like NOx.

Because the physics are so complex, a Mild Combustion CFD Validation is essential. We must prove that our computer models can match real-world experiments. The goal of this project is to simulate the combustion of syngas fuel and validate our results against the experimental data from the research paper by Huang et al. [1]. For those who want to learn the basics of reacting flows, please check our combustion tutorials.

• Reference [1]: Huang, Ming-ming, et al. “Effect of fuel injection velocity on MILD combustion of syngas in axially-staged combustor.” Applied thermal engineering1-2 (2014): 485-492.

Figure 1: The geometry and structured grid from the reference paper [1] used for validation.

Simulation Process: Chemkin Mechanism in Fluent

To perform a successful Mild Combustion Fluent simulation, we first built the geometry. The combustion chamber is symmetric, so we only needed to model a slice of it. This saves computational time. We then created a high-quality structured grid in ANSYS Meshing. A good mesh is critical for validation studies because it minimizes numerical errors.

The core of this Mild Combustion Simulation is the chemistry setup. Standard combustion models are often not detailed enough for MILD regimes. Therefore, we used the Species Transport model and coupled it with a Chemkin Mechanism. Using Chemkin Mechanism fluent integration allows us to import a detailed file containing all the specific chemical species and reaction pathways for syngas. This level of detail is necessary to accurately predict the slow, distributed oxidation process that characterizes Mild Combustion.

Figure 2: The structured mesh created for the combustion chamber domain.

Post-processing: Validating the Flameless Regime

The most critical part of this study is the validation against experimental results. We compared our Mild Combustion ANSYS fluent simulation data directly with the measurements from the reference paper. Figure 3 shows a graph of the oxygen mass fraction along the centerline of the burner. The agreement is remarkable. We can see that the oxygen level drops smoothly from an initial value of about 0.187 down to near zero as it is consumed by the reaction. This gradual consumption is the signature of the MILD regime. When we compare our curve (the solid line) with the experimental points (the dots), the maximum error is less than 5%. This proves that the Chemkin Mechanism is accurately calculating the reaction rates.

Figure 3: Validation graph comparing the simulated oxygen mass fraction with experimental data (Error < 5%).

Figure 4 provides a visual confirmation of the “flameless” nature of the combustion. In a standard flame, you would see a sharp, thin line where the reaction happens. However, our Mild Combustion CFD results show a very different pattern. The oxygen mass fraction contour reveals a large, spread-out reaction zone. The oxygen concentration changes gradually across the chamber, indicating that the reaction is distributed over a wide volume. Specifically, this reaction zone begins approximately 15 to 20 jet diameters downstream from the fuel inlet. We compared this location and the size of the zone with the experimental findings, and they match with an accuracy of within 3%. This high level of precision confirms that our Validation of CFD combustion models is successful. It demonstrates that by using the right detailed chemistry solvers in ANSYS Fluent, we can reliably predict the complex behavior of clean, flameless combustion systems.

Figure 4: Oxygen mass fraction contour showing the distributed reaction zone.

Key Takeaways & FAQ

• Q: What defines Mild Combustion?
  • A: Mild Combustion is defined by a distributed reaction zone, low peak temperatures, and no visible flame front. It is achieved by preheating the air and recirculating exhaust gases, which dilutes the oxygen and slows down the reaction rate.
• Q: Why use a Chemkin Mechanism in Fluent?
  • A: Standard “fast chemistry” models assume fuel burns instantly. In Mild Combustion, the reaction is slower and depends on specific intermediate species. A Chemkin Mechanism provides the detailed chemical kinetics needed to accurately predict ignition delay and pollutant formation in ANSYS Fluent.
• Q: How accurate is this validation study?
  • A: This Mild Combustion CFD Validation is highly accurate. The simulation results for oxygen mass fraction matched the experimental data with a maximum error of less than 5%, and the location of the reaction zone matched within 3%.