The process of hydrogen-enriched methane mild combustion is when hydrogen is mixed with methane to make its combustion more efficient and lower the amount of pollution that is released. This method uses mild combustion conditions with lower peak temperatures and less NOx production, which makes it a better choice for the climate. In this 12th lesson of the ANSYS Fluent course for beginners, we will look at how to use ANSYS Fluent to simulate this process using computational fluid dynamics (CFD). The study is based on a reference paper from the FUEL journal called “Effect of hydrogen-blending ratio and wall temperature on establishment, NO formation, and heat transfer of hydrogen-enriched methane MILD combustion.” This paper shows how changing the hydrogen ratio and wall temperature can change the characteristics of combustion.

• Reference [1]: Xu, Shunta, et al. “Effect of hydrogen-blending ratio and wall temperature on establishment, NO formation, and heat transfer of hydrogen-enriched methane MILD combustion.” Fuel369 (2024): 131787.

Figure 1: Computational grid of MILD Combustion CFD Simulation

Simulation process

To properly model the hydrogen-enriched methane MILD combustion process, the simulation method includes a number of important steps. First, the exact geometrical configuration data is extracted using the Plotdigitizer software. Next, an ANSYS Design Modeler model is made with specific size factors. The shape is then carefully meshed with ANSYS Meshing to make a structured grid for computing. The species transport model is used to look at the volumetric reactions, and the Eddy Dissipation Concept (EDC) model is used to show how the turbulence and chemistry interacts. The Discrete Ordination (DO) model is used to look at radiative heat transfer, and a refined Weighted Sum of Gray Gases Model (WSGGM) is added to make the estimates for thermal radiation more accurate. Because the system is symmetrical, an axisymmetric planar mode is used to make the computer study of the basic reaction easier:

CH4 + H2 + O2 → CO2 + H2O

Figure 2: structured grid for Hydrogen-enriched Methane MILD Combustion CFD Simulation | ANSYS Fluent Tutorial

Post-processing

The velocity and species distribution trends show unique features of the hydrogen-enriched methane MILD combustion system that works with a fuel mix of 25% H2 and 75% CH4 and an equivalence ratio (φ) of 0.86. With fuel entering at 17.2 m/s and air entering at 16.1 m/s, the velocity contours show a well-developed flow field. The CH4 mass fraction contours show that the fuel is being used rapidly in the reaction zone. The concentration is largest near the inlet and gradually decreases as you move downstream. This confirms that the mixing and burning processes are working well, which is typical of MILD combustion regimes.

Figure 3: Flame velocity inside combustion chamber

The contours of the temperature distribution show a reaction zone that is uniform and spread out, with peak temperatures that stay in the MILD combustion range. This pattern of temperatures is typical of the MILD regime; there is no clear flame front, just a reaction zone that changes temperatures slowly. There should not be any sharp temperature differences and there should be a volumetric reaction zone. These are signs of good MILD combustion establishment, which is made even better by adding hydrogen to the fuel mixture. The temperature field shows that the heat is evenly spread throughout the combustion chamber. This suggests that the heat is working well and that NOx formation is less likely because the temperature peaks are mild, which is typical of MILD combustion.

Figure 4: Flame temperature distribution inside combustion chamber – Mild combustion