Oxygen is an essential reactant in the processes of combustion. Oxygen supply affects combustion efficiency, flame temperature, combustion completeness, and emission production. Insufficient oxygen can cause incomplete combustion, releasing carbon monoxide and unburned hydrocarbons. Conversely, abundant oxygen can boost combustion efficiency but also increase thermal NOx generation. To maximize performance and reduce environmental impact, combustion oxygen concentration must be understood and optimized.

In this study, we investigate the oxygen effect on combustion by means of ANSYS Fluent. The study intends to seek two case results in which there is a 10% oxygen difference. Thus, both are assumed to complete reactions.

Figure 1: Combustion chamber schematic

Simulation process

The cylindrical shape of the combustion chamber is produced by considering a 2D section of it revolving around the axis of rotation, utilizing an Axisymmetric tool. The generation of a structured grid has helped create a high-quality grid.

The volumetric reaction of methane and air is modeled by Species Transport module. The combustion turbulence model is Eddy Dissipation. Fuel and air are injected from two different nozzles and then meet each other in the combustion chamber.

Post-processing

Through two cases with a 10% oxygen difference, the CFD modeling results show how important oxygen concentration is to the combustion process. The O2 mass fraction outlines show clear patterns of how oxygen is distributed and used in the combustion chamber. In the base case, the oxygen mass fraction is between 0.000 and 0.2591, which shows that the areas where the fuel and air streams meet are well mixed and respond. You can clearly see the reaction zone in the middle of the chamber. It’s marked by a slow drop from high to low oxygen levels, which shows that oxygen is being used up in the combustion process.

Figure 2: Oxygen mass fraction in two cases

Figure 3: Temperature field in two cases – with & without 10% excess air

Temperature contours, which show that the reaction zone has highest temperatures of about 1805K, provide more proof of the oxygen effect. When you compare the two cases, you can see that the case with 10% extra air has higher oxygen levels in the exit area because the extra oxygen flows through the combustion chamber without joining the reaction. Even though this extra air makes sure that the fuel burns completely, the high temperatures (above 1600K) in the combustion zone could cause more NOx to form. The study effectively shows that while enough oxygen is necessary for complete combustion, too much air may not improve combustion efficiency and may instead lead to the production of unwanted emissions. This suggests that the right amount of oxygen should be kept at all times for optimal performance.