Experimental and Numerical Study of Water Vapor Diluted Methane-Oxygen Flames in a Micro-Mixing Injector

The semi-closed pure oxygen combustion cycle, utilizing water vapor as both the diluent and working medium, represents a crucial technology supporting future zero-emission advanced gas turbines. This study experimentally and numerically investigated the effect of water vapor H₂O(g) dilution on combustion characteristics and stability mechanism in an axial-tangential micro-mixing swirl nozzle. Firstly, the experimental results showed that the methane-oxygen flame was more sensitive to the flow rate of water vapor than the equivalence ratio. The increase in water vapor flow rate caused the flame to undergo the following transition process: anchored inside the nozzle, anchored at the nozzle exit, liftoff, and blowoff. Then, mixing characteristics were studied using Large Eddy Simulation (LES) code Boundary Fitted Flow Integrator (BOFFIN), which considered differential diffusion effects by incorporating species individual molecular diffusivities. The simulations demonstrated very good agreement with the experiments in terms of temperature and OH radical distributions. The results showed that with the increase of axial velocity, the flame was prone to liftoff, and even extinction due to the influence of the local extinction stretch rate. The difference in Damköhler numbers became noticeable as the mixing distance changes. It was also found that the flame could stabilize at the position, where the local radial velocity matched the global laminar flame speed. Additionally, the addition of water vapor changed the chain reaction of methane combustion, enhancing the production and consumption rate of O and H atoms, OH radical, thereby significantly influencing the laminar burning velocity and flame temperature.