Numerical investigation of the near-injector flow of a CH4-O2 flame using finite-rate chemistry modeling

Kurzfassung

Methane–oxygen propulsion is gaining attention for reusable launch systems and in-space applications. It offers favorable storage properties, system-level benefits, and the potential for lower lifecycle costs. Accurate prediction of flame stabilization and anchoring is essential for high-pressure rocket combustion. Small shifts in flame attachment strongly affect engine stability, injector durability, and thermal loads at the combustor inlet. These effects influence the entire engine cycle including turbopump behavior.

Predicting flame anchoring and stabilization behavior remains challenging, as high-pressure methane–oxygen combustion involves strong real-gas effects, steep density gradients, and complex turbulence–chemistry interactions. These challenges are further amplified in methane–oxygen systems compared to hydrogen, as methane shows slower chemical reactivity, exhibits narrower extinction limits, and is more sensitive to mixing and thermal conditions. As a result, reliable modeling of flame anchoring is substantially more difficult for methane.

This work numerically investigates flame anchoring and near-injector flow physics for a recessed, non-tapered shear-coaxial CH₄/O₂ element under high-pressure, cryogenic inlet conditions representative of rocket engines. Although, this injector type is widely adopted in modern propulsion systems for its effective atomization and mixing performance (largely rooted in hydrogen injector heritage), it shows a more complex flame-stabilization behavior when operated with methane. Since classical flamelet models fail to capture the relevant anchoring physics, a finite-rate chemistry formulation is employed, increasing computational cost.

Simulations are performed using the DLR TAU solver with the Zhukov–Kong high-pressure methane mechanism. Real-gas effects are modeled using the Soave–Redlich–Kwong equation of state, and turbulence is described by a two-equation RANS model with appropriate near-wall treatment. To enable broader parameter study at manageable cost, the configuration is reduced to a 2D axisymmetric setup. The subsequent parametric study examines the influence of methane and oxygen injection temperatures, chamber pressure, mixture ratio, LOX-post thickness, and injector-tip thermal condition. The evaluation is based on OH mass-fraction fields, radial density and temperature profiles, and metrics describing flame attachment and the dominant swirl center.

The results show that methane injection temperature and mixture ratio exert the strongest impact on anchoring behavior. Very cold fuel promotes unsteadiness and localized lift-off, whereas sufficiently hot fuel can lead to full flame detachment. Increasing the mixture ratio—at either fixed total mass flow or fixed LOX mass flow—thickens the reaction zone and shifts the anchoring position upstream toward the LOX post. A larger LOX-post thickness consistently moves the flame core downstream by enlarging the recirculation region. In contrast, oxygen injection temperature within practical limits and the post-tip wall temperature have only minor influence on overall flame structure. Higher chamber pressure strengthens flame attachment and reduces diffusion length scales.

These findings represent the initial stage of a broader investigation aimed at developing a more comprehensive understanding of methane-oxygen flame anchoring under realistic rocket-engine conditions. Ongoing work extends the analysis towards three-dimensional configurations, transient behavior, more complex geometries and multi-injector interactions.