Numerical Investigation of Boundary Conditions on the Near-Injector Flow of a CH4-O2 Flame Using Finite-Rate Chemistry Modeling

Abstract

Methane–oxygen propulsion is increasingly attractive for reusable launch systems and in-space applications thanks to its storability, system-level benefits and lifecycle cost reduction. Yet, accurate prediction of flame stabilization and anchoring remains challenging due to strong real-fluid effects, steep density gradients and complex kinetics at supercritical pressures typical of the operating conditions of these engines. The flame’s position, whether attached to the injector or detached within the chamber, can profoundly impact engine stability, cooling efficiency and component lifetime.

This research numerically investigates flame anchoring and near-injector flow physics for a recessed, non-tapered shear-coaxial CH4/O2 element under high-pressure, cryogenic conditions typical of rocket engine operation, widely adopted in modern propulsion systems for its effective atomization and mixing characteristics. A finite-rate chemistry model is implemented within the DLR TAU solver. The ZhukovKong skeletal high-pressure methane combustion mechanism is employed and the Soave–Redlich–Kwong equation of state is applied to the injected propellants, while turbulence is modeled with a two-equation RANS approach and appropriate near-wall treatment. The study builds a dedicated 2D axisymmetric mesh to resolve anchoring and shear-layer gradients and then performs a parametric campaign varying methane and oxygen injection temperatures, chamber pressure, mixture ratio, LOX post thickness and tip thermal condition. A consistent analysis framework, based on OH mass fraction fields, density/temperature radial profiles and metrics for flame attachment and main swirl center, quantifies sensitivity across operating regimes.

The results highlight that methane injection temperature and mixture ratio dominate anchoring behavior. Very cold fuel promotes unsteadiness and localized lift-off, whereas sufficiently hot fuel can cause complete detachment. Raising the mixture ratio, at either fixed total mass flow or fixed LOX mass flow, thickens the reaction zone and shifts the anchoring point upstream toward the post. Post thickness systematically pushes the flame core downstream via a larger recirculation region, while oxygen injection temperature within practical ranges and the post-tip wall temperature have only minor effects on global topology. Rising chamber pressure strengthens attachment and shortens diffusion scales. To conclude, a comparative analysis between finite-rate chemistry and flamelet models reveals that the finite-rate approach produces sharper flame profiles with more accurate temperature and OH predictions, while the flamelet model tends to underestimate these quantities.