Computational investigation of real fluid effects in cryogenic laminar premixed CH₄–O₂ flames

This study investigates the effects of real fluid thermodynamic and transport properties on the structure and propagation of canonical planar, unstretched, premixed methane-oxygen flames at transcritical conditions, which is relevant to the ignition phase in a liquid rocket engine. The ideal gas law is inapplicable at such extreme conditions, and real fluid thermodynamic and transport properties are required to accurately model the combustion physics at supercritical conditions. The computational framework used in this study integrates real fluid property estimation into the steady-state, freely-propagating flame solver available in the Cantera combustion suite [Goodwin et al., 2017]. The Peng-Robinson equation of state provides closure for the thermodynamic terms in the governing equations. High-pressure thermal conductivity and mass diffusivity coefficients are obtained using the Chung and Takahashi correlations, respectively. A mixture-averaged mass diffusion model is assumed, while neglecting the Soret and Dufour effects. The reaction rate terms are modeled with the standard Arrhenius ideal gas kinetics model using the CH₄–O₂ mechanism from Lindstedt. The effects on laminar flame structure are presented. It is found that the choice of chemical mechanism has a much greater influence on mass burning rates than the real fluid effects. Real fluid properties produce lower mass burning rates by just ∼10% near the critical region, while the laminar flame speeds are reduced by a factor of ∼5. It is shown that using dilute gas transport properties in combination with a real fluid equation of state and differential diffusion effects may cause a qualitatively different, non-monotonic density profile in the flame.